Content:
Presentation type:
OS – Ocean Sciences

EGU24-2879 | Orals | OS3.2 | Highlight | Fridtjof Nansen Medal Lecture

Future trends and climate feedbacks of the biological carbon pump 

Stephanie Henson

The biological carbon pump is a series of processes that transfers organic carbon from the surface ocean into the deep ocean.  Without it, atmospheric CO2 levels would be ~ 50 % higher than pre-industrial levels.  Despite its importance, we currently struggle to understand how the strength and efficiency of the biological carbon pump varies temporally and spatially.  This makes it difficult to observe, and therefore model the pump, so our knowledge of how this important component of the global carbon cycle might respond to climate change is poor.  In this talk I’ll present recent progress on using autonomous vehicles to quantify variability in the biological carbon pump, discuss the current limitations in our understanding of the pump, and the implications of those knowledge gaps for robust modelling of the current and future pump. 

How to cite: Henson, S.: Future trends and climate feedbacks of the biological carbon pump, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2879, https://doi.org/10.5194/egusphere-egu24-2879, 2024.

EGU24-10184 | ECS | Orals | OS1.6 | Highlight | OS Division Outstanding ECS Award Lecture

The global influence of ice-ocean interactions in Antarctica 

Alessandro Silvano

In this seminar, I will explore the oceanic processes that drive melting of the Antarctic Ice Sheet, and consequent global sea level rise. Different processes lead certain areas of the Antarctic Ice Sheet to be more susceptible to rapid ocean-driven melting, while other areas to be more resilient. I will also show the emergence of a feedback between the ice sheet and Southern Ocean: increased melting leads to warming of the oceanic waters surrounding Antarctica, with consequences for future sea level rise. I will conclude by describing how increased melting of the Antarctic Ice Sheet as well as changes in sea ice affect the global ocean abyss and its ability to store anthropogenic heat and carbon.

How to cite: Silvano, A.: The global influence of ice-ocean interactions in Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10184, https://doi.org/10.5194/egusphere-egu24-10184, 2024.

OS1 – Ocean Circulation and Climate

EGU24-775 | ECS | Posters on site | OS1.1

The rapid life of Arctic sea-ice ridge consolidation and melt 

Evgenii Salganik, Benjamin Lange, Christian Katlein, Ilkka Matero, Dmitry Divine, Polona Itkin, Knut Høyland, and Mats Granskog

In this study, we cover observations of the rapid consolidation and enhanced melt of Arctic sea-ice ridges. During the freezing period, the consolidated part of sea ice ridges is usually up to 1.6–1.8 times thicker than surrounding level ice. Meanwhile, during the melt season, ridges are often observed to be fully consolidated, but this process is not fully understood. We present the evolution of the morphology and temperature of a first-year ice ridge studied during MOSAiC from its formation to advanced melt. From October to May, the draft of first-year ice at the MOSAiC coring site increased from 0.3 m to 1.5 m, while from January to July, the consolidated layer thickness in the ridge reached 3.9 m. We observed several types of ridge consolidation. From the beginning of January until mid-April, the ridge consolidated slowly through heat loss to the atmosphere, with a total consolidated layer growth of 0.7 m. From mid-April to mid-June, there was a rapid increase in ridge consolidation rates, despite conductive heat fluxes not increasing. In this period, the mean thickness of the consolidated layer increased by 2.2 m. We also estimated a substantial snow mass fraction (6%–11%) of ridges using analysis of oxygen isotope composition. Our observations suggest that this sudden change was related to the transport of snow-slush inside the ridge keel via adjacent open leads that decreased ridge macroporosity, which could result in more rapid consolidation.

During the summer season, sea ice melts from the surface and bottom. The melt rates substantially vary for sea ice ridges and undeformed first- and second-year ice. Ridges generally melt faster than undeformed ice, while the melt of ridge keels is often accompanied by further summer growth of their consolidated layer, which increases their survivability. We examined the spatial variability of ice melt for different types of ice from in situ drilling, coring, and multibeam sonar scans of the remotely operated underwater vehicle. Six sonar scans performed from 24 June to 21 July were analyzed and validated using seven ice drilling transects. The area investigated by the sonar (0.4 km by 0.2 km) consisted of several ice ridges, surrounded by first- and second-year ice. We show a substantial difference in melt rates for sea ice with a different draft. We also show how ridge keels decay depending on the keel draft, width, steepness, and location relative to the surrounding ridge keel edges. We also use temperature buoy data to distinguish snow, ice surface, and bottom melt rates for both ridges and level ice. These results are important for quantifying ocean heat fluxes for different types of ice during the advanced melt and for estimating the ridge contribution to the total ice mass and summer meltwater balances of the Arctic Ocean.

How to cite: Salganik, E., Lange, B., Katlein, C., Matero, I., Divine, D., Itkin, P., Høyland, K., and Granskog, M.: The rapid life of Arctic sea-ice ridge consolidation and melt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-775, https://doi.org/10.5194/egusphere-egu24-775, 2024.

EGU24-929 | ECS | Posters on site | OS1.1 | Highlight

Potential effects of summer Cryosat-2 sea ice thickness observations on sea ice forecast 

Ruizhe Song, Longjiang Mu, Xianyao Chen, Frank Kauker, Svetlana Loza, and Martin Losch

Skillful Arctic sea ice forecast for the melting season remains a great challenge because there is no reliable pan-Arctic sea ice thickness (SIT) data set for the summertime. A new summer Cryosat-2 SIT observation data set based on an artificial intelligence algorithm may mitigate the situation. We assess the impact of this new data set on the initialization of both short-term and long-term sea ice forecasts in the melting seasons of 2015 and 2016 in a sea-ice couple model with data assimilation. We find that the assimilation of the new summer CryoSat-2 SIT observations can reduce the summer ice edge prediction error. Further, adding SIT observations to an established forecast system with sea ice concentration assimilation leads to a more realistic short-term summer ice edge forecast in the Arctic Pacific sector. The long-term Arctic-wide SIT prediction is also improved especially before the onset of freezing. In spite of remaining uncertainties,  summer CryoSat-2 SIT observations have the potential to enhance Arctic sea ice forecast on multiple time scales.

How to cite: Song, R., Mu, L., Chen, X., Kauker, F., Loza, S., and Losch, M.: Potential effects of summer Cryosat-2 sea ice thickness observations on sea ice forecast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-929, https://doi.org/10.5194/egusphere-egu24-929, 2024.

The Arctic region is experiencing a notable increase in precipitation, known as Arctic wetting, amidst the backdrop of Arctic warming. This phenomenon has implications for the Arctic hydrological cycle and numerous socio-ecological systems. However, the ability of climate models to accurately simulate changes in Arctic wetting has not been thoroughly assessed. In this study, we analyze total precipitation in the Arctic using station data, multiple reanalyses, and 35 models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). By employing the moisture budget equation and an evaluation method for model performance with ERA5 reanalysis as a reference, we evaluated the models’ capability to reproduce past Arctic wetting patterns. Our findings indicate that most reanalyses and models are able to replicate Arctic wetting. However, the CMIP6 models generally exhibit an overestimation of Arctic wetting during the warm season and an underestimation during the cold season from 1979 to 2014 when compared to the ERA5 reanalysis. Further investigation reveals that the overestimation of wetting during the warm season is largest over the Arctic Ocean’s northern part, specifically the Canadian Arctic Archipelago, and is associated with an overestimation of atmospheric moisture transport. Conversely, the models significantly underestimate wetting over the Barents-Kara Sea during the cold season, which can be attributed to an underestimation of evaporation resulting from the models’ inadequate representation of sea ice reduction in that region. The models with the best performance in simulating historical Arctic wetting indicate a projected intensification of Arctic wetting, and optimal models significantly reduce uncertainties in future projections compared to the original models, particularly in the cold season and oceanic regions. Our study highlights significant biases in the CMIP6 models’ simulation of Arctic precipitation, and improving the model’s ability to simulate historical Arctic precipitation could reduce uncertainties in future projections.

How to cite: Cai, Z. and You, Q.: Arctic wetting: Performances of CMIP6 models and projections of precipitation changes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1673, https://doi.org/10.5194/egusphere-egu24-1673, 2024.

EGU24-1892 | ECS | Orals | OS1.1

Observations of deep near-inertial internal waves in the Eurasian Basin. 

Joel Bracamontes Ramírez and Maren Walter

The Arctic Ocean has a less energetic internal wave climate than other oceans, mainly due to the thick sea ice cover which inhibits wind interaction with the surface. With the continued decrease in summer sea ice extent and the increase in seasonal ice-free areas, wind-driven internal waves, especially in the near-inertial range, are becoming more energetic. Coupled with the fact that most of the Arctic Ocean lies north of the critical latitude for semi-diurnal tides, the shift in ice dynamics implies an increase in the importance of near-inertial waves (NIW) for the internal wave climate. In particular, increased NIW amplitude and kinetic energy in the Canadian Basin and enhanced wind-driven vertical heat fluxes and dissipation rates in the Eurasian Basin have already been observed in the upper column. In the deep ocean beyond the critical latitude, NIWs are expected to drive mixing in the interior, but it is unclear to what extent. Here, we present innovative and unprecedented deep current observations from a mooring in the Gakkel Ridge in the Eurasian Basin at 82.53°N. The presence of barotropic diurnal and semi-diurnal tides and semi-diurnal harmonics enriches the complex interplay of internal waves. By comparing the observed downward and upward NIW kinetic energy with wind speed, sea ice properties and numerical simulations, we discuss the likely surface origin of the NIW. In particular, there is a lagged correlation of <26 days between ice drift speed and downward NIW energy, and of ~15 days between wind factor and downward NIW energy. In addition, the buoyancy frequency is weaker than the local Coriolis frequency, effectively limiting NIW propagation. Evidence for wave reflection is found and also discussed, with a focus on the implications for NIW coming from the surface.

How to cite: Bracamontes Ramírez, J. and Walter, M.: Observations of deep near-inertial internal waves in the Eurasian Basin., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1892, https://doi.org/10.5194/egusphere-egu24-1892, 2024.

The Arctic is one of Earth’s regions most susceptible to climate change. Climate models show that in a warming climate, the Arctic Ocean warms much faster than the global ocean mean, mainly due to the rapid warming of the Atlantic layer, which is called 'Arctic Ocean Amplification.' However, climate models still encounter challenges with large biases and considerable inter-model spread in the Arctic Ocean. For example, the Atlantic layer in the Arctic Ocean, simulated by the climate models, is too thick and too deep. This leads to the warming trend, and inter-annual variability of the simulated Atlantic Water that are too small compared to the observations. Here, we present Arctic Ocean dynamical downscaling simulations and projections based on a high-resolution ice-ocean coupled model, FESOM, and a climate model, FIO-ESM. The historical results demonstrate that the root mean square errors of temperature and salinity in the downscaling simulations are much smaller than those from CMIP6 climate models. The common biases, such as the overly deep and thick Atlantic layer in climate models, are significantly reduced by dynamical downscaling. Dynamical downscaling projections show that the Arctic Ocean may warm faster than the projections made by CMIP6 fully-coupled climate models.

How to cite: Shu, Q. and Wang, Q.: Dynamical downscaling simulations and future projections of the Arctic Ocean based on FESOM and FIO-ESM., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1921, https://doi.org/10.5194/egusphere-egu24-1921, 2024.

EGU24-1922 | Posters on site | OS1.1

Estimation of Sea Ice Production in the North Water Polynya Based on Ice Arch Duration in Winter During 2006–2019 

Fengming Hui, Haiyi Ren, Mohammed Shokr, Xiao Cheng, Xinqing Li, and Zhilun Zhang

The North Water Polynya (NOW) is the largest recurrent Arctic coastal polynya. The formation of the NOW is critically dependent on the development of an ice arch that defines its northern boundary. In this study, high-resolution ENVISAT Advanced Synthetic Aperture Radar data, Sentinel-1A data, and Moderate Resolution Imaging Spectroradiometer data were employed to identify the spatio-temporal characteristics of the ice arch during 2006–2019. Polynya pixels were identified based on the thin ice thickness (TIT), using a threshold of TIT <0.2 m, from which the polynya extent, heat flux, and ice production (IP) were estimated. The results show the different locations of the ice arch in different years, with a mean duration of 132 ± 69 days. The average annual polynya extent over the 14 years is ∼38.8 ± 8 × 103 km2, and we found that it is more closely correlated with wind speed during the winter and air temperature during early spring. The average heat flux drops from about 248 W/m2 in the winter months to about 34 W/m2 in May. The average accumulated IP varies significantly every year, with an average of 144 ± 103 km3, and peak values in March in most years. No apparent interannual trends are shown for the polynya area, heat flux, and IP during 2006–2019. The results also show that IP calculated based on the ice arch data is approximately 25% lower than that obtained by assuming a fixed time, location, and duration for the polynya.

How to cite: Hui, F., Ren, H., Shokr, M., Cheng, X., Li, X., and Zhang, Z.: Estimation of Sea Ice Production in the North Water Polynya Based on Ice Arch Duration in Winter During 2006–2019, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1922, https://doi.org/10.5194/egusphere-egu24-1922, 2024.

Sea ice in the high latitude is an indicator of climate change and has undergone dramatic changes because of recent global warming. Synthetic aperture radar (SAR) is a relatively practical tool for sea ice monitoring because of its low sensitivity to clouds, rain, and fog, as well as its capability for high-resolution earth observation in daylight or darkness. With the progression in SAR systems from single-pol to dual-pol, quad-pol and hybrid-pol, large numbers of parameters have been proposed for sea ice classification. Even though a large number of SAR characteristics have been used to classify sea ice, it remains unclear which parameters are the most effective for different regions and seasonal or environmental conditions. Meanwhile, classification studies for fine sea ice with high spatial resolution and many sub-types of sea ice, particularly in the case of rapidly changing first-year ice (FYI), which includes new ice (NI), young ice (YI), and FYI, are rather few. NI and YI have comparatively thinner thickness, and are often classified as FYI in these studies[1].

A new method of sea ice classification based on feature selection from Gaofen-3 polarimetric SAR observations is proposed. The new approach classifies sea ice into four categories: open water, NI, YI, and FYI. Seventy parameters that have previously been applied to sea ice studies are re-examined for sea ice classification in the Okhotsk Sea near the melting point on 28 February 2020. The ‘separability index’ is used for the selection of optimal features for sea ice classification. Full polarization (σohh, SEi, Ks) and hybrid polarization parameters (σorl, CPSEirh-rv, αs) are determined as optimal. The selected parameters are used to classify NI, YI, and FYI using a SVM machine learning classifier; and classification results are validated by manually interpreted ice maps derived from Landsat-8 data.

 


[1]Sea ice: types and forms - Canada.ca

How to cite: Li, H. and Yang, K.: Fine resolution classification of new ice, young ice, and first-year ice based on feature selection from Gaofen-3 quad-polarization SAR, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2666, https://doi.org/10.5194/egusphere-egu24-2666, 2024.

EGU24-2699 | Posters on site | OS1.1

Changes in ocean circulation and dissolved oxygen/nutrient distributions in the Canadian Basin 

Shigeto Nishino, Jinyoung Jung, Kyoung-Ho Cho, William Williams, Amane Fujiwara, Akihiko Murata, Motoyo Itoh, Eiji Watanabe, Mariko Hatta, Michiyo Yamamoto-Kawai, Takashi Kikuchi, Eun Jin Yang, and Sung-Ho Kang

The Arctic Ocean is facing dramatic environmental and ecosystem changes. To obtain the current baseline data, a coordinated multiship and multination pan-Arctic ship-based sampling campaign was implemented for the period between 2020 and 2022 under the project of Synoptic Arctic Survey (SAS). During the 2020 survey, unusually low dissolved oxygen and acidified water (salinity = 34.5) were found in a high-seas fishable area of the western (Pacific-side) Arctic Ocean. The data showed that the Beaufort Gyre (BG) shrunk to the east of the Chukchi Plateau (CP) and formed a front between the water within the gyre and the water from the eastern (Atlantic-side) Arctic. That phenomenon triggered a frontal northward flow along the CP. This flow likely transported the low oxygen and acidified water toward the high-seas fishable area; similar biogeochemical properties had previously been observed only on the shelf-slope north of the East Siberian Sea (ESS). Northward flows were also predominant west of the CP associated with the penetration of the water from the eastern Arctic. The northward flows would transport nutrient-rich shelf water (salinity = 32.5) from the ESS to the southwestern Canadian Basin (CB). Furthermore, the northeastward flow of the shrunk BG during the SAS period (2020-2022) could spread the nutrient-rich ESS shelf water to the northeastern CB. As a result, the nutrient concentration there during the SAS period was higher than the period when the BG enlarged to the west of CP, because the westward flow of the BG that overshot the CP inhibited the northward transport of the nutrient-rich ESS shelf water toward the southwestern CB. As a future study, we would like to combine the data from the Atlantic gateway because the ocean circulation and dissolved oxygen/nutrient distributions in the CB are largely influenced by the penetration of the water from the eastern Arctic. This is a reason why we have applied to present in this session.

How to cite: Nishino, S., Jung, J., Cho, K.-H., Williams, W., Fujiwara, A., Murata, A., Itoh, M., Watanabe, E., Hatta, M., Yamamoto-Kawai, M., Kikuchi, T., Yang, E. J., and Kang, S.-H.: Changes in ocean circulation and dissolved oxygen/nutrient distributions in the Canadian Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2699, https://doi.org/10.5194/egusphere-egu24-2699, 2024.

EGU24-3080 | Posters on site | OS1.1

No emergence of deep convection in the Arctic Ocean across CMIP6 models 

Céline Heuzé and Hailong Liu

As sea ice disappears, the emergence of open ocean deep convection in the Arctic, which would enhance ice loss, has been suggested. Here, using 36 state-of-the-art climate models and up to 50 ensemble members per model, we show that Arctic deep convection is rare under the strongest warming scenario. Only 5 models have convection by 2100, while 11 have had convection by the middle of the run. For all, the deepest mixed layers are in the eastern Eurasian basin. When the models convect, that region undergoes a salinification and increasing wind speeds; it is freshening otherwise. The models that do not convect have the strongest halocline and most stable sea ice, but those that lose their ice earliest -because of their strongly warming Atlantic Water- do not have a persistent deep convection: it shuts down mid-century. Halocline and Atlantic Water changes urgently need to be better constrained in models.  

How to cite: Heuzé, C. and Liu, H.: No emergence of deep convection in the Arctic Ocean across CMIP6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3080, https://doi.org/10.5194/egusphere-egu24-3080, 2024.

EGU24-3635 | Orals | OS1.1

Drivers of interannual salinity variability in the Arctic Ocean 

Antoine Hochet, Camille Lique, Florian Sévellec, and William Llovel

Accurate projections and attributions of Arctic ocean changes in climate models require a good understanding of the mechanisms underlying interannual salinity variability in the region. Although some mechanisms have been extensively studied in idealized settings, in particular for the dynamics of the Beaufort Gyre (BG), their applicability to the more complex system remains unclear. This study introduces a new diagnostic based on the salinity variance budget to robustly assess the mechanisms of salinity variations. The diagnostic is then applied to the Estimating the Circulation and Climate of the Ocean state estimate. 
The results indicate that the advection of salinity anomalies in the direction of the mean salinity gradient, produced by velocity anomalies is the primary source of interannual salinity variability. These velocities are primarily caused by fluctuating winds via Ekman transports.
Fluctuating surface freshwater fluxes from the atmosphere and sea ice are the second most important source of variability and cannot be neglected. The two sinks of interannual salinity variance are associated with the erosion of large-scale  mean circulation gradients by eddies and to a lesser extent to the diffusive terms. Over continental shelves, particularly over the East Siberian Shelf (ESS), ocean surface freshwater fluxes and diffusion play a more important role than in the deep basins.
We also report a strong intensification of all sources and sinks of interannual salinity variability in the BG and an opposite weakening in the ESS in the second decade of the analysis (2004-2014) with respect to the first (1993-2003). 

How to cite: Hochet, A., Lique, C., Sévellec, F., and Llovel, W.: Drivers of interannual salinity variability in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3635, https://doi.org/10.5194/egusphere-egu24-3635, 2024.

EGU24-3752 | ECS | Posters on site | OS1.1

Latitudinal distribution of biomarkers across the western Arctic Ocean and the Bering Sea: an approach to assess sympagic and pelagic algal production 

Youcheng Bai, Marie-Alexandrine Sicre, Jian Ren, Vincent Klein, Haiyan Jin, and Jianfang Chen

The drastic decline of Arctic sea ice due to global warming and polar amplification of environmental changes in the Arctic basin profoundly alter primary production with consequences for polar ecosystems and the carbon cycle. In this study, we use highly branched isoprenoids (HBIs), brassicasterol, dinosterol and terrestrial biomarkers (n-alkanes and campesterol) in surface sediments to assess sympagic and pelagic algal production with changing sea-ice conditions along a latitudinal transect from the Bering Sea to the high latitudes of the western Arctic Ocean. Suspended particulate matter (SPM) was also collected in surface waters at several stations of the Chukchi Sea to provide snapshots of phytoplankton communities under various sea-ice conditions for comparison with underlying surface sediments. Our results show that sympagic production (IP25 and HBI-II) increased northward between 62°N and 73°N, with maximum values at the sea-ice edge in the Marginal Ice Zone (MIZ) between 70°N and 73°N in southeastern Chukchi Sea and along the coast of Alaska. They were consistently low at northern high latitudes (>73°N) under extensive summer sea-ice cover and in the Ice-Free Zone (IFZ) of the Bering Sea. Enhanced pelagic sterols and HBI-III occurred in the IFZ across the Bering Sea and in southeastern Chukchi Sea up to 70°N-73°N in the MIZ conditions that marks a shift of sympagic over pelagic production. In surface water SPM, pelagic sterols display similar patterns as Chl a, increasing southwards with higher amounts found in the Chukchi shelf pointing out the dominance of diatom production. Higher cholesterol values were found in the mid-Chukchi Sea shelf where phytosterols were also abundant. This compound prevailed over phytosterols in sediments, compared to SPM, reflecting efficient consumption of algal material in the water column by herbivorous zooplankton.

How to cite: Bai, Y., Sicre, M.-A., Ren, J., Klein, V., Jin, H., and Chen, J.: Latitudinal distribution of biomarkers across the western Arctic Ocean and the Bering Sea: an approach to assess sympagic and pelagic algal production, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3752, https://doi.org/10.5194/egusphere-egu24-3752, 2024.

EGU24-3959 | ECS | Posters on site | OS1.1

Arctic Wintertime Sea Ice Lead Detection from Sentinel-1 SAR Images 

Shiyi Chen, Mohammed Shokr, Lu Zhang, Zhilun Zhang, Fengming Hui, Xiao Cheng, and Peng Qin

Leads in sea ice cover are almost linear fractures within the pack ice, and are commonly observed in the polar regions. In winter, leads promote energy flux from the underlying ocean to the atmosphere. Synthetic aperture radar (SAR) can monitor leads with a fine spatial resolution, regardless of solar illumination and atmospheric conditions. In this paper, we present an approach for automatic sea ice lead detection (SILDET) in the Arctic wintertime using Sentinel-1 SAR images. SILDET is made up of four modules: 1) a segmentation module; 2) a balance module; 3) an optimization module; and 4) a mask module. The validation results presented in this paper show that SILDET has the capability of detecting open and frozen leads at different stages of freezing. The lead map obtained from SILDET was compared to a lead dataset based on Moderate Resolution Imaging Spectroradiometer (MODIS) data and validated by the use of Sentinel-2 images. This shows that SILDET can provide a more detailed distribution of leads and better estimation of lead width and area. Compared with visual interpretation of Sentinel-1 images, the overall detection accuracy is 97.80% and the Kappa coefficient is 0.88 (for all types). The pyramid scene parsing network (PSPNet) in the segmentation module shows a better performance in detecting frozen leads, compared with the deep learning methods of UNet and DeepLabv3+. The optimization module utilizing shape features also improves the precision in detecting frozen leads. SILDET was applied to present the Arctic lead distribution in January and April 2023 with a spatial resolution of 40 m. The Arctic-wide lead width distribution follows a power law with an exponent of 1.64 ± 0.07. SILDET can be expected to provide long-term high-resolution lead distribution records.

How to cite: Chen, S., Shokr, M., Zhang, L., Zhang, Z., Hui, F., Cheng, X., and Qin, P.: Arctic Wintertime Sea Ice Lead Detection from Sentinel-1 SAR Images, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3959, https://doi.org/10.5194/egusphere-egu24-3959, 2024.

EGU24-4291 | ECS | Orals | OS1.1

Arctic sea ice drift fields extraction based on feature tracking to MODIS imagery 

Yan Fang, Xue Wang, Gang Li, Zhuoqi Chen, Fengming Hui, and Xiao Cheng

Moderate-resolution optical imagery holds great potential in deriving Arctic sea ice drift fields because of its higher resolution than microwave radiometers and scatterometers, as well as its larger swath widths than most other optical and synthetic aperture radar (SAR) images. However, the application of such imagery is hindered by cloud influences and a lack of texture. In this study, we propose a method of deriving Arctic sea ice drift fields based on applying feature tracking to Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. To enhance the quality of the feature tracking step, a bundle of digital image processing techniques is first introduced, including histogram equalization which is based on the Cumulative Distribution Function (CDF) of the sea ice area, and Laplacian filtering which enhances image texture. Various MODIS bands and A-KAZE parameter settings are subsequently compared to balance the quality of sea ice drifting fields and calculation efficiency. Three pairs of MODIS images observed in different zones of the Arctic Ocean are selected to evaluate the performance of the proposed method. International Arctic Buoy Programme (IABP) buoy data are employed for validating the derived drift vectors with MODIS imagery. The results show that our proposed method effectively increases the number of vectors and their coverage rates of the sea ice drift fields extracted with MODIS images. The coverage rates of sea ice drift fields in three regions increase from 4.8%, 2.3%, and 2.5% to 56.5%, 23.5%, and 53.0% compared to using the A-KAZE algorithm directly, respectively. The MAEs of the derived sea ice motion vectors are 707 m/d in speed and 6.4° in direction, superior to the sea ice drift products based on the Advanced Very High Resolution Radiometer (AVHRR) imagery. The proposed method enables MODIS and other medium-resolution optical data to be utilized in deriving Arctic sea ice drift fields, which is of great significance to the long-term and large-scale Arctic environment, climate, and oceanography research in the future. 

How to cite: Fang, Y., Wang, X., Li, G., Chen, Z., Hui, F., and Cheng, X.: Arctic sea ice drift fields extraction based on feature tracking to MODIS imagery, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4291, https://doi.org/10.5194/egusphere-egu24-4291, 2024.

EGU24-6019 | ECS | Orals | OS1.1

Variability in the Arctic Ocean currents during 1990-2100 

Xiaoyan Wei, Chris Wilson, Sheldon Bacon, and Benjamin Barton

The Arctic Ocean is changing rapidly due to climate change, with significant impacts on subpolar ocean dynamics and mid-latitude regional weather. By utilizing a global, 1/12 degree, ocean sea-ice model (NEMO-SI3), which is forced at its surface by an Earth System Model, UKESM1.1, and simulates from 1981 to 2100 under scenario SSP3-7.0, we will demonstrate significant differences in the spatial structure and energy spectrum of Arctic Ocean currents among the past, the present, and the future. We will then explore the implications of changes in Arctic Ocean currents on mass transport pathways within the Arctic and transport across its boundaries. Subsequently, our study will identify the dominant physical drivers of these changes, such as sea ice melting, freshwater discharge, wind stresses, surface heat fluxes, and tides.

How to cite: Wei, X., Wilson, C., Bacon, S., and Barton, B.: Variability in the Arctic Ocean currents during 1990-2100, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6019, https://doi.org/10.5194/egusphere-egu24-6019, 2024.

EGU24-6330 | Orals | OS1.1

Recent estimates of the sea ice volume and solid freshwater flux across the Arctic’s major export passageways 

Stephen Howell, David Babb, Jack Landy, and Mike Brady

Sea ice export from the Arctic Ocean is important to the ice mass balance and freshwater budget of the Arctic Ocean and the delivery of freshwater to the North Atlantic. Historically, estimates of the sea ice volume and solid freshwater flux across the Arctic’s major export passageways were temporally limited in terms of available ice thickness data together with low spatial resolution satellite derived sea ice motion data. However, observational advances now provide year-round estimates of ice thickness from CryoSat-2 and high spatiotemporal estimates of sea ice motion can be derived from Senitnel-1 and the RADARSAT Constellation Mission (RCM) synthetic aperture radar (SAR) satellites. In this presentation, we present the results of merging these datasets that provide new high-quality annual and monthly estimates of the sea ice volume flux across the Arctic’s major export passageways of Fram Strait, Nares Strait, Davis Strait and the Canadian Arctic Archipelago from 2016-2022. Over our study period, the annual average volume export at Fram Strait was 1586 km3 that agrees with its longer-term decline. The annual average volume export at Nares Strait and the Canadian Arctic Archipelago was 160 km3, and 43 km3, respectively that is in agreement with longer-term increases and indicates a divergent trajectory compared to Fram Strait. The annual average sea ice volume flux through Davis Strait was 816 km3, nearly double previous estimates. Annually, a total of 1912 km3 of solid freshwater was delivered to the North Atlantic from the passageways of Fram Strait and Davis Strait. Overall, our new high-quality estimates of these sea ice variables provide updated quantities for understanding recent changes in ice mass balance and freshwater budget of the Arctic Ocean and the freshwater balance of the North Atlantic, where overturning is critical to the global climate.

How to cite: Howell, S., Babb, D., Landy, J., and Brady, M.: Recent estimates of the sea ice volume and solid freshwater flux across the Arctic’s major export passageways, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6330, https://doi.org/10.5194/egusphere-egu24-6330, 2024.

The surface Arctic Ocean is subject to rapidly changing freshwater inputs, from increasing ice melt and riverine inputs. Close monitoring of inflow waters from the Pacific and Atlantic is also needed for understanding the balance of geochemical cycles and making future predictions in the Arctic.  However, our knowledge of ocean biogeochemical data is very limited, necessitating an expansion of spatial and temporal coverage. However, the acquisition of ocean samples is hindered by the intricate sampling and analytical procedures employed both at sea and on land.

In our recent work [Hatta et al, 2021; 2023], a miniaturized, automated, microfluidic analyzer for nutrient analysis was developed using the programmable flow injection (pFI) technique.  This innovative system achieves accurate measurements with minimal reagent use, computer-controlled manipulations, and auto-calibration techniques, thus it is a promising oceanographic tool for increasing sample acquisition and determination, as well as minimizing human error.  For the pFI technique, the traditional silicate (Si) molybdenum blue method was modified by combining oxalate and ascorbic acid into a single reagent. This new method obtained a limit of detection of 514 nM Si, r.s.d. 2.1%, sampling frequency rate of 40 samples per hour, reagent consumption of 700 microliters per sample, and use of deionized (DI) water as a carrier solution. Phosphate (P) does not interfere significantly in this technique if the Si:P ratio is 4:1 or larger. Additionally, since there is no salinity influence, samples collected from the open ocean, coastal areas, or rivers can all be determined accurately using a DI water-based standard calibration covering a single small range by diluting samples to fall within this limited range.

In this contribution, this new shipboard method using programmable Flow Injection will be presented along with high-resolution Si data from the Chukchi shelf.  These data were obtained every 10-20 minutes by directly connecting the pFI platform to the underway water sampling system during the RV Mirai summer cruises. This new analytical platform will allow us to significantly expand our database and thus help to constrain and quantify geochemical processes and budgets in the Arctic Ocean.

How to cite: Hatta, M., Davis, M., and Measures, C.: Surface silicate distribution in the Chukchi-shelf region during the Arctic summer cruises using programmable flow injection technique, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6998, https://doi.org/10.5194/egusphere-egu24-6998, 2024.

EGU24-7486 | Orals | OS1.1

Sea-ice lead dynamics in the Arctic Ocean and associated drivers 

Sascha Willmes, Günther Heinemann, and Michelle Rasic

Based on a novel sea-ice lead climatology derived from thermal-infrared satellite imagery we identify drivers of wintertime sea-ice dynamics in the Arctic Ocean. ERA-5 atmospheric reanalyses and large-scale sea-ice drift data are used to investigate the causes for prominent spatial patterns and for the inter-annual variability in the occurrence of sea-ice leads. We can show that large-scale atmospheric circulation patterns and the Arctic Oscillation determine where and to which extent leads form on weekly to monthly timescales. Events with strong lead openings can directly be associated with pronounced anomalies in wind divergence and sea-ice drift. We also show the dominant modes in the Arctic sea-ice lead variability and their relation to atmospheric circulation. Moreover, the role of ocean processes in shaping long-term spatial lead patterns in the Arctic Ocean is presented. Implications for sea-ice modelling, forecasts and future trends are discussed.

How to cite: Willmes, S., Heinemann, G., and Rasic, M.: Sea-ice lead dynamics in the Arctic Ocean and associated drivers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7486, https://doi.org/10.5194/egusphere-egu24-7486, 2024.

EGU24-8264 * | Orals | OS1.1 | Highlight

Still Arctic? - The changing Barents Sea 

Sebastian Gerland, Randi B. Ingvaldsen, Marit Reigstad, Arild Sundfjord, Bjarte Bogstad, Melissa Chierici, Tor Eldevik, Haakon Hop, Paul E. Renaud, Lars H. Smedsrud, Leif Christian Stige, and Marius Årthun

The Barents Sea is one of the Polar regions where current climate and ecosystem change is most pronounced. In a recent review (DOI: 10.1525/elementa.2022.00088) as a part of the cross-disciplinary Norwegian research project “The Nansen Legacy”, the current state of knowledge of the physical, chemical and biological systems in the Barents Sea is described. Here, we present some of the key findings from this review. Physical conditions in this area are characterized by large seasonal contrasts between partial sea-ice cover in winter and spring versus predominantly open water in summer and autumn. Observations over recent decades show that surface air and ocean temperatures have increased, sea-ice extent has decreased, ocean stratification has weakened, and water chemistry and ecosystem components have changed. In general changes can be described as “Atlantification” and “borealisation,” with a less “Arctic” appearance. In consequence, only the northern part of the Barents Sea can be still called “Arctic”. The temporal and spatial changes have a wider relevance reaching beyond the Barents Sea, such as in the context of large-scale climatic (air, water mass and sea-ice) transport processes. The observed changes also have socioeconomic consequences, such as for fisheries and other human activities. Recent Barents Sea mooring data shows stronger inflow of warm water from the north during winter, affecting the sea ice locally. “The Nansen Legacy” has significantly reduced Barents Sea observation- and knowledge gaps, especially for winter months when field observations and sample collections have been sparse until recent.

How to cite: Gerland, S., Ingvaldsen, R. B., Reigstad, M., Sundfjord, A., Bogstad, B., Chierici, M., Eldevik, T., Hop, H., Renaud, P. E., Smedsrud, L. H., Stige, L. C., and Årthun, M.: Still Arctic? - The changing Barents Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8264, https://doi.org/10.5194/egusphere-egu24-8264, 2024.

EGU24-8828 | Posters on site | OS1.1

Changes of mesoscale eddy activity in the Eurasian Basin from 1-km simulations 

Vasco Müller, Qiang Wang, Nikolay Koldunov, Sergey Danilov, Xinyue Li, Caili Liu, and Thomas Jung

Mesoscale eddies play a crucial role in shaping the dynamics of the Arctic Ocean, making them essential for understanding future Arctic changes and the ongoing 'Atlantification' of the region. In this study, we use simulations generated by the unstructured-mesh Finite volumE Sea ice-Ocean Model (FESOM2) with a 1-km horizontal resolution in the Arctic Ocean.

Our investigation includes multiple simulations, namely a seven-year run representing the present-day climate and a slice simulation for the end of the 21st century, representative for a 4°C warmer world. Through these simulations, we evaluate changes in Eddy Kinetic Energy (EKE) within the Eurasian Basin and analyze their correlation with factors like sea-ice cover, baroclinic conversion rate, and stratification. To deepen our understanding, we combine Eulerian properties like EKE and baroclinic conversion rate with Lagrangian properties obtained from an algorithm that automatically identifies and tracks eddies using vector geometry.

Our findings from the end-of-century slice simulation indicate a significant increase in Arctic eddy activity in the future, accompanied by retreating sea ice. The present-day simulation reveals that the seasonality of EKE is mainly influenced by changes in sea ice, with distinct drivers at different depth levels for monthly anomalies. The mixed layer shows a robust connection to sea ice variability, while deeper levels, protected by stratification, are more significantly influenced by baroclinic conversion.

How to cite: Müller, V., Wang, Q., Koldunov, N., Danilov, S., Li, X., Liu, C., and Jung, T.: Changes of mesoscale eddy activity in the Eurasian Basin from 1-km simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8828, https://doi.org/10.5194/egusphere-egu24-8828, 2024.

EGU24-9183 | ECS | Orals | OS1.1 | Highlight

Thin Ice, Large Impact: Temporal and spatial trends of Arctic thermodynamic and dynamic sea ice thickness change 

Luisa von Albedyll and Robert Ricker

The Arctic Ocean's transition from perennial sea ice to more ice-free summers has halved sea ice thickness in the last six decades, significantly impacting the Arctic climate and ecosystem. Recent trends show a slowing in ice thickness and volume decline, prompting a need to investigate the underlying seasonal and long-term feedback mechanisms of sea ice thickness change. To do so, we use a Lagrangian drift-aware sea ice thickness product (DA-SIT), combined with extensive data on thermodynamic growth conditions and sea ice deformation, to quantify thermodynamic and dynamic thickness change in selected Arctic regions. A key focus is the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, which provided regional-scale analysis of seasonal sea ice thickness change using airborne ice thickness measurements, sea ice deformation, and in-situ snow and thermodynamic growth data. Our study extends these findings to larger temporal and spatial scales, evaluating their pan-Arctic applicability using long-term satellite datasets. We compare the MOSAiC trajectory with different dynamic regimes and ask how representative the conditions were for the “old” and the “new” Arctic. This analysis is key to understanding future sea ice thickness change, which is of great relevance for many climate and ecosystem processes.

How to cite: von Albedyll, L. and Ricker, R.: Thin Ice, Large Impact: Temporal and spatial trends of Arctic thermodynamic and dynamic sea ice thickness change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9183, https://doi.org/10.5194/egusphere-egu24-9183, 2024.

EGU24-9293 | Posters on site | OS1.1

Analyzing Mesoscale Eddy Impact on the West Spitsbergen Current in the Fram Strait 

Hwa Chien, Yen-Chen Chen, Huang-Meng Chang, Ke-Hsien Fu, and Bo-Shian Wang

Accelerated melting of Arctic sea ice, a consequence of global warming, is being exacerbated by increased freshwater inputs. This has led to a significant reduction in the halocline layer within the Fram Strait, enhancing ocean stratification and creating a feedback loop that further accelerates sea ice loss. This process is critical for the formation of North Atlantic Deep Water (NADW), where the West Spitsbergen Current (WSC) plays an essential role in recirculation.

Our study delves into the characteristics and influences of mesoscale eddies in the Fram Strait, particularly focusing on their impact on the WSC recirculation and NADW formation. Conducted over three years (2021-2023) during the months of minimal sea ice cover (August to October), the research involved deploying 36 specialized surface mini buoys across the strait. Analytical methods such as horizontal dispersion coefficients, finite-size Lyapunov exponents (FSLE), Lagrangian eddy identification, and sea surface temperature (SST) e-folding time were employed to assess WSC surface dynamics, eddy activities, and air-sea heat exchange.

Notably, we observed WSC bifurcation and intense mesoscale seawater mixing in the southwest Yermak Plateau and east of Molloy Deep (MD), areas marked by a rise in SST e-folding scale time gradient and considerable heat loss to the atmosphere (approximately 120 W/m²). Surface water convergence and sinking were detected near the western side of Molloy Deep and the Hovgaard (HG) regions, coinciding with high vorticity zones. Analysis of the buoy trajectories identified 682 eddy samples, forming the basis for a statistical examination of their size, period, intensity, and cyclonic features. This analysis was complemented by correlating eddy trajectories with sea surface height anomaly (SSHA) data, showing notable alignment.

Our results reveal a predominance of anticyclonic eddies in the Fram Strait, accounting for nearly 65% of the total eddies. Further, a consistency analysis between these eddies and wind stress curl indicated that about 66% of the anticyclonic eddies in the Molloy Deep region correlate with wind stress curl patterns, suggesting wind influence in their formation

How to cite: Chien, H., Chen, Y.-C., Chang, H.-M., Fu, K.-H., and Wang, B.-S.: Analyzing Mesoscale Eddy Impact on the West Spitsbergen Current in the Fram Strait, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9293, https://doi.org/10.5194/egusphere-egu24-9293, 2024.

EGU24-9304 | ECS | Orals | OS1.1

Distribution and characteristics of subsurface eddies in the Aleutian Basin, Bering Sea 

Kun Zhang, Haibin Song, and Linghan Meng

The subarctic Bering Sea, situated between the Pacific Ocean and Arctic Ocean, stands as one of the world's most productive oceanic regions. While the role of oceanic eddies in material transport and energy transfer has been extensively studied, surface eddies have dominated these investigations owing to advancements in remote sensing technology. Recently, attention has shifted to subsurface eddies for their influence on enhancing oceanic mixing. However, challenges persist in delineating the distribution and characteristics of subsurface eddies in the Bering Sea due to the limited effectiveness of satellite methods and the scarcity of field observation.

Multichannel seismic (MCS) data can provide high-resolution acoustic images of subsurface thermohaline fine structures, known as seismic oceanography. In this study, we integrate MCS with concurrent vessel-mounted Acoustic Doppler Current Profiles (vmADCP), expendable bathythermograph (XBT), and expendable Conductivity Temperature Depth (XCTD) data collected during cruise MGL1111, along with Argo and Copernicus Marine Service Global Ocean Physics Reanalysis data to investigate the distribution and characteristics of subsurface eddies in the Aleutian Basin.

The results underscore the presence of 44 subsurface eddies in the Aleutian Basin, primarily submesoscale with diameters averaging around 20 km. Eddy thickness spans 71.14 - 416.57 m, with eddy core depths ranging from 69.96 - 657.24 m, predominantly concentrated in the 100 - 200 m depth range; only 5 eddies exhibit core depths below 300 m. The cumulative volume of these eddies reaches approximately 434.38 × 109 m3, with the majority exhibiting anticyclonic characteristics, as corroborated by concurrent ADCP data. Analysis of historical CTD data, along with concurrent XBT and XCTD data from cruise MGL1111, delineates distinct water masses—Bering Sea Upper Water (BUW), Bering Sea Intermediate Water (BIW), and Bering Sea Deep Water (BDW)—in the study area. Most identified eddies are characterized as cold core, facilitating the transport of BIW. Trajectory assessments, incorporating concurrent Argo and Copernicus Marine Service Global Ocean Physics Reanalysis data, suggest an eastern and southern origin for these eddies, predominantly propagating westward. Assuming a propagating velocity of 1 cm/s, the estimated total transport of these eddies is approximately 1.76 Sv.

We believe that these findings will contribute essential insights to the fields of marine ecology, and climate studies, enhancing our knowledge of ocean dynamics in this critical region.

How to cite: Zhang, K., Song, H., and Meng, L.: Distribution and characteristics of subsurface eddies in the Aleutian Basin, Bering Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9304, https://doi.org/10.5194/egusphere-egu24-9304, 2024.

EGU24-9400 | ECS | Posters on site | OS1.1

Spatial and temporal distribution of all Arctic Polynyas since 1979 

Hau Man Wong, Céline Heuzé, Luisa Ickes, and Lu Zhou

Polynyas, open water regions within the sea ice cover, have been observed by satellites intermittently in the Arctic region over the past few decades. Their formation is complex, requiring various drivers to precondition and trigger the opening, which then influences local and regional weather significantly. Therefore, understanding Arctic polynyas’ spatial and temporal distribution is crucial to studying the polynyas’ impacts on climate. To date, most research is local and short-term, focusing on the major active Arctic polynyas or specific events; there is a need for pan-Arctic, long-term studies of all polynyas. Here, we use all available sea ice satellite data products to investigate all Arctic polynya events since 1979, in particular their locations for each day. The location preciseness and robustness are examined by sensitivity tests, varying the sea ice concentration (20 – 40%) and thickness (10 – 30 cm) thresholds. In the meantime, polynyas’ daily area extent, event duration, and recurrence are also obtained. The results indicate that the Franz-Josef Land, Eastern Kara Sea, and Nares Strait are the most active polynya formation-prone regions during wintertime. In addition, there is an increasing trend of polynya formation across the observation period. In future work, we plan to use the retrieved locations to determine whether thermodynamics or dynamic forcings contribute most to the Arctic polynyas’ opening.

How to cite: Wong, H. M., Heuzé, C., Ickes, L., and Zhou, L.: Spatial and temporal distribution of all Arctic Polynyas since 1979, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9400, https://doi.org/10.5194/egusphere-egu24-9400, 2024.

EGU24-10445 | ECS | Posters on site | OS1.1

Temporal and spatial variability of the oceanic front between the Atlantic Water and adjacent water masses north of Svalbard 

Stian Vikanes, Frank Nilsen, and Ragnheid Skogseth

The inflow of warm Atlantic Water (AW) into the Arctic Ocean is controlled by
oceanic fronts and air-ocean interactions north of Svalbard. The warm AW has
a significant impact on the sea ice extent and marine ecosystems in the region.
Therefore, it is crucial to understand the variability of the oceanic fronts offshore and
onshore of the AW, including their temporal and spatial characteristics, as well as
the mechanisms that govern them, such as atmospheric forcing, frontal instabilities,
and advection. However, our current understanding of the variability of these fronts
and AW north of Svalbard is limited due to lack of observational data. In this study,
we will use historical and more recent hydrographic data to analyze and describe
surface and subsurface fronts, both offshore and onshore of the AW core, in terms of
their dominant water masses along the continental slope north of Svalbard. We will
also determine the strength and position of these fronts by examining the horizontal
gradients in temperature, salinity, and density, and connect it to known changes in
the wind forcing.

How to cite: Vikanes, S., Nilsen, F., and Skogseth, R.: Temporal and spatial variability of the oceanic front between the Atlantic Water and adjacent water masses north of Svalbard, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10445, https://doi.org/10.5194/egusphere-egu24-10445, 2024.

The polar sea ice cover exhibits narrow bands of increased deformation, resulting in the formation of leads and pressure ridges. They are referred to as linear kinematic features (LKFs). They are important features of the sea ice field as they directly influence the heat and momentum exchange between ocean and atmosphere. By doing so, they influence the development of not only the sea ice cover but also the ocean and the local climate. Conversely, LKFs are also influenced by climate changes as the sea ice cover will be affected by changes in atmospheric and ocean temperature. In this work, the LKFs in the Arctic sea ice cover in current climate are compared to those in a warmer world. An LKF detection and tracking algorithm will be used to create a climate change signal. For this, the number of LKFs as well as their lifetimes are taken into account. The data analyzed in this work is created by the ocean-sea ice model FESOM. As LKFs are highly localized features, using a high spatial resolution is crucial. The resolution used in the analyzed runs is 1km. They span over five years starting at 2010, 2050, and 2090.

How to cite: Gärtner, J.: Detecting Linear Kinematic Features in Arctic Sea Ice in a Warmer World Using High Resolution Model Output, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10598, https://doi.org/10.5194/egusphere-egu24-10598, 2024.

EGU24-11226 | Posters on site | OS1.1

Weak signals of dense shelf water cascading in 2020 during a persisting phase of sporadic Atlantic water intrusions into the deep layer of the SW-Svalbard slope 

Patrizia Giordano, Manuel Bensi, Vedrana Kovacevic, Aniello Russo, and Leonardo Langone

The intensifying influence of warmer Atlantic Ocean waters in the Arctic, known as Arctic Atlantification, amplifies climate change effects by accelerating sea ice melting and altering ecosystems. Long-term data series are indispensable for discerning nuances in climate changes, especially when occurring in the deep ocean. They also provide a crucial temporal foundation for accurate modeling, useful to predict future scenarios and formulate effective strategies to address the challenges of climate change.

Here, we present oceanographic data collected from June 2014 to June 2023 at mooring site S1 (76°N, 14°E, 1040 m water depth), above the continental slope on the southwestern margin of Svalbard Archipelago (Fram Strait). There, the main branch of the West Spitsbergen Current transports Atlantic Water (in the upper layer) and Norwegian Sea Deep Water (below 900 m depth) poleward into the Arctic Ocean. Site S1 strategically lies at the convergence of Atlantic waters, the Arctic Ocean heat source, with waters from Storfjorden (Spitsbergen largest fjord) and shelf waters from the West Spitsbergen continental shelf. The oceanographic mooring S1 is part of the SIOS marine infrastructure network (Svalbard Integrated Arctic Earth Observing System, https://sios-svalbard.org/), and has undergone progressive instrument improvement over time, adding data collection at the intermediate layer since the summer 2022.

We focus on exploring short-term and seasonal variations in thermohaline properties, ocean currents, and particulate fluxes recorded in the deep layer over the last nine years. This analysis is undertaken in conjunction with meteorological conditions and trends in sea ice concentration. Oceanographic mooring data together with repeated Conductivity-Temperature-Depth (CTD) casts during summer surveys, show that the period 2014-2021 was characterized by the absence of dense shelf water exported at the near bottom on the slope, probably due to a limited production of dense water in the fjords, while the wind-induced vertical mixing and the resulting internal oscillations were probably favoured. During this period, a gradual decline in sea ice cover in winter is observed in the S1 area and adjacent fjords. The only exception is the winter 2020, when the sea ice extent returned apparently to pre-2013 levels, and at 1000m depth there were weak signals of cascading of dense shelf water, probably originated in the Storfjorden polynya.

Contrary to what is clearly evident in the literature regarding the increasing propagation of Atlantic waters northwards, temperature and salinity at mooring S1 showed no, or very little, positive trends over the investigated period. However, sporadic intrusions of relatively warm and saline water into the deep layer were observed. These occur most frequently in winter and are associated with the passage of internal waves that promote turbulent mixing of intermediate Atlantic waters with deep waters, facilitating the heat diffusion into the ocean depths.

How to cite: Giordano, P., Bensi, M., Kovacevic, V., Russo, A., and Langone, L.: Weak signals of dense shelf water cascading in 2020 during a persisting phase of sporadic Atlantic water intrusions into the deep layer of the SW-Svalbard slope, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11226, https://doi.org/10.5194/egusphere-egu24-11226, 2024.

EGU24-11917 | Orals | OS1.1 | Highlight

Atlantic Water warming increases melt below Northeast Greenland's last floating ice tongue 

Claudia Wekerle, Rebecca McPherson, Wilken-Jon von Appen, Qiang Wang, Ralph Timmermann, Patrick Scholz, Sergey Danilov, and Torsten Kanzow

Rising sea level poses a significant challenge and threat to our societies, given that coastal regions are densely populated. The Greenland ice sheet has been a major contributor to global sea level rise in the last decades, particularly its marine-terminating glaciers and their extensions into the ocean. The 79 North Glacier (79NG) features Greenland's largest floating ice tongue, stretching over 80 km in length in a 20 km wide fjord. The 79NG and its neighboring glacier, the Zachariæ Isstrøm, drain the Northeast Greenland Ice Stream which covers 12% of the Greenland Ice Sheet area. Its complete melt would lead to a 1.1-m global sea level rise. Though the extent of the 79NG has not changed significantly in recent years, observations have indicated a major thinning of its ice tongue from below.  Both ocean warming and an increase in subglacial discharge from the ice sheet induced by atmospheric warming could increase the basal melt; however, available observations alone cannot tell which of these is the main driver.

In this study, we present a setup of the Finite-volumE Sea ice-Ocean Model (FESOM2.1) which explicitly resolves the ocean circulation in the cavity of the 79NG with 700 m resolution. With this novel methodology, we seamlessly connect the global and regional ocean circulation to the circulation in the cavity. Our simulation with realistic bathymetry and ice shelf geometry covers the period 1970-2021, allowing us to disentangle the drivers of the upward trend and interannual variability of basal melt. We find that ocean warming in the subsurface Atlantic Intermediate Water layer that enters the cavity below the 79NG has played a dominant role in the basal melt rate over the past 50 years. The temperature variability can be traced back across the continental shelf of Northeast Greenland to the eastern Fram Strait with a lag of 3 years, implying a predictability of the basal melt of the 79NG. In contrast, subglacial discharge has a relatively small contribution to the interannual variation of the basal melt.

How to cite: Wekerle, C., McPherson, R., von Appen, W.-J., Wang, Q., Timmermann, R., Scholz, P., Danilov, S., and Kanzow, T.: Atlantic Water warming increases melt below Northeast Greenland's last floating ice tongue, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11917, https://doi.org/10.5194/egusphere-egu24-11917, 2024.

EGU24-12163 | ECS | Orals | OS1.1

Atlantification at the gateway of the Arctic Ocean over the last thousand years 

Francesco De Rovere, Angelo Rubino, and Davide Zanchettin

Atlantification is a major process driving rapid changes in the Arctic Ocean, e.g., sea-ice loss, warming and
salinification of the near-surface, enhanced mixing and changes in the ecosystem structure. The recent
scientific literature highlights the importance of the transport of Atlantic water as a cause of Atlantification,
but fundamental climatic processes driving this phenomenon are far from being fully understood.
Moreover, most studies focused on the analysis of recent observational data covering the last decades,
while recent studies showed that Atlantification had started in the 19th century.


In this contribution, we illustrate scope and progress of the Italian funded project “ATTRACTION: Atlantification dRiven by polAr-subpolar ConnecTIONs in a changing climate”. The project aims to provide a historical perspective on Atlantification by integrating observational evidence over the last decades, paleo-reconstructions and numerical paleoclimate simulations covering the past several centuries. We assess the capability of available tools to robustly describe coupled dynamics at the gateway of the Arctic Ocean (Fram Strait and Barents Sea), and their variations over multi-centennial periods. Furthermore, we provide a solid past reference for attribution of the ongoing Atlantification and discuss how paleoclimate simulations could support the identification of key locations for proxy-based reconstruction of the Atlantification. Toward a mechanistic understanding of Atlantification-like events over the last millennium, our assessment focus on the role of heat and salt redistribution by sub-polar dynamics by its major controls, including the Atlantic Multidecadal Overturning Circulation, the Sub-Polar Gyre and the Greenland Sea Gyre.

How to cite: De Rovere, F., Rubino, A., and Zanchettin, D.: Atlantification at the gateway of the Arctic Ocean over the last thousand years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12163, https://doi.org/10.5194/egusphere-egu24-12163, 2024.

EGU24-12174 | ECS | Posters on site | OS1.1

Feasibility of using C-band Synthetic Aperture Radar datasets for long term (1991-present) sea ice monitoring: towards multi-decadal analysis of sea ice type changes in the Atlantic sector of the Arctic Ocean 

Wenkai Guo, Anthony Paul Doulgeris, Johannes Lohse, Malin Johansson, Polona Itkin, Torbjorn Eltoft, Jack Landy, and Shiming Xu

We present a feasibility assessment of using several publicly available C-band wide-swath SAR datasets to derive sea ice type maps in the Atlantic sector of the Arctic Ocean from 1991 to present. This region is characterized by highly variable and dynamic sea ice conditions, and temporally consistent, large-scale monitoring of sea ice parameters is only possible through satellite remote sensing. We use data from C-band sensors including Sentinel-1, RADARSAT-2, Envisat ASAR and ERS-1/2, which have similar central frequencies and spatial resolution, to cover the study period. We evaluate comparative image classification performances and classification consistency using these datasets with common training datasets in geographically and temporally overlapping scenes and a sea ice classifier correcting for per-class incidence angle (IA) effects. Through this evaluation, we demonstrate the differences in these datasets affecting sea ice classification and the feasibility of using legacy sensors including Envisat ASAR and ERS-1/2 to extend the time series of sea ice type maps back to 1991 in our study area. This study provides theoretical support for the establishment of a multi-decadal SAR-based sea ice type product, which will contribute to the assessment of seasonal and inter-annual sea ice variations, especially the variability in new ice formation, which strongly influences physical and biogeochemical processes across the ocean-ice-atmosphere interface. This study is part of the collaborative project INTERAAC (air-snow-ice-ocean INTERactions transforming Atlantic Arctic Climate) between Norway and China, which aims at generating a reconciled multi-mission Climate Data Record (CDR) for Atlantic Arctic sea ice.

How to cite: Guo, W., Doulgeris, A. P., Lohse, J., Johansson, M., Itkin, P., Eltoft, T., Landy, J., and Xu, S.: Feasibility of using C-band Synthetic Aperture Radar datasets for long term (1991-present) sea ice monitoring: towards multi-decadal analysis of sea ice type changes in the Atlantic sector of the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12174, https://doi.org/10.5194/egusphere-egu24-12174, 2024.

In this study, we investigate the interannual variability of the sea ice area (SIA) in the Barents-Kara Sea (BKS) region. We explore the contributing factors to this variability, primarily focusing on oceanic influences evident in the Barents Sea Opening (BSO). The BSO, characterized by eastward warm Atlantic Water (AW) inflow, plays a crucial role in shaping the BKS SIA. While the inflow has been extensively studied, the westward-directed outflow known as Bear Island Slope Current (BISC), remains insufficiently observed. Being fed by relatively warm recirculating modified AW (mAW), the BISCs impact on the overall ocean heat transport (OHT) variability is uncertain.

 

Utilizing the global Finite Volume Sea Ice and Ocean Model (FESOM2.1), we derive estimates of the interannual volume transport and temperature variability of the BISC, filling the observational gap. We find that whereas the variability of BSO inflow/BISC volume transport is similar in magnitude, the temperature variability of the BISC exceeds the BSO inflow temperature variability. By linking the simulated BISC variability to BKS SIA, our findings reveal a yet unknown, strong co-variation between the volume transport of the BISC and the BKS SIA at the end of the freezing season, with a short lead time of zero to three months. We thus further examine the role of the BISC in generating interannual anomalies in the BKS SIA. Our model simulations illustrate that the volume transport of the BISC can be modified by the emergence of a secondary mAW recirculation downstream the northern AW path through the BS in the months preceding anomalously large BKS SIA. This secondary mAW recirculation is thereby increasing the volume transport of mAW leaving the BS via the BISC, reducing the amount of AW reaching the northern Barents Sea ice edge downstream. Additionally, we identify a connection between the atmospheric forcing pattern associated with the volume transport variability of the BISC and anomalous sea ice advection into the BKS as a second cause for the BISC volume transport/BKS SIA co-variability.

In general, our study emphasizes the co-variability between BKS SIA and the BISC. We highlight the role of the mAW recirculations in altering the amount of AW, and consequently ocean heat, reaching the ice edge in the northwestern Barents Sea.

How to cite: Heukamp, F. and Wekerle, C.: Variability of the Barents-Kara Sea Sea Ice Area and its Correlation with Atlantic Water Recirculation through the Barents Sea Opening, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12688, https://doi.org/10.5194/egusphere-egu24-12688, 2024.

EGU24-13067 | Orals | OS1.1 | Highlight

Intra- and interannual variability in the Atlantic Water inflow region north of Svalbard: sea ice, hydrography, nutrients and the potential for primary production 

Angelika Renner, Arild Sundfjord, Marit Reigstad, Allison Bailey, Øyvind Lundesgaard, Randi Ingvaldsen, Melissa Chierici, Elizabeth Jones, and Agnieszka Beszczynska-Möller

Atlantic Water (AW) is the major source of heat and nutrients to the Arctic Ocean. Changing AW inflow promotes sea ice decline and borealisation of marine ecosystems and affects primary production in the Eurasian Arctic. North of Svalbard, the AW inflow dominates oceanographic conditions along the shelf break and hence the distribution of heat and nutrients in the region. However, interaction with sea ice and Polar Surface Water determines nutrient supply to the euphotic layer. Using a combination of multidisciplinary approaches such as ship-based measurements and sampling, moored sensors, remote sensing and numerical modelling, we have been monitoring and studying the AW boundary current north of Svalbard since 2012. In this presentation, I will show some of our findings with particular focus on repeated measurements from a transect across the AW inflow at 31°E, 81.5°N. Large interannual variability in hydrography, nutrients and chl a indicates varying levels of nutrient drawdown by primary producers over summer. Sea ice conditions impact surface stratification, light availability, and wind-driven mixing, with a strong potential for steering chl a concentration over the productive season. In early winter, nutrient re-supply through vertical mixing varied in efficiency, again related to sea ice conditions. The autumn re-supply elevated nutrient concentrations sufficiently for primary production but likely happened too late as high-latitude light levels limited potential autumn blooms. Multidisciplinary observations are key to gain insight into the interplay between physical, chemical, and biological drivers and to understand ongoing and future changes. They are particularly important in regions like north of Svalbard that can indicate what we can expect in the central Arctic Ocean in the future.

How to cite: Renner, A., Sundfjord, A., Reigstad, M., Bailey, A., Lundesgaard, Ø., Ingvaldsen, R., Chierici, M., Jones, E., and Beszczynska-Möller, A.: Intra- and interannual variability in the Atlantic Water inflow region north of Svalbard: sea ice, hydrography, nutrients and the potential for primary production, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13067, https://doi.org/10.5194/egusphere-egu24-13067, 2024.

EGU24-13914 | ECS | Orals | OS1.1

Sea Ice Drift Retrieval based on Fengyun-3D Multi-Sensor Data 

Xue Wang, Zhuoqi Chen, Zhizhuo Xu, Ruirui Wang, Ran Lu, Fengming Hui, and Xiao Cheng

Under the background of global warming, sea ice changes rapidly. Sea ice drift is an important indicator for sea ice flux, atmospheric and ocean circulation, and ship navigation. Currently, the large-scale observed sea ice drift datasets are mainly obtained based on single-sensor remotely sensed data, which suffer low spatial resolution or poor spatial continuity. Considering that passive microwave radiometer and medium-resolution optical sensor complement each other in terms of spatial resolution and continuity, this study proposed a novel sea ice drift retrieval method based on Fengyun-3D (FY-3D) multi-sensor data. The proposed method is summarized as follows. First, low resolution sea ice drift fields were obtained from FY-3D Microwave Radiation Imager (MWRI) data based on the normalized cross-correlation pattern-matching method. Then, fine resolution vectors were extracted from FY-3D Medium-Resolution Spectral Imager (MERSI) data based on A-KAZE feature-tracking method. Finally, the low resolution pattern-matching vectors and fine resolution feature-tracking vectors were merged together based on Co-Kriging algorithm to obtain the final sea ice drift result. The proposed method was evaluated by comparing the buoy displacements obtained from the International Arctic Buoy Program (IABP) with the retrieved merged vectors from FY-3D remotely sensed images collected in the Beaufort Sea, the East Siberian Sea, and the Fram Strait on 2020. The results showed that the proposed method can retrieve accurate, fine resolution and spatial continuous sea ice motion fields.

How to cite: Wang, X., Chen, Z., Xu, Z., Wang, R., Lu, R., Hui, F., and Cheng, X.: Sea Ice Drift Retrieval based on Fengyun-3D Multi-Sensor Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13914, https://doi.org/10.5194/egusphere-egu24-13914, 2024.

EGU24-14215 | ECS | Posters on site | OS1.1

Potential of diatoms in sediments as seeds for autumn blooms in the Pacific Arctic shelf 

Yuri Fukai, Amane Fujiwara, Shigeto Nishino, Kohei Matsuno, and Koji Suzuki

The Pacific gateway to the Arctic has a vast continental shelf spanning the northern Bering and the Chukchi Seas. Within this shelf region, diatoms are crucial in sustaining high primary production and facilitating the sinking particulate organic carbon flux from spring to summer. Consequently, the bottom sediments have abundant viable diatoms, including resting stages. Despite the importance of diatoms, our understanding of the dynamics of this organism in sediments and their capacity to initiate primary production in the Pacific Arctic shelf remains limited.

In this study, we delved into the photophysiological capabilities of diatoms in the surface sediments collected from the Chukchi Sea in autumn through a laboratory incubation experiment at 3°C under the light conditions of 300 or 30 µmol photons m-2 s-1 for seven days. This experiment revealed that diatoms, mainly Chaetoceros, quickly resumed photosynthesis after light exposure and reached the maximum photosynthetic carbon fixation rates within only several days. These results suggest that diatoms in sediments have a significant potential to function as “seeds” for bloom formation in the sunlit water column. We further examined diatom communities, including resting spores, in the water column of the Chukchi Sea during autumn using scanning electron microscopy (SEM) and DNA metabarcoding techniques, as well as environmental parameters. Consequently, intense winds and subsequent water turbulence in the shallow Chukchi caused the predominance of Chaetoceros resting spores, probably derived from the sediments, in the diatom assemblages. As speculated from the incubation experiment mentioned above, diatom resting spores from the sediments can germinate immediately in the water column. Thus, settled diatoms could work as seeds for subsequent autumn blooms by being supplied from the seafloor along with nutrient-rich water.

The recent delayed sea ice formation in the autumn Arctic leads to increased storm occurrence over open water and enhanced vertical mixing, resulting in more frequent autumn blooms. Therefore, diatoms in sediments could be one of the critical contributors to autumn blooms in the shallow Pacific Arctic.

How to cite: Fukai, Y., Fujiwara, A., Nishino, S., Matsuno, K., and Suzuki, K.: Potential of diatoms in sediments as seeds for autumn blooms in the Pacific Arctic shelf, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14215, https://doi.org/10.5194/egusphere-egu24-14215, 2024.

EGU24-14505 | ECS | Posters on site | OS1.1

Winter to summer evolution of pCO2 in surface water of northern Greenland fjords  

Camille Akhoudas, Christian Stranne, Karl Adam Ulfsbo, Brett Thornton, and Martin Jakobsson

Ocean acidification induced by the absorption of anthropogenic CO2 and its consequences pose a potential threat to marine ecosystems around the globe. The Arctic Ocean, particularly vulnerable to acidification, provides an ideal region to investigate the progression and impacts of acidification before they manifest globally. Recent documentation of undersaturated surface waters in carbonate minerals in the Sherard Osborn fjord in northwest Greenland, a region visited for the first time in summer 2019, reveals inherent variability in biogeochemical processes. Associated with highly acidic surface waters, the partial pressure of CO2 (pCO2) was undersaturated relative to the atmosphere, indicating this study area as a CO2 sink. To comprehend variations in pCO2 in the northwest Greenland fjords and identify its drivers, we conducted a comparative study between two fjords in the region (Petermann and Sherard Osborn fjords) and used carbonate system data from the temperature minimum layer to examine the winter-to-summer evolution of pCO2 and influencing factors. Additionally, we evaluated pCO2 variations (δpCO2) concerning temperature, freshwater inputs, biological activity, and air-sea CO2 uptake to quantitatively assess the seasonal influencing factors on surface ocean pCO2. In the Sherard Osborn fjord, despite a substantial increase in surface temperature from winter to summer potentially increasing pCO2 and causing CO2 supersaturation relative to the atmosphere, freshwater inflow and biological activity reduced pCO2, resulting in CO2 undersaturation relative to the atmosphere. In the Petermann fjord, pCO2 remained lower than atmospheric levels due to a slight seasonal variation in surface temperature and significant biological activity, reducing pCO2 in surface water.

How to cite: Akhoudas, C., Stranne, C., Ulfsbo, K. A., Thornton, B., and Jakobsson, M.: Winter to summer evolution of pCO2 in surface water of northern Greenland fjords , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14505, https://doi.org/10.5194/egusphere-egu24-14505, 2024.

EGU24-15778 | Orals | OS1.1

Genesis and Decay of Baroclinic Eddies in the Seasonally Ice-Covered Arctic Ocean 

Gianluca Meneghello, John Marshall, Camille Lique, Pål Erik Isachsen, Edward Doddridge, Jean-Michel Campin, Heather Regan, and Claude Talandier

We explore the origin and evolution of mesoscale eddies in the seasonally ice-covered interior Arctic Ocean. Observations of ocean currents show a curious, and hitherto unexplained, vertical and temporal distribution of mesoscale activity. A marked seasonal cycle is found close to the surface: strong eddy activity during summer, observed from both satellites and moorings, is followed by very quiet winters. In contrast, subsurface eddies persist all year long within the deeper halocline and below.

We find that the surface seasonal cycle is controlled by friction with sea ice, dissipating existing eddies and preventing the growth of new ones. In contrast, subsurface eddies, enabled by interior potential vorticity gradients and shielded by a strong stratification at a depth of approximately 50 m, can grow independently of the presence of sea ice. 

We address possible implications for the transport of water masses between the margins and the interior of the Arctic basin, and for climate models’ ability to capture the fundamental difference in mesoscale activity between ice-covered and ice-free regions.

How to cite: Meneghello, G., Marshall, J., Lique, C., Isachsen, P. E., Doddridge, E., Campin, J.-M., Regan, H., and Talandier, C.: Genesis and Decay of Baroclinic Eddies in the Seasonally Ice-Covered Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15778, https://doi.org/10.5194/egusphere-egu24-15778, 2024.

EGU24-16897 | Orals | OS1.1

Assessing changes in winter sea ice deformation – from MOSAiC to the Fram Strait 

Polona Itkin and Dmitry Divine

During winter, sea ice is moving in cohesive clusters of ice plates. These clusters – hereafter named ‘Coherent Dynamic Elements’ (CDE) are composed of several areas of deformed and level ice, that slide coherently along active sea ice fractures. The largest sea ice fractures detectable from medium resolution Synthetic Aperture Radar (SAR) satellites (about 50 m spatial resolution) are the Linear Kinematic Features (LKFs). Sea ice deformation information can be estimated from the strain rates in the LKFs and as well from the geometrical characteristics of the CDEs. However, there is a sudden seasonal transition, at the point where the sea ice warms and loses its internal strength. After this transition the delineation of LKFs and CDEs from SAR becomes challenging. In this contribution we will analyze sea ice deformation during the drift of the MOSAiC expedition from October 2019 to July 2020. During this time, the expedition drifted the entire length of the Transpolar drift from the northern Laptev Sea into the Fram Strait and the sea ice surrounding it underwent numerous deformation events. The MOSAiC sea ice deformation data and the onset of the melt period is compared to the data over the Fram Strait, where the sea ice deformation can was estimated from SAR and upward looking sonar devices on fixed moorings for the period of 2010-2023. We will present the data on the changes in the onset of the melt period and show that MOSAiC year was a typical year representative for the sea ice deformation of the recent decade.

How to cite: Itkin, P. and Divine, D.: Assessing changes in winter sea ice deformation – from MOSAiC to the Fram Strait, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16897, https://doi.org/10.5194/egusphere-egu24-16897, 2024.

EGU24-17166 | ECS | Orals | OS1.1

The Northeast Water Polynya, Greenland; Climatology, Atmospheric Forcing and Ocean Response 

Miriam Bennett, Ian Renfrew, David Stevens, and Kent Moore

The Northeast Water Polynya is a significant annually recurring summertime Arctic polynya, located off the coast of Northeast Greenland. It is important for marine wildlife and affects local atmospheric and oceanic processes. In this study, over 40 years of observational and reanalysis products (ERA5 and ORAS5) are analysed to characterise the polynya's climatology and ascertain forcing mechanisms. The Northeast Water Polynya has high spatiotemporal variability; its location, size and structure vary interannually, and the period for which it is open is changing. We show this variability is largely driven by atmospheric forcing. The polynya extent is determined by the direction of the near-surface flow regime, and the relative locations of high and low sea-level pressure centers over the region. The surface conditions also impact the oceanic water column, which has a strong seasonal cycle in potential temperature and salinity, the amplitude of which decreases with depth. The ocean reanalyses also show a significant warming trend at all depths and a freshening near the surface consistent with greater ice melt, but salinification at lower depths (~ 200 m). As the Arctic region changes due to anthropogenic forcing, the sea-ice edge is migrating northwards and the Northeast Water Polynya is generally opening earlier and closing later in the year. This could have significant implications for both the atmosphere and ocean in this complex and rapidly changing environment.

How to cite: Bennett, M., Renfrew, I., Stevens, D., and Moore, K.: The Northeast Water Polynya, Greenland; Climatology, Atmospheric Forcing and Ocean Response, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17166, https://doi.org/10.5194/egusphere-egu24-17166, 2024.

EGU24-18118 | Posters on site | OS1.1

Arctic Ocean simulations in two high-resolution coupled climate models 

Chuncheng Guo, Mats Bentsen, Aleksi Nummelin, Mehmet Ilicak, Alok Gupta, and Andreas Klocker

Large model spread and biases exist in simulating the Arctic Ocean water mass and circulations from the latest CMIP6 coupled and ocean-sea ice-only simulations. This can be at least partly attributed to large uncertainties due to unresolved key processes in this region, and it is hoped that high resolution can - to a certain extent - come to the rescue.

In this work, we first examined two high-resolution simulations by two CMIP6-class models: 1) a multi-centennial integration of CESM (CESM-HR; ocean resolution 1/10-deg), and 2) a 50-year integration of NorESM (NorESM-MX; ocean resolution 1/8-deg). The two models show clear signs of improvements in simulating the Arctic Ocean compared to their standard 1-deg resolution counterparts, but certain biases remain, such as the incorrect pathway of the Atlantic Water and the too-deep mixed layer depth in NorESM-MX.

We then performed and analysed a similar NorESM-MX simulation, but this time with a newly developed hybrid vertical coordinate (z-density) in the ocean model (the default is isopycnal/density coordinate). ​​Experience from hybrid coordinate testing runs in standard 1-deg resolution shows e.g. much-improved water masses and sea ice extent in the Southern Ocean, mixed layer depths, and importantly more rapid equilibration to energy balance in coupled simulations. When applied in the high-resolution NorESM-MX configuration, the results with the new coordinate show a much-improved representation of the pathway of Atlantic water and the distribution of mixed layer depth in the Arctic Ocean. 

How to cite: Guo, C., Bentsen, M., Nummelin, A., Ilicak, M., Gupta, A., and Klocker, A.: Arctic Ocean simulations in two high-resolution coupled climate models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18118, https://doi.org/10.5194/egusphere-egu24-18118, 2024.

EGU24-18702 | ECS | Posters on site | OS1.1

Observation of temporal and spatial variability of deep near-inertial waves in the western Arctic Ocean 

Chanhyung Jeon, Samuel Boury, Kyoung-Ho Cho, Eun-Joo Lee, Jae-Hun Park, and Thomas Peacock

Near‐inertial waves are waves propagating in the interior of the ocean. Created by surface storms, they have the potential to influence the ocean environment by inducing vertical mixing. Compared to other oceans, the Arctic Ocean has low near-inertial wave activity, but might be changing. It is a challenge, however, to predict near-inertial wave activity in the Arctic Ocean due to its intricate vertical salinity and temperature stratification. Our in-situ campaign has obtained the first direct deep current measurements revealing notable temporal and spatial variability of deep near-inertial waves in the western Arctic Ocean. These observations are an important step towards a clearer depiction of the evolving energy budget, and concomitant mixing, associated with potentially high impact near-inertial wave activity in an increasingly ice-free Arctic Ocean.

How to cite: Jeon, C., Boury, S., Cho, K.-H., Lee, E.-J., Park, J.-H., and Peacock, T.: Observation of temporal and spatial variability of deep near-inertial waves in the western Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18702, https://doi.org/10.5194/egusphere-egu24-18702, 2024.

EGU24-18990 | Posters on site | OS1.1

Extreme sea ice motion -- analysis of ice drifter buoy data in the Gulf of Bothnia  

Henri Vuollekoski, Mikko Lensu, and Jari Haapala

Sea ice and particularly its motion are problematic for vessels and structures in water areas that experience sea ice. For example, several offshore wind farms are planned to be installed in the Gulf of Bothnia, but uncertainty related to extreme sea ice motion is likely to worry potential investors. Winter navigation, particularly in the coastal boundary zone, can be difficult. While climate change is likely to decrease the average ice concentration, extrema may become more severe. 

The motion of sea ice is affected by wind, currents and internal dynamics of the ice field, which are highly complex and inadequately understood. In this study we analyze time-series of data from ice drifter buoys deployed in the Gulf of Bothnia, Baltic Sea, during 2012 - 2023. The combination of data from multiple buoys, ice charts as well as other observations and model forecasts on the atmosphere-sea-ice interaction allows for estimating various parameters for the respective ice fields, such as shear, divergence and deformation, as well as temporal and spatial variability.

How to cite: Vuollekoski, H., Lensu, M., and Haapala, J.: Extreme sea ice motion -- analysis of ice drifter buoy data in the Gulf of Bothnia , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18990, https://doi.org/10.5194/egusphere-egu24-18990, 2024.

Currently freshwater anomaly is building up in the Beaufort gyre of the Arctic Ocean. There is a risk that this freshwater may discharge into the North Atlantic, disrupting the Atlantic Meridional Overturing Circulation (AMOC). Recent changes in Beaufort gyre size and circulation suggest this may occur soon or has already started: the North Atlantic has recently experienced its largest freshening for the last 120 years. In contrast, so far there is only limited evidence of Arctic fresh water impacting freshwater accumulation in the Labrador Sea. The North Atlantic is a region of high variability on interannual to decadal timescales, potentially affecting European and global climates.

The study focuses on changes in oceanic transports through the Arctic gateways under the Carbon Dioxide Removal (CDR) es-SSP5-3.4-ov CMIP6 emission–driven scenario 2015-2100 and analyses UKESM1 simulations. We examine historical and projected periods and compare the model results to the long-term observations in the key Arctic straits. The difference between the present-day and future model transports is in their partitioning between Fram Strait: in the future most of the Atlantic model inflow occurs via the Barents Sea (5.2 Sv northwards); model 2000-2020s and 2040-2090s Fram Strait transports are 2.4 Sv and 4.6 Sv southwards. It is worth noting that the observed Fram Strait volume transport estimates bear a large uncertainty, from 2.0±2.7 Sv southwards from moorings to 1.1±1.2 Sv from inverse modelling and 0.8 ±1.5 Sv from geostrophic analysis.

The model results show that during the increase of CO2 in the 2040s–2060s, the Beaufort Gyre is getting stronger, whereas the North Atlantic Subpolar Gyre (SPG) weakens. At the carbon dioxide removal phase (2060s–2090s) the Beaufort Gyre is strengthened while SPG weakened further. However, the cyclonic gyres in the Nordic Seas (Greenland, Iceland and Norwegian) become stronger. This points to a potential future change in the oceanic pathways between the Arctic and the North Atlantic. The corresponding heat transports due to overturning and gyres present different trends in the North Atlantic and the Arctic Ocean and different reversibility at latitudes between 26°N and 80°N, suggesting loss of immediate oceanic connectivity between the Atlantic and the Arctic via Nordic Seas. The simulations show a hysteresis in the AMOC: AMOC does not recover to the same level as before the mitigation even if the atmospheric CO2 concentration does.

Acknowledgement: We acknowledge funding from the EC Horizon Europe project OptimESM “Optimal High Resolution Earth System Models for Exploring Future Climate Changes”, grant 101081193 and UKRI grant 10039429, from the project EPOC “Explaining and Predicting the Ocean Conveyer”, EU grant 101059547 and UKRI grant 10038003, as well as from NERC highlight topics 2023 project “Interacting ice Sheet and Ocean Tipping - Indicators, Processes, Impacts and Challenges (ISOTIPIC)”. For the EU projects the work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.

How to cite: Aksenov, Y. and Rynders, S.: Transports through the Arctic gateways linked to the ocean gyres in the Carbon Dioxide removal (CDR) CMIP6 simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19583, https://doi.org/10.5194/egusphere-egu24-19583, 2024.

EGU24-19973 | ECS | Posters on site | OS1.1

Exploring links between Mixed-Layer depth and Sea Ice concentration variability in the Greenland Sea. 

Sonia Domingo, Joan Mateu Horrach, Alfredo Izquierdo, and Ángel Rodriguez

The Greenland Sea is a key player in the Atlantic Meridional Overturning Circulation (AMOC), crucial for forming dense waters through open-water convection and influencing global climate dynamics. Recent changes, such as decreasing sea ice concentration (SIC) and the shoaling of the mixed layer depth (MLD), have spurred detailed research into their impact on the AMOC. Our study, using the latest TOPAZ reanalysis, explores these changes from 1991 to 2021.

To strengthen our findings, we meticulously compare a 10-year observational dataset, validating TOPAZ's ability to reproduce processes like dense water formation and MLD evolution in the Greenland Sea. We find notable agreement, with the MLD reaching intermediate depths, and TOPAZ's overflow water density aligning with observations. Results show a decrease in SIC and a shallowing of the MLD, linked to rising surface water temperatures.

While our results indicate a similar trend, we're not ready to draw final conclusions. Further analysis is needed to understand how observational data compares to TOPAZ findings. Although reanalysis data provides valuable insights, it's crucial to validate everything with observational data. The comprehensive dataset and almost daily temporal resolution of our observational platforms significantly bolster the reliability of our conclusions.

Understanding Greenland Sea variability is vital not only for decoding its role in the AMOC but also for grasping broader implications for the global climate system. By highlighting the intricate relationship between SIC, MLD, temperature, and salinity, our research contributes to the ongoing dialogue on climate change dynamics.

 

How to cite: Domingo, S., Horrach, J. M., Izquierdo, A., and Rodriguez, Á.: Exploring links between Mixed-Layer depth and Sea Ice concentration variability in the Greenland Sea., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19973, https://doi.org/10.5194/egusphere-egu24-19973, 2024.

EGU24-20222 | ECS | Orals | OS1.1

Surface Controls of Freshwater Export through Denmark Strait  

Emma Boland, Yavor Kostov, and Dani Jones

Denmark Strait is a key route for the export of freshwater from the Arctic. Understanding the controls on the amount of freshwater entering the Subpolar North Atlantic is key for understanding the implications of rapid changes in the region, such as recent observed freshening of the Arctic Ocean. We present the results of an adjoint modelling study, which uses the ECCOv4 ocean state estimate to produce a reconstruction of the freshwater transport at Denmark Strait from 1992 to 2017. The reconstruction is formed of contributions from surface fluxes of buoyancy and momentum. We investigate the relative importance of these different contributions on different spatial and temporal scales. We find that surface wind stress at up to 2 years lag dominates variability. We also find a seasonally varying pattern in the dominant lags, with winter fluxes showing peak correlations with contributions from lags of up to 4 years, whereas spring fluxes showing a peak correlations on the scale of weeks.

How to cite: Boland, E., Kostov, Y., and Jones, D.: Surface Controls of Freshwater Export through Denmark Strait , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20222, https://doi.org/10.5194/egusphere-egu24-20222, 2024.

EGU24-20285 | Orals | OS1.1

Export of Greenland Sea Water across the Mohn Ridge as Measured by a Mooring during 2016–2018 

Jinping Zhao, Xusiyang Shen, and Tore Hattermann

Cold and dense water from the Greenland Sea, which has been found in the Lofoten Basin in the Norwegian Sea, is an important contributor to the Greenland–Scotland Ridge overflow, which feeds the deep and bottom waters in the North Atlantic. These two basins are divided by the Mohn Ridge, but there is no clear current connecting them. The aim of this study is to investigate how the Greenland Sea water enters the Lofoten Basin. We deployed a mooring on the western flank of the Mohn Ridge to measure the potential transport across the ridge during two periods: 2016/17 and 2017/18. The observation results indicate that the water above 1500 m in the Greenland Sea can be intermittently transported to the Lofoten Basin. In addition, we observed periods of flow reversal, which indicate bidirectional exchange between the two basins across the ridge. Our data from three consecutive seasons indicate that such inflows in August–September are a typical feature of the exchange across the Mohn Ridge. Net exports during these two periods into the Lofoten Basin were eltimated to be 5.86 Sv and 3.00 Sv, exhibiting noticeable interannual variations. We propose two possible mechanisms that could be driving the export. One is due to passing cyclones, which lower the sea level height along the Mohn Ridge and drive outflow. The second is due to the sudden weakening of the wind in summer, which results in outflow from the Greenland Sea through temporary geostrophic deviation.

How to cite: Zhao, J., Shen, X., and Hattermann, T.: Export of Greenland Sea Water across the Mohn Ridge as Measured by a Mooring during 2016–2018, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20285, https://doi.org/10.5194/egusphere-egu24-20285, 2024.

EGU24-20571 | Posters on site | OS1.1

A New Sea Ice Type Concentration Retrieval Algorithm from Microwave Remote Sensing Data 

Yufang Ye, Yanbing Luo, Mohammed Shokr, Zhuoqi Chen, and Xiao Cheng

Sea ice types, e.g., first-year ice (FYI) and multi-year ice (MYI), can be discriminated based on their radiometric and scattering signatures. However, changes in ice surfaces caused by factors such as ice deformation and melt-refreeze events can lead to extensive ice type misclassification. To solve this problem, a new sea ice type concentration (SITC) algorithm from microwave observations (SITCAM) is proposed in this study. It builds upon a previous algorithm, namely ECICE, but improves from two perspectives. Firstly, a new cost function is employed, with weights indicating the separation efficiencies of microwave parameters. Secondly, a pre-classification scheme is incorporated to account for the bimodal distributions in microwave characteristics. With SITCAM, daily Arctic SITCs are retrieved for the winters of 2002–2011 using passive (AMSR-E) and active (QuikSCAT and ASCAT) microwave data. The results are compared with a sea ice age product (SIA) and evaluated with ice type samples and SAR images. Overall, SITCAM performs well on mitigating the misclassifications induced by the aforementioned factors. The Arctic MYI area agrees well with that from SIA. Compared to ECICE, the retrieval accuracy for MYI and FYI samples increases to 96% and 90%, respectively (increasing by 5% and 15%, respectively), in SITCAM. The bias in MYI concentration between the SITC retrievals and SAR-based results has reduced from 15% to 4%. Furthermore, instead of being limited to specific observations (e.g., Ku-band scatterometer data), SITCAM performs well with various combinations of microwave data, even solely passive microwave data. This universality allows for a long-term record of SITC, which enables the potential of dating SITC back to late 1970s.

How to cite: Ye, Y., Luo, Y., Shokr, M., Chen, Z., and Cheng, X.: A New Sea Ice Type Concentration Retrieval Algorithm from Microwave Remote Sensing Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20571, https://doi.org/10.5194/egusphere-egu24-20571, 2024.

EGU24-1253 | ECS | Orals | OS1.2

AMOC weakening and its association with increased dynamic sea level in recent decades  

Emmanuel Eresanya, Gerard McCarthy, Jennifer MecKing, He Yinghui, and Adekunle Osinowo

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial mechanism of poleward heat transport in the ocean and climate system. It modulates the redistribution of heat and carbon in the northern hemisphere. The state of AMOC in recent decades has revealed a slowdown compared to the industrial era. Its state is linked to a number of physical factors, including sea level. Along the eastern seaboard of North America, on long timescales, the imprint of the AMOC is projected onto sea level patterns. The relationship between AMOC weakening and sea level is not clearly understood. This study investigates the state of the AMOC in recent decades and its link to the regional sea level using CMIP6 and RAPID datasets.

One of the most critical questions in ocean science is whether climate models and observations of the state of the AMOC in recent decades are consistent. If these datasets show significant differences, it could lead to a bias in our projected long-term climate knowledge. This study shows the potential of sea level data to inform the evolution of the AMOC to constrain and improve future projections.

How to cite: Eresanya, E., McCarthy, G., MecKing, J., Yinghui, H., and Osinowo, A.: AMOC weakening and its association with increased dynamic sea level in recent decades , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1253, https://doi.org/10.5194/egusphere-egu24-1253, 2024.

Ocean reanalyses covering many decades, including those with few observations, are needed to understand climate variability and to initialize and assess interannual to decadal climate predictions. The Met Office Statistical Ocean Re-Analysis (MOSORA) exploits long-range covariances to generate full-depth reanalyses of monthly ocean temperature and salinity even from sparse observations. The latest version of MOSORA presented here is for the first time an ensemble that samples uncertainties in these long-range covariances. The ensemble is created by using initial covariances from different perturbed-physics historical model runs and these are then improved with observations using an iterative process.

We demonstrate that covariances are mostly improved by iteration, and that this procedure, using very sparse observations typical of the 1960s, captures many features of analyses benefiting from modern observation density. We investigate the ensemble spread and find that salinity trends in the covariances from model runs can introduce unexpected changes in the reanalyses. In the Gulf of Guinea, there are insufficient observations to constrain the model covariances, which vary due to different model representations of Antarctic Intermediate Water. If models are improved in this region, this could lead to a better analysis of temperature and salinity.

We nudge the reanalyses into an ensemble of coupled climate models to produce estimates of the Atlantic Meridional Overturning Circulation (AMOC) back to 1960. At 26°N, the AMOC shows decadal variability consistent with observations at this latitude and shows signs of strengthening in recent years. The ensemble spread in AMOC reconstructions at this latitude increases with time as more observations interact with uncertain covariances. More observations should be able to better constrain these covariances.

At 45°N, the amount of decadal variability in the AMOC varies between members. The uncertainty of our reconstruction at this latitude varies through time partly related to the number of observations made on the western boundary, just off the Grand Banks of Newfoundland. This shows potential for targeted and sustained observations to constrain the transport into the North Atlantic subpolar gyre.

How to cite: Hermanson, L., Dunstone, N., Eade, R., and Smith, D.: An ensemble reconstruction of ocean temperature, salinity, and the Atlantic Meridional Overturning Circulation 1960–2021, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2037, https://doi.org/10.5194/egusphere-egu24-2037, 2024.

EGU24-2054 | Posters on site | OS1.2

Multidecadal Variability of Ocean Climate and Circulation of the North Atlantic Ocean 

Alexey Mishonov, Dan Seidov, and James Reagan

The North Atlantic's surface has been heating up for decades. There was concern that the thermohaline circulation and essential climate variables, such as the seawater temperature and salinity, could endure substantial changes in response to this surface warming. The Atlantic Meridional Overturning Circulation (AMOC) has changed noticeably over the last century and possibly slowed down in recent decades. Therefore, concerns about the trajectory of the North Atlantic Ocean climate are warranted. The key to understanding the North Atlantic current climate trajectory is to identify how the decadal climate responds to ongoing surface warming.  We address this issue using objectively analyzed in-situ data from the World Ocean Atlas covering 1955-2017 and from the Simple Ocean Data Assimilation reanalysis data for 1980-2019 as fingerprints of the North Atlantic three-dimensional circulation and AMOC’s dynamics. We have found that although the entire North Atlantic is systematically warming, the climate trajectories in different sub-regions of the North Atlantic reveal diverse regional decadal variability, although the thermohaline geostrophic circulation in the North Atlantic during the most recent decade has slowed down. The warming trends in the subpolar North Atlantic lag behind the subtropical gyre and Nordic Seas warming by at least a decade. The climate and circulation in the North Atlantic remained steady from 1955 to 1994, while the last two decades (1995-2017) demonstrated a noticeable reduction in AMOC strength, which may be closely linked to changes in the geometry and strength of the Gulf Stream system.

How to cite: Mishonov, A., Seidov, D., and Reagan, J.: Multidecadal Variability of Ocean Climate and Circulation of the North Atlantic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2054, https://doi.org/10.5194/egusphere-egu24-2054, 2024.

EGU24-2104 | ECS | Orals | OS1.2

Recalibration of extreme multi-decadal trends in the North Atlantic Oscillation. 

Rosemary Eade, David B. Stephenson, Adam A. Scaife, and Doug M. Smith

The historical variability of the winter mean North Atlantic Oscillation (NAO) has featured periods with large multi-decadal trends which are not well represented by coupled general circulation models (CGCMs), consistent with a lack of autocorrelation in the winter mean NAO index series. Post-processing “reddening” methods are proposed, using stochastic model theory to make the autocorrelation structure of the CGCM NAO index match that of the observed NAO. Using CGCMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6), these recalibration methods are shown to successfully improve the autocorrelation structure of the NAO and in turn the simulation of extreme trends. The 1963-1993 NAO trend is the maximum 31-year trend in the historical period, but without reddening the CGCMs underestimate the likelihood of this trend by a factor of ten.

 

CMIP6 future projections show a small systematic increase in long-term (2024-2094) NAO ensemble mean trends relative to the magnitude of the radiative forcing from ‑0.09 to 0.16 hPa/decade (range for low to high radiative forcing scenarios). This range is doubled after reddening, becoming ‑0.24 to 0.35 hPa/decade. There is also a related shift in the distribution of extreme 31-year NAO trends, which is more clearly apparent after reddening. Near-term projections of the next 31 years (2024-2054) are less sensitive to the future scenario. After reddening they still show weak-to-no forced trend in the models but have a 74% larger ensemble range (around +/- 1 standard deviation per decade). This level of internal variability could increase or decrease regional climate change signals in the Northern Hemisphere by magnitudes that are greatly underestimated when using raw climate model output.

How to cite: Eade, R., Stephenson, D. B., Scaife, A. A., and Smith, D. M.: Recalibration of extreme multi-decadal trends in the North Atlantic Oscillation., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2104, https://doi.org/10.5194/egusphere-egu24-2104, 2024.

EGU24-2646 | ECS | Orals | OS1.2

On the Formation and Maintenance of the Interannual Variability of the North Atlantic Oscillation 

Yang Yang, X. San Liang, and Wei-Bang He

Motivated by the observation that the interannual variability of the North Atlantic Oscillation (NAO) is associated with the ensemble emergence of individual NAO events occurring on the intraseasonal time scale, one naturally wonders how the intraseasonal processes cause the interannual variability, and what the dynamics are underlying the multiscale interaction. Using a novel time-dependent and spatially localized multiscale energetics formalism, this study investigates the dynamical sources for the NAO events with different phases and interannual regimes. For the positive-phase events (NAO+), the intraseasonal-scale kinetic energy (K1) over the North Atlantic sector is significantly enhanced for NAO+ occurring in the negative NAO winter regime (NW), compared to those in the positive winter regime (PW). It is caused by the enhanced inverse cascading from synoptic transients and reduced energy dispersion during the life cycle of NAO+ in NW. For the negative-phase events (NAO), K1 is significantly larger during the early and decay stages of NAO in NW than that in PW, whereas the reverse occurs in the peak stage. Inverse cascading and baroclinic energy conversion are primary drivers in the formation of the excessive K1 during the early stage of NAO in NW, whereas only the latter contributes to the larger K1 during the decay stage of NAO in NW compared to that in PW. The barotropic transfer from the mean flow, inverse cascading and baroclinic energy conversion are all responsible for the strengthened K1 in the peak stage of NAO in PW.

How to cite: Yang, Y., Liang, X. S., and He, W.-B.: On the Formation and Maintenance of the Interannual Variability of the North Atlantic Oscillation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2646, https://doi.org/10.5194/egusphere-egu24-2646, 2024.

EGU24-2686 | ECS | Posters on site | OS1.2

Deep Atlantic Multidecadal Variability 

Jiajun Yang, Jianping Li, and Qirong An

Investigating deep‐sea temperature variability is essential for understanding deep‐sea variability and its profound impacts on climate. The first mode in the Atlantic is referred to as Deep Atlantic Multidecadal Variability (DAMV), characterized by a north‐south dipole pattern in the mid‐high latitudes with a quasi‐period of 20‐50 years. The DAMV and Atlantic Multidecadal Variability, despite a statistical discrepancy, may be different responses to ocean heat transport (OHT) driven by the Atlantic Meridional Overturning Circulation (AMOC) at distinct depths separately. The relationship between the DAMV and the AMOC is established, indicating the AMOC is likely to transport surface heat downwards by deep convection and contribute to such dipole pattern in the deep Atlantic. Furthermore, meridional OHT proves the AMOC can explain the DAMV variation as a dynamic driver. These results reinforce the importance of deep‐sea studies concerning the Atlantic climate system.

How to cite: Yang, J., Li, J., and An, Q.: Deep Atlantic Multidecadal Variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2686, https://doi.org/10.5194/egusphere-egu24-2686, 2024.

A new thermodynamic potential of seawater is found with the temperature variable being Conservative Temperature.  From this thermodynamic potential all the thermodynamic variables of seawater can be calculated.  This thermodynamic potential adds to the two other thermodynamic potentials, the Gibbs function and the Helmholtz function, which have been known for more than a century.  Because of the advantages of using Conservative Temperature instead of in situ temperature, it is expected that the new thermodynamic potential will replace the Gibbs function in oceanography.  The new thermodynamic potential can be expressed as the sum of two parts, one depending on enthalpy and the other on entropy, and it is shown that there is a clean separation between the thermodynamic properties such as specific volume and sound speed that depend only on enthalpy, and those that depend also on enthalpy such as in situ temperature. 

How to cite: McDougall, T.: The new thermodynamic potential of seawater in terms of Conservative Temperature , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2756, https://doi.org/10.5194/egusphere-egu24-2756, 2024.

Our recent research underscores the pivotal roles of the Tibetan Plateau and Antarctica in the development of the Atlantic Meridional Overturning Circulation (AMOC). This study rigorously investigates how these two regions collectively influence the AMOC, using coupled model’s sensitive experiments that sequentially introduce the Tibetan Plateau followed by Antarctica (TP2AT), and then in the reverse order (AT2TP). The rise of the Tibetan Plateau markedly alters atmospheric moisture transport patterns in the Northern Hemisphere, leading to a fresher North Pacific and a saltier North Atlantic. This change is the key to shifting deep-water formation from the North Pacific to the North Atlantic, thereby initiating the AMOC. Antarctica’s contribution is primarily linked to its impact on the strength and position of atmospheric westerlies over the high latitudes of the Southern Hemisphere, which strengthens the AMOC by enhancing Ekman upwelling and Agulhas leakage in the Southern Ocean. The synergistic effect of the Tibetan Plateau and Antarctica is instrumental in forming the contemporary pattern of the AMOC. The TP2AT scenario is more effective in establishing the AMOC compared to AT2TP. In the latter scenario, a strong Pacific Meridional Overturning Circulation (PMOC) exists before the introduction of the Tibetan Plateau. The rise of the Tibetan Plateau must first terminate the PMOC before initiating the AMOC.

How to cite: Tong, M., An, F., and Yang, H.: The Dominant Role of the Tibetan Plateau and the Antarctic in Establishing the Atlantic Meridional Overturning Circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3019, https://doi.org/10.5194/egusphere-egu24-3019, 2024.

EGU24-3216 | Orals | OS1.2

Florida Current: four decades of steady state at 27°N 

Denis Volkov, Ryan Smith, Rigoberto Garcia, Molly Baringer, William Johns, Benjamin Moat, and David Smeed

The Florida Current (FC) provides the majority of the northward volume and heat transports for both the meridional overturning and the horizontal gyre circulations in the subtropical North Atlantic. A unique, sustained observing system in the Florida Straits at about 27°N, consisting of voltage measurements recorded from a submarine telecommunication cable installed between Florida and Grand Bahama Island, paired with regular calibration and validation cruises, was established in 1982. Since then, the recorded cable voltage time series has enabled over 40 years of quasi-continuous, daily estimates of the FC volume transport. The cable data constitutes the longest observational record of any boundary current and a key component of the Atlantic Meridional Overturning Circulation (AMOC) in existence. By this measure, it can be representative of the AMOC weakening, suggested by climate models and proxy-based reconstructions.

Here, we reassess the record-long change in the FC strength by revising the processing of voltages measured on the submarine cable. With the increased length of the cable record, we show that it has become necessary to account for the secular change in the Earth’s geomagnetic field, especially when studying processes on decadal and longer time scales. We calculate the corrected estimates of the FC volume transport and show that (i) the FC strength has not declined as reported recently, but has remained remarkably stable since 1982, and (ii) with the corrected FC record, the AMOC at ~26.5°N exhibits a decadal-scale variability rather than a long-term decline.

The results of this study indicate that, if climate models are correct that the AMOC is slowing or will soon slow down, this slowdown has not yet been reflected in the FC, or the observational record is still too short to detect it with confidence. The existing records are just starting to resolve decadal-scale signals relevant to climate variability. Continued observations are thus necessary for detection and mechanistic understanding of climate-related changes and for validating and improving ocean and climate models.

How to cite: Volkov, D., Smith, R., Garcia, R., Baringer, M., Johns, W., Moat, B., and Smeed, D.: Florida Current: four decades of steady state at 27°N, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3216, https://doi.org/10.5194/egusphere-egu24-3216, 2024.

EGU24-3406 | Orals | OS1.2

Summer fresh layers and winter mixed layers in the western Subpolar Gyre 

Femke de Jong and Nora Fried

The strength of the Atlantic Meridional Overturning Circulation has been tied to deep convection in the subpolar North Atlantic. The depth of convection in winter, and the density of its product, depends on the balance between the water column stratification at the end of summer and the buoyancy removed through cooling in winter. As climate change progresses, ocean stratification is expected to increase as a result of warming and increasing fluxes of freshwater from the Arctic and Greenland, which in turn may weaken convection. Recently, a large freshwater anomaly has been seen to go round the Subpolar Gyre and has been speculated to increase stratification to the point where it inhibited convection in the Irminger Sea in 2019. However, less is known about near surface salinity in other years.

Both the extent of the upper ocean summer fresh layer and the winter mixed layers are investigated using Argo profiles and gridded salinity products. Particularly the westernmost basins of the North Atlantic Subpolar Gyre are characterized by a strong seasonal cycle in near surface salinity. Fresh layers of around 50 m depth form over spring and summer and are diluted through mixing with deeper, more saline waters in winter. Larger fresh anomalies are seen in recent years, but Argo profiles show that this upper ocean freshwater can still be mixed over the water column if winter cooling is strong enough. This diminishes the fresh signal in amplitude, while spreading it over a much thicker layer. In the Labrador Sea and south of Greenland this can be seen in mixed layers over 1000 m deep, but even in the Irminger Sea fresh mixed layers down to 800 m were recorded in the winter of 2021-2022. Concomitantly, the western Subpolar Gyre has exhibited a freshening of the upper to intermediate water column that may partly be related to this spreading of freshwater over the water column. Documenting the strength and variability of the near surface summer fresh layer, and the extent to which it can be incorporated into winter mixed layers or not, will help project how deep convection may transition to a less frequent or weaker state in the future.

How to cite: de Jong, F. and Fried, N.: Summer fresh layers and winter mixed layers in the western Subpolar Gyre, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3406, https://doi.org/10.5194/egusphere-egu24-3406, 2024.

We use Voluntary Observing Ship (VOS) observations available form the ICOADS collection for estimating surface fluxes in the North Atlantic for the period from 1900-2022. One problem of the use of VOS observations for deriving long-term air-sea flux time series is associated with inhomogeneous in space and time sampling, especially during the period prior WW2. Another problem is associated with systematic biases in a number of VOS state variables (first of all cloud cover) for the first part of 20th century. To derive surface flux anomalies we first reconstruct turbulent heat fluxes from 1900 onwards for the whole North Atlantic from EQ to 70 N. To homogenize sampling density and obtaine more robust estimates we use the procedure of sub-sampling for the earlier decades and then integrate computed turbulent heat fluxes in the coordinates of steering parameters (vertical surface temperature and humidity gradients on one hand and wind speed on the other). Biases in cloud cover are associated with changes in the observational practices of in the early 1950s when WMO implemented new standardized coding system. These biases have the effect of systematic underestimation of total cloud cover during 1900-1940 compared to the past WW2 period ranging from 0.3 to 1 octa and imply biases in short- and long-wave radiation of up to 10 W/m2 and 4 W/m2 respectively. We explored all sources of these biases using direct analysis of early 20th century log-books and performed correction of cloud cover using cloud cover probability density functions. Then short- and long-wave radiative fluxes were computed using state of the art bulk parameterizations. Thus, we obtained long-term time series of turbulent heat fluxes and radiative fluxes for 120-yr period 1900-2022. Analysis of centennial trends shows upward change in sensible plus latent flux ranging from 3 to 14 W/m2 during 120 years, while the increase over the last 40 years amounts to 6-7 W/m2 with the major growth during the 1990s and early 2000s. Radiative fluxes demonstrated increase in short-wave radiation (positive directed to the ocean) of 3-5 W/m2 in the Atlantic subtropics and mid latitudes and weak or close to zero trends in long-wave radiation. While changes in radiative fluxes partially compensate opposite trends in turbulent fluxes, the upward tendency in ocean heat budget (atmosphere gains) remains significant with magnitude of 2-6 W/m2 over 120-yr period. Interdecadal variability of surface turbulent fluxes is of an order of magnitude stronger compared to the radiative fluxes (10-20 W/m2 vs 0.5-2 W/m2), thus implying the dominant role of turbulent fluxes on forming long-term changes of the ocean heat budget. Further interdecadal variability of surface heat budget is discussed in the context of the North Atlantic multidecadal variations.

Research is funded by RSF project # 23-47-00030.

How to cite: Gulev, S. and Aleksandrova, M.: Revealing long-term changes in the North Atlantic air-sea fluxes from provisionally corrected VOS observations (1900-2022), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3420, https://doi.org/10.5194/egusphere-egu24-3420, 2024.

EGU24-3457 | Orals | OS1.2

A glimpse into the future: The 2023 temperature extremes in the North Atlantic in the context of longer-term climate change 

Till Kuhlbrodt, Ranjini Swaminathan, Paulo Ceppi, and Thomas Wilder

In the year 2023, we have seen extraordinary extrema in high sea-surface temperature (SST) in the North Atlantic which are outside the 4-sigma envelope of the 1982-2011 daily timeseries. Here we take a first look at the large-scale, longer-term drivers of these extrema. Earth’s net global energy imbalance (in the 12 months up to September 2023) amounts to +1.9 W/m2 as part of a remarkably large upward trend, ensuring continuous heating of the ocean. However, the regional radiation budget over the North Atlantic does not show signs of a significant step increase from less negative aerosol forcing since 2020 as was suggested elsewhere. While the temperature in the top 100 m of the global ocean has been rising in all basins since about 1980, specifically the Atlantic basin has continued to further heat up since 2016. Similarly, salinity in the top 100 m of the ocean has increased in recent years specifically in the Atlantic basin. Outside the North Atlantic, around 2015 a substantial negative trend for sea-ice extent in the Southern Ocean has begun, leading to record low sea-ice extent in 2023. We suggest analysing the 2023 temperature extremes in the North Atlantic in the context of these recent global-scale ocean changes. Analysing climate and Earth System model simulations of the future, we find that the extreme SST in the North Atlantic and the extreme in Southern Ocean sea-ice extent in 2023 lie at the fringe of the expected mean climate change for a global surface-air temperature warming level (GWL) of 1.5°C, and closer to the average at a 3.0°C GWL. Understanding the regional and global drivers of these extremes is indispensable for assessing frequency and impacts of similar events in the coming years.

How to cite: Kuhlbrodt, T., Swaminathan, R., Ceppi, P., and Wilder, T.: A glimpse into the future: The 2023 temperature extremes in the North Atlantic in the context of longer-term climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3457, https://doi.org/10.5194/egusphere-egu24-3457, 2024.

EGU24-3534 | ECS | Posters on site | OS1.2

Perturbation Potential Energy Bridging North Atlantic Ocean Forcing to Atmospheric Multidecadal Variability in the North Atlantic  

Hongyuan Zhao, Jianping Li, Yuan Liu, Emerson Delarme, and Ning Wang

The North Atlantic sea surface temperature anomalies (SSTA) are considered an important origin of the North Atlantic atmospheric multidecadal variability. Employing the perturbation potential energy (PPE) theory, we analyzed the energetics linking North Atlantic Ocean forcings to atmospheric multidecadal variability. Supporting the previous model results, a cyclic pattern involving the Atlantic multidecadal oscillation (AMO) and North Atlantic tripole (NAT) is observed: positive AMO phase (AMO+, similarly hereafter) →NAT→AMO→NAT+, with a phase lag of approximately 15~20 years. An atmospheric mode characterized by basin-scale sea level pressure anomaly in the North Atlantic is associated with the AMO, which is termed as the North Atlantic uniformity (NAU). The AMO+ induces positive uniform PPE anomalies over the North Atlantic through precipitation heating, leading to decreased energy conversion to perturbation kinetic energy (PKE) and a large-scale anomalous cyclone. For the NAT+, tripolar SSTA result in tripolar PPE anomalies through accumulated tripolar precipitation. Anomalous energy conversions occur where the PPE anomaly gradient is large, which is explained by an energy balance derived from thermal wind relationship. The PKE around 15°N and 50°N (25°N and 75°N) increases (decreases), forming the anomalous anticyclone and cyclone at subtropical and subpolar region, respectively, known as the North Atlantic Oscillation (NAO). The reverse holds for the NAT and AMO. As the phases of the ocean modes alternate, the energetics induce the NAU, NAO, NAU+, and NAO+ in sequence. The SSTA-PPE-PKE energetics processes contribute a comprehensive understanding of how the ocean influences atmosphere in the North Atlantic.

How to cite: Zhao, H., Li, J., Liu, Y., Delarme, E., and Wang, N.: Perturbation Potential Energy Bridging North Atlantic Ocean Forcing to Atmospheric Multidecadal Variability in the North Atlantic , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3534, https://doi.org/10.5194/egusphere-egu24-3534, 2024.

EGU24-3655 | ECS | Posters on site | OS1.2

Shoaled glacial Atlantic Ocean Circulation despite vigorous tidal Dissipation: Vertical Stratification matters 

Yugeng Chen, Pengyang Song, Xianyao Chen, and Gerrit Lohmann

During the Last Glacial Maximum (LGM), tidal dissipation was about three times higher than today, which could have led to a considerable increase in vertical mixing. This would enhance the glacial Atlantic Meridional Overturning Circulation (AMOC), contradicting the shoaled AMOC as indicated by paleo proxies. Here, we conduct ocean model simulations to investigate the impact of background climate conditions and tidal mixing on the AMOC during LGM. Our results show that the shoaled glacial AMOC is mainly due to strong glacial ocean stratification and enhanced glacial Antarctic Bottom Water (AABW), irrespective of enhanced tidal dissipation. Enhanced tides only play an important role if they are applied to a present background climate with relatively weak ocean stratification. Given the critical role of AMOC in (de-)glacial climate evolution, our results highlight the complex interactions of ocean stratification and tidal dissipation that have been neglected so far.

How to cite: Chen, Y., Song, P., Chen, X., and Lohmann, G.: Shoaled glacial Atlantic Ocean Circulation despite vigorous tidal Dissipation: Vertical Stratification matters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3655, https://doi.org/10.5194/egusphere-egu24-3655, 2024.

EGU24-3719 | ECS | Posters on site | OS1.2

An observation-based estimate of the Atlantic meridional freshwater transport from 2004 to 2012 

Huayi Zheng, Lijing Cheng, Yuying Pan, and Chenyu Zhu

Meridional freshwater transport in the Atlantic Ocean (AMFT) plays a vital role in the Atlantic meridional overturning circulation and global climate change, but an accurate estimate of AMFT time series remains challenging.

This study uses an indirect approach that combines the observation of ocean salinity, surface evaporation and precipitation observations to derive AMFT and its uncertainty from 2004 to 2012, by solving the ocean freshwater budget equation. The method provides an independent estimation of AMFT, complementary to array observation and model/reanalysis data. The climatology, interannual and trend of AMFT based on indirect method are analyzed.

Climatologically, there is a strong southward AMFT between 18.5°S and 33.5°S, and a shift to northward from 18.5°S to 66.5°N. The highest transport occurs at 3.5°S (-0.29±0.09 Sv) and 39.5°N (-0.52±0.08 Sv). The estimation based on direct observation and reanalysis data are compared to give a clear understanding of AMFT climatology.

The interannual variability of AMFT exhibits meridional coherence from 33.5°S to 66.5°N, except for the lag propagation near 44ºN, the boundary of the subpolar and subtropical North Atlantic. The peaks and valleys of AMFT align with El Niño-Southern Oscillation (ENSO) variation. In the south of 44.5ºN, a southward anomalous AMFT appears during the La Nina events, such as January 2006 (-0.13 Sv), January 2008 (-0.16 Sv), and November 2010 (0 Sv) for 20ºS-44.5ºN mean. Conversely, northward AMFT increases when ONI peaks, 0.07Sv and 0.17Sv for 20ºS-44.5ºN mean in November 2008 and January 2010, respectively. The corresponding relationship between ENSO and AMFT suggest a potentially remote impact of ENSO on the Atlantic Ocean.

The derived time series indicates that, throughout the Atlantic Ocean, there is an increasing trend of northward AMFT from 2004 to 2012 when AMOC weaken, resulting in a freshwater divergence in the South Atlantic and subtropical North Atlantic, as well as a freshwater convergence in the subpolar North Atlantic.

Additionally, we discuss the definition of freshwater transport, considering its dependence on reference salinity. Analyzing the impact of reference salinity on MFT estimation based on a theoretical model, we find that the choice of reference salinity has little impact when there is no net volume transport. Therefore, reference salinity does not significantly affect the AMFT discussed in this study.

How to cite: Zheng, H., Cheng, L., Pan, Y., and Zhu, C.: An observation-based estimate of the Atlantic meridional freshwater transport from 2004 to 2012, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3719, https://doi.org/10.5194/egusphere-egu24-3719, 2024.

EGU24-3813 | ECS | Orals | OS1.2

Wind Steering of Mid-latitude Eastern Pathway of AMOC 

Sifan Gu, Zhengyu Liu, Sijia Zou, Shaoqing Zhang, Yangyang Yu, and Chengfei He

The spreading pathway of the North Atlantic Deep Water (NADW), which is the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), determines how climate change signals are transported throughout the global ocean. NADW is suggested to be transported from the subpolar Atlantic to the subtropics in the western basin by the deep western boundary current and the eddy-driven interior pathway west of the Mid-Atlantic Ridge (MAR). However, much less attention has been paid to AMOC cross-gyre transport in the eastern basin. Here, combining hydrographic observations and reanalysis, we identify a robust mid-depth Eastern Pathway located east of the MAR, which is further corroborated by model simulations with various resolutions, including eddy-resolving simulations. The Eastern Pathway accounts for half of the NADW transport across the intergyre boundary. Sensitivity experiments suggest that the mid-depth Eastern Pathway is formed by basin-scale ocean circulation dynamics due to wind steering on the intergyre communicating window instead of bottom topography. Our results provide a new paradigm for the AMOC pathway and call for further investigations on the climate response and variabilities associated with different AMOC pathways.

How to cite: Gu, S., Liu, Z., Zou, S., Zhang, S., Yu, Y., and He, C.: Wind Steering of Mid-latitude Eastern Pathway of AMOC, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3813, https://doi.org/10.5194/egusphere-egu24-3813, 2024.

EGU24-3819 | ECS | Posters on site | OS1.2

Ocean warming acceleration in Atlantic tied to the changes in ocean heat transport 

Yuying Pan and Lijing Cheng

Change in ocean warming rate is essential for evaluating the current climate change and predict future climate conditions. It has been confirmed that in the context of accelerated warming of the Earth climate system, the global oceans have been warming, especially since the 21st century, with a certain rate of acceleration. Because the local ocean heat content (OHC) changes are mainly balanced by the net sea surface heat flux (FS) and the oceanic heat divergence/convergence (OHD), the acceleration of ocean warming is closely related to the trend of the latter two. In this study, we first calculate the oceanic meridional heat transport (MHT) as a residual of energy budget including OHC, FS, and heat related to sea ice volume changes (Qice), and then adjust the discrepancy caused by systematic errors in different data and mismatch between them on a monthly basis. Our estimated MHT is compared to the results from RAPID observations, which shows good agreement between the two, with a correlation coefficient of 0.73 in the time series during January 2009 - December 2020. Based on the multiple datasets, we further evaluate the accelerated/decelerated changes in Atlantic OHC associated with the ocean and air-sea energy flow changes. The results show that during 1985-2016, in the north Atlantic Ocean, the ocean warming is slowing down, which are mainly dominated by the decreased OHD, while the southern Atlantic Ocean is accelerating warming mainly caused by the strengthened OHD. Therefore, MHT changes accompanied by the energy flow within the ocean play a more important role to the regional ocean warming acceleration than the changes in regional sea air heat exchange. The methodology we use here provides a method to estimate the heat transports, and can be used to analysis the ocean warming rates and Earth’s energy changes, and to detect the future climate variability.  

How to cite: Pan, Y. and Cheng, L.: Ocean warming acceleration in Atlantic tied to the changes in ocean heat transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3819, https://doi.org/10.5194/egusphere-egu24-3819, 2024.

EGU24-4035 | Posters on site | OS1.2

The past and projected future freshwater flux from Arctic land ice 

Jonathan Bamber, Zelu Zhang, and Adam Igneczi

We have developed a freshwater flux (FWF) time series aimed at providing a benchmark data set for testing the sensitivity of ocean and coupled GCMs to realistic, plausible future FWF forcing alongside a 70 year reconstruction of past fluxes. Here we build on previous work that reconstructed the freshwater flux (FWF) from Arctic glaciers and the Greenland Ice Sheet from reanalysis (Bamber et al., 2018). First, we use ERA5 reanalyses, a regional climate model and satellite observations to reconstruct the FWF for all Arctic land ice from 1950-2021, partitioned into solid and liquid phases around the coastline of glaciated sectors of the Arctic. We then project the FWF forward until 2100 using estimates of Greenland Ice Sheet melt derived from a structured expert judgement assessment for two temperature scenarios that approximate business as usual and a Paris Agreement limit to warming (Bamber et al., 2019; Bamber et al., 2022). Fluxes from glaciers and ice caps (GIC) are derived from GIC projections for equivalent temperature scenarios. We develop projections for both the median and 95th percentile melt estimates to provide FWF forcing that encompasses the plausible future range from Arctic land ice. To achieve this, we assumed a linear increase in mass loss from 2021 onward such that the integral up to 2100 matches the estimates in the structured expert analysis. The geographic distribution of melt anomalies are scaled according to present-day anomalies in runoff and solid ice discharge from the ice sheet. For the high end case (business as usual, 95th percentile) this equates to a FWF anomaly from the Greenland Ice Sheet of about 0.16 Sv by mid century and 0.3 Sv by 2100, representing an unlikely but plausible FWF entering, primarily, the sub-polar North Atlantic.

 

Bamber, J. L., M. Oppenheimer, R. E. Kopp, W. P. Aspinall, and R. M. Cooke (2019), Ice sheet contributions to future sea-level rise from structured expert judgment, Proc. Nat. Acad. Sci., 116(23), 11195-11200, doi:10.1073/pnas.1817205116.

Bamber, J. L., M. Oppenheimer, R. E. Kopp, W. P. Aspinall, and R. M. Cooke (2022), Ice Sheet and Climate Processes Driving the Uncertainty in Projections of Future Sea Level Rise: Findings From a Structured Expert Judgement Approach, Earth's Future, 10(10), e2022EF002772, doi:https://doi.org/10.1029/2022EF002772.

Bamber, J. L., A. J. Tedstone, M. D. King, I. M. Howat, E. M. Enderlin, M. R. van den Broeke, and B. Noel (2018), Land Ice Freshwater Budget of the Arctic and North Atlantic Oceans: 1. Data, Methods, and Results, Journal of Geophysical Research: Oceans, 123(3), 1827-1837, doi:10.1002/2017jc013605.

 

How to cite: Bamber, J., Zhang, Z., and Igneczi, A.: The past and projected future freshwater flux from Arctic land ice, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4035, https://doi.org/10.5194/egusphere-egu24-4035, 2024.

EGU24-4366 | ECS | Orals | OS1.2

North Atlantic subtropical mode water formation controlled by Gulf Stream fronts 

Jingjie Yu, Bolan Gan, Lixin Wu, Gokhan Danabasoglu, R. Justin Small, Allison H. Baker, Fan Jia, Zhao Jing, Xiaohui Ma, Haiyuan Yang, and Zhaohui Chen

The North Atlantic Ocean hosts the largest volume of global subtropical mode waters (STMWs), serving as heat, carbon, and oxygen silos in the ocean interior. STMWs are formed in the Gulf Stream region where thermal fronts are pervasive with strong feedbacks to atmosphere. However, their roles in the STMW formation have been overlooked. Using eddy-resolving global climate simulations, we find that suppressing local frontal-scale ocean-to-atmosphere (FOA) feedback leads to STMW formation being reduced almost by half. This is because FOA feedback enlarges STMW outcropping, attributable to the mixed layer deepening associated with cumulative excessive latent heat loss due to higher wind speeds and greater air-sea humidity contrast driven by the Gulf Stream fronts. Such enhanced heat loss overshadows the stronger restratification induced by vertical eddy and turbulent heat transport, making STMW colder and heavier. With more realistic representation of FOA feedback, the eddy-present/rich coupled global climate models reproduce the observed STMWs much better than the eddy-free ones. Such improvement in STMW production cannot be achieved even with the oceanic resolution solely refined but without coupling to the overlying atmosphere in oceanic general circulation models. Our findings highlight the need to resolve FOA feedback to ameliorate the common severe underestimation of STMW and associated heat and carbon uptakes in earth system models.

How to cite: Yu, J., Gan, B., Wu, L., Danabasoglu, G., Small, R. J., Baker, A. H., Jia, F., Jing, Z., Ma, X., Yang, H., and Chen, Z.: North Atlantic subtropical mode water formation controlled by Gulf Stream fronts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4366, https://doi.org/10.5194/egusphere-egu24-4366, 2024.

EGU24-4367 | ECS | Orals | OS1.2

Intensified Atlantic Multidecadal Variability in a warming climate 

Shujun Li, Lixin Wu, and Yiting Wang

The Atlantic Multidecadal Variability (AMV) is a basin-scale natural mode of the sea surface temperature (SST) in the North Atlantic, exerting a global impact, including contribution to the multidecadal Sahel drought and subsequent recovery and the post-1998 global warming hiatus. How greenhouse warming affects AMV remains unclear. Here, using models with multi-century-long outputs of future climate, we find an intensified AMV under greenhouse warming. Surface warming and freshwater input from sea ice melt increase surface buoyancy, leading to a slowdown of Atlantic Meridional Overturning Circulation (AMOC). Reduced vertical mixing associated with the suppressed oceanic deep convection results in a thinned mixed layer and its variability, favoring stronger AMV SST variability. Further, a weakened AMOC and associated meridional heat advection prolong the lifespan of the AMV, providing a long time for the AMV to grow. Thus, multidecadal global surface fluctuations and the associated climate extremes are likely to be more intense.  

How to cite: Li, S., Wu, L., and Wang, Y.: Intensified Atlantic Multidecadal Variability in a warming climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4367, https://doi.org/10.5194/egusphere-egu24-4367, 2024.

The Gulf Stream is a vital limb of the North Atlantic circulation that influences regional climate, sea level, and hurricane activity. Given the Gulf Stream's relevance to weather and climate, many studies have attempted to estimate trends in its volumetric transport from various datasets, but results have been inconclusive, and no consensus has emerged whether it is weakening with climate change. Here we use Bayesian analysis to jointly assimilate multiple observational datasets from the Florida Straits to quantify uncertainty and change in Gulf Stream volume transport since 1982. We find with virtual certainty (probability P>99%) that Gulf Stream volume transport through the Florida Straits declined by 1.2 ± 1.0 Sv in the past 40 years (95% credible interval). This significant trend has emerged from the dataset only over the past ten years, the first unequivocal evidence for a recent multidecadal decline in this climate-relevant component of ocean circulation.

How to cite: Piecuch, C. and Beal, L.: Robust weakening of the Gulf Stream during the past four decades observed in the Florida Straits, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4666, https://doi.org/10.5194/egusphere-egu24-4666, 2024.

EGU24-4707 | Orals | OS1.2

Twenty years of observing the Atlantic Meridional Overturning Circulation (AMOC) at 26N 

Ben Moat, David Smeed, William Johns, Shane Elipot, Darren Rayner, Ryan Smith, Denis Volkov, Jules Kajtar, Tillys Petit, and Julie Collins

The RAPID-MOCHA-WBTS (hereafter RAPID) array is an observing system designed to study the Atlantic Meridional Overturning Circulation (AMOC). It is an international collaboration between the National Oceanography Centre, University of Miami, and NOAA. The primary goals of the RAPID array are to observe and understand changes in the AMOC over time, and improve our understanding of how changes in the ocean circulation system may influence regional and global climate patterns. The array consists of a network of moored instruments, which measure ocean temperature, salinity, dissolved oxygen, and flow velocities.

The AMOC at 26◦N has now been continuously measured by the RAPID array over the period April 2004 to present (20 years of observing). This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic ship sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced overturning circulation.

We will present the history of the RAPID observational array and its contribution to AMOC science.

How to cite: Moat, B., Smeed, D., Johns, W., Elipot, S., Rayner, D., Smith, R., Volkov, D., Kajtar, J., Petit, T., and Collins, J.: Twenty years of observing the Atlantic Meridional Overturning Circulation (AMOC) at 26N, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4707, https://doi.org/10.5194/egusphere-egu24-4707, 2024.

EGU24-5278 | ECS | Orals | OS1.2

The role of a weakening AMOC in shaping future Euro-Atlantic atmospheric circulation 

Andrea Vito Vacca, Katinka Bellomo, Federico Fabiano, and Jost von Hardenberg

Climate change simulations predict a weakening of the Atlantic Meridional Overturning Circulation (AMOC). In the North Atlantic, where the deep convection occurs, the AMOC has a particularly marked influence. Here, the AMOC decline could have significant implications for the evolution of weather patterns, resulting in societal risks for densely populated areas of Europe. 

We employ the Weather Regime framework to analyse the change in the daily variability of large-scale atmospheric circulation in three coordinated experiments from the CMIP6 archive (i.e., ssp2-4.5, ssp5-8.5 and abrupt-4xCO2). We find that models that simulate a larger AMOC decline feature a net increase in NAO+ regime frequency and persistence compared to models that simulate a smaller AMOC decline. We show that this is due to the influence of a reduced warming of the subpolar North Atlantic (SPNA) on mean geopotential height, caused by the AMOC weakening. We further show that this also causes the storm track to strengthen due to an increased baroclinicity of the atmosphere in the region, with possible consequences on future extreme events.

Overall, our results suggest that the evolution of the Euro-Atlantic atmospheric circulation depends on the AMOC decline. We conclude that ocean circulation is a main driver of NAO variability in projections of future climate change, in addition to previously known drivers. 

How to cite: Vacca, A. V., Bellomo, K., Fabiano, F., and von Hardenberg, J.: The role of a weakening AMOC in shaping future Euro-Atlantic atmospheric circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5278, https://doi.org/10.5194/egusphere-egu24-5278, 2024.

EGU24-5412 | ECS | Orals | OS1.2

Role of Ocean Memory in Subpolar North Atlantic Decadal Variability 

Hemant Khatri, Richard Williams, Tim Woollings, and Doug Smith

The decadal variability in the subpolar North Atlantic Ocean heat content is significantly influenced by the atmosphere. The impact of seasonal-annual atmospheric perturbations lasts for many years in the oceans due to the ocean's long memory. The anomalous air-sea heat fluxes and winds associated with atmospheric perturbations first rapidly modify upper ocean temperatures, initiating a short-term or local ocean response. Subsequently, these modifications can alter meridional heat transport rates, leading to anomalous heat convergence persisting for several years—a long-term or far-field ocean response—in the subpolar ocean (Khatri et al., 2022, Geophys Res Lett).

We propose a novel technique that incorporates these two ocean responses to evaluate ocean memory and examine its role in driving decadal ocean variability. Here, we combine heat budget analysis with linear response theory to examine how the North Atlantic Oscillation (NAO), which captures about 40% of atmospheric variability, controls the decadal variability in upper ocean temperatures and quantify the associated ocean memory. Utilising CMIP6 climate model outputs and observations, our estimations suggest ocean memory for the subpolar North Atlantic to be between 10 to 20 years. Furthermore, we find that the NAO strongly influences long-term ocean variability, explaining 30% to 40% of subpolar ocean heat content variability on decadal timescales. Specifically, the impact of seasonal atmospheric events on the ocean persists for more than a decade through a combination of local and far-field ocean responses. The proposed ocean memory-based framework, integrating local and far-field ocean effects into a single metric, can be utilised to analyse how relatively short-timescale atmospheric variability drives changes in the ocean state over decadal timescales.

How to cite: Khatri, H., Williams, R., Woollings, T., and Smith, D.: Role of Ocean Memory in Subpolar North Atlantic Decadal Variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5412, https://doi.org/10.5194/egusphere-egu24-5412, 2024.

EGU24-6046 | Orals | OS1.2

The role of subtropical mode waters in the variability of meridional heat transport in the North Atlantic subtropical gyre 

David A. Smeed, William E. Johns, Ryan H. Smith, Daren Rayner, Denis L. Volkov, Shane Elipot, Tillys Petit, Jules B. Kajtar, Elaine L. McDonagh, and Ben Moat

The AMOC is usually defined as the maximum of the overturning streamfunction.    The time series produced by the RAPID-MOCHA-WBTS observing array uses a streamfunction calculated in depth space.     Using data from the RAPID-MOCHA-WBTS array along with additional data from the WBTS sections in the Florida Straits and other hydrographic data, we have made a time series of the overturning streamfunction calculated in density space.  The streamfunction in density space reveals the shallow overturning cell associated with subtropical mode waters (STMW) that is obscured in the depth-space streamfunction.    The time series of the data also reveal that inter-annual variability in the amount of STMW in the Florida Straits is linked to changes in meridional heat transport.

How to cite: Smeed, D. A., Johns, W. E., Smith, R. H., Rayner, D., Volkov, D. L., Elipot, S., Petit, T., Kajtar, J. B., McDonagh, E. L., and Moat, B.: The role of subtropical mode waters in the variability of meridional heat transport in the North Atlantic subtropical gyre, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6046, https://doi.org/10.5194/egusphere-egu24-6046, 2024.

EGU24-6116 | ECS | Orals | OS1.2

Early warning signals of AMOC collapse from North Atlantic array observations 

Emma Smolders, René van Westen, and Henk Dijkstra

The Atlantic Meridional Overturning Circulation (AMOC), one of the most prominent climate tipping elements on Earth, can potentially collapse as a consequence of surface freshwater input in the North Atlantic. A collapse from its current strong northward overturning state would have major impacts for the global climate system. Although available reconstructions appear to indicate a gradual weakening of the AMOC over the last century, the proximity of the climate system to a potential future collapse of the AMOC remains unknown. Here, we use the results of the first AMOC tipping event modelled in a state-of-the-art Global Climate Model, the Community Earth System Model (CESM), to identify regions and variables that play a key role in a forthcoming AMOC collapse and can therefore serve as early-warning signals (EWS). We analyse the statistical EWS properties using two steady state simulations with the same CESM version, the steady state simulations differ in the distance to the AMOC tipping point. These results will subsequently be used to assess the usefulness of observations from the SAMBA, RAPID and OSNAP arrays to determine whether the present-day AMOC is approaching a tipping point.

How to cite: Smolders, E., van Westen, R., and Dijkstra, H.: Early warning signals of AMOC collapse from North Atlantic array observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6116, https://doi.org/10.5194/egusphere-egu24-6116, 2024.

EGU24-6506 | ECS | Orals | OS1.2

Eighteen Degree Water Dynamics Viewed from an Ensemble  

Luolin Sun, Takaya Uchida, Thierry Penduff, Bruno Deremble, and William Dewar

The subtropical mode water in the North Atlantic, often referred to as ‘eighteen-degree water’ (EDW), has been investigated based on observational and theoretical studies. We here discuss the mechanism of EDW by using an ensemble-based approach which offers the advantage of separating the eddy field from the mean flow without making implicit assumptions on the temporal or spatial scales of the eddies. We employ an ensemble of North Atlantic Ocean simulations partially coupled with the atmosphere at mesoscale permitting resolution (1/12°), and determine EDW as a pool of the Ertel potential vorticity (PV) lower than the surroundings. Our results suggest that the maintenance of EDW can be explained by the down-gradient eddy PV fluxes balancing the mean flow: the low PV in the formation region is transported by the eddy fluxes to the pool and mixes with the surrounding high PV.  

How to cite: Sun, L., Uchida, T., Penduff, T., Deremble, B., and Dewar, W.: Eighteen Degree Water Dynamics Viewed from an Ensemble , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6506, https://doi.org/10.5194/egusphere-egu24-6506, 2024.

EGU24-6918 | Posters on site | OS1.2

Asymmetries between phases of Atlantic Multi-decadal Variability in the CMIP6 multi models 

Haedo Baek, Dong Eun Lee, Yeong-Ho Kim, Young-Gyu Park, Hye-jI Kim, and Eun Young Lee

The Atlantic Multidecadal Variability (AMV) is a phenomenon in which North Atlantic Sea Surface Temperature Anomalies (SSTAs) occur almost simultaneously in the subpolar and tropical regions, imprinting their impact not only on neighboring countries but also on the global climate system.Due to its long lifespan, the natural variability associated with AMV seriously amplifies the uncertainty of future climate projections, as the exact mechanisms of the AMV remain unknown despite numerous previous studies.In this study, we investigate the asymmetry in two opposite phases of AMV in different models using preindustrial control experiments from 46 different models participating in the Coupled Model Intercomparison Project 6 (CMIP6). Overall, we find a well-fitted positive linear relationship for tropical Atlantic SSTAs with respect to subpolar SSTAs among 46 models. However, when investigating the model sensitivity between two opposite AMV phases in each model, we find that the strength and phase preference in terms of the tropical SSTA sensitivity to subpolar SSTA widely vary, resulting in AMV+ preferred groups, AMV- preferred groups, or symmetric AMV groups.Among the three groups, the characteristics of models in the AMV+ preferred group are found to be most distinctive. It is most notable with the AMV+ preferred models that the net surface heat flux in the subpolar Atlantic adds heat from the atmosphere into the ocean during the positive AMV phase due to a robust hemispheric reduction of the Westerlies and the Trades.In contrast, it is clearly indicated with the AMV+ preferred model during negative phases of AMV, or with all other model groups during both AMV phases, that subpolar SSTAs associated with AMV originate from the ocean, rather than the atmosphere.This contrast in subpolar A-O interaction found in the AMV+ preferred model can be partially explained as the result of competition between subpolar and tropical SST influences, involving surface ocean feedback in the Tropical Atlantic. As the AMV+ positive group shows a significantly larger weakening of the westerlies and trade winds during AMV+, the vertical cold advection due to Ekman divergence becomes significantly weaker during positive AMV, resulting in warm SSTAs. In addition to the Wind-Evaporation-SST feedback, this Wind-upwelling-SST feedback associated with equatorial convergence further intensifies SSTAs and the tropical positive feedback. Further investigation reveals that the reason for the asymmetric AMV+ preference is in the nonlinear feedback mechanism: positive SST anomalies strengthen the stratification to help local warming driven by anomalous downwelling, whereas negative SST anomalies weaken the stratification and hinder local cooling driven by anomalous upwelling.

How to cite: Baek, H., Lee, D. E., Kim, Y.-H., Park, Y.-G., Kim, H., and Lee, E. Y.: Asymmetries between phases of Atlantic Multi-decadal Variability in the CMIP6 multi models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6918, https://doi.org/10.5194/egusphere-egu24-6918, 2024.

EGU24-7739 | Orals | OS1.2

Non-stationarity in the NAO–Gulf Stream SST front interaction 

Alessio Bellucci, Luca Famooss Paolini, Nour-Eddine Omrani, Panos Athanasiadis, Paolo Ruggieri, Casey Patrizio, and Noel Keenlyside

The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of non-stationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible non-stationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal timescale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multi-annual timescales but only during 2005–2015. The non-stationarity in the decadal NAO–GSF co-variability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–1990 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal timescale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–1990. Here it is suggested that the non-stationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal timescale to differ between the two time periods.

How to cite: Bellucci, A., Famooss Paolini, L., Omrani, N.-E., Athanasiadis, P., Ruggieri, P., Patrizio, C., and Keenlyside, N.: Non-stationarity in the NAO–Gulf Stream SST front interaction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7739, https://doi.org/10.5194/egusphere-egu24-7739, 2024.

EGU24-7850 | Posters on site | OS1.2

Subinertial Variability in Four Southeast Greenland Fjords in Realistic Numerical Simulations 

Renske Gelderloos, Thomas Haine, and Mattia Almansi

Natural variability at subinertial frequencies (time scale of several days) plays an important role in the interaction between Greenland’s fjords, the continental shelf, and shelf-break exchange with the deep basins. In this study we identified the nature and driving mechanisms of this variability in four fjords in Southeast Greenland, in three high-resolution numerical simulations. We find two dominant frequency ranges in along-fjord velocity, volume transport of Atlantic Water, and along-fjord heat transport: one around 2–4 days and one around 10 days. The higher frequency is most prominent in the two smaller fjords (Sermilik Fjord and Kangerdlugssuaq Fjord), while the lower frequency peak dominates in the larger fjords (Scoresby Sund and King Oscar Fjord). The cross-fjord structure of variability patterns is determined by the fjord's dynamic width, while the vertical structure is determined by the stratification in the fjord. The dominant frequency range is a function of stratification and fjord length, through the travel time of resonant internal Kelvin waves. We find that the subinertial variability is the imprint of Coastal Trapped Waves, which manifest as Rossby-type waves on the continental shelf and as internal Kelvin-type waves inside the fjords. Between 50% and 80% of the variability in the fjord is directly forced by Coastal Trapped Waves propagating in from the shelf, with an additional role played by alongshore wind forcing on the shelf.

How to cite: Gelderloos, R., Haine, T., and Almansi, M.: Subinertial Variability in Four Southeast Greenland Fjords in Realistic Numerical Simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7850, https://doi.org/10.5194/egusphere-egu24-7850, 2024.

EGU24-7929 | ECS | Posters on site | OS1.2

Observed variability of AMOC transport components at 11°S 

Anna Christina Hans, Rebecca Hummels, Peter Brandt, and Rodrigue Anicet Imbol Koungue

The Atlantic meridional overturning circulation (AMOC) is a key feature of the oceanic circulation and has a big impact on regional weather and global climate. As the characteristics of the northward return flow of the AMOC crossing the equator are crucial for deep water formation at high latitudes in the North Atlantic, the AMOC variability in the South Atlantic is of particular interest. Here, we present observations of several components of the upper branch of the AMOC at 11°S taken from the Tropical Atlantic Circulation and Overturning at 11°S (TRACOS) array. We focus on the transport time series and seasonal to interannual variability of the North Brazil Undercurrent at the western boundary, the Angola Current at the eastern boundary and the upper layer AMOC transport composed of the geostrophic interior and the Ekman transports. The two boundary currents are derived from 10 years of direct moored current measurements. For the geostrophic interior transport, transport anomalies are derived from 10 years of bottom pressure measurements at the eastern and western continental margin at 300 m and 500 m depth and from sea level anomaly data. In all three analysed time series, no long-term trend is visible, and seasonal to interannual variability dominates. Water mass characteristics of the NBUC show a salinification in the central water range.

How to cite: Hans, A. C., Hummels, R., Brandt, P., and Imbol Koungue, R. A.: Observed variability of AMOC transport components at 11°S, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7929, https://doi.org/10.5194/egusphere-egu24-7929, 2024.

A part of the uncertainties in global climate model projections over Europe arise from their underestimation of multidecadal variability in the winter-time North Atlantic Oscillation (NAO). This underestimation, however, remains poorly understood. Past studies have linked the weak multidecadal NAO variability in models to an underestimated atmospheric response to North Atlantic sea surface temperature variability. Using the CMIP6 large ensemble of climate models, we explore statistical relationships with physical drivers that may contribute to intermodel spread in NAO variability. We find a significant intermodel correlation between multidecadal NAO variability and multidecadal stratospheric polar vortex variability, as well as a stratosphere-troposphere coupling parameter that quantifies the relationship between stratospheric winds and the NAO. Models with the lowest NAO variance are associated with weaker polar vortex variability and a weaker stratosphere-troposphere coupling parameter. The identification of this relationship suggests that modelled spread in multidecadal NAO variability has the potential to be reduced by improved knowledge of observed multidecadal stratospheric variability, although observational records are currently too short to provide a robust constraint on these indices.

How to cite: Maycock, A., Bonnet, R., and McKenna, C.: Model spread in the multidecadal variability of the winter North Atlantic Oscillation connected to stratosphere-troposphere coupling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8426, https://doi.org/10.5194/egusphere-egu24-8426, 2024.

EGU24-8474 | ECS | Posters on site | OS1.2

Assessing North Sea carbon and nutrient cycle responses to regional and global climate change 

A.C. (Cuun) Koek, R. (Richard) Bintanja, and W.H. (Willem) Van de Poll

The North Sea is a very productive and heavily exploited continental shelf sea that absorbs considerable quantities of atmospheric CO2. The fraction of absorbed CO2 1) flowing out towards the North Atlantic and 2) buried in sediments, is highly uncertain, rendering future changes of the system difficult to predict. As part of the NoSE (North Sea-Atlantic Exchange) project, this study focuses on the present-day and future roles of the North Sea within the wider carbon and biogeochemical systems of the Atlantic Ocean. Specifically, in this study we will assess the response of carbon and nutrient cycling in the North Sea and the adjacent North Atlantic Ocean to regional and global climate change.

            The carbon cycle configuration of state-of-the-art Earth System Model EC-Earth3, EC-Earth3-CC (atmosphere: IFS36r4; land surface: HTESSEL; Ocean: NEMO3.6; Sea ice: LIM3; Dynamic vegetation: LPJ-GUESS; Atmospheric composition: TM5; Ocean biogeochemistry: PISCES) was used to simulate both present-day (1981 – 2020) and future (2071 – 2100) climate, marine biogeochemistry, ocean primary production and nutrient distributions. Here, we present a validation of the EC-Earth3-CC present-day climatologies in the North Sea and adjacent parts of the North Atlantic Ocean, using existing observational datasets. We also compare the EC-Earth3-CC results to other global (CMIP6) and regional climate models to infer how EC-Earth3-CC biases compare to deficiencies in other models. Furthermore, we will address the response of the North Sea carbon and nutrient fluxes and budgets to regional and global climate change by comparing the present-day and future climatologies.

            This study will reveal new insights into the cycling of carbon and nutrients in the North Sea, their exchange with the Atlantic Ocean, and how these processes may evolve in the future.

How to cite: Koek, A. C. (., Bintanja, R. (., and Van de Poll, W. H. (.: Assessing North Sea carbon and nutrient cycle responses to regional and global climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8474, https://doi.org/10.5194/egusphere-egu24-8474, 2024.

EGU24-8498 | Posters on site | OS1.2

Where does the AMOC peak? Assesssing regional variations in North Atlantic Overturning from GLORYS12  

Caroline Katsman, David Oldenhuis, Dennis Vermeulen, and Renske Gelderloos

The Atlantic Meridional Overturning Circulation (AMOC) transports vast amounts of heat to high latitudes, and is largely responsible for Western Europe’s relatively mild climate. Climate models project the AMOC will weaken substantially over the 21st century, which impacts weather, climate, sea level and the oceanic carbon cycle. In many studies, the AMOC state is described in a condensed two-dimensional view or even by means of a single metric, which leaves many aspects of its complex 3D-structure underexposed. By revealing the sharp contrast in overturning strength between the western and eastern subpolar gyre (SPG), the recent OSNAP observations emphasized the importance of considering the AMOC in 3D.

In this study, we explore this further by analyzing the characteristics of the overturning in density space in the North Atlantic SPG on a regional scale, and over time periods ranging from seasons to decades. For this, we use model data from the high-resolution GLORYS12 reanalysis, spanning the period 1993-2020. Following the approach applied in OSNAP, the overturning is assessed from alongstream changes in boundary current transport in specific density classes. This analysis is performed for the entire SPG, for its major basins (Iceland Basin, Irminger Sea, and Labrador Sea) and for smaller segments along the boundary currents, thus providing detailed insights in variations of the overturning varies along the entire SPG boundary.

The mean overturning from GLORYS12 for 1993-2020 is 23.8 Sv, distributed as 41%, 29%, and 30% for the Iceland Basin, Irminger Sea, and Labrador Sea respectively, and peaking at increasingly higher densities in alongstream direction. Within each basin, a pronounced seasonal cycle can be identified, with the maximum overturning occurring in March and the minimum in September. Over the entire reanalysis period, the overturning strength in both the Iceland Basin and Irminger Sea exhibits a weak decreasing trend, whereas the Labrador Sea displays a weak increasing trend

The subdivision in shorter segments reveals large spatial differences in overturning, both with regard to its overall strength and its distribution over density classes. However, these outcomes are less robust than the analyses on the scale of the major basins, as the flow is highly variable and numerical uncertainties associated with offline overturning calculations become more prominent.

Further research is needed to properly interpret these regional variations, and thereby improve our understanding of the AMOC dynamics and its sensitivity to changing oceanic and atmospheric forcing conditions. Linking them to local processes known to govern the overturning (i.e., formation of dense waters in the interior of marginal seas and their export, formation of dense waters within the boundary current system itself and the exchange of waters via overflows) seems a viable route.

How to cite: Katsman, C., Oldenhuis, D., Vermeulen, D., and Gelderloos, R.: Where does the AMOC peak? Assesssing regional variations in North Atlantic Overturning from GLORYS12 , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8498, https://doi.org/10.5194/egusphere-egu24-8498, 2024.

EGU24-8528 | ECS | Posters on site | OS1.2

The RAPID-Evolution Project: Towards a low-cost and sustainable observing system of the AMOC at 26°N 

Tillys Petit, Ben Moat, Adam Blaker, Chris Cardwell, Shane Elipot, James Harle, Matthieu Le Henaff, Nick Higgs, William Johns, Jules Kajtar, Darren Rayner, Bablu Sinha, David Smeed, Ryan Smith, and Denis Volkov

Direct measurements of the Atlantic Meridional Overturning Circulation (AMOC) and meridional heat transport (MHT) are necessary to better understand the impact of anthropogenic greenhouse gas emissions for the global climate system. The RAPID-MOCHA-WBTS array at 26°N is the only trans-Atlantic observing system to provide 20 years of continuous measurements of the AMOC and MHT. While the design of the array has continuously evolved as our understanding of the AMOC has advanced and as new technologies have become available, the goal of the RAPID-evolution project is now to design a lower cost and sustainable observing system to continue the measurements at the accuracy required by users. Using the dataset gathered since 2004 and ocean reanalysis, a first objective seeks to evaluate the sensitivity of the AMOC estimate to the choice of methodology and data included in the calculation. The project includes the development of a new high-resolution ocean model to identify the short and longer term impacts of incorporating these datasets in the AMOC estimation. Recent technological developments also enable new approaches that could provide better and more cost-effective calculation of the AMOC. The RAPID-Evolution project investigates these approaches and develops methodologies to make use of them, including a new variation of the stepping method using glider deployments and the telemetry of mooring data via an autonomous vehicle.

How to cite: Petit, T., Moat, B., Blaker, A., Cardwell, C., Elipot, S., Harle, J., Le Henaff, M., Higgs, N., Johns, W., Kajtar, J., Rayner, D., Sinha, B., Smeed, D., Smith, R., and Volkov, D.: The RAPID-Evolution Project: Towards a low-cost and sustainable observing system of the AMOC at 26°N, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8528, https://doi.org/10.5194/egusphere-egu24-8528, 2024.

EGU24-8598 | Orals | OS1.2

Drivers and Impacts of Changing Subpolar North Atlantic Surface Temperature and Salinity 

Simon Josey, Jeremy Grist, and Bablu Sinha

Two aspects of Subpolar North Atlantic variability are explored using observations and model analysis. The first aspect is the autumn-winter seasonal reduction of sea surface temperature (SST). In a climate change simulation with the HadGEM3-GC3.1-HM model, a strong increase in the magnitude of the seasonal temperature reduction (STR) is found in sea-ice affected regions and the subpolar gyre. Similar results are obtained from an observational analysis using the HadISST dataset. In both cases, the STR has increased in magnitude by up to 0.3 ºC per decade over 1951-2020. The primary driver for the increased STR is a greater sensitivity of SST to heat loss due to increased surface stratification brought about predominantly by warming of the northern ocean regions. The increase in STR, leads to a greater winter meridional SST gradient, with potential consequences for increasing winter storminess. The second aspect is an investigation of the atmospheric impacts of surface salinity anomalies through modification of mixed layer properties and the surface heat exchange. For this analysis, the seasonal evolution of two 20-member ensembles of HadGEM3-GC3.1-HM have been undertaken with and without an imposed initial winter salinity anomaly in the western Subpolar North Atlantic that is similar in magnitude to the Great Salinity Anomaly. The evolution of the perturbed model runs will be examined with a focus on the consequences for European spring-summer climate conditions.

How to cite: Josey, S., Grist, J., and Sinha, B.: Drivers and Impacts of Changing Subpolar North Atlantic Surface Temperature and Salinity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8598, https://doi.org/10.5194/egusphere-egu24-8598, 2024.

Pressure on the ocean's "sidewalls" - the global continental slope - is strongly dynamically constrained by the steep topography. As a result we find that, even in an eddy-rich ocean model, its variability exhibits coherence over many thousands of kilometres. Here, we examine the time-mean pressures and show how they reflect a combination of global wind-driven signals, interaction with the Antarctic Circumpolar Current and the AMOC, which is seen in the development of pressure around the boundary of the North Atlantic. The need for pressure to be single-valued around the global continental slope ensures that these factors must come to a consistent balance, which shows that two remote factors together must come into a balance with the AMOC. We elucidate how these factors interact, and illustrate them with diagnostics from a 1/12 degree ocean model.

How to cite: Hughes, C. and Gururaj, S.: Remote influence of (or on?) the Atlantic Meridional Overturning Circulation: A boundary pressure perspective., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8720, https://doi.org/10.5194/egusphere-egu24-8720, 2024.

EGU24-8826 | ECS | Posters on site | OS1.2

Deep Circulation in the North Atlantic from Ocean Bottom Seismometer Noise: Insights from the UPFLOW/iReverb Project 

Afonso Loureiro, Maria Tsekhmistrenko, Alex Saoulis, Carlos Corela, Rui Vieira, Jesus Reis, Rui Caldeira, Miguel Miranda, and Ana Ferreira

Ocean Bottom Seismometers (OBS) face unique challenges in recording seismic events due to their exposure to harsh oceanic conditions. The UPFLOW project deployed 50 OBS of various instrument types in the North Atlantic Ocean. The iReverb project aims to investigate the tidally-modulated current-induced noise generated by water flow around the instrument's frame.

This study presents an analysis of seasonal variations in tidal-induced noise on different OBS types across the Azores, Madeira and Canaries region. 

In some instances, the detected harmonics allow the identification of individual frame components contributing to the noise, offering, on the one hand, insights into potential mitigation solutions for future deployments. On the other hand, our project's main focus - large-scale detection of non-seismic or current-induced reverberation events on OBS - provides valuable data for mapping resonances and tracking ocean currents. 

Our study uses machine learning/deep learning algorithms, automating the mapping of resonances across large datasets and obtaining a proxy for Ocean Bottom Circulation (OBC) patterns.

Here, we present a brief overview of our methodology, describe our results and compare them to classical oceanographic methods to determine ocean currents.

This project was funded by the UPFLOW project (ERC grant 101001601), and by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (DOI: 10.54499/UIDB/50019/2020), UIDP/50019/2020 (DOI: 10.54499/UIDP/50019/2020) and LA/P/0068/2020 (DOI: 10.54499/LA/P/0068/2020).

How to cite: Loureiro, A., Tsekhmistrenko, M., Saoulis, A., Corela, C., Vieira, R., Reis, J., Caldeira, R., Miranda, M., and Ferreira, A.: Deep Circulation in the North Atlantic from Ocean Bottom Seismometer Noise: Insights from the UPFLOW/iReverb Project, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8826, https://doi.org/10.5194/egusphere-egu24-8826, 2024.

EGU24-9171 | ECS | Orals | OS1.2

Observing the volume and property changes of the Water Masses in the Nordic Seas 

Lucas Almeida, Nicolas Kolodziejczyk, and Camille Lique

The Nordic Seas, where cold and fresh Arctic waters mix with warmer and saltier North Atlantic waters, play a crucial role in the ocean circulation system. This region is also the place of intense water mass transformations, with a conversion of lighter waters into denser waters that contribute to the lower limb of the Atlantic Meridional Overturning Circulation. In recent years, the region has experienced Atlantification, characterized by an increased contribution of Atlantic waters, leading to a warming in the upper layers. This study aims to investigate the impact of Atlantification on the properties of water masses in the Nordic Seas. We have used ISAS, an optimal interpolation from ARGO data with a monthly time series spanning 2002 to 2020, the ANDRO dataset for computing geostrophic velocities from ARGO float drift, and the ERA5 dataset for air-sea flux exchanges. The Nordic Seas are divided into four basins: the Greenland Sea (GS), the Icelandic Plateau (IP) in the west, and the Lofoten Basin and Norwegian Basin in the east. The water column is divided into three water masses based on potential density (𝞼0): surface (𝞼0 < 27.8 kg m-3), intermediate (27.8 < 𝞼0 < 28.0 kg m-3), and deeper water mass (28.0 < 𝞼0 < 28.07 kg m-3). Based on the observational datasets, we estimate the variations of the volume of each water mass, the transport within and outside the basins, and the surface-forced Water Mass Transformation (WMT). The eastern basins are experiencing surface warming, particularly after 2013, accompanied by an increase in the volume of the same water mass. Moreover, the volume of intermediate water masses is decreasing. In the Norwegian Basin, surface-forced transformations dominate the volume changes, while the Lofoten Basin experiences a significant influence from both surface-forced transformation and the import of warm waters from the south. In the western basins, both the intermediate and deeper water masses are increasing in volume encompassing a larger depth range , with a smaller trend in the Icelandic Plateau. In the Greenland Sea, the WMT are dominating these changes and the region is mostly exporting denser waters. In contrast, in the Icelandic Plateau the intermediate water is mostly explained by differences in the transports, and the deeper water masses by the surface transformation. We conclude that the changes observed in the Nordic Seas water masses result from a combination of local changes driven by air-sea fluxes and the advection of warmer waters. Monitoring the relative contributions of remote and local processes involved in WMT will help us to better understand and anticipate the ongoing and future shifts in the Nordic Seas conditions. 

How to cite: Almeida, L., Kolodziejczyk, N., and Lique, C.: Observing the volume and property changes of the Water Masses in the Nordic Seas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9171, https://doi.org/10.5194/egusphere-egu24-9171, 2024.

EGU24-9943 | ECS | Orals | OS1.2

Emerging impacts of enhanced Greenland melting on Labrador Sea dynamics 

Ilana Schiller-Weiss, Torge Martin, and Franziska Schwarzkopf

Meltwater input to the subpolar North Atlantic from the Greenland ice sheet has been steadily increasing in the past decades due to global warming. To identify the impacts of this enhanced freshwater input since the late 1990s, we use output from the eddy-rich model VIKING20X (1/20˚) running two nearly identical simulations from 1997–2021 only differing in the freshwater input from Greenland: one with realistic interannually varying runoff increasing in the early 2000s and the other continued after 1997 using the local, grid-cell climatology of 1961–2000 maintaining the mean seasonal runoff cycle. Here, runoff is based on the JRA55-do reanalysis (Tsujino et al., 2018, Ocn.Mod.), which includes the Bamber et al. (2018, JGR-O) Greenland runoff and calving record, where liquid and solid discharge is combined into a single liquid flux entering the ocean through the surface and coast. Apart from this, atmospheric forcing is identical between the two runs. To our knowledge this is the first set of twin experiments with a most realistic, well validated, eddy-rich ocean model to assess the impact of the current, observed increase in Greenland ice sheet mass loss. 

We find that the majority of the additional freshwater remains within the boundary current. This enhances the density gradient between the fresh and cool slope current and the warm and salty waters of the interior Labrador Sea and leads to a small (.01 m/s) but significant increase in boundary current speed in our experiment. Both, the faster slope current and the enhanced shelf–interior density gradient increase the potential for intensified eddy shedding into the interior Labrador Sea. This more dynamic regime fosters the eddy-driven import of fresh boundary current waters (Polar Water and meltwater) into the nearby deep convection regions. Lastly, our experiments indicate a role of enhanced Greenland runoff in the eastward shift of deep convection reported by Rühs et al. (2021, JGR-O) for the recent period 2015–2018. The experiment with realistically increased runoff exhibits meltwater tracer mixed only to shallower depths before transferred east into the Irminger Sea leading to a weaker stratification in the upper to mid-depth Irminger Sea than in the experiment with less, climatological runoff, which would enable or at least support deep convection southeast of Greenland.

How to cite: Schiller-Weiss, I., Martin, T., and Schwarzkopf, F.: Emerging impacts of enhanced Greenland melting on Labrador Sea dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9943, https://doi.org/10.5194/egusphere-egu24-9943, 2024.

EGU24-10107 | Orals | OS1.2

Modelling the North Atlantic: How parameterizations affect model biases and uncertainties 

Stephan Juricke, Ekaterina Bagaeva, Sergey Danilov, and Nikolay Koldunov

In this presentation, we discuss the role of a variety of parameterizations for simulating ocean dynamics in the North Atlantic and how they contribute to biases and model uncertainties. Their effect is analyzed via a range of diagnostics and model setups.

Many of the crucial processes in the ocean still need to be parameterized in state-of-the-art global ocean and climate models. Among those processes are mesoscale ocean eddies and mixed layer dynamics which cannot be fully resolved in most multidecadal simulations. However, they play a crucial role in setting the dynamic and hydrographic conditions in the North Atlantic and the global oceans. Increasing resolution tends to improve some of the long-standing ocean biases, but is very costly and makes it difficult to disentangle which specific processes or boundary conditions are driving certain improvements.

A consequence of imperfect process parameterizations are systematic errors resulting in large model biases. Furthermore, they can lead to inaccurate representation of the chaotic evolution of the ocean system, leading to insufficient representations of forecast uncertainties via ensemble simulations. In the North Atlantic, both of these consequences play a large role, leading to strong model biases and a general underdispersion of ensemble forecasts.

Classical biases of ocean models at so called eddy-permitting resolution, where mesoscale eddies are barely resolved, are related to overdissipation of kinetic energy and enhanced diffusion of tracers. We introduce a set of parameterizations that tackle the overdissipation of kinetic energy via specific viscosity schemes, including schemes that reinject some of the overdissipated energy back into the system. A combination of such schemes reduces classical ocean biases such as the North Atlantic cold bias by enhancing eddy activity and improving the path of mean currents such as the Gulf Stream. In addition, we demonstrate how stochastic methods can be used to account for parameterization uncertainties in the North Atlantic, quantifying the role of parameterization errors in ocean and climate simulations. These new schemes come at a small additional computational cost, especially compared to higher resolution simulations, and provide a means of understanding the origin of model biases and uncertainties.

How to cite: Juricke, S., Bagaeva, E., Danilov, S., and Koldunov, N.: Modelling the North Atlantic: How parameterizations affect model biases and uncertainties, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10107, https://doi.org/10.5194/egusphere-egu24-10107, 2024.

EGU24-11088 | ECS | Posters on site | OS1.2

Pathways of Glacial Meltwater from the Hudson Strait into the North Atlantic Ocean: Insights from Eddy-Resolving Model Simulations 

Sara Martin Alis, Olivier Marchal, Alan Condron, and Sean (Si-Yuan) Chen

A long-standing question in paleoclimate research concerns the fate and consequences of the glacial water released into the ocean from the Laurentide Ice Sheet (LIS) during the last deglaciation. In this presentation, we will describe detailed simulations of the pathways of glacial meltwater released from the LIS which have been obtained from an eddy-resolving, regional configuration of the general circulation model of the MIT (MITgcm) coupled with a sea-ice model. Emphasis will be placed on glacial meltwater discharged from Hudson Strait into the Labrador Sea and on its interaction with the North Atlantic Current (NAC). Our regional configuration of the MITgcm represents the glacial Atlantic between 34.5oN and 67oN at a horizontal resolution of 1/20o, with 61 vertical levels (21 levels in the upper 100 m), and with continental shelves removed (sea level lowered by 130 m). The relatively fine spatial grid permits the simulation of the mesoscale eddy field and of the baroclinic structure of the buoyant current produced by the meltwater inflow. Surface forcing is provided by the atmospheric conditions during the last glacial maximum which have been simulated by a global climate model (Community Climate System Model v.3). Our preliminary results show that the meltwater current from Hudson Strait flows to the SE along the continental slope of Labrador and Newfoundland and sheds anticyclonic eddies which carry offshore meltwater and are entrained by the NAC near the Grand Banks. In turn, the meltwater influences the NAC through its effect on seawater density, suggesting a new mechanism by which glacial water fluxes may change large-scale circulation in the North Atlantic. In our presentation, attention will be paid on the influence of the meltwater on the strength and structure of the NAC near and downstream of the Grand Banks.

How to cite: Martin Alis, S., Marchal, O., Condron, A., and Chen, S. (.-Y.: Pathways of Glacial Meltwater from the Hudson Strait into the North Atlantic Ocean: Insights from Eddy-Resolving Model Simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11088, https://doi.org/10.5194/egusphere-egu24-11088, 2024.

Wintertime variability of both the strength of the jet stream and the North Atlantic Oscillation (NAO) index have been correlated in decadal time scale. Both have positive trends since the 1960s which have been recently proposed to be connected to anthropogenic global warming. At the same time there is a rich literature explaining both the observed variability and also the discrepancy with circulation models in which the variability is usually much smaller. Among the proposed mechanisms were “tug-of-war” between the tropics and the Arctic lower troposphere and surface temperatures, Arctic amplification, polar vortex strength. However, none of those forcing can not explain the trends in all the studied period.

 

The motivation behind the present study is to find a mechanism which can explain the variability and trend in the whole period of accelerated global warning, that is since the middle of the previous century. One possible candidate can be warming of the troposphere and cooling of the stratosphere, both well established results of the increase in greenhouse gas forcing. Together with the lowering of the tropopause altitude with increasing latitude, this results in warming south of the jet stream and cooling north of it, increasing the very gradient which sustains a thermal wind such as the jet stream.

 

The results of early analysis show that the greenhouse related tropospheric warming / stratospheric cooling is a plausible candidate for the driver of changes in the wintertime jet stream strength and related NAO changes supporting the notion that NAO may head towards constant positive values. However the question remains why such changes are only visible in the Atlantic sector and not elsewhere in the mid-latitudes of the Northern Hemisphere. The multidecadal wintertime NAO changes seemed related with the AMO/AMV variability of North Atlantic SST values at least until the 1990s. This leaves the possibility that both Atlantic SSTs and greenhouse gas forcing are drivers of the variability in the wintertime jet stream strength.

 

How to cite: Piskozub, J.: Anthropogenic influence on wintertime jet stream strength in the Atlantic sector. Is it real? Is it Atlantic SST mediated?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11572, https://doi.org/10.5194/egusphere-egu24-11572, 2024.

EGU24-11980 | ECS | Posters on site | OS1.2

How does tropical Atlantic Multidecadal Variability develop? 

Balaji Senapati, Christopher H. O’Reilly, and Jon Robson

Atlantic Multidecadal Variability (AMV) has been linked to climate variability in many regions across the globe. However, the mechanisms through which the AMV develops remain unclear. Modelling studies show that global teleconnections from the AMV are sensitive to how the tropical branch is represented, though understanding how the decadal Sea Surface Temperature (SST) anomalies develop in this region has received little attention. Here, we present a quantitative examination of the generation of tropical AMV using SST restoring experiments. In contrast to the generally proposed mechanisms of wind-flux-SST or cloud feedback, this study provides new insight into the dominance and crucial role of upper ocean dynamics, particularly concerning the mixed layer depth. Given the sensitivity of tropical AMV on global implications, the accurate simulation of the upper ocean dynamics in coupled climate models becomes imperative.

How to cite: Senapati, B., O’Reilly, C. H., and Robson, J.: How does tropical Atlantic Multidecadal Variability develop?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11980, https://doi.org/10.5194/egusphere-egu24-11980, 2024.

EGU24-12078 | ECS | Posters on site | OS1.2

Understanding the influence of Atlantic Meridional Overturning Circulation interannual variability on European cold extremes 

Eduardo Alastrué de Asenjo, Jana Sillmann, and Johanna Baehr

Changes in the Atlantic Meridional Overturning Circulation (AMOC) impact the redistribution of heat across the climate system and can therefore influence surface temperatures over land. A large AMOC weakening, frequently analysed through idealised model simulations (e.g., freshwater hosing experiments), would lead to a strong cooling over the Northern Hemisphere. This cooling is most pronounced for winter months, suggesting a potential influence on cold extreme events; and for Europe, this influence has been hinted at. However, whether a more realistic interannual variability in the AMOC, rather than an idealised long-term weakening, also influences European mean temperatures and cold extremes is thus far unknown.

To unravel this issue, we use the historical simulations of the 50-member MPI-ESM1.2-LR large ensemble, whose size is particularly suitable for analysing extremes. In these simulations, we categorise European temperatures based on their preceding interannual AMOC strengths. For yearly mean temperatures in a pre-industrial climate, we find that the distribution of temperatures following weak interannual AMOC strengths is significantly shifted towards colder values compared to years preceded by strong interannual AMOC strengths. Among all seasons, this shift is largest in winter; and spatially it is accentuated for northern latitudes. When considering present-day climate, the same shift still occurs, although less pronounced and strongest now for Eastern Europe. For daily extreme cold temperatures, the distribution of events is again colder following years of prevalent weak AMOC strengths; and this difference also becomes less clear and moves south-eastward in present-day climate. We complete the analysis by looking at the potential chain of physical atmospheric mechanisms that explains not only the connection between AMOC strengths and European extreme cold temperatures but also the evolution of this connection in the recent past.

How to cite: Alastrué de Asenjo, E., Sillmann, J., and Baehr, J.: Understanding the influence of Atlantic Meridional Overturning Circulation interannual variability on European cold extremes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12078, https://doi.org/10.5194/egusphere-egu24-12078, 2024.

EGU24-12614 | ECS | Orals | OS1.2

The influence of tides on the AMOC in an eddy-permitting global general circulation model 

Federica Borile, Paola Cessi, Doroteaciro Iovino, and Nadia Pinardi

The energy budget of the global ocean circulation highlights the importance of winds and tides as primary energy sources. Tidal influence extends throughout the water column, particularly in regions of rough topography where internal waves are generated, leading to the conversion of energy from barotropic to baroclinic high-frequency modes. Our study explores the impact of tidal forcing on the general circulation using different experiments of a mesoscale-permitting global ocean model, with the addition of a topographic wave drag parametrization for unresolved scales. The focus is specifically on the Atlantic meridional overturning circulation (AMOC). Our findings reveal that tides interact with mesoscale structures, either reinforcing or weakening the mean circulation based on the dynamic conditions of the flow. On a basin scale, we find that the meridional circulation is weakened by tides on multidecadal time scales, despite robust interannual variability. We analyze these impacts in the momentum balance, concentrating on the role of tides in altering the AMOC geostrophic balance.

How to cite: Borile, F., Cessi, P., Iovino, D., and Pinardi, N.: The influence of tides on the AMOC in an eddy-permitting global general circulation model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12614, https://doi.org/10.5194/egusphere-egu24-12614, 2024.

EGU24-12743 | ECS | Posters on site | OS1.2

Holocene Variability of the AMOC as derived from 231Pa/230Th 

Lukas Gerber, Jörg Lippold, Finn Süfke, Ole Valk, Manuel Ehnis, Saskia Tautenhahn, Lars Max, Cristiano M. Chiessi, Marcel Regelous, Sönke Szidat, and Frerk Pöppelmeier

Climate models and paleo-reconstructions suggest that alterations in the Atlantic Meridional Overturning Circulation (AMOC) are not only indicators but also drivers of climate changes. Therefore, the AMOC is considered a critical tipping element within Earth’s climate system. Many lines of evidence indicate that the last glacial termination was characterised by large swings in AMOC strength, yet proxy evidence remains ambiguous about centennial-scale fluctuations during the Holocene. Inconsistencies persist regarding the timing, spatial pattern, and intensity of North Atlantic deep-water production. This study evaluates the variability of the AMOC during the Holocene based on several marine sediment cores covering the North Atlantic in high temporal resolution. For this, we exploit the 231Pa/230Th proxy, which indicates the bottom water advection strength. Additionally, past particle fluxes were reconstructed to determine a possible influence of particle composition and particle rain rate on the 231Pa/230Th signal. This study thus aims to extend existing paleo-circulation reconstructions of the AMOC from the last deglacial period with more recent analyses. Five new high-resolution 231Pa/230Th down-core records from different oceanographic settings and water depths in the North Atlantic consistently exhibit low variability throughout the entire Holocene. The 231Pa/230Th records generally display deviations of ± 10% from their respective Holocene mean. A generalised additive model (GAM) was fitted to the timeseries to detect mean North Atlantic trends within the different Holocene-normalised datasets. This model exhibits a virtually constant 231Pa/230Th level throughout the Holocene, interrupted by two time periods of slightly increased ratios, indicative of a weaker AMOC. The first time period is within the timeframe of the 8.2 ka event, characterised by a sudden cold spell across parts of the Northern Hemisphere. During this interval, four of the five timeseries show slightly elevated 231Pa/230Th ratios, although two records within this period hold a reduced sampling resolution. This limited temporal resolution and the shortness of the event make it challenging to decidedly conclude on the magnitude of the AMOC weakening during this time. The second period of higher 231Pa/230Th coincides with the 4.2 ka event and is only evident from the ODP 1063 data (Bermuda Rise). However, these higher 231Pa/230Th ratios can be explained by increased bottom scavenging of 231Pa presumably caused by benthic storms, induced by the transfer of eddy kinetic energy from the surface to the deep ocean. Consequently, atmospheric forcing during the 4.2 ka event seems to be a more plausible explanation than a paleoceanographic cause for the observed higher 231Pa/230Th. In conclusion, our study suggests that deep ocean circulation in the North Atlantic did not exhibit high variability on sub-millennial time scales, but has remained relatively stable throughout the Holocene.

How to cite: Gerber, L., Lippold, J., Süfke, F., Valk, O., Ehnis, M., Tautenhahn, S., Max, L., Chiessi, C. M., Regelous, M., Szidat, S., and Pöppelmeier, F.: Holocene Variability of the AMOC as derived from 231Pa/230Th, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12743, https://doi.org/10.5194/egusphere-egu24-12743, 2024.

Thermal variability in the subpolar North Atlantic Ocean may be understood in terms of opposing fast and slow responses to atmospheric events, such as involving the response to the North Atlantic Oscillation (NAO). What is unclear is the associated ocean carbon response to atmospheric events, and how that response differs from the thermal response? Here, we diagnose the output from a full Earth system model, UKESM1 piControl simulation integrated over 1100 years, and analyse the transient response to a composite NAO event, derived from combining 270 NAO+ and 246 NAO- individual events. The carbon response is then separated into a fast and slow response to the onset of a single NAO event. During a NAO+ event, there is an initial local response extending over the first one to two years involving anomalous surface cooling and air-sea uptake of carbon in the subpolar gyre. Consequently, there is a reduction in heat storage and an increase in ocean dissolved inorganic carbon (DIC), together with enhanced mixed-layer entrainment of nutrients leading to an increase in biological export of carbon. There is then a delayed response extending for a further 10 years, involving an influx of warm and salty waters through ocean advection, which also carries an increase in both alkalinity and dissolved inorganic carbon. Hence, the ocean thermal and carbon responses  involve  a combination of fast, local responses to atmospheric  forcing (involving air-sea exchange, entrainment and biological export) plus a slow, far-field response to prior atmospheric events (involving ocean redistribution of heat, salt, alkalinity and carbon together with continued air-sea exchange). The thermal and carbon responses differ in that the thermal response involves opposing signs in the fast and slow contributions, while the carbon response involves reinforcing fast and slow contributions. This asymmetry is primarily due to opposing signs in the fast contributions with surface cooling leading to a reduction in heat storage, but an increase in carbon storage. Hence, the ocean memory of an atmospheric event is greater for carbon than for heat. 

How to cite: Williams, R. and Khatri, H.: Reinforcing fast and slow carbon responses to atmospheric events in the subpolar North Atlantic , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12828, https://doi.org/10.5194/egusphere-egu24-12828, 2024.

EGU24-13381 | ECS | Posters on site | OS1.2

Imperfect emergency brake: Can delayed Solar Radiation Modification revert AMOC and SPG weakening?  

Claudia Wieners, Daniel Pflüger, Leo van Kampenhout, René Wijngaard, and Henk Dijkstra

 

Solar Radiation Modification (SRM) is a collection of hitherto hypothetical methods that would reflect a small fraction of incoming solar radiation, thereby cooling the Earth and reducing the impact of greenhouse gas forcing, albeit imperfectly.  The best-researched method so far is Stratospheric Aerosol Injection (SAI), which would work by injecting a reflective aerosol (e.g. sulphate) or a precursor gas (e.g. SO2) into the stratosphere.

Previous studies (e.g., Tilmes et al, 2018, 2020, Xie et al., 2022) have shown that SAI and other SRM methods can reduce or even prevent Atlantic Meridional Overturning Circulation (AMOC) weakening. No dedicated study has however been done on the effect of SRM on the Subpolar Gyre (SPG). Also, most SRM modelling studies focus on present-day (2020) or at least speedy initialization of SRM. In reality, SRM might only begin many decades from now, if at all. In our study, we investigate whether delaying SRM will cause irreversible changes to the AMOC and the SGP.

 

To this end we compare three scenarios in the CESM2 model:

  • Control: An extreme warming scenario (RCP8.5) without SAI
  • SAI2020: As Control, but keeping global mean surface temperature constant by means of SAI from 2020 onwards
  • SAI2080: As Control, but starting SAI from 2080 such as to bring global mean surface temperature to 2020 levels and keeping it constant thereafter.

These are extreme scenarios, not intended to represent plausible policy choices but meant to investigate whether irreversibility can occur in principle.

We find that in Control AMOC weakens from 16 Sv in 2020 to 7 Sv in 2100, while in SAI2020, it only weakens to 12 Sv. In SAI2080, AMOC stops weakening after 2080, but does not recover (at least till 2100) to the strength it has in SAI2020. Thus, delayed SAI cannot quickly revert AMOC weakening, if at all. This has effects on the local climate, in particular overcooling around the North Atlantic, and even the interhemispheric temperature gradient.

In addition, we find for Control, that deep convection (i.e. deep mixed layers in winter) ceases in the Labrador sea around 2050 and south of Iceland around 2070. Under SAI2020, deep convection remains active south of Iceland. Under SAI2080, deep convection does not recover by 2100.

We conclude that SAI is not a perfect “emergency brake” for global warming: If action is delayed, changes in ocean circulation persist at least for several decades. However, we stress that other, including political, factors must be taken into account when considering (near-term) SAI, and that phasing out greenhouse gas emissions must remain the primary tool of climate policy. 

How to cite: Wieners, C., Pflüger, D., van Kampenhout, L., Wijngaard, R., and Dijkstra, H.: Imperfect emergency brake: Can delayed Solar Radiation Modification revert AMOC and SPG weakening? , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13381, https://doi.org/10.5194/egusphere-egu24-13381, 2024.

EGU24-13748 | Posters on site | OS1.2

Surface factors controlling the volume of accumulated Labrador Sea Water 

Yavor Kostov, Marie-José Messias, Herlé Mercier, David P. Marshall, and Helen L. Johnson

We explore historical variability in the volume of Labrador Sea Water (LSW) using ECCO, an ocean state estimate configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The model’s adjoint, a linearization of the MITgcm, is set up to output the lagged sensitivity of the watermass volume to surface boundary conditions. This allows us to reconstruct the evolution of LSW volume over recent decades using historical surface wind stress, heat, and freshwater fluxes. Each of these boundary conditions contributes significantly to the LSW variability that we recover, but these impacts are associated with different geographical fingerprints and arise over a range of time lags. We show that the volume of LSW accumulated in the Labrador Sea exhibits a delayed response to surface wind stress and buoyancy forcing outside the convective interior of the Labrador Sea, at important locations in the North Atlantic Ocean. In particular, patterns of wind and surface density anomalies can act as a “traffic controller” and regulate the North Atlantic Current’s (NAC) transport of warm and saline subtropical water masses that are precursors for the formation of LSW. This propensity for a delayed response of LSW to remote forcing allows us to predict a limited yet substantial and significant fraction of LSW variability at least a year into the future.  Our analysis also enables us to attribute LSW variability to different boundary conditions and to gain insight into the major mechanisms that drive volume anomalies in this deep watermass. We point out the important role of key processes that promote the formation of LSW both in the Irminger and Labrador Seas: buoyancy loss and preconditioning along the NAC pathway, in the Iceland Basin, the Irminger Sea, and the Nordic Seas.

How to cite: Kostov, Y., Messias, M.-J., Mercier, H., Marshall, D. P., and Johnson, H. L.: Surface factors controlling the volume of accumulated Labrador Sea Water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13748, https://doi.org/10.5194/egusphere-egu24-13748, 2024.

The North Atlantic Subpolar Gyre (SPG) plays an important role in climate predictability and influences climate variability due to its complex coupling with the atmospheric circulation in the North Atlantic and the Atlantic Meridional Overturning Circulation (AMOC). In this study, we investigate the impact of sea surface temperature (SST) variability in the SPG on atmospheric circulation patterns and climate extremes. We use the EC-Earth3 model (T255~80 km) and perform four sets of AMIP-type ensemble experiments with four different prescribed SST anomalies, each with 10 members and spanning 35 years from 1980 to 2014. The experimental design allows the climatic impact of SPG SST variability to be isolated from other global SST modes. Our results show that SPG SST anomalies directly influence atmospheric circulation between 30-75°N, causing zonally oriented wave-like anomalies. Notably, a warm SST anomaly in the subpolar gyre causes strong low-pressure anomalies over the North Atlantic and North Pacific, leading to warming of regions mainly between 45-60°N and cooling of regions mainly between 60-75°N. We find that the anomalous temperatures are particularly pronounced over the North American continent. We also investigate the indirect effects of SPG variability through its synergy with the North Atlantic and North Pacific SSTs, as well as the atmospheric teleconnections and extreme events associated with SPG variability. The results underline the importance of the SPG for the atmospheric circulation, the teleconnections, the regional climate and the extreme events.

How to cite: Karami, M. P., Koenigk, T., and Schenk, F.: Unravelling the impact of subpolar gyre variability on climate extremes and variability:  Insights from an ensemble atmospheric model study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15292, https://doi.org/10.5194/egusphere-egu24-15292, 2024.

EGU24-15604 | Orals | OS1.2

Detecting climatic change in AMOC observations 

Gerard McCarthy, Guillaume Hug, David Smeed, and Ben Moat

The detection of trends and variations in the Atlantic Meridional Overturning Circulation (AMOC) is an important and at times controversial topic. On average, CMIP6 models project a 1 Sv/decade decrease in the strength of the AMOC in response to anthropogenic climate change. Atlantic subpolar decadal sea surface temperature variations of 0.5º indicate an associated change in AMOC strength of 2 Sv. These are challenging thresholds of signal detection for AMOC observing.

 

Estimates of the AMOC streamfunction, such as those from the RAPID array, have a number of sources of variability ranging from short term Ekman transport to variations in the strength of North Atlantic Deep Water associated with deep water formation that have a slower timescale. Climate model studies have shown that Ekman transport contributes little to the signal of future AMOC decline.

 

We look at the nearly 20 years of data from the RAPID array from a signal to noise perspective. Fluctuations associated with Ekman transport are the largest contribution to noise in the AMOC estimates and hold no signal of low frequency change. Deeper layers show more of the low frequency signal. We amplify this low frequency signal by removing the impact of noise derived from the Ekman transport on the deep temperature and salinity. Finally, we show that the best place for detection of low frequency, climatic changes in AMOC is in the deepest North Atlantic Deep Water, with the noise of the wind removed.

How to cite: McCarthy, G., Hug, G., Smeed, D., and Moat, B.: Detecting climatic change in AMOC observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15604, https://doi.org/10.5194/egusphere-egu24-15604, 2024.

EGU24-15696 | Orals | OS1.2

Enhanced northward ocean transport of anthropogenic carbon through recovery of overturning circulation may be affecting North Atlantic CO2 uptake efficiency 

Pete Brown, Elaine McDonagh, Richard Sanders, Ben Moat, Eleanor Frajka-Williams, Brian King, Lidia Carracedo, Andrew Watson, Ute Schuster, Anita Flohr, William Johns, and Molly Baringer

Of the additional carbon dioxide added to the atmosphere by human activities the ocean absorbs approximately a quarter, with a disproportionate fraction accumulating at depth in the North Atlantic due to the combined action of northward ocean transport (through the meridional overturning circulation) and strong air-sea fluxes. Combining repeat hydrography with circulation estimates from the RAPID mooring array at 26N it was found that between 2004 and 2012 these two processes were roughly equal in magnitude, but decreasing ocean transports were tipping the balance more towards air-sea uptake over time as the AMOC weakened. New observations from 2012 to 2022 show that this process has now reversed - a recovering AMOC combined with increasing loadings of carbon is now transporting substantially greater quantities of anthropogenic carbon northwards into the North Atlantic. Changes in regional air-sea fluxes suggests that the increased northward ocean carbon transport may be affecting CO2 uptake capacity downstream.

How to cite: Brown, P., McDonagh, E., Sanders, R., Moat, B., Frajka-Williams, E., King, B., Carracedo, L., Watson, A., Schuster, U., Flohr, A., Johns, W., and Baringer, M.: Enhanced northward ocean transport of anthropogenic carbon through recovery of overturning circulation may be affecting North Atlantic CO2 uptake efficiency, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15696, https://doi.org/10.5194/egusphere-egu24-15696, 2024.

EGU24-15728 | ECS | Orals | OS1.2

AMOC representation in the North Atlantic in a forced ocean model 

Simon Wett, Monika Rhein, Arne Biastoch, and Eleanor Frajka-Williams

The Atlantic Meridional Overturning Circulation (AMOC) plays a vital role in the climate of Europe and the North Atlantic region by redistributing heat and freshwater in the Atlantic. Climate model studies project an AMOC decline under global warming in the 21st century. However, they disagree on the magnitude and timescales of the weakening. Thus, assessing model performance regarding the representation of the AMOC remains essential. Observational estimates can serve as important benchmarks to understand AMOC variability in ocean models. AMOC observations at different monitoring arrays in the North Atlantic have shown strong variability on multiple time scales and no long-term trend. We analyze the AMOC at the North Atlantic Changes (NOAC) array line at 47°N in the high-resolution forced VIKING20X model simulation from 1980 to 2021. The mean AMOC strength is within the range of the NOAC observations. However, the VIKING20X AMOC exhibits a decreasing trend from the mid-1990s until 2010. This decrease coincides with significant cooling and freshening in the subpolar North Atlantic in VIKING20X. In agreement with NOAC observations, VIKING20X shows meridional connectivity between the NOAC and RAPID AMOC when the NOAC AMOC leads by about one year, though less distinct. This agreement indicates a common mechanism, determining the meridional connectivity in observations and VIKING20X. These mechanisms must be understood and represented in climate models to make informed projections of the future AMOC and its role in the climate system. Furthermore, ocean models and gridded observational data sets could help complement new approaches to monitoring the AMOC at key locations using novel methods and instrumentation, such as drift-free bottom pressure sensors, which could help resolve the geostrophic reference level.

How to cite: Wett, S., Rhein, M., Biastoch, A., and Frajka-Williams, E.: AMOC representation in the North Atlantic in a forced ocean model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15728, https://doi.org/10.5194/egusphere-egu24-15728, 2024.

EGU24-16016 | ECS | Orals | OS1.2

Mid-20th Century Atlantic Circulation informed by Modern Observations and Models  

Guillaume Hug, Gerard McCarthy, Ben Moat, and Emma Worthington

The Atlantic Meridional Overturning Circulation (AMOC) is a driving force in the redistribution of heat on our planet and has a particularly large impact on the climate of the Northern Hemisphere and Europe. Reliability of coupled model projections has been questioned due to a body of evidence that the multi-model mean of climate models disagree with observational proxies for the AMOC, in particular in the mid-20th century. In turn, the reliability of these observational proxies has been questioned as they are not direct observations of the AMOC.

In order to study the variations of AMOC during the 20th century, we have developed layered models based on a limited number of time series: Ekman transport and the Florida Strait, as well as the density time series of the Thermocline, Antarctic Intermediate Waters (AAIW), Upper North Atlantic and Lower North Atlantic Deep Waters (UNADW, LNADW). These models, using the deep AMOC branches, are trained with modern RAPID measurements at 26N and compared to each other.

We use these models to predict, from hydrographic profiles, an estimate of the strength of the AMOC during the (mid) 20th century. Locations where EN4 profiles may be relevant to the reconstruction are identified using ocean model data that correlate temperature and salinity with the location of the RAPID measurement. The linear contribution of wind stress is also removed from the density time series using simple linear regression. Our aim is to provide, in the light of modern direct observations, an answer on the reliability of AMOC reconstructions and historical climate simulations during the mid-20th century.

How to cite: Hug, G., McCarthy, G., Moat, B., and Worthington, E.: Mid-20th Century Atlantic Circulation informed by Modern Observations and Models , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16016, https://doi.org/10.5194/egusphere-egu24-16016, 2024.

EGU24-16264 | ECS | Orals | OS1.2

Greenland Tip Jet in the future: Declining Surface Heat Loss in a High-Resolution CESM Simulation (2015-2099) 

Aleksandr M. Fedorov, Claudia E. Wieners, Marieke Femke de Jong, and Henk A. Dijkstra

The Greenland Tip Jet is a strong westerly wind generated by the interaction between the synoptic Icelandic Low and the steep Greenland orography. Tip Jets were not extensively explored until the beginning of 2000s when gridded atmospheric products reached temporal and spatial resolution high enough to resolve such mesoscale wind events. This mesoscale wind affects surface heat and freshwater content in the area to the southeast of Greenland and then it causes intensification of deep water formation in the Irminger Sea. Through this increase in deep convection intensity, Tip Jets can potentially affect the large scale Atlantic Meridional Overturning Circulation (AMOC) transport on daily-centennial time scales. Given Tip Jets’ role in deep convection, the research question arises: Will the influence of Tip Jets on AMOC change in the future? In the current research, we aim to fill the gap on the Tip Jet variability in the 21st century using the high resolution (0.25°) CESM 1.3 future climate simulation forced with RCP 8.5 for 2015-2099. We identify Tip Jets, estimate future composite anomalies of the surface heat flux and wind stress associated with Tip Jet events, and define the leading factors of their variability in the 21st century. Our analysis reveals no significant trends in Tip Jet frequency or wind stress for 2015-2099. Although no long-term changes are modelled in Tip Jets and wind stress, upward surface heat flux decreases both during Tip Jet days and during the whole winter season (DJFM) in the area to the southeast of Greenland. We attribute this decrease in surface cooling to changes in air-sea temperature difference (Ta – SST). To the east of Cape Farewell, the atmosphere is warming faster than water, causing Ta – SST to shrink during the 21st century. The observed trend in Ta – SST subsequently appears in surface latent and sensible heat fluxes growth for 2015-2099. Therefore, the more rapid warming of the atmosphere compared to the ocean leads to an increase in background latent and sensible heat, resulting in less cold being transported to the central Irminger Sea during Tip Jets. We showed that Tip Jets will likely continue to affect heat and freshwater content in the Irminger sea, however, the character of this influence will be different with climate change during the 21st century. 

How to cite: Fedorov, A. M., Wieners, C. E., de Jong, M. F., and Dijkstra, H. A.: Greenland Tip Jet in the future: Declining Surface Heat Loss in a High-Resolution CESM Simulation (2015-2099), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16264, https://doi.org/10.5194/egusphere-egu24-16264, 2024.

The subpolar North Atlantic assumes a key role in ventilating the ocean’s interior as it is a primary site for deep water formation. Dissolved oxygen concentrations exhibit high sensitivity to climate variability and changes due to the interplay between sea-surface temperature fluctuations and ocean stratification. This relationship not only affects the solubility of dissolved oxygen but also modulates its transport from the near-surface ocean to the interior, known as ventilation. We collected sixty years of observations, spanning from 1960 to 2022, from three different datasets: GLODAPV2, WOD18 and BGC-Argo. These data underwent rigorous secondary quality control process, which adjusted biases between GLODAPV2 and WOD18, as well as BGC-Argo to minimize systematic errors. We conducted an in-depth analysis of the long-term changes and interannual variability in dissolved oxygen, apparent oxygen utilization (AOU), oxygen utilization rate (OUR) and water mass ages within the upper 2000 meters of the water column. Our specific focus encompassed the Subpolar Mode Water (SPMW), Intermediate Water (IW) and Labrador Sea Water (LSW). The computation of OUR and water mass ages in particular relied on tracer data such as chlorofluorocarbons (CFCs) and Sulphur hexafluoride (SF6) to estimate ventilation ages via the Transit Time Distribution (TTD) method. OUR provides insights into local oxygen consumption due to remineralization of organic matter, while the total AOU is the integrated OUR along the pathway of the water parcel. Therefore, identifying these parameters enables to distinguish between the primary drivers behind oxygen variations in the subpolar North Atlantic, namely air-sea gas exchanges, ocean circulation, and marine biology.

How to cite: Stendardo, I. and Steinfeldt, R.: Ventilation changes in the Subpolar North Atlantic: Insights from Six Decades of Oxygen Observations and Tracer-Based Age Analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16353, https://doi.org/10.5194/egusphere-egu24-16353, 2024.

EGU24-16587 | ECS | Orals | OS1.2

Simulated Atlantic Meridional Overturning Circulation in a warmer climate and the linkage with the North Atlantic convection using EC-Earth-HR 

René Gabriel Navarro Labastida, Mehdi Pasha Karami, Torben Koenigk, Agatha de Boer, and Marie Sicard

This study aims to analyze the effect of increasing atmospheric CO2 concentrations on the Atlantic Meridional Overturning Circulation (AMOC) and its dependence on convection in the  Labrador (LAB) and Greenland (GIN) Seas. We have used EC-Earth3-HR, the high-resolution version of the global coupled climate model EC-Earth3 in this study. EC-Earth3-HR has a resolution of about 0.25 degrees in the ocean and 40 km in the atmosphere. In contrast to the HighResMIP-protocol, EC-Earth3-HR has undergone a tuning process and a multi-centennial spin-up has been performed. The set of experiments analyzed here consists of a pre-industrial control simulation (piControl), a one percent per year increase in CO2 experiment (1pctCO2) branching from year 250 of our piControl simulation, and two experiments with fixed CO2 concentrations (400.9 ppm and 551.5 ppm) branch off from two points corresponding to global temperature anomalies of around 1°C and 2°C in the 1pctCO2 experiment. Here we have defined deep convection as the mean mixed volume in March, with deep convection equal to zero when the mixed-layer is shallower than a critical depth. Our preliminary results suggest that as the climate warms, the North Atlantic waters become warmer and fresher, promoting the weakening of the North Atlantic deep convection and a subsequent reduction in AMOC strength (up to 20% reduction). The simulated overturning circulation weakening seems to be dominated by changes in LAB deep convection with GIN convection contributing less. Circulation changes in the pre-industrial and the different CO2 concentration experiments are dominated by a strong decadal variability. Compared to the standard resolution EC-Earth3-version, the use of a high resolution leads to deeper ocean mixing in LAB and GIN. More analysis has to be done on the way to clarify to what extent increased resolution affects our results in comparison with previous studies.

How to cite: Navarro Labastida, R. G., Karami, M. P., Koenigk, T., de Boer, A., and Sicard, M.: Simulated Atlantic Meridional Overturning Circulation in a warmer climate and the linkage with the North Atlantic convection using EC-Earth-HR, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16587, https://doi.org/10.5194/egusphere-egu24-16587, 2024.

EGU24-17617 | Orals | OS1.2

Atlantic meridional nutrient transport 2004-2018 timeseries: insights into inorganic nutrient pool reorganization by the AMOC  

Lidia I. Carracedo, Elaine McDonagh, Richard Sanders, Mark Moore, Herlé Mercier, Pete Brown, Sinhué Torres-Valdés, Edward W. Mawji, Molly Baringer, David Smeed, and Gabriel Rosón

North Atlantic (NA) biological productivity and resulting carbon uptake (Biological Carbon Pump, BCP) are supported by the northward transport of nutrients by the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). Changes in the strength of the AMOC are subject to influence ocean nutrient cycling and the efficiency of the BCP. In this study, we present evidence for non-steady state behaviour based on 14 years of observations (2004-2018) at 26.5°N. Our results show significant (>80%) nutrient transport variability tightly related to AMOC alongside predominantly net southward nutrient transport exceeding total nutrient sources. Changes over the observational period indicate: i) increasing NA BCP efficiency (remineralized:preformed ratio); ii) decreasing NA nutrient inventory, except towards the end of the period when the system was closer to balance.

How to cite: Carracedo, L. I., McDonagh, E., Sanders, R., Moore, M., Mercier, H., Brown, P., Torres-Valdés, S., Mawji, E. W., Baringer, M., Smeed, D., and Rosón, G.: Atlantic meridional nutrient transport 2004-2018 timeseries: insights into inorganic nutrient pool reorganization by the AMOC , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17617, https://doi.org/10.5194/egusphere-egu24-17617, 2024.

EGU24-17710 | ECS | Orals | OS1.2

Circulation of freshwater over the Labrador shelf and into the interior subpolar North Atlantic 

Elodie Duyck and Eleanor Frajka-Williams

Increasing freshwater input from Greenland and the Arctic could potentially affect the stratification of the water column in the Labrador Sea, and weaken deep convection. While freshwater export from the West Greenland shelf to the interior Labrador Sea is well-documented, little to no exchange is believed to take place off the Labrador Shelf.
In this study, we use drifters deployed on the Greenland and Labrador shelves since 2019 to deepen our understanding of the Labrador shelf surface circulation and cross-shelf exchanges. Trajectories confirm that fresh surface waters from Baffin Bay, Hudson Bay, and the West Greenland Current join to form the Labrador Current with two distinct velocity cores: one at the shelf break and a second inshore coastal core. The recent drifter observations provide further detail about the shelf circulation including topographically-steered exchanges between the main core and the coastal core of the Labrador Current, and confirm the absence of direct connection between Baffin and Hudson Bays, and the interior Labrador Sea. Instead, substantial export takes place between Flemish Cap and the tail of the Grand Banks, with the export location dependent on upstream circulation.
Freshwater originating from the Baffin and Hudson Bays, and the west Greenland ice sheet, is unlikely to directly impact the Labrador Sea deep convection region. Their mixing and diluting along this longer pathway complicate their potential influence on deep convection in the Subpolar North Atlantic.

How to cite: Duyck, E. and Frajka-Williams, E.: Circulation of freshwater over the Labrador shelf and into the interior subpolar North Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17710, https://doi.org/10.5194/egusphere-egu24-17710, 2024.

EGU24-19638 | ECS | Posters on site | OS1.2

Relating excess and redistributed temperature to the Turner Angle in the subtropical North Atlantic using GO-SHIP observations and Machine Learning 

Matthew Clark, Dafydd G. Evans, Elaine McDonagh, and Fatma Jebri

The ocean takes up 93% of the warming in the climate system. Here, we develop methods to isolate this warming signature using multidecadal observations in the North Atlantic. As part of GO-SHIP, repeat ship-based CTD hydrographic observations have been made across the A05 section in the North Atlantic at 24.5˚N. These are climate quality observations of relatively high spatial resolution along the section, providing a unique opportunity to monitor the state of Atlantic physical properties and biogeochemistry. The A05 section has been occupied approximately every 5 years since 1992. Temperature and salinity variability across A05 is influenced by several factors, including air-sea interaction and the effects of anthropogenically driven climate change. Excess temperature is a measure of the amount of extra temperature in the ocean due to post-industrial atmospheric CO2, whereas redistributed temperature quantifies the reorganisation of ocean temperature structure by ocean circulation and mixing. Existing methods to decompose the excess and redistributed temperature changes rely on estimates of the anthropogenic carbon change. The Turner angle, which represents the angle between the theta-s curve and an isopycnal in theta-s space, provides an index of the relative contributions of temperature and salinity on stratification, and thus, on water column stability. Using data from A05, we explore how temporal shifts in temperature and salinity affect the Turner angle, with the aim of using this relationship to separate the excess and redistributed components of change without relying on estimates of anthropogenic carbon. We will establish the relationship between excess and redistributed temperature and Turner angle using Machine Learning tools and the known link between anthropogenic carbon and excess temperature. This approach will enable the use of the Turner angle-based method in areas without any carbon data.

How to cite: Clark, M., Evans, D. G., McDonagh, E., and Jebri, F.: Relating excess and redistributed temperature to the Turner Angle in the subtropical North Atlantic using GO-SHIP observations and Machine Learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19638, https://doi.org/10.5194/egusphere-egu24-19638, 2024.

Two major trans-basin mooring arrays, the Rapid Climate Change-Meridional Overturning Circulation and Heatflux Array (RAPID) at 26.5°N since 2004 and the Overturning in the Subpolar North Atlantic Program (OSNAP) situated at 53°–60°N since 2014, have been continuously monitoring the Atlantic
Meridional Overturning Circulation (AMOC). This study explores the connectivity of AMOC across these two mooring lines from a novel adiabatic perspective utilizing a model-based data set. The findings unveil significant in-phase connections facilitated by the adiabatic basinwide redistribution of water between the two lines on a monthly timescale. This adiabatic mode is a possible cause for the observed subpolar AMOC seasonality by OSNAP. Furthermore, the Labrador Sea was identified as a hotspot for adiabatic forcing of the overturning circulations, primarily attributed to its dynamic isopycnal movements.

How to cite: Han, L.: AMOC Connectivity Between the RAPID and OSNAP Lines Revealed by a Model-Based Dataset, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20241, https://doi.org/10.5194/egusphere-egu24-20241, 2024.

EGU24-313 | ECS | Orals | OS1.3 | Highlight

Multi-decadal changes in water mass properties of the South Indian Ocean along 110°E 

Meng Han, Helen Phillips, Nathan Bindoff, Ming Feng, and Ramkrushnbhai Patel

Two hydrographic voyages separated by 56 years reveal significant changes in the watermass properties in the southeast Indian Ocean along 110°E. The observations from the International Indian Ocean Expedition in 1963 and the reoccupation of the line in 2019 covered the full ocean depth from 40°S to 11°S, measuring physical, chemical, and biological properties. We focus on the physical and biogeochemical properties in watermass layers of the global meridional overturning circulation and the Indian Ocean’s shallow overturning cells.  The subtropical high salinity water (STHW), which forms the lower branch of the shallow overturning cells, has warmer and increased salinity. Subantarctic Mode Water has cooled and freshened on density levels and Antarctic Intermediate Water (AAIW) has warmed and increased in salinity. Both the SAMW and AAIW watermasses have decreased dissolved oxygen content but increased concentrations of nitrate and phosphate. The results show that changes within watermasses follow their northward pathways, suggesting influences from their formation regions, modified by interior mixing along the overturning pathways.

How to cite: Han, M., Phillips, H., Bindoff, N., Feng, M., and Patel, R.: Multi-decadal changes in water mass properties of the South Indian Ocean along 110°E, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-313, https://doi.org/10.5194/egusphere-egu24-313, 2024.

EGU24-727 | ECS | Orals | OS1.3 | Highlight

Indian Summer Monsoon Rainfall trends over 1979-2022 driven by ocean warming and anomalous wind patterns. 

Ligin Joseph, Nikolaos Skliris, Dipanjan Dey, and Robert Marsh

India receives 80% of its annual rainfall during the Indian Summer Monsoon (ISM) season from June to September. The climate model simulations of Coupled Model Intercomparison Project 6 (CMIP6) robustly indicate a strengthening of the Indian summer monsoon rainfall in a warming climate, despite a reduced land-sea thermal contrast. In this study, we analysed the ISM precipitation trend over India from 1979 to 2022 using rain gauge, satellite-derived, and atmospheric re-analysis data. The results show a broad-scale increasing precipitation trend over major parts of India. However, there is strong spatial variability, with a pronounced precipitation increase over Western India and decreasing precipitation in parts of north-eastern India. The precipitation trend pattern is associated with sea surface temperature (SST) and wind anomalies over the Indian Ocean. Observations indicate a basin-scale warming of the Indian Ocean (IO) that is more prominent in the west equatorial region and Arabian Sea (AS), altering the east-west SST gradient over this period, which is associated with increased equatorial winds during the summer monsoon period. Evaporation correspondingly increases over the Indian Ocean, with widespread increases along the typical atmospheric moisture transport pathway over the western Indian Ocean during the summer monsoon, driven by both ocean surface warming and increasing winds. Increased evaporation results in more moisture being available in the atmosphere over the western Indian Ocean, which subsequently feeds ISM precipitation. Furthermore, a strong correlation between the AS moisture transport and the ISM rainfall has been noticed over the central and western parts of India, where increased precipitation trends exist. A moisture budget trend analysis over Western India suggests that the large increase in moisture convergence in this area is driven by increased moisture entering from the AS concomitant with strongly reduced outgoing moisture transport through the eastern and northern boundaries. A detailed analysis shows that the increased moisture convergence in Western India is predominantly attributed to changes in the wind pattern driven by anomalously reduced winds in the northern part of the peninsula. In addition, the teleconnections between ISM rainfall and large-scale natural climate variability modes such as ENSO and IOD were also shown to modulate precipitation variations over India during the considered period at inter-annual to multi-decadal scales. 

How to cite: Joseph, L., Skliris, N., Dey, D., and Marsh, R.: Indian Summer Monsoon Rainfall trends over 1979-2022 driven by ocean warming and anomalous wind patterns., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-727, https://doi.org/10.5194/egusphere-egu24-727, 2024.

The Indian Ocean dipole (IOD) has a significant impact on the global atmospheric circulation and contributes to determining important aspects of local and global environments. Although the IOD events can significantly cause SST anomalies and chlorophyll fluctuations in the western Indian Ocean, there is still very little known about the interannual variability of the Arabian Sea oxygen minimum zone (ASOMZ) under the influence of these remote forcing processes. In this study, a coupled physical-biogeochemical numerical model was used to investigate the dynamical response of the ASOMZ to extreme negative (2016) and positive (2019) IOD events. Our findings revealed that the suboxic area of the ASOMZ reduced (expanded) by about 27% (about 28%) after the negative (positive) IOD event. Compared to the 2019 pIOD event, approximately 2.5 times more oxygen-rich water was delivered into the Arabian Sea during the 2016 nIOD event, replenishing dissolved oxygen (DO) consumed by intensified upwelling-induced enhanced remineralization of particulate organic matter (POM), thereby increasing the DO concentration in the Gulf of Aden. Conversely, more POM from the upwelling regions in the western Arabian Sea was transported to the central Arabian Sea, leading to a subsequent decrease in DO concentration there. These findings contributed to our understanding of the ASOMZ's response to IOD events, which is essential for studying the Arabian Sea's marine ecosystem.

How to cite: Zhang, Z.: Dynamical Response of the Arabian Sea Oxygen Minimum Zone to the Extreme Indian Ocean Dipole Events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1410, https://doi.org/10.5194/egusphere-egu24-1410, 2024.

EGU24-1583 | ECS | Orals | OS1.3

The Influence of Freshwater Input on the Evolution of the 1995 Benguela Niño 

Leo Costa Aroucha, Joke Lübbecke, Mareike Körner, Rodrigue Anicet Imbol Koungue, and Founi Mesmin Awo

Benguela Niño events are characterized by strong warm sea surface temperature (SST) anomalies off the Angolan and Namibian coasts. In 1995, the strongest event in the satellite era took place, impacting fish availability in both Angolan and Namibian waters. In this study, we use direct observations, satellite data, and reanalysis products to investigate the impact that the up-until-now unnoticed mechanism of freshwater input from Congo River discharge (CRD) and precipitation had on the evolution of the 1995 Benguela Niño. Before the onset phase of the event, anomalous rainfall in November/December 1994 at around 6ºS, combined with a high CRD, generated a low salinity plume. The plume was advected into the Angola-Namibia region in the following February/March 1995 by an anomalously strong poleward surface current generated by the relaxation of the southerly winds and shifts in the coastal wind stress curl. The presence of this low surface salinity anomaly of about -2 psu increased ocean stability by generating barrier layers, thereby reducing the turbulent heat loss, since turbulent mixing acted on a weak vertical temperature gradient. A mixed layer heat budget analysis demonstrates that southward advection of Angolan waters drove the warming at the onset of the event, while reduced mixing played the main role at the event’s peak. We conclude that a freshwater input contributed to the SST increase in this exceptionally strong event and suggest that this input can influence the SST variability in Angola-Namibia waters through a combination of high CRD, precipitation, and the presence of a strong poleward surface current.

How to cite: Costa Aroucha, L., Lübbecke, J., Körner, M., Imbol Koungue, R. A., and Awo, F. M.: The Influence of Freshwater Input on the Evolution of the 1995 Benguela Niño, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1583, https://doi.org/10.5194/egusphere-egu24-1583, 2024.

EGU24-2595 | Orals | OS1.3

Emergence of the Central Atlantic Niño 

Lei Zhang, Chunzai Wang, Weiqing Han, Michael McPhaden, Aixue Hu, and Wen Xing

The Atlantic Niño is characterized by sea surface warming in the equatorial Atlantic, which can trigger La Niña - the cold phase of El Niño-Southern Oscillation (ENSO). Although observations show that the Atlantic Niño has weakened by approximately 30% since the 1970s, its remote influence on ENSO remains strong. Here we show that this apparent discrepancy is due to the existence of two types of Atlantic Niño with distinct patterns and climatic impacts, which we refer to as the central and eastern Atlantic Niño. Our results show that with equal strength, the central Atlantic Niño has a stronger influence on tropical climate than its eastern counterpart. Meanwhile, the eastern Atlantic Niño has weakened by approximately 50% in recent decades, allowing the central Atlantic Niño to emerge and dominate the remote impact on ENSO. Given the distinct climatic impacts of the two types, it is necessary to distinguish between them and investigate their behaviors and influences on climate in future studies.

How to cite: Zhang, L., Wang, C., Han, W., McPhaden, M., Hu, A., and Xing, W.: Emergence of the Central Atlantic Niño, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2595, https://doi.org/10.5194/egusphere-egu24-2595, 2024.

EGU24-2956 * | ECS | Orals | OS1.3 | Highlight

Future Indian Ocean warming patterns 

Sahil Sharma, Kyung-Ja Ha, Ryohei Yamaguchi, Keith B. Rodgers, Axel Timmermann, and Eui-Seok Chung

Most future projections conducted with coupled general circulation models simulate a non-uniform Indian Ocean warming, with warming hotspots occurring in the Arabian Sea (AS) and the southeastern Indian Ocean (SEIO). But little is known about the underlying physical drivers. Here, we are using a suite of large ensemble simulations of the Community Earth System Model 2 to elucidate the causes of non-uniform Indian Ocean warming. Strong negative air-sea interactions in the Eastern Indian Ocean are responsible for a future weakening of the zonal sea surface temperature gradient, resulting in a slowdown of the Indian Ocean Walker circulation and the generation of southeasterly wind anomalies over the AS. These contribute to anomalous northward ocean heat transport, reduced evaporative cooling, a weakening in upper ocean vertical mixing and an enhanced AS future warming. In contrast, the projected warming in the SEIO is related to a reduction of low-cloud cover and an associated increase in shortwave radiation. Therefore, the regional character of air-sea interactions plays a key role in promoting future large-scale tropical atmospheric circulation anomalies with implications for society and ecosystems far outside the Indian Ocean realm.

How to cite: Sharma, S., Ha, K.-J., Yamaguchi, R., Rodgers, K. B., Timmermann, A., and Chung, E.-S.: Future Indian Ocean warming patterns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2956, https://doi.org/10.5194/egusphere-egu24-2956, 2024.

EGU24-3250 | Orals | OS1.3 | Highlight

Indian Ocean Dipole Intensifies Benguela Niño Through Congo River Discharge 

Michael McPhaden, Sreelekha Jarugula, Leo Aroucha, and Joke Luebbecke

Benguela Niños are periodic episodes of unusual El Niño-like warming in the upwelling zone off the coast of southwest Africa with significant impacts on marine ecosystems, coastal fisheries and regional weather variability.  The strongest Benguela Niño in the past 40 years occurred in February-April 1995 with areal average sea surface temperature (SST) anomalies of 2°C and local anomalies up to 4°C off the coast of Angola and Namibia.  Benguela Niños are generated through a combination of remote and regional wind-forced dynamical processes originating within the Atlantic basin. However, a recent study has argued that the extraordinary warming observed in early 1995 resulted from southward advection of unusually high fresh water discharge from the Congo River, which led to the formation of thin mixed layers that trapped heat near the surface to boost coastal SSTs. 

The purpose of this presentation is to show that a strong Indian Ocean Dipole (IOD) that peaked in September-November 1994 was the reason for the high Congo River discharge in early 1995. IOD events are roughly the Indian Ocean equivalent of El Niño and La Niña events in the Pacific, which are generated though anomalous coupled interactions between surface winds and SSTs. It has been previously demonstrated that the IOD can affect eastern tropical Atlantic sea surface salinity through Congo River basin hydrology.  In particular, positive IOD events (warm SSTs in the western Indian Ocean and cold SSTs in the east) like that which occurred in 1994 lead to elevated Congo River discharge and subsequently lower eastern tropical Atlantic sea surface salinity.  However, it has not been previously shown how these the end-to-end processes originating with IOD development can affect Benguela Niños.

We use a variety of data sets and reanalyses (both oceanic and atmospheric) to show how during the 1994 IOD event, moisture was transported through the atmosphere from the western Indian Ocean to the Congo River basin where it converged and rained out to increase Congo River discharge.  The freshwater discharge in turn was advected southward in early 1995 which resulted in formation of thin surface mixed layers atop thick barrier layers that arrested the entrainment of cold subsurface waters, thereby amplifying Benguela Nino SSTs. We further show that this sequence of events has occurred at other times, as for example during a weak 2015 IOD and subsequent 2016 Benguela Niño.  These results suggest that the significant temporal lags between IOD development, Congo River basin rainfall, river discharge, and offshore accumulation of freshwater offer opportunities for improved seasonal forecasting of Indian Ocean impacts on the Atlantic through ocean-atmosphere-land interactions.

How to cite: McPhaden, M., Jarugula, S., Aroucha, L., and Luebbecke, J.: Indian Ocean Dipole Intensifies Benguela Niño Through Congo River Discharge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3250, https://doi.org/10.5194/egusphere-egu24-3250, 2024.

EGU24-3828 | ECS | Posters on site | OS1.3

Equatorial wave diagnosis for the Atlantic Niño with an ocean reanalysis 

Qingyang Song

There has been a long-standing need for a rapid-detection method for waves using simulation data for Atlantic Niño events. This study addresses this by utilizing an ocean reanalysis.  The proposed method firstly decomposes the climatological values and anomalies at each grid point are decomposed into the first four baroclinic modes based on their local density profiles, then the wave energy flux is calculated by means of a group-velocity-based scheme.  In the instance during the 2019 Niño event, the decomposed geopotential can well reproduce the displacement of the thermocline during the event. The obtained wave energy fluxes confirm the significant influence of subseasonal Kelvin waves on the event and also suggest that wave energy from off-equatorial regions likely preconditioned the event. This study is thus a useful tool for diagnosing the equatorial waveguide and can support the warning systems for Atlantic Niño events.

How to cite: Song, Q.: Equatorial wave diagnosis for the Atlantic Niño with an ocean reanalysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3828, https://doi.org/10.5194/egusphere-egu24-3828, 2024.

EGU24-6857 | ECS | Orals | OS1.3

Seasonality of Mixing at Tropical Instability Wave Fronts in the Atlantic Ocean  

Mia Sophie Specht, Johann Jungclaus, and Jürgen Bader

Tropical Instability Waves (TIWs) in both Pacific and Atlantic Ocean have been shown to play a role in modulating upper ocean mixing. However, previous studies on the modulation of TIW related mixing are based on small numbers of TIWs. These approaches do not allow for the consideration of temporal variability, which can lead to discrepancies in the findings. In this study, we analyze 12-years of simulation output from the comprehensive, global, high-resolution ocean model ICON, to show for the first time that deep reaching mixing at TIW fronts in the Atlantic Ocean follows a distinct seasonal cycle. We find that, regardless of whether TIWs are present earlier in the year, mixing primarily occurs in boreal summer when the vertical shear of the mean zonal currents also reaches its maximum. Our results suggest that in the Atlantic Ocean, shear at the TIW fronts related to the wave itself is generally not large enough to trigger deep reaching mixing. Instead, the background shear in addition to the TIW related shear also needs to be sufficiently large to generate mixing. This additional background shear is strongly modulated by the seasonality of the South Equatorial Current (SEC). Hence, the SEC and its temporal variability contribute to the generation and modulation of deep reaching mixing at TIW fronts in the Atlantic Ocean.

How to cite: Specht, M. S., Jungclaus, J., and Bader, J.: Seasonality of Mixing at Tropical Instability Wave Fronts in the Atlantic Ocean , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6857, https://doi.org/10.5194/egusphere-egu24-6857, 2024.

EGU24-7178 | Orals | OS1.3 | Highlight

Winter Convective Mixing Mediating Coupling of N-gain and -loss in the Arabian Sea 

Arvind Singh, Himanshu Saxena, Deepika Sahoo, Sipai Nazirahmed, Niharika Sharma, Deepak Kumar Rai, and Sanjeev Kumar

Marine dinitrogen (N2) fixation fuels primary production and thereby influences the Earth’s climate. Yet, its geographical distribution and controlling environmental parameters remain debatable. We measured N2 fixation rates from the two spatially and physicochemically contrasting regions of the Arabian Sea during the winter monsoon: (a) the colder and nutrient-rich waters in the northern region owing to winter convection and (b) the warmer and nutrient-poor waters in the southern region unaffected by winter convection. We found higher N2 fixation rates at the surface of northern region due to convective mixing driven supply of phosphate (intuitively iron also) from the underlying suboxic waters, whereas the lower rates in the southern region are attributable to the limited supply of iron. N2 fixation was favoured by high nutrients concentration in the euphotic waters, whereas remained unaffected by nutrients availability in the aphotic waters. We conclude that diazotrophs dwelling in the euphotic zone chose phosphate and iron over fixed nitrogen-poor waters. However, we found that among oligotrophic waters, anticyclonic eddy extremes the barrier of fixed nitrogen supply and thereby elevates N2 fixation. While the Arabian Sea loses about 20 to 40% of the global ocean fixed nitrogen, we estimate that N2 fixation in the Arabian Sea offsets only up to 42% of its fixed nitrogen-loss by denitrification, but this offset could be higher if diazotrophic activity is further examined up to the deeper depths of the Arabian Sea.

How to cite: Singh, A., Saxena, H., Sahoo, D., Nazirahmed, S., Sharma, N., Rai, D. K., and Kumar, S.: Winter Convective Mixing Mediating Coupling of N-gain and -loss in the Arabian Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7178, https://doi.org/10.5194/egusphere-egu24-7178, 2024.

EGU24-7353 | ECS | Orals | OS1.3

Mechanisms of the Indian Ocean surface warming pattern in CMIP5 and 6 models 

Gopika Suresh, Sadhvi Kwatra, Jérôme Vialard, Vincent Danielli, Neetu Suresh, and Matthieu Lengaigne

The latest assessment report of the Intergovernmental Panel on Climate Change highlights an accelerated warming of the Indian Ocean (IO) compared to the global average. Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5/6) projections also indicate a distinct warming pattern, reminiscent of the Indian Ocean Dipole (IOD), characterized by enhanced warming in the Arabian Sea and western Indian Ocean alongside a reduction in the IO branch of the Walker Cell. This study uses an SST heat budget adapted from Zhang and Li (2014, hereafter ZL14) across 46 CMIP5/6 simulations, to examine the drivers of the IO mean warming and its spatial distribution, for both the multi-model mean (MMM) and inter-model diversity.

Differing from the prior ZL14 approach, this study incorporates feedback related to downward longwave heat fluxes. While ZL14 highlighted downward longwave fluxes as the main driver of the IO average warming, our results reveal a dominant role of latent heat flux changes for both the MMM and diversity. These changes are further related to a basin-scale wind speed reduction, linked to the winter monsoon & IO Walker cell branch weakening.

Regarding the spatial pattern, our results emphasize a key role in the Bjerknes feedback in driving the IOD-like pattern for both the MMM and inter-model diversity. There is indeed a strong relationship across models between the IOD-like warming pattern, rainfall increase over the western IO, weakened equatorial easterlies, an east-west dipole in thermocline anomalies and the contribution of oceanic processes to surface warming. In the Arabian Sea, the enhanced warming is controlled by a seasonally varying balance, with the evaporative cooling feedback dominating during spring and summer when upwellings are strong, and the wind speed reduction associated with the winter monsoon weakening dominating later in the year.

Overall, these results call for more comprehensive process-oriented studies with more sophisticated approaches (ocean or coupled model sensitivity experiments) to unravel the IO warming mechanisms.

Keywords: Indian Ocean warming, Air-Sea Interaction, IOD-like warming, Walker cell weakening, Arabian sea warming, Coupled model intercomparison project (CMIP)

How to cite: Suresh, G., Kwatra, S., Vialard, J., Danielli, V., Suresh, N., and Lengaigne, M.: Mechanisms of the Indian Ocean surface warming pattern in CMIP5 and 6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7353, https://doi.org/10.5194/egusphere-egu24-7353, 2024.

EGU24-8544 | Posters on site | OS1.3

Changes in the Variability and Teleconnections of the Northeastern Tropical Atlantic Upwelling Region around 2000 

Joke Lübbecke, Belén Rodríguez-Fonseca, Marta Martin-Rey, Teresa Losada, Elsa Mohino, and Irene Polo

Sea Surface Temperatures (SST) in the Northeastern Tropical Atlantic upwelling region off Senegal and Mauritania feature pronounced variability on interannual time scales with impacts on the marine ecosystem. While part of this variability results from wind stress and wind stress curl-driven changes in local upwelling, the roles of air-sea heat fluxes, horizontal advection and potentially remotely forced thermocline variations have also been discussed. Here the relative roles of these forcing mechanisms and how they change over the time period from 1958 to 2020 are investigated utilizing reanalysis products as well as output from a general ocean circulation model (NEMO) forced by the atmospheric JRA55-do forcing. In the configuration analyzed (VIKING20X), oceanic resolution is increased to 1/20º over the Northern Atlantic via a two-way nesting approach, allowing for a better representation of the near-coastal upwelling region.

Interestingly, while interannual SST variability in the eastern equatorial Atlantic and the Angola Benguela region has decreased since 2000 and is projected to further decrease in the future, there is an increase of SST variability in the Northeastern Tropical Atlantic. To understand this increase, we address the roles of changes in local wind forcing and the connection to the equatorial region via the propagation of equatorial and coastal trapped waves. Along with the altered SST variability, teleconnection patterns related the Northeastern Tropical Atlantic, in particular with the El Niño – Southern Oscillation, also changed.    

How to cite: Lübbecke, J., Rodríguez-Fonseca, B., Martin-Rey, M., Losada, T., Mohino, E., and Polo, I.: Changes in the Variability and Teleconnections of the Northeastern Tropical Atlantic Upwelling Region around 2000, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8544, https://doi.org/10.5194/egusphere-egu24-8544, 2024.

EGU24-8829 | ECS | Orals | OS1.3

An assessment of equatorial Atlantic interannual variability in OMIP simulations 

Arthur Prigent and Riccardo Farneti

The eastern equatorial Atlantic (EEA) seasonal cycle and interannual variability of the sea surface temperature strongly influence the climate of the surrounding continents. It is thus crucial that models used in both climate predictions and future climate projections are able to simulate them accurately. In that context, the EEA seasonal cycle and interannual variability are evaluated over the period 1985-2004 in models participating to the Ocean Model Intercomparison Project Phases 1 and 2 (OMIP1 and OMIP2). The main difference between OMIP1 and OMIP2 simulations is their atmospheric forcing: CORE-II and JRA55-do, respectively. Seasonal cycles of the equatorial Atlantic zonal winds, sea level anomaly and sea surface temper- ature in OMIP1 and OMIP2 are comparable to reanalysis datasets. Yet, some discrepancies exist in both OMIP ensembles: the thermocline is too diffusive and there is a lack of cooling during the development of the Atlantic cold tongue. In addition, the vertical ocean velocity in the eastern equatorial Atlantic in boreal summer is larger in OMIP1 than in OMIP2 simulations. The EEA interannual sea surface temperature variability in the OMIP1 ensemble mean is found to be 51% larger (0.62 ± 0.04 ˚C) than the OMIP2 ensemble mean (0.41 ± 0.03 ˚C). Sensitivity experiments demonstrate that the discrepancy in interannual sea surface temperature variability between OMIP1 and OMIP2 is mainly attributed to their wind forcing. While the April-May- June zonal wind variability in the western equatorial Atlantic is similar in both forcing, the zonal wind variability peaks in April for JRA55-do and in May for CORE-II. Differences in surface heat fluxes between the two atmospheric forcing datasets have no significant impacts on the simulated interannual SST variability.

How to cite: Prigent, A. and Farneti, R.: An assessment of equatorial Atlantic interannual variability in OMIP simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8829, https://doi.org/10.5194/egusphere-egu24-8829, 2024.

EGU24-10500 | ECS | Posters on site | OS1.3

Decadal Variability of the Indonesian Throughflow’s Vertical Structure and the Impact on Heat and Salinity Transport 

Daniel Waitzmann, Shouyi Wang, Delia W. Oppo, and Caroline C. Ummenhofer

The Indonesian Throughflow, a low-latitude passage of the global conveyor belt, transfers water from the tropical Pacific to the Indian Ocean, modulating the properties of both oceans. Observational and modelling studies have shown that the interannual and decadal variability of the Indonesian Throughflow is closely linked to the leading climate modes of the tropical Pacific, namely the El Niño Southern Oscillation and the Interdecadal Pacific Oscillation; further, it is modulated by variability in the Indian Ocean, especially in the outflow region. The Indonesian Throughflow volume transport variability affects salinity and temperature transport and ocean-atmosphere exchange in the Indo-Pacific warm pool. The Makassar Strait transport represents about 80% of the total Indonesian Throughflow transport and is, therefore, a good proxy for the Indonesian Throughflow transport. Observations from the Indonesian Seas have been used to explain the variability on seasonal to interannual time scales. However, due to the lack of long observational time series in the region, assessing the variability and driving mechanisms on longer time scales is challenging. Here, we use transient runs of a high-resolution coupled ocean-atmosphere model to address the decadal variability of the Indonesian Throughflow and its change under global warming over the time period 1850-2102. We assess how heat content, salinity, and volume transport in the Makassar Strait region change on these timescales and how they contribute to the heat and freshwater transport changes. In addition, we investigate the vertical structure of the Indonesian Throughflow variability and its driving mechanisms. This involves understanding how Indonesian Throughflow variability is connected more broadly to large-scale conditions in the Pacific and Indian Oceans. The results presented here may motivate further analysis using multiple simulations of the high-resolution model configurations conducted as part of HighResMIP to assess the forced changes to the Indonesian Throughflow under RCP8.5 forcing in a highly dynamic ocean region that plays a pivotal role in global heat and freshwater transport.

How to cite: Waitzmann, D., Wang, S., Oppo, D. W., and Ummenhofer, C. C.: Decadal Variability of the Indonesian Throughflow’s Vertical Structure and the Impact on Heat and Salinity Transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10500, https://doi.org/10.5194/egusphere-egu24-10500, 2024.

EGU24-12377 | Posters on site | OS1.3 | Highlight

Developing a 3.5-million-year benchmark record of Indian Ocean Dipole variability  

Stefanie Kaboth-Bahr, Oliver Kern, and André Bahr

The Indian Ocean Dipole (IOD) is the primary mode of interannual sea surface temperature variability (SST) in the tropical Indian Ocean. The climatic effects of the IOD are diverse and geographically widespread. Extreme flood events in eastern Africa, weakened summer monsoon intensity over India and Southeast Asia, and severe droughts in Australia are among the most significant societal consequences of IOD variability. These extreme climate events caused by the IOD are predicted to become more common as greenhouse gas emissions increase. However, despite its significance, surprisingly little is known about IOD variability during the geological past, which would allow for a better assessment of its sensitivity to atmospheric CO2 level changes in the future. This study presents the first insights into the spatio-temporal complexity of the IOD over the past 3.5 million years. We utilize geochemical proxy data (XRF core scanning, stable oxygen, and carbon isotopes, as well as Mg/Ca paleothermometry of planktonic foraminifera) derived from Site ODP 709, situated in the western equatorial Indian Ocean - a critical region for IOD forcing.

How to cite: Kaboth-Bahr, S., Kern, O., and Bahr, A.: Developing a 3.5-million-year benchmark record of Indian Ocean Dipole variability , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12377, https://doi.org/10.5194/egusphere-egu24-12377, 2024.

EGU24-14029 | Orals | OS1.3

Exploring 6-month lead predictability of the Atlantic zonal mode in CMIP6 

Ingo Richter, Tomoki Tozuka, Yu Kosaka, Shoichiro Kido, and Hiroki Tokinaga

Skillful prediction of the equatorial Atlantic zonal mode (AZM) remains challenging, with many prediction systems dropping below an anomaly correlation coefficient (ACC) of 0.5 beyond a lead time of 3 months. Since the El Niño-Southern Oscillation (ENSO) is well known to have global impacts, it could be expect to be a useful predictor of the AZM but its influence on the adjacent equatorial Atlantic basin is inconsistent. This is perhaps best exemplified by the fact that the extreme 1982 and 1997 El Niño events were followed by Atlantic zonal mode (AZM) events of the opposite sign.

Here we re-examine the potential role of ENSO in the predictability of the AZM using pre-industrial control simulations (piControl) from the Coupled Model Intercomparison Phase 6 (CMIP6). The observed correlation between boreal winter (DJF) sea-surface temperature (SST) in the Niño 3.4 region and the following summer (JJA) SSTs in the ATL3 region is close to zero, indicative of the inconsistent relation between the two. Individual models, however, exhibit a wide range of behaviors with correlations ranging from about -0.5 to +0.5. While the influence of ENSO on equatorial Atlantic SST is inconsistent, the influence of ENSO on surface winds over the equatorial Atlantic is rather robust. All models show a negative correlation between DJF Niño 3.4 SST and boreal spring (MAM) surface winds over the western equatorial Atlantic. In addition, we find that SSTs in the South Atlantic act as a precursor to AZM events. Based on these relations, we construct a multi-linear regression model to predict AZM events in JJA based on Pacific and Atlantic SST in DJF. In most climate models, this simple scheme can predict AZM events with an ACC above 0.5 during ENSO years. We will discuss to what extent these insights may help in the prediction of real-world AZM events.

How to cite: Richter, I., Tozuka, T., Kosaka, Y., Kido, S., and Tokinaga, H.: Exploring 6-month lead predictability of the Atlantic zonal mode in CMIP6, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14029, https://doi.org/10.5194/egusphere-egu24-14029, 2024.

EGU24-14272 | ECS | Orals | OS1.3

Anomalous Seawater Radiocarbon Depletion Event during Glacial Interval in the Equatorial Indian Ocean Thermocline 

Sanjit Kumar Jena, Ravi Bhushan, Partha Sarathi Jena, Nisha Bharti, Sudheer Athiyarath Krishnan, Ajay Shivam, and Ankur Dabhi

The role of intermediate water mass in ocean circulation is well acknowledged from the global oceanographic and climatic perspectives. Abnormal depletions in the upper oceanic radiocarbon concentrations during the last deglaciation have been attributed to the southern ocean sourced aged CO2 ventilations via Antarctic intermediate waters. However, the fundamental origin and nature of the source, and its spatio-temporal variability still remains a question.

The present study reconstructs the radiocarbon records of the upper Equatorial Indian Ocean (EIO) over the last 44 ka using the radiocarbon dating of depth-specific planktonic foraminifers. The results reveal an extremely depleted radiocarbon interval in the EIO thermocline between 25-34 ka during the Marine Isotopic Stage 3 – Marine Isotopic Stage 2 (MIS3-MIS2) transition. The Reunion hotspot and/or the Amsterdam Island appear to be the responsible source(s) of contemporaneous hydrothermal dead carbon supply into the EIO thermocline. However, the deglacial thermocline radiocarbon depletions were primarily caused by the southern ocean sourced aged CO2 ventilations only. The radiocarbon records also indicate a well stratified upper oceanic condition prevailing over the EIO during the last 44 ka.

How to cite: Jena, S. K., Bhushan, R., Jena, P. S., Bharti, N., Athiyarath Krishnan, S., Shivam, A., and Dabhi, A.: Anomalous Seawater Radiocarbon Depletion Event during Glacial Interval in the Equatorial Indian Ocean Thermocline, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14272, https://doi.org/10.5194/egusphere-egu24-14272, 2024.

EGU24-14616 | ECS | Posters on site | OS1.3

Volcanic ash likely triggers N2 fixation in the Andaman Sea  

Himanshu Saxena, Deepika Sahoo, Ajayeta Rathi, Sipai Nazirahmed, Sanjeev Kumar, and Arvind Singh

Marine N2 fixation fuels the growth of primary producers, drives marine carbon export fluxes, and in turn, influence the Earth’s climate. While the Bay of Bengal is at least explored, the Andaman Sea, which is adjacent to the only active volcano of the south Asia and is separated from the Bay of Bengal by the Andaman and the Nicobar Islands to its west, has never been explored for its viability to N2 fixation. The warm and oligotrophic surface waters and suboxic subsurface waters of these two basins may provide suitable stimulus for N2 fixation. We investigated N2 fixation in the euphotic and the oxygen minimum zones of the Bay of Bengal and the Andaman Sea during the autumn inter-monsoon. We found that N2 fixation is about an order of magnitude higher in the surface waters of the Andaman Sea than the Bay of Bengal, attributable to the relatively high iron input associated with volcanic ash deposition in the Andaman Sea. We underscored that N2 fixation at the immediate sea surface (sampled manually through a bucket) is largely four times higher than the subsurface waters at 10 m depth (sampled through CTD) in the northeastern Indian Ocean. Our findings imply that the traditional CTD rosette sampling is unable to capture the surface N2 fixation activity, and therefore, previously reported N2 fixation rates in the global ocean are likely to be massively underestimated.

How to cite: Saxena, H., Sahoo, D., Rathi, A., Nazirahmed, S., Kumar, S., and Singh, A.: Volcanic ash likely triggers N2 fixation in the Andaman Sea , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14616, https://doi.org/10.5194/egusphere-egu24-14616, 2024.

EGU24-15235 | ECS | Posters on site | OS1.3

Bayesian optimization of ocean mixed layer parameterizations 

Marta Mrozowska, Markus Jochum, James Avery, Ida Stoustrup, and Roman Nuterman

Global climate is highly sensitive to tropical sea surface temperature. Accurately representing the tropical SST remains a significant challenge for general circulation and climate models. One of the largest sources of uncertainty is the vertical turbulent mixing. To accurately represent the distribution of ocean mixed layer depths, turbulence closure schemes necessitate careful tuning. This is most commonly done manually by comparing with mixed layer depth climatologies. Advancements in machine learning research introduce a new strategy: automated tuning. Veropt, an add-on to the python ocean model Veros, uses Gaussian processes to emulate an objective function in a multi-dimensional parameter space. We present a surprising combination of changes to the default parameters of the commonly used turbulent kinetic energy (TKE) closure scheme that minimise the model bias in tropical mixed layer depth.

How to cite: Mrozowska, M., Jochum, M., Avery, J., Stoustrup, I., and Nuterman, R.: Bayesian optimization of ocean mixed layer parameterizations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15235, https://doi.org/10.5194/egusphere-egu24-15235, 2024.

EGU24-17728 | ECS | Orals | OS1.3

Merging Process of the Great Whirl and the Socotra Gyre in 2019 

Lingxing Dai, Xingwei Jiang, Yifan Xia, Minyang Wang, Shilin Tang, and Yan Du

The Great Whirl (GW) and the Socotra Gyre (SoG), two prominent anticyclonic eddies in the western Arabian Sea, exhibit strong dynamic interactions. This study reports a case of the merging of the GW and the SoG recorded by Argo floats in September 2019. Combined with satellite observations and a state-of-the-art ocean reanalysis, we show that the merging process was first detected at the subsurface layer (~150 m depth) rather than the surface. As the original water inside the GW is cooler than the SoG, the merging created a baroclinic structure between the eddies. The density gradients associated with the baroclinic structure drive strong subsurface geostrophic currents following the thermal wind relationship, leading to the fast merging at 100-200 m depth. Energy analysis shows that the predominant energy source for the merged eddy was the barotropic and baroclinic instability. The dissipative processes caused the rapid decay of the merged eddy. The merging process induced sub-mesoscale activities and promoted ocean vertical exchanges south of Socotra Island.

How to cite: Dai, L., Jiang, X., Xia, Y., Wang, M., Tang, S., and Du, Y.: Merging Process of the Great Whirl and the Socotra Gyre in 2019, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17728, https://doi.org/10.5194/egusphere-egu24-17728, 2024.

The tropical Indian Ocean presents a distinctive opportunity to investigate monsoon-induced changes in primary productivity and ocean hydrography. Planktic foraminifera, with their unique ecological preferences, are well-suited for reconstructing past environmental conditions. Different species of planktic foraminifera exhibit varied responses to changes in the physico-chemical parameters of the ambient water. This study presents a high-resolution planktic foraminiferal assemblage from the marine sediment core SSD004 GC03 for the last 24,000 years from the tropical Indian Ocean. The record includes 24 planktic foraminifera species with G. bulloides, G. glutinata, G. ruber, G. sacculifer, N. dutertrei and G. menardii  being the most abundant. The species are categorized into eutrophic, oligotrophic, mixed layer, and thermocline assemblages. Notably, during the last glacial maximum (LGM; 19.0-23.0 ka), a significant abundance of mixed layer assemblage is observed between 21.0-19.0 kyr. Heinrich stadial 1 (~15.0-18.0 ka) and the Younger Dryas (~11.-12.9 ka) periods exhibit a lower mixed layer assemblage and a higher thermocline assemblage. The Bølling-Allerød (~12.9-15.0 ka) period is characterized by a sudden increase in mixed-layer assemblages. The abundance of eutrophic species G. bulloides and G. glutinata during the LGM and Holocene indicates increased surface productivity influenced by the Northeast Monsoon and the strong Southwest Monsoon, respectively. The results underscore the unique and intricate dynamics of the studied region, primarily influenced by both the southwest and northeast monsoons.

How to cite: Rai, S. and Singh, D. P.: Planktic foraminifera reflects surface productivity and hydrographic changes in the tropical Indian Ocean during the last 24,000 years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17818, https://doi.org/10.5194/egusphere-egu24-17818, 2024.

EGU24-17832 | ECS | Posters on site | OS1.3

The Atlantic sibling: a reconciling vision on the nature of El Niño’s “little brother”  

Cosimo Enrico Carniel, Gian Luca Borzelli, Aniello Russo, and Sandro Carniel

The Atlantic Niño, also referred to as Atlantic zonal mode, equatorial Atlantic mode or, sometimes, El Niño’s little brother, is an important source of the year-to-year variability of the tropical Atlantic, consisting in an irregular oscillation of the Sea Surface Temperature (SST) in the eastern part of the basin. The physical mechanism underlying the activation of the oscillation is a matter of debate; some theories, termed dynamical, explain the Atlantic Niño as an ENSO-like phenomenon initiated by internal waves excited by the relaxation of easterly winds in the western tropical Atlantic and/or by the reflection of Rossby waves impinging the western Atlantic boundary. Some other theories, called thermodynamic, attribute the eastern tropical Atlantic SST variability to thermodynamic processes induced by off equatorial heat fluxes. Here, by using Sea Surface Height (SSH) data provided by orbiting altimeters and heat fluxes deduced from horizontal currents and Temperature-Salinity (TS) profiles provided by the Copernicus project, we show that, at least in the period Jan 1993-Dec 2021, both mechanisms were active and two sub-periods can be identified: the first, between Jan 1993 and Dec 2009, in which the eastern tropical Atlantic temperature variability can be explained reasonably well in terms of heat advected from the south by horizontal currents and, another period, between Jan 2010 and Dec 2021, in which the temperature variability of the eastern tropical Atlantic is explained by displacements of the thermocline induced by internal Kelvin waves propagating along the equatorial wave-guide. Finally, by using daily SST anomaly data over the period Jan 1940-Dec 2022, we show that the SST variability in the eastern tropical Atlantic and in the Angola-Benguela upwelling region are well correlated with each other with a lag slightly lower than a month and the SST in the Angola-Benguela region leading, suggesting a positive feedback between off equatorial heat availability and increasing SST in the eastern tropical Atlantic.

How to cite: Carniel, C. E., Borzelli, G. L., Russo, A., and Carniel, S.: The Atlantic sibling: a reconciling vision on the nature of El Niño’s “little brother” , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17832, https://doi.org/10.5194/egusphere-egu24-17832, 2024.

EGU24-19767 | Orals | OS1.3 | Highlight

Impact of the Atlantic Niño on California Ecosystem predictability 

Belen Rodríguez-Fonseca, Mercedes Pozo, Jerome Fiechter, Steven Bograd, and Mike Jacox

The Atlantic Niño is the dominant mode of sea surface temperature variability in the tropical Atlantic at interannual time scales. In the last decades this mode of variability has been identified as a driver of the Pacific Niño, increasing its predictability. The mechanism involved in the relation between the Atlantic Niño and ENSO is through the modification of the Walker Cell, altering surface winds in the western Pacific and triggering oceanic kelvin waves. These kelvin waves propagate to the east in the equatorial Pacific and along the north and South American coasts, altering the structure of the water column. The impact of this teleconnection on eastern boundary current upwelling systems has not been analyzed so far. This work demonstrates, for the first time, the impact of the Atlantic Niño on physical and biogeochemical processes in the California Current ecosystem, by the alteration of wind-driven coastal upwelling and the modification of upwelled source water properties. The mechanism relates an Atlantic Niño with enhanced production due to the uplifting of isopycnals, which that supplies more nutrients to the euphotic zone and enhances primary production and subsequent vertical export and remineralization at depth. In addition, statistical prediction is performed, indicating strong predictability of California Current biogeochemical variability from the equatorial Atlantic anomalous SSTs more than one year ahead.

 

How to cite: Rodríguez-Fonseca, B., Pozo, M., Fiechter, J., Bograd, S., and Jacox, M.: Impact of the Atlantic Niño on California Ecosystem predictability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19767, https://doi.org/10.5194/egusphere-egu24-19767, 2024.

EGU24-1534 * | ECS | Orals | OS1.5 | Highlight

Increased future ocean heat uptake constrained by Antarctic sea ice extent 

Linus Vogt, Casimir de Lavergne, Lester Kwiatkowski, Jean-Baptiste Sallée, Thomas L. Frölicher, and Jens Terhaar

The ocean is the major sink of excess heat from anthropogenic climate change, and has so far prevented global warming from already surpassing the limits set by the Paris Agreement. This warming of the ocean impacts metabolic processes in marine species and causes sea level rise, more frequent extreme events, and ocean deoxygenation. The current generation of Earth system models has large uncertainties in projections of historical and future ocean heat uptake. Reducing this uncertainty is paramount for informing climate mitigation and adaptation measures.
Here we demonstrate that the amount of future global ocean heat uptake is strongly linked to present day Antarctic sea ice extent, so that satellite observations of sea ice can be used to reduce the uncertainty of future ocean heat uptake. Antarctic sea ice extent serves as an indicator of the baseline climate state of the Southern Ocean, and is linked to ocean heat uptake through hemispheric-scale cloud feedbacks. Climate models typically simulate insufficient Antarctic sea ice, a warm bias in Southern Ocean surface temperatures and insufficient Southern Hemisphere low cloud concentrations, negatively biasing future ocean heat uptake. Using present day Antarctic sea-ice extent observations as an emergent constraint allows to reassess the cumulative ocean heat uptake from 2024 to 2100 under a high-emissions scenario, yielding an increased estimate with reduced uncertainty of 2596 ± 216 ZJ.
Our findings indicate that ocean heat uptake and its associated impacts will likely be greater than previously estimated, and underline the climatic significance of recent observed changes in Antarctic sea ice, which may foreshadow changes in oceanic and atmospheric warming rates.

How to cite: Vogt, L., de Lavergne, C., Kwiatkowski, L., Sallée, J.-B., Frölicher, T. L., and Terhaar, J.: Increased future ocean heat uptake constrained by Antarctic sea ice extent, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1534, https://doi.org/10.5194/egusphere-egu24-1534, 2024.

EGU24-1702 | ECS | Posters on site | OS1.5

The Effects of Mesoscale Eddies on Southern Ocean Carbon and Biogeochemistry 

Lydia Keppler, Matthew Mazloff, Ariane Verdy, Sarah Gille, Lynne Talley, Yassir Eddebbar, Veronica Tamsitt, and Nicola Guisewhite

The Southern Ocean modulates global biogeochemical (BGC) cycles substantially, affecting biological production and the global air-sea balance of carbon dioxide and interior dissolved oxygen content. Concurrently, the Southern Ocean is rich in highly dynamic mesoscale eddies. These eddies have the potential to alter local carbon, nutrient, and oxygen distributions through eddy pumping, stirring, and trapping. Additionally, the strong westerly winds could result in significant eddy-induced Ekman pumping counteracting the eddy pumping effects. However, the impact of mesoscale eddies on upper-ocean Southern Ocean biogeochemistry has not been quantified observationally at a regional scale.

We now have nearly a decade of BGC observations from Argo floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM). In addition, the Mesoscale Eddy Trajectory Atlas, version 3.2, delayed time (Meta3.2DT) database provides us with a robust assessment of eddies as detected by satellite altimeter measurements. Together, the two datasets allow us to investigate the three-dimensional structure of the biogeochemistry in Southern Ocean eddies. Here, we co-locate Southern Ocean eddies with BGC Argo floats to characterize composite vertical and horizontal structures of dissolved inorganic carbon (DIC), oxygen, and nitrate inside anticyclonic and cyclonic eddies compared to the mean climatological fields. We conduct this analysis in several subregions with different dominant processes. We find positive DIC and nitrate anomalies in cyclonic eddies, which we attribute to upward eddy pumping. We also find positive oxygen anomalies near the surface, which we attribute to upwelled nutrients that enhance biological production, leading to enhanced photosynthesis. At depth, we find negative oxygen anomalies in cyclonic eddies, which may be driven both by enhanced respiration due to increased biological production as well as the heaving of isopycnals via eddy pumping. The opposite is true for anticyclonic eddies due to downward eddy pumping (negative DIC and nitrate anomalies; negative oxygen anomalies near the surface and positive oxygen anomalies at depth). The magnitudes of the eddy imprints on biogeochemistry vary by region, indicating that stratification and other background signals influence the magnitude of the effect of eddies in a region. Our findings can help us to interpret the influence of mesoscale eddies on the Southern Ocean carbon fluxes and biogeochemistry, including assessing the relative dominance of eddy pumping and eddy-induced Ekman pumping in different subregions of the Southern Ocean.

How to cite: Keppler, L., Mazloff, M., Verdy, A., Gille, S., Talley, L., Eddebbar, Y., Tamsitt, V., and Guisewhite, N.: The Effects of Mesoscale Eddies on Southern Ocean Carbon and Biogeochemistry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1702, https://doi.org/10.5194/egusphere-egu24-1702, 2024.

EGU24-3332 | ECS | Posters on site | OS1.5

Comparing float pCO2 profiles in the Southern Ocean to ship data reveals discrepancies 

Chuqing Zhang, Yingxu Wu, Peter J. Brown, David Stappard, Amavi N. Silva, and Toby Tyrrell

The Southern Ocean plays a crucial role in the global carbon cycle. Recently, the utilization of biogeochemical (BGC) Argo float data has provided valuable insights into the uptake and release of carbon dioxide (CO2) by this region. However, significant uncertainty remains regarding the accuracy of pCO2 (partial pressure of CO2) values derived from float data. In this study, we compared pCO2 estimates obtained from float pH data with those from ship-collected data across the Southern Ocean, employing pCO2-depth, pCO2-O2 and CO2-O2 vssaturation plots to assess the degree of agreement between these two datasets. Our findings reveal significant systematic differences. A preliminary analysis, ignoring other factors, found that the float data is consistently higher, on average, than the ship data at equivalent depths and oxygen levels. We tested the hypothesis that inaccurate float pH data or float pCO2 correction process is the main cause of the pCO2 difference, by quantifying other factors that could produce systematic differences, including: (i) spatial sampling bias, (ii) seasonal bias, (iii) errors in estimated alkalinity, (iv) errors in carbonate system constants, and (v) higher levels of anthropogenic CO2 in float data. However, none of the other factors were found to be able to fully account for the discrepancies, suggesting issues with float pH data quality and/or the float pCO2 correction process. Additional analysis included refinements to ship-based and float-based pCO2 before intercomparison. Overall, we estimate that, in the Southern Ocean, surface pCO2 from floats is biased high by, on average, at least 10 μatm.

How to cite: Zhang, C., Wu, Y., Brown, P. J., Stappard, D., Silva, A. N., and Tyrrell, T.: Comparing float pCO2 profiles in the Southern Ocean to ship data reveals discrepancies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3332, https://doi.org/10.5194/egusphere-egu24-3332, 2024.

EGU24-3601 | ECS | Posters on site | OS1.5

Deep winter mixed layer anchored by the meandering Antarctic Circumpolar Current: Cross-basin variations 

Zihan Song, Shang-Ping Xie, Lixiao Xu, Xiao-Tong Zheng, Xiaopei Lin, and Yu-Fan Geng

The Southern Ocean features some of the deepest winter mixed layers on Earth, crucial for water mass formation and the storage of anthropogenic heat. The winter mixed layer depth (MLD) significantly varies across basins, exceeding 300 m in the Indian and Pacific sectors but less than 150 m in the Atlantic. Current climate models simulate a distribution that is too broad and struggle to accurately represent this inter-basin variation. Using observational data and a global atmospheric model, this study investigates the contribution of surface buoyancy flux and background stratification to inter-basin MLD variations.

The surface heat flux is decomposed into broad-scale and frontal-scale variations, both of which are influenced by the Antarctic Circumpolar Current’s (ACC) structure. At the broad-scale, the meandering ACC path is accompanied by a zonal wavenumber-1 structure of sea surface temperature with a warmer Pacific than Atlantic; under the prevailing westerly winds, this temperature contrast results in larger surface heat loss facilitating deeper MLD in the Indian and Pacific than the Atlantic. At the frontal-scale, intensified ACC fronts in the Indian sector further strengthen heat loss to the north. Surface freshwater flux pattern largely follows that of evaporation and reinforces the heat flux pattern, especially in the southeast Pacific.

Background stratification also significantly varies across the Southern Ocean, influencing MLD pattern. In the Atlantic and western Indian oceans where the ACC is at a low latitude (45°S), solar heating, intrusions of subtropical gyres and energetic mesoscale eddies together maintain strong stratification. In the southeast Pacific, in comparison, the ACC reaches its southernmost latitude (56°S), far away from the Subtropical Front. This creates a weaker stratification that allows deep mixed layers to form.

How to cite: Song, Z., Xie, S.-P., Xu, L., Zheng, X.-T., Lin, X., and Geng, Y.-F.: Deep winter mixed layer anchored by the meandering Antarctic Circumpolar Current: Cross-basin variations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3601, https://doi.org/10.5194/egusphere-egu24-3601, 2024.

EGU24-3930 | ECS | Orals | OS1.5 | Highlight

Emergent constraint on future anthropogenic carbon and excess heat uptake in the Southern Ocean 

Timothée Bourgeois, Nadine Goris, Jörg Schwinger, and Jerry F. Tjiputra

The Southern Ocean is a major sink of anthropogenic carbon and excess heat. In this region, the Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread. This contributes significantly to the large uncertainties in the overall climate sensitivity and remaining carbon budgets for ambitious climate targets. Hence, a reduction in the uncertainty of the future Southern Ocean carbon and heat sinks is urgently needed.

Globally, Bronselaer and Zanna (2020) identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emissions scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. Here, we use this approach for the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake. We found that, in this region, the variations in the models’ contemporary water-column stability over the first 2000 m is highly correlated to both their future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data of water-column stability, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Independent studies have found similar constraints in the Southern Ocean and globally, strengthening our findings (Liu et al., 2023; Newsom et al., 2023; Terhaar et al., 2021, 2022), and pinpointing that a better representation of water-column stratification in Earth system models is essential to improve future anthropogenic climate change projections.

Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J. F.: Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S, Nat Commun, 13, 340, https://doi.org/10.1038/s41467-022-27979-5, 2022.

Bronselaer, B. and Zanna, L.: Heat and carbon coupling reveals ocean warming due to circulation changes, Nature, 584, 227–233, https://doi.org/10.1038/s41586-020-2573-5, 2020.

Liu, M., Soden, B. J., Vecchi, G. A., and Wang, C.: The Spread of Ocean Heat Uptake Efficiency Traced to Ocean Salinity, Geophys. Res. Lett., 50, e2022GL100171, https://doi.org/10.1029/2022GL100171, 2023.

Newsom, E., Zanna, L., and Gregory, J.: Background Pycnocline Depth Constrains Future Ocean Heat Uptake Efficiency, Geophys. Res. Lett., 50, e2023GL105673, https://doi.org/10.1029/2023GL105673, 2023.

Terhaar, J., Frölicher, T. L., and Joos, F.: Southern Ocean anthropogenic carbon sink constrained by sea surface salinity, Sci. Adv., 7, eabd5964, https://doi.org/10.1126/sciadv.abd5964, 2021.

Terhaar, J., Frölicher, T. L., and Joos, F.: Observation-constrained estimates of the global ocean carbon sink from Earth system models, Biogeosciences, 19, 4431–4457, https://doi.org/10.5194/bg-19-4431-2022, 2022.

How to cite: Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J. F.: Emergent constraint on future anthropogenic carbon and excess heat uptake in the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3930, https://doi.org/10.5194/egusphere-egu24-3930, 2024.

Subantarctic Mode Water is a water mass with nearly vertically homogeneous physical properties in the Southern Ocean, which exhibits variability at various time scales. This study investigates the low-frequency variability of upper-ocean temperature in the Central Pacific Subantarctic Mode Water (CPSAMW) formation region since the 1980s using an eddy-resolving ocean model and two observation-based products. It is found that the CPSAMW core layer temperature has significant low-frequency variability, with an unusually cold period around 2000 and warm periods around 2005 and 2015, respectively. This low-frequency variability is closely related to the change in local mixed layer temperature, which in turn is mainly attributed to the change in surface latent heat flux resulting from the change in wind speed. Further analysis indicates that the low-frequency variability of wind speed in the CPSAMW formation region is dominated mainly by the Interdecadal Pacific Oscillation (IPO) and to a lesser extent by the Southern Annular Mode (SAM). This study reveals the relationship in the low-frequency variability of CPSAMW temperature with the IPO and SAM, and provides insight into the remote influence of Pacific decadal variability on SAMW variability.

How to cite: Jing, W., Luo, Y., and Zhang, R.: Low-frequency variability of upper-ocean temperature in the Central Pacific Subantarctic Mode Water formation region since the 1980s, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4301, https://doi.org/10.5194/egusphere-egu24-4301, 2024.

EGU24-5853 | ECS | Posters on site | OS1.5

A physiological approach to parameterising variable Diatom Si:N ratios using a quota model to reproduce nutrient addition experiments in the Southern Ocean 

Jacob Harper, Mark Moore, Ben Ward, Adrian Martin, and Toby Tyrrell

The Si:N ratio of diatoms in the Southern Ocean (SO) is increased in response to iron limitation resulting in enhanced removal of Si from the surface waters that are entrained into the Subantarctic Mode Water (SAMW) leaving it Si deficient. Biogeochemical models usually employ direct parameterisations to represent this phenomenon empirically, however minor differences in how this process is parameterised can lead to large disparities in model results due to the large influence of SAMW on productivity in the lower latitudes. We explore the complexities of parameterisation using data from a recent cruise of the ‘Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth’ (CUSTARD) project. This project undertook a series of factorial nutrient addition experiments including Fe, Mn and Si, along 89° W between 54° S and 59.99° S. Experimental results reinforced that Si:N ratio is dependent not only on Fe but also Si availability. To properly reproduce this data, a quota model was altered to allow phytoplankton to uptake and store luxury quantities of nutrients to then be used for growth from internal pools. This model was able to successfully reproduce quantitative patterns of nutrient limitation and the Fe-Si dependency of diatom Si:N ratios through an explicit physiological approach without the need for a direct parameterisation. Such methodology is both more authentic to the natural control of diatom stoichiometry and may avoid the potential for artificial responses created by direct parameterisations.

How to cite: Harper, J., Moore, M., Ward, B., Martin, A., and Tyrrell, T.: A physiological approach to parameterising variable Diatom Si:N ratios using a quota model to reproduce nutrient addition experiments in the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5853, https://doi.org/10.5194/egusphere-egu24-5853, 2024.

EGU24-6438 | ECS | Orals | OS1.5

The Evolving Relative Role of Stratospheric Ozone and Greenhouse Gasses in Modifying the Southern Ocean Carbon Sink from 1950-2100 

Tereza Jarníková, Corinne Le Quéré, Steven Rumbold, and Colin Jones

Southern Ocean winds have strengthened and moved poleward in the latter half of the 20th century, which has been attributed to the depletion of stratospheric ozone and to climate warming from rising greenhouse gas concentrations. Both ozone recovery and changing greenhouse gas concentrations are expected to continue modulating wind structure throughout the 21st century. Here, we quantify the relative roles of ozone and greenhouse gases on Southern Ocean wind structure from 1950-2100 using the UK Earth System Model (UKESM1) model output, with a combination of three scenarios of ozone and two scenarios of greenhouse gas evolution. Both ozone depletion and increases in greenhouse gas concentration act to increase wind speed over the Southern Ocean. The influence of ozone is predominant in summer winds, while the influence of greenhouse gases acts in all seasons. We show that wind speeds return close to their original levels by the end of the 21st century under a low-greenhouse gas scenario with ozone recovery. The influence of ozone on wind speed was dominant in the 1950-2000 time-period, but not in the 21st century when the influence of greenhouse gases becomes two to three times larger than that of ozone, even in the low emissions scenario. We find significant effects of both ozone scenario and greenhouse gas emissions on physical-oceanographic variables (sea surface temperature, mixed layer depth, and overturning circulation). Finally, we quantify the relative contributions of these physical changes to the evolving carbon sink of the Southern Ocean, and discuss how wind-induced physical changes can alter ecosystem processes and the associated carbon export to the deep ocean.

How to cite: Jarníková, T., Le Quéré, C., Rumbold, S., and Jones, C.: The Evolving Relative Role of Stratospheric Ozone and Greenhouse Gasses in Modifying the Southern Ocean Carbon Sink from 1950-2100, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6438, https://doi.org/10.5194/egusphere-egu24-6438, 2024.

EGU24-7355 | ECS | Posters on site | OS1.5

Coupled atmosphere-sea-ice-ocean feedback accelerates rapid sea ice decline in Weddell Sea in high-resolution global climate model 

Dae-Won Kim, Thomas Jung, Navajyoth Puthiyaveettil, Wonsun Park, Tido Semmler, Axel Timmermann, and Martina Zapponini

Sea ice extent around the Antarctic exhibits a high level of variability on interannual and longer timescales, characterized by a positive trend since the satellite era and interruptions due to e.g., the emergence of the Maud Rise Polynya in 2016. Given the relatively short period of observational data and the high level of natural variability, it has remained challenging to unequivocally identify the anthropogenic fingerprint in Antarctic sea ice. Moreover, to properly study the Antarctic sea ice and its response to future warming, it is necessary to capture important dynamics, such as polynyas, the Antarctic slope current, and coastal leads. Many models within the CMIP6 model portfolio do not even have the spatial resolution to adequately resolve these features. This implies that their Antarctic projections may not be as trustworthy and robust as those for the Arctic Ocean.

In this study we employ the high-resolution OpenIFS-FESOM (AWI-CM3) coupled general circulation (nominally 30 km atmosphere and 4-25 km ocean resolutions) to investigate the Antarctic sea ice response to greenhouse warming, following a SSP5-8.5 greenhouse gas emission scenario. Our simulation exhibits a sudden decline of Antarctic sea ice in the Weddell Sea (WS) which can be explained by a combination of physical processes that involve continued strengthening of westerlies, increased atmosphere-ocean momentum transfer due to sea ice decline, a spin-up of the Weddell-Sea Gyre and slope current and corresponding vertical and horizontal supply of heat into the Weddell Sea. The resulting decrease of sea ice further leads to heat accumulation in austral summer due to the absorption of short-wave radiation, which can further weaken winter sea ice extent and intensify the momentum transfer and associated heat transport into the Weddell Sea gyre.  

Our study highlights the relevance of positive atmosphere-sea ice-ocean feedbacks in triggering the abrupt decline in Antarctic sea ice.  

How to cite: Kim, D.-W., Jung, T., Puthiyaveettil, N., Park, W., Semmler, T., Timmermann, A., and Zapponini, M.: Coupled atmosphere-sea-ice-ocean feedback accelerates rapid sea ice decline in Weddell Sea in high-resolution global climate model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7355, https://doi.org/10.5194/egusphere-egu24-7355, 2024.

EGU24-7520 | ECS | Posters on site | OS1.5

Intensification of the Antarctic slope current due to freshwater forcing in a warmer climate 

Myeong-Hyeon Kim, Gyuseok Yi, June-Yi Lee, Axel Timmermann, Wonsun Park, and Sun-Seon Lee

The Antarctic slope current (ASC) flows westward along the Antarctic coastlines and influences heat exchange across the Antarctic continental shelf. Therefore, it could play an important role in regulating the Southern Ocean circulation by affecting processes such as ice melting and water mass formation. However, clarifying the mechanism and change of ASC in future climate using high-resolution climate model is still challenging. We showthat ASC is projected to accelerate in response to CO2 increases by comparing present-day and CO2 increased simulations (2×CO2 and 4×CO2)conducted with the fully coupled ultra-high-resolution Community Earth System Model. The intensification of ASC was attributable to an increase in the gradient of sea surface height due to a decrease in salinity through geostrophic balance. This freshening was dominated by sea ice melting, while increases in runoff and precipitation minus evaporation played a minor role with regional and seasonal dependence. These results increased understanding about the future change of ASC using high-resolution simulations and have important implications for changes in mesoscale ocean circulation and the climate of Southern Ocean.

How to cite: Kim, M.-H., Yi, G., Lee, J.-Y., Timmermann, A., Park, W., and Lee, S.-S.: Intensification of the Antarctic slope current due to freshwater forcing in a warmer climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7520, https://doi.org/10.5194/egusphere-egu24-7520, 2024.

EGU24-9682 | Orals | OS1.5 | Highlight

The Imprint of Sea Ice Cover on the Biological Carbon Pump in the Southern Ocean 

Moritz Holtappels and Marwa Baloza

The Seasonal Ice Zone (SIZ) around Antarctica covers an area of 16 Mio km2 and is considered the largest biogeochemical province in the Southern Ocean. Despite a well-documented control of sea ice on primary production, its large-scale effect on the biological carbon pump, i.e. the sinking of organic carbon into deep waters and ultimately to the sediments, remains poorly constrained. Here we demonstrate that the degree of sea ice cover during the growth season is a strong predictor for carbon remineralization rates in underlying sediments. We compiled the available benthic rate measurements for the SIZ and found that more than 80% of the variability can be explained by only two environmental factors: long-term occurrence of moderate sea ice cover in the summer season, and water depth. The empirical model was used to map the benthic carbon remineralization for the entire SIZ, showing elevated rates especially at the Antarctic Peninsula and the Amundsen Sea in West Antarctica, and the D’Urville Sea, Davis Sea and Prydz Bay in East Antarctica. Altogether, benthic remineralization in the entire SIZ summed up to 46 Tg C per year, of which 71% can be assigned to shelf sediments. Applying an empirical function for the burial rate, the total organic carbon supply to the sediments was estimated to be 52 Tg C per year and the carbon export from the euphotic zone (<100m) was calculated to be ~500 Tg C per year. In summary, the results illustrate the dominant influence of sea ice dynamics on the biological carbon pump and suggest that anticipated changes in Antarctic sea ice will have a significant effect on the biological carbon sequestration in the Southern Ocean.

How to cite: Holtappels, M. and Baloza, M.: The Imprint of Sea Ice Cover on the Biological Carbon Pump in the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9682, https://doi.org/10.5194/egusphere-egu24-9682, 2024.

EGU24-9949 | ECS | Posters on site | OS1.5

Long-term temperature trends in Antarctic water masses across the New Zealand–Antarctica chokepoint  

Antonino Ian Ferola, Yuri Cotroneo, Giorgio Budillon, Pasquale Castagno, Pierpaolo Falco, Giannetta Fusco, Enrico Zambianchi, and Giuseppe Aulicino

A 29-year time series of summer Expendable Bathythermographs (XBT) data collected along the New Zealand-Antarctica 'chokepoint' of the Antarctic Circumpolar Current (ACC) was used to analyse the temperature variability of the surface and intermediate layers of the Southern Ocean (SO) from 1994 to 2023. Our findings confirm previous studies, showing an overall warming of the SO over the past 30 years and that the northernmost portion of the ACC exhibits significant warming, while areas south of the Polar Front experience no significant temperature trends.
Additionally, as different masses across the Antarctic Circumpolar Current can be representative of different regions of the SO on a variety of spatial and temporal scales, we focused on the estimation of the temperature trend associated. Our analysis reveals strong warming trends of approximately 0.27°C/decade and 0.13°C/decade respectively for Sub Antarctic Mode Water - SAMW and Antarctic Intermediate Water - AAIW over the study period, while Antarctic Surface Water - AASW and Circumpolar Deep Water - CDW show negligible and/or not significant trends.

How to cite: Ferola, A. I., Cotroneo, Y., Budillon, G., Castagno, P., Falco, P., Fusco, G., Zambianchi, E., and Aulicino, G.: Long-term temperature trends in Antarctic water masses across the New Zealand–Antarctica chokepoint , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9949, https://doi.org/10.5194/egusphere-egu24-9949, 2024.

EGU24-10622 | ECS | Orals | OS1.5

 Reconstructing the 2003-2022 Sea Level Anomalies field in ice-covered regions of the Southern Ocean 

Cosme Mosneron Dupin, Jean-Baptiste Sallée, Pierre Veillard, Casimir de Lavergne, Pierre Prandi, and Yannice Faugère

Despite its pivotal role in the climate system, subpolar circulation in the Southern Ocean remains poorly observed, primarily owing to the physical limitations of conventional satellite altimetry in ice-covered regions. However, recent progress in processing methods now enables precise SLA (Sea Level Anomalies) estimations within sea-ice fractures and leads.

Thanks to these advances, previous studies were able to construct altimetry maps with a complete Southern Ocean coverage over the period 2011-2019. It allowed for the quantification of its full SLA seasonal cycle. Here, we introduce a novel SLA product that encompasses both open and ice-covered oceanic domains, and covers a larger temporal expanse amounting to two decades (2003-2022). Employing an optimal interpolation approach, multiple satellite missions, namely Envisat, Cryosat, SARAL/AltiKa, and Sentinel-3, are combined together, improving spatial resolution and increasing temporal range. A newly developed algorithm ensures the seamless continuity of observations, bridging the observational disconnect between open-ocean and leads data points.

The robustness of the derived SLA product is corroborated against in-situ data from moorings and bottom pressure recorders. The observed seasonal cycle aligns consistently with the existing literature. Overall, the temporal extent of this dataset provides, for the first time, the opportunity to investigate the interannual variability of the whole Southern Ocean circulation through observational data.

How to cite: Mosneron Dupin, C., Sallée, J.-B., Veillard, P., de Lavergne, C., Prandi, P., and Faugère, Y.:  Reconstructing the 2003-2022 Sea Level Anomalies field in ice-covered regions of the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10622, https://doi.org/10.5194/egusphere-egu24-10622, 2024.

EGU24-10878 | Posters on site | OS1.5

Water Mass Changes and Carbon Uptake by Subantarctic Pacific Waters 

Maribel I. García-Ibáñez, Paula C. Pardo, Peter J. Brown, Gareth Lee, Adrian Martin, Sophy Oliver, Katsia Pabortsava, Pablo Trucco-Pignata, and Dorothee C.E. Bakker

The Southern Ocean (SO) is a critical component of the global carbon cycle, acting as a significant sink for atmospheric carbon dioxide (CO2). Understanding the intricate processes governing CO2 uptake in the SO is paramount for comprehending the global carbon budget and predicting future climate scenarios. Recent observations suggest that changes in SO water masses, driven by climate-induced alterations in temperature and circulation patterns, can significantly impact CO2 uptake. Understanding these feedbacks is crucial for predicting the SO's future role as a carbon sink and its broader implications for climate mitigation efforts. In this work, we determine changes in the water mass composition and their characteristics, including their CO2 content, along the CUSTARD transect (54ºS-59ºS 90ºW) in Subantarctic Pacific waters. The CUSTARD transect crosses a region of formation of mode and intermediate waters. We use an extended Optimum Multiparameter (eOMP) analysis and data from three repeats of the CUSTARD transect in 1993 (expocode 316N19930222; data from GLODAPv2.2023), 2005-2006 (316N20050821 and 316N20060130; from GLODAPv2.2023), and 2019-2020 (74EQ20191202; the CUSTARD cruise). We observe isopycnal heaving in the southern part of the transect from 1993 to 2020. In the upper ocean (neutral density (γn) < 27.2 kg m-3), isopycnal heaving is linked to a temperature decrease of up to -2ºC and a salinity decrease of up to -0.15 between 1993 and 2005, extending to γn < 27.5 kg m-3 in 2019-2020. The physicochemical changes in the upper ocean are linked to changes in the water mass composition, including an increase in the volume of Antarctic Surface Water and Antarctic Intermediate Water and a decrease in the volume of SubAntarctic Mode Water over the 18-year study period. These water mass changes are accompanied by decreases in concentrations of oxygen, dissolved nutrients, and total alkalinity, along with an increase in total dissolved inorganic carbon of up to 40 µmol kg-3  for γn < 27.5 kg m-3 from 1993 to 2019-2020. For 27.5 kg m-3 < γn <28.2 kg m-3, salinity increased by 0.05 from 1993 to 2005 and by 0.15 over the 18-year studied period in the southern part of the transect. This salinity increase extends northward in 2019-2020. These changes in salinity are linked to an increase in Circumpolar Deep Water volume. In the deep layer (γn > 28.2 kg m-3), Ross Sea Bottom Water replaces Adélie Bottom Water from 1993 to 2019-2020. The changes in water mass composition observed along the CUSTARD transect indicate circulation variations linked to the Southern Annular Mode (SAM), with a prevalent positive phase since 1995. Positive SAM pahses increase upwelling south of the Antarctic Polar Front and downwelling in the Subantarctic Zone. Due to these circulation changes, the SO’s uptake of atmospheric CO2 decreases during positive SAM phases, which are predicted to intensify with climate change.

How to cite: García-Ibáñez, M. I., Pardo, P. C., Brown, P. J., Lee, G., Martin, A., Oliver, S., Pabortsava, K., Trucco-Pignata, P., and Bakker, D. C. E.: Water Mass Changes and Carbon Uptake by Subantarctic Pacific Waters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10878, https://doi.org/10.5194/egusphere-egu24-10878, 2024.

EGU24-11356 | Posters on site | OS1.5

Stronger Southern Ocean carbon uptake in high-resolution ocean biogeochemistry simulations 

Lavinia Patara, Judith Hauck, Jan Klaus Rieck, Malin Ödalen, Andreas Oschlies, and Özgür Gürses

It is increasingly recognized that the way Southern Ocean mesoscale eddies are represented in ocean models influences air-sea CO2 fluxes and their response to climate change. In this study, we assess the Southern Ocean carbon uptake since the 1960s in a hierarchy of global ocean biogeochemistry models (GOBMs) based on the NEMO-MOPS and FESOM-REcoM models. The horizontal resolutions of the GOBMs range from 1° and 0.5° resolutions (“eddy-parameterized”) to 0.25° and 0.1° resolutions (“eddy-rich”, where eddies are explicitly represented). We find that the “eddy-rich” models have steeper density surfaces across the ACC with respect to “eddy-parameterized” models, in better agreement with observations. A larger amount of deep waters low in anthropogenic carbon (Cant) is thereby transported to the surface, leading to a 10% higher Cant uptake and storage. Natural CO2 (Cnat), which integrated over the whole Southern Ocean is directed into the ocean, shows a somewhat higher ingassing in the “eddy-rich” models. As a result, the net CO2 uptake is about 14% higher in the “eddy-rich” with respect to the “eddy-parameterized” models. Trends over the 1958-2018 period reveal a gradual wind-driven reduction of Cnat uptake in all configurations, but this trend is about 40% weaker in the 0.1° model with respect to the lower resolution models. At the same time, the upward trend in the residual meridional overturning circulation (MOC) is weaker in the 0.1° model, supporting the hypothesis of a more pronounced “eddy-compensation” of the wind-driven Cnat trends. Our study suggests that GOBMs using standard eddy parameterizations may underestimate the net and anthropogenic CO2 uptake by about 10%, and emphasizes the importance of adequately simulating mesoscale eddies for better constraining the Southern Ocean carbon uptake in changing climate conditions.

How to cite: Patara, L., Hauck, J., Rieck, J. K., Ödalen, M., Oschlies, A., and Gürses, Ö.: Stronger Southern Ocean carbon uptake in high-resolution ocean biogeochemistry simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11356, https://doi.org/10.5194/egusphere-egu24-11356, 2024.

EGU24-12418 | ECS | Orals | OS1.5

The Weddell Sea atmospheric CO2 uptake: An overview of its seasonal cycle and relationship to sea ice 

Elise Droste, Mario Hoppema, Dorothee Bakker, Oliver Huhn, and Peter Landschützer

The Weddell Sea has previously been estimated to be a net atmospheric CO2 sink, transporting anthropogenic CO2 to deeper parts of the ocean. However, a paucity of spatial and temporal observational data coverage hinders a complete understanding of its (seasonal and interannual) variability, how it is affected by seasonal sea ice cover, and how it may change with rapidly changing Antarctic sea ice regimes. We provide a status overview of all available partial pressure CO2 (pCO2) observations and estimates in the Weddell Sea, including SOCAT, GLODAP, and SOCCOM float datasets. We identify a particular lack of data on the continental shelves. Floats fill the wintertime-gap by obtaining year-round data, but are restricted to the open ocean and water depths of at least 2000 m. The collated dataset illustrates a seasonal cycle for the Weddell Sea, in which the summertime CO2 uptake can be strong with a mean of -1.2 mol m2 yr-1, but extremely variable (± 2.2 mol m2 yr-1). Some of the summertime CO2 uptake is compensated by wintertime CO2 outgassing, particularly in the northern Weddell Sea where sea ice cover is lowest and wind speeds are high. We use additional reanalysis and observational data-based products to perform a further analysis of differences between subregions within the Weddell Sea. Results show that most regions have a strong seasonal cycle in the sea-air CO2 gradient, with mean amplitudes ranging between 27 µatm (Northern Weddell Sea) and 100 µatm (eastern Peninsula shelf regions). However, wintertime outgassing is largely restricted by sea ice cover in all regions. The central Weddell Sea seems to be a particularly important region for net CO2 uptake, which is partly explained by the timing of wintertime sea ice advance before the surface pCO2 oversaturates with respect to atmospheric CO2. These results imply that the timing of sea ice advance or retreat can have high impact on the net CO2 uptake of the Weddell Sea.

How to cite: Droste, E., Hoppema, M., Bakker, D., Huhn, O., and Landschützer, P.: The Weddell Sea atmospheric CO2 uptake: An overview of its seasonal cycle and relationship to sea ice, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12418, https://doi.org/10.5194/egusphere-egu24-12418, 2024.

EGU24-12655 | ECS | Posters on site | OS1.5

Temporal scales of mesoscale eddy-induced horizontal and vertical transport of carbon, heat and oxygen in the Southern Ocean.  

Mariana Salinas-Matus, Nuno Serra, Fatemeh Chegini, and Tatiana Ilyina

The Southern Ocean (SO) has been identified as one of the most widespread mesoscale eddy fields observed in the ocean. However, historically in the SO, eddy effects on the carbon cycle have been poorly understood, especially quantitatively, due to sparse observations in the SO and limited computational resources restricting model resolution. Recently, the importance of representing mesoscale eddies in the SO for generating reliable transient simulations and global climate projections has been suggested. This work focuses on comprehending and quantifying the vertical and horizontal eddy-induced transport of carbon, heat, and oxygen in the upper ocean (first 300 m) across time scales ranging from inter-annual to high frequency. It aims to elucidate the impact of these processes on the ocean's uptake of carbon, heat, and oxygen. Studying these three components helps to distinguish  the role of biogeochemical and physical processes, due to the shared and distinct mechanisms that affect them. As the main tool, we used simulations made with the ocean component of the ICON model, coupled with the biogeochemical model HAMOCC. We employed a hierarchy of model resolutions, ranging from eddy-parameterized to eddy-resolved resolutions, to elucidate the role of representing eddies in facilitating/impeding air-sea fluxes.

How to cite: Salinas-Matus, M., Serra, N., Chegini, F., and Ilyina, T.: Temporal scales of mesoscale eddy-induced horizontal and vertical transport of carbon, heat and oxygen in the Southern Ocean. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12655, https://doi.org/10.5194/egusphere-egu24-12655, 2024.

EGU24-13132 | Posters on site | OS1.5

Variability in upper ocean properties around the South Orkney Islands, Antarctica 

Angelika Renner, Sebastian Menze, Elizabeth Jones, Emma Young, Sally Thorpe, and Eugene Murphy

The South Orkney Islands region is a highly productive environment situated between the Weddell Sea to the south and Scotia Sea to the north. Complex bathymetry around the island plateau strongly influences circulation and water mass exchanges. While the general, large-scale patterns in currents and hydrography are fairly well described, more detailed studies into spatial and temporal variability are mostly lacking, especially for the upper water column. In this study, we present hydrographic and ocean current observations from two surveys across the plateau conducted in January 2016 and 2019. The data confirm the dominant, topographically steered boundary current associated with the Weddell Front, which follows the continental slope around the southern edge of the South Orkney Plateau towards its northeastern side. During this passage, core characteristics of Weddell Sea water masses become eroded through interaction with other water masses. Where the Weddell Front first meets the plateau on its western side, large variability in currents is observed, possibly due to eddy activity and likely promoting mixing and water mass transformation. Differences in water mass characteristics between the two years are likely related to very different climatic conditions in the months prior to the surveys with opposing sea ice states, and large differences in regional winds, and air and sea surface temperatures. On the northwestern South Orkney Plateau, two canyons are particular hotspots for Antarctic krill, and the larger canyon was surveyed with high resolution, repeat transects. These repeated observations show high day-to-day variability in both currents and hydrography, possibly forced by short-term wind variability driving or restricting water exchange between the canyon and the deeper ocean. This suggests that the elevated krill abundance associated with the canyons may be due to several mechanisms, including retention by the local currents, interactions between the currents and krill behaviour, and potentially increased phytoplankton growth due to additional nutrient availability driven by the highly dynamic environment.

How to cite: Renner, A., Menze, S., Jones, E., Young, E., Thorpe, S., and Murphy, E.: Variability in upper ocean properties around the South Orkney Islands, Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13132, https://doi.org/10.5194/egusphere-egu24-13132, 2024.

EGU24-13207 * | ECS | Orals | OS1.5 | Highlight

Unprecedented changes in the Southern Ocean detected by satellites 

Alessandro Silvano, Rafael Catany, Estrella Olmedo, Veronica González-Gambau, Antonio Turiel, Carolina Gabarró, Aina García-Espriu, Cristina González-Haro, F. Alexander Haumann, Aditya Narayanan, Alberto Naveira Garabato, and Roberto Sabia

The Southern Ocean has experienced unprecedented changes since 2016. Most notably are 1) a reduction in sea ice cover and 2) the appearance of offshore polynyas not seen since the 1970s. Several hypotheses have been put forward to explain both these events, including atmospheric (e.g. winds, atmospheric rivers) and oceanic (e.g. upwelling) drivers. To help explain what has occurred over the past decade we use the first regional product of sea surface salinity (SSS) in the Southern Ocean derived by satellites as part of the SO-FRESH project. We combine this new dataset with sea ice observations from satellites as well as with in situ observations and models to show that both atmospheric and oceanic processes are involved in the observed changes, highlighting the complexity of the ice-ocean-atmosphere system in the Southern Ocean.

 

How to cite: Silvano, A., Catany, R., Olmedo, E., González-Gambau, V., Turiel, A., Gabarró, C., García-Espriu, A., González-Haro, C., Haumann, F. A., Narayanan, A., Naveira Garabato, A., and Sabia, R.: Unprecedented changes in the Southern Ocean detected by satellites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13207, https://doi.org/10.5194/egusphere-egu24-13207, 2024.

EGU24-13382 | ECS | Orals | OS1.5

Southern Ocean upwelling: Climatology and long-term trends 

Fanglou Liao, Kunde Yang, Yaping Wang, Guandong Gao, Peng Zhan, Daquan Guo, Zipeng Li, and Ibrahim Hoteit

The Southern Ocean upwelling is the most globally significant upwelling branch, and it plays a crucial role in redistributing water, heat, salt, and carbon on a global scale. The aim of this study is to enhance the understanding of this upwelling system, focusing primarily on the climatology and long-term trends of the Southern Ocean upwelling, both historical and projected, using global climate models. The simulated large-scale upwelling in the Southern Ocean is ~0.5 m/day. Although the spatial distribution pattern of the simulated Southern Ocean upwelling appears similar across different models, the strength of the upwelling is highly sensitive to resolution, generally showing stronger upwelling in eddy-permitting and eddy-resolving models. The most intense upwelling is predominantly concentrated around five major topographic features; this finding is consistent with those of previous studies. Our analysis of an eddy-resolving climate model shows no discernible trend during a historical period (1850–2005) and under a business-as-usual emission scenario in the 21st century (2006-2100). However, significant multidecadal variations are evident from this eddying model, which may be related to the low-frequency variations in the wind-stress curl and eddy kinetic energy. Notably, two lower-resolution climate models cannot very well simulate this multidecadal variations, and there is no consensus regarding its intensification or weakening. Our results suggest that wind stress is likely to increase under a scenario of comparatively high greenhouse gas emissions in the future; however, elevated vertical stratification of seawater may act as a barrier to the intensification of the upwelling.

How to cite: Liao, F., Yang, K., Wang, Y., Gao, G., Zhan, P., Guo, D., Li, Z., and Hoteit, I.: Southern Ocean upwelling: Climatology and long-term trends, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13382, https://doi.org/10.5194/egusphere-egu24-13382, 2024.

EGU24-13752 | ECS | Posters on site | OS1.5

Evolution of the Subantarctic Mode Water in the Southern Ocean 

Yu Hong

Powerful westerlies in the Southern Ocean drive the Antarctic Circumpolar Current (ACC), the northward Ekman flow, and the associate upwelling of the Circumpolar Deep Water (CDW). The upwelled CDW is transported northward by the Ekman flow. Upon reaching the north side of the Subantarctic Front (SAF), these waters undergo intensive vertical mixing driven by cooling of the atmosphere in winter, forming the Subantarctic Mode Water (SAMW) with vertically homogenous properties. The SAMW is then transported eastward with the ACC and northward with the subtropical gyre, completing the ventilation of the Southern Hemisphere oceans. As a part of the upper limb of the Southern Ocean overturning circulation, the formation and transport of the SAMW play essential roles in the heat, freshwater, carbon, oxygen, and nutrient budgets both regionally and globally. Changes in its physical properties also provide good indications of global climate change. The SAMW has significant natural variability on different time scales, mainly regulated by factors such as sea surface buoyancy flux, the Ekman transport and pumping, and eddies. Under global warming, research has presented different conclusions regarding changes in the volume and properties of the SAMW, hindering our further understanding of its climate impacts.

By analyzing gridded Argo observations in the past decades and future warming model simulation from the Coupled Model Intercomparison Project (CMIP), we found that the volume of the SAMW is generally decreasing. This volume decreasing is mainly determined by the change in surface buoyancy flux. The volume of the SAMW slowly increases after the radiative forcing stabilized in the future warming simulation. We also found there is an opposite change in volume between different density layers, representing changes in properties of the SAMW. The opposite volume change is mainly determined by the change in the depth and position of the winter deep mixed layer. Meanwhile, the observed average temperature and salinity of the SAMW in the South Indian Ocean are increasing. But the freshening in the formation area and the southward shift of the isopycnal surfaces weaken the trend of the average temperature and salinity increase of the SAMW. In future warming simulations, the cooling and freshening on the isopycnal surfaces cause the minimum warming and strong freshening in the depth of SAMW. These conclusions deepen our understanding of the evolution of the SAMW in the Southern Ocean and its underlying physical mechanisms, providing a new perspective on the climate response and impact of water masses in the Southern Ocean.

How to cite: Hong, Y.: Evolution of the Subantarctic Mode Water in the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13752, https://doi.org/10.5194/egusphere-egu24-13752, 2024.

EGU24-14592 | ECS | Posters on site | OS1.5

Unraveling Southern Ocean dynamics: Insights into Antarctic gyre circulation and slope current through direct numerical simulations 

Bajrang Chidhambaranathan, Bishakhdatta Gayen, and Catherine Vreugdenhil

The Southern Ocean holds a distinctive and pivotal position globally, connecting major ocean basins via its intricate circulation network. This makes it a central hub of oceanic transport. Despite numerous studies, the precise mechanisms governing the local and regional dynamics influencing the rapid poleward heat transport and Antarctic ice melting, aided by the Southern Ocean, remains elusive. Thus, to enhance our comprehension of the role of important regional-scale circulation dynamics like the Weddell, Ross and Kerguelen gyre circulations, high-fidelity direct numerical simulations employing simplified Antarctic geographical features were performed. These simulations were solely forced by the latitudinally varying sea surface temperature. The results obtained were found to closely mirror the real-world system, showcasing phenomena such as the Antarctic circumpolar current, slope current, bottom water formation and polar gyres, even without accounting for the wind forcing. This approach extends solutions from the small turbulence scales to larger planetary processes through down-scaling by employing principles of dynamic similarity, producing energy-conserving flow models. While limited by the absence of salinity and wind forcings, the study demonstrated the viability of direct numerical simulations in comprehending Southern Ocean dynamics, including polar gyres and slope currents. This groundwork lays the foundation for integrating further complexities to fine-tune the system for a more accurate analysis of the Southern Ocean’s physical dynamics - an endeavor of significant importance in a dynamically changing climate landscape.

How to cite: Chidhambaranathan, B., Gayen, B., and Vreugdenhil, C.: Unraveling Southern Ocean dynamics: Insights into Antarctic gyre circulation and slope current through direct numerical simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14592, https://doi.org/10.5194/egusphere-egu24-14592, 2024.

EGU24-17291 | Posters on site | OS1.5

Modelling of multiyear variability of oceanographic variables in the area surrounding South Georgia, Southern Ocean 

Ragnhild Daae, Ingrid Ellingsen, and Cian Kelly

The area surrounding South Georgia in the Southern Ocean is a highly productive area. This study seeks to interpret the oceanographic processes within this area by using high resolution model data. Emphasizing multiyear variability, our investigation centres on hydrography, currents, mesoscale eddies, upwelling phenomena, and their profound impact on vertical mixing.

Using data from two distinct model domains, our study encompasses the finer details facilitated by both larger and smaller resolution scales. The larger 'mother' domain with 4 km horizontal resolution, and its smaller counterpart at 800 m resolution, offer nuanced perspectives on the region's dynamics and their resolution-dependency.

This modelling initiative forms an integral part of a larger project, SFI Harvest, aimed at developing a coupled physical-biological model specifically for understanding primary production dynamics in the Southern Ocean. SFI Harvest is a long-term centre for research-based innovation, where our part is to better understand the spatiotemporal variability for sustainable harvesting of krill in the Southern Ocean.

How to cite: Daae, R., Ellingsen, I., and Kelly, C.: Modelling of multiyear variability of oceanographic variables in the area surrounding South Georgia, Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17291, https://doi.org/10.5194/egusphere-egu24-17291, 2024.

EGU24-17353 | ECS | Posters on site | OS1.5

Impact of Wind Stress Curl on the Eddy Saturation of the Antarctic Circumpolar Current from a Barotropic Perspective 

Sima Dogan, Caroline Muller, Louis-Philippe Nadeau, and Antoine Venaille

Eddy saturation, a phenomenon where east-west transport remains insensitive to changes in wind stress, is believed to play a crucial role in explaining the behavior of the Antarctic Circumpolar Current (ACC). Two distinct mechanisms are known to lead to eddy saturation: (i) baroclinic instability in stratified flows and (ii) topographic-barotropic instability in unstratified flows. This study focuses on eddy saturation resulting from topographic-barotropic instability in a doubly periodic domain. Previous findings have shown that topographic-barotropic instability, typically occurring within a specific range of wind stress, is significantly influenced by the geometry of the topography. We investigate how the introduction of wind stress curl affects the occurrence and the dynamics of eddy saturation. Our findings demonstrate that wind stress curl and its interaction with topography is crucial in understanding the eddy saturation and, consequently, for determining the zonal transport of the ACC. In the doubly periodic domain, a dependence is observed between the zonal transport and the wind stress variations in relation to mean wind stress,  associated with the form stress composed by the interaction with bottom topography with singular and multiple ridges.

How to cite: Dogan, S., Muller, C., Nadeau, L.-P., and Venaille, A.: Impact of Wind Stress Curl on the Eddy Saturation of the Antarctic Circumpolar Current from a Barotropic Perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17353, https://doi.org/10.5194/egusphere-egu24-17353, 2024.

The Southern Ocean considerably influences the global climate by exchanging heat and carbon between the deep ocean and the surface. Historically, it mitigated surface warming by absorbing 70% of excess heat and over 10% of human-induced CO2 emissions. The future of this role is strongly linked to salinity changes, as salinity controls, through its influence on the density stratification, the vertical exchange of water masses, heat and carbon.  A strong freshening of the Southern Ocean surface waters in the decades before 2016 has resulted in increased surface density stratification all around Antarctica. This enhanced stratification reduces the mixing between deep and surface waters, and in particular the vertical mixing of carbon-rich deep waters into the surface layer. By comparing post-2010 hydrographic sections in the GLODAP database to the climatology, we observe consistent and significant anomalies in the biogeochemical properties of the top 500 m of all the sectors of the Southern Ocean. While the surface layer is freshening, salinity, temperature, dissolved inorganic carbon (DIC) and total alkalinity (TA) increase in the subsurface layer. We find that this increase results from the shallowing of upper circumpolar deep water south of 50°S. We investigate the variability in properties of the surface and subsurface layers over the last decade, as well as the impact of such changes on the potential fugacity of CO2 to better understand how the change in stratification may impact the air-sea CO2 flux.

How to cite: Olivier, L. and Haumann, A.: Changes in salinity driven stratification and impacts on the deep-water CO2 ventilation in the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17470, https://doi.org/10.5194/egusphere-egu24-17470, 2024.

Recent observations show that mass loss from Antarctic ice sheets and ice shelves is accelerating and is projected to increase further in the coming decades. The resulting freshwater input from melting of the grounded ice sheet and ice shelves is expected to have significant impacts on Southern Ocean dynamics that could also feedback onto the global climate system and its future changes. However, most state-of-the-art coupled models do not include interactive ice sheets and shelves, resulting in large uncertainty in future climate projections. Additionally, the physical response within the Southern Ocean and beyond remains elusive and largely dependent on the model used and the experimental design.

Here, we present results from a model participating in the Southern Ocean Freshwater Input from Antarctica (SOFIA) initiative, an international model intercomparison project in which freshwater is added to the ocean surrounding Antarctica to simulate the otherwise missing ice-sheet mass loss. Contrary to most models participating in SOFIA, we have performed our experiments with an ocean-sea ice model in which sea surface salinity restoring is deactivated and previously computed restoring-induced surface fluxes are provided at the ocean surface in order to keep a stable climate. Besides the missing atmospheric feedback, an ocean-only SOFIA experiment allows the investigation of the ocean's response to Antarctic freshwater discharge and, through the comparison with SOFIA coupled models, the quantification of the role of atmospheric feedbacks.

Preliminary results, based on a suite of experiments using varying strengths of the freshwater perturbation, are presented for both Southern Ocean physics and dynamics, with implications for the global circulation.

How to cite: Farneti, R.: Southern Ocean response and sensitivity to idealized freshwater perturbation experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17598, https://doi.org/10.5194/egusphere-egu24-17598, 2024.

EGU24-17657 | ECS | Posters on site | OS1.5

Global ocean ventilation: a comparison between a general circulation model and data-constrained inverse models 

Bruno Millet, Casimir de Lavergne, William Gray, Mark Holzer, and Didier Roche

Ocean ventilation, or the transfer of tracers from the surface boundary layer into the ocean interior, is a critical process in the climate system. Here, we assess steady-state ventilation patterns and rates in three models of ocean transports: a 1° global configuration of the Nucleus for European Modelling of the Ocean (NEMO), version 2 of the Ocean Circulation Inverse Model (OCIM), and the Total Matrix Intercomparison (TMI). We release artificial dyes in six surface regions of each model and compare equilibrium dye distributions as well as ideal age distributions. We find good qualitative agreement in large-scale dye distributions across the three models. However, the distributions indicate that TMI is more diffusive than OCIM, itself more diffusive than NEMO. NEMO simulates a sharp separation between bottom and intermediate water ventilation zones in the Southern Ocean, leading to a weaker influence of the latter zone on the abyssal ocean. A shallow bias of North Atlantic ventilation in NEMO contributes to a stronger presence of the North Atlantic dye in the mid-depth Southern Ocean and Pacific. This isopycnal communication between the North Atlantic surface and the mid-depth Pacific is very slow, however, and NEMO simulates a maximum age in the North Pacific about 900 years higher than the data-constrained models. Possible causes of this age bias are interrogated with NEMO sensitivity experiments. Implementation of an observation-based 3D map of isopycnal diffusivity augments the maximum age, due to weaker isopycnal diffusion at depths. We suggest that tracer upwelling in the subarctic Pacific is underestimated in NEMO and a key missing piece in the representation of global ocean ventilation in general circulation models.

How to cite: Millet, B., de Lavergne, C., Gray, W., Holzer, M., and Roche, D.: Global ocean ventilation: a comparison between a general circulation model and data-constrained inverse models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17657, https://doi.org/10.5194/egusphere-egu24-17657, 2024.

EGU24-18195 | Posters on site | OS1.5

RoSES: The Role of the Southern Ocean in the Earth System 

Elaina Ford and Romy Hall

The Southern Ocean is a key component in the Earths global carbon cycle and associated climate dynamics, as a primary hotspot for the oceanic sink of anthropogenic carbon dioxide (CO2). However, our understanding of the vital processes in this area was limited. In recognition of this critical knowledge gap, the Natural Environment Research Council (NERC) invested £7 million in a pioneering research programme spanning five years, under the Role of the Southern Ocean in the Earth System (RoSES) programme. The overarching objective of this ambitious endeavour was twofold: to substantially reduce uncertainty in 21st-century global climate change projections and to lay a robust scientific foundation to guide international climate policy.

The strength of the RoSES programme is built on the synergies between five distinct but interwoven projects:-

  • SONATA focussed on the Southern Ocean's biological and physical processes, and the relationship between oceanic currents and marine life that ultimately influences carbon sequestration.
  • SARDINE assessed the Southern Ocean's role in regulating global nutrient cycles, a key aspect with impacts on carbon dynamics and, consequently, climate patterns.
  • PICCOLO focussed on phytoplankton and their role as carbon consumers, examining how these tiny organisms contribute to the Southern Ocean's carbon sink.
  • CUSTARD investigated the Southern Ocean's role in atmospheric CO2 uptake, further enriching the understanding of this vast region's carbon dynamics.
  • CELOS focussed on the Southern Ocean's contribution to the global ocean overturning circulation, a critical component in the Earth's climate system.

Each of these projects focussed on different element within the Southern Ocean domain, collectively seeking to uncover its carbon dynamics and unravel the complexities associated with anthropogenic CO2.

This poster will provide an overview of the delivery and management of the RoSES programme and will signpost to the outputs and dissemination activities of those associated with the programme here at EGU2024.

How to cite: Ford, E. and Hall, R.: RoSES: The Role of the Southern Ocean in the Earth System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18195, https://doi.org/10.5194/egusphere-egu24-18195, 2024.

EGU24-19204 | ECS | Posters on site | OS1.5

Eddy effects on South Atlantic Ventilation Pathways using Lagrangian trajectories   

Simon Schäfers, Alexa Griesel, and Manita Chouksey

The Southern Ocean takes up a significant amount of anthropogenic CO2 emissions by subduction, as dense water masses get displaced northward and form the Antarctic Intermediate Water (AAIW).  Subducting oceanic water masses encapsulate dissolved atmospheric gases and retard anthropogenic climate change by taking up about a quarter of industrial CO2 emissions, nearly half of which in the Southern Ocean. However, the processes that control the water mass formation and their ventilation pathways, relevant to climate change remain actively researched. Southern Ocean dynamics are strongly influenced by mesoscale eddies which are likely to intensify in a warmer global climate, raising the question on the role and importance of eddies in shaping the ventilation pathways and in oceanic tracer and heat uptake. Previous results using Lagrangian backtracking and tracer simulations in the South Atlantic Ocean indicate heterogeneous source regions and pathways for AAIW, suggesting the potential role of eddies in addition to wind stress-induced Ekman Transport. Here we investigate the impact of mesoscale eddies on the subduction timescales and ventilation pathways of the AAIW in the South Atlantic Ocean.  
   
We use an eddy-resolving 1/10 degree ocean model (Parallel Ocean Program) with a Lagrangian particle tracking algorithm (Parcels) following discrete particles from the interior of the South Atlantic AAIW backward in time until they reach the mixed layer after tracking them for 100 years. In total 105 particles were released in the South Atlantic Ocean between 15° and 40°S in depths that meet the density criteria of 26.8 to 27.4 kg/m3  for the AAIW. The experiment was performed on a repeated year velocity field with daily mean data from 1990. For comparability, we performed the same experiment on a decadal mean state, eliminating mesoscale eddy activity. The Transit Time Distributions (TTD) inferred from the backtracking of Lagrangian trajectories aid in quantifying eddy effects on the advection time scales, source regions, and pathways of the AAIW. We expect eddy effects to alter the position and time scales of the subduction process and affect the importance of specific routes, such as the cold and warm water routes.

How to cite: Schäfers, S., Griesel, A., and Chouksey, M.: Eddy effects on South Atlantic Ventilation Pathways using Lagrangian trajectories  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19204, https://doi.org/10.5194/egusphere-egu24-19204, 2024.

EGU24-291 | ECS | Orals | OS1.6 | Highlight

Oceanic gateways to Antarctic grounding lines - Impact of critical access depths on sub-shelf melt 

Lena Nicola, Ronja Reese, Moritz Kreuzer, Torsten Albrecht, and Ricarda Winkelmann

Melting underneath the floating ice shelves surrounding the Antarctic continent is a key process for the stability of the Antarctic Ice Sheet and therefore its current and future mass loss. Troughs and sills on the continental shelf play a crucial role in modulating sub-shelf melt rates, as they can allow or block the access of relatively warm, modified Circumpolar Deep Water to ice-shelf cavities.

In our study (Nicola et al., subm.), we identify potential oceanic gateways that could allow the access of warm water masses to Antarctic grounding lines based on critical access depths inferred from high-resolution bathymetry data. We analyse the properties of water masses that are currently present in front of the ice shelf and that might intrude into the respective ice-shelf cavities in the future. We use the ice-shelf cavity model PICO to estimate an upper limit of melt rate changes in case all warm water masses up to a certain depth level gain access to the cavities. The identification of oceanic gateways is thus valuable for assessing the potential of ice-shelf cavities to switch from a 'cold' to a 'warm' state, which could result in widespread ice loss from Antarctica.

How to cite: Nicola, L., Reese, R., Kreuzer, M., Albrecht, T., and Winkelmann, R.: Oceanic gateways to Antarctic grounding lines - Impact of critical access depths on sub-shelf melt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-291, https://doi.org/10.5194/egusphere-egu24-291, 2024.

EGU24-609 | ECS | Orals | OS1.6

High resolution pH measurements at the edge of the Dotson Ice shelf using state-of-the-art autonomous technologies. 

Daisy Pickup, Karen Heywood, Dorothee Bakker, Emily Hammermeister, Socratis Loucaides, Yixi Zheng, Gareth Lee, Patricia Yager, and Rob Hall

The global ocean takes up about a quarter of anthropogenic carbon dioxide emissions, with the Southern Ocean playing a disproportionately large role. This uptake has led to changes in the Southern Ocean's carbonate chemistry, reducing pH through ocean acidification. The Amundsen Sea, West Antarctica, is surrounded by rapidly melting ice shelves, that may be impacting the carbonate balance of this coastal region. Near the Dotson Ice Shelf, we collected the first high-resolution, full-depth pH dataset using a Lab-on-Chip spectrophotometric sensor attached to an autonomous profiling ocean glider. The sensor collected data within 10 km of the Dotson Ice Shelf over a 19-day period in January/February 2022 and captured the variability that results from summertime biogeochemical and physical processes. In the upper 150 m, net primary production dominates variation in pH, producing a maximum pH of 8.34 (on the total hydrogen scale) in front of Dotson Ice Shelf, where chlorophyll fluorescence also peaks. Below 150 m, pH is generally lower, likely as a result of net respiration. The inflow of modified Circumpolar Deep Water near the east side of Dotson Ice Shelf exhibits a slightly elevated pH (0.05 units) compared to surrounding deep waters. The meltwater-laden outflow that exits on the west side of the ice shelf at depths between 300 - 500 m displays a lower pH (0.1 units) relative to the surrounding waters, which shoals and mixes, reducing pH in the overlying surface waters. In the coastal current along Dotson Ice Shelf, an unusual subsurface maximum in pH (0.1 units at 150 m, compared to surrounding waters) is observed and is also associated with increased chlorophyll fluorescence. Possible explanations for the observed features are discussed. These high-resolution findings reveal the potential of pH measurements on an autonomous vehicle for investigating difficult to access regions with glacial melt.

How to cite: Pickup, D., Heywood, K., Bakker, D., Hammermeister, E., Loucaides, S., Zheng, Y., Lee, G., Yager, P., and Hall, R.: High resolution pH measurements at the edge of the Dotson Ice shelf using state-of-the-art autonomous technologies., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-609, https://doi.org/10.5194/egusphere-egu24-609, 2024.

The ice sheets flowing into the Amundsen Sea, West Antarctica, are losing mass faster than most others about the continent due to rapid basal melting of their floating ice shelf extensions. A key oceanographic control of the rate of ice-shelf basal melting is a warm eastward undercurrent that flows along the continental shelf break and eventually towards the ice shelves. On monthly timescales surface winds drive fast barotropic variability in the undercurrent. On decadal timescales, however, undercurrent variability is not well understood. We present model results that show that on decadal timescales undercurrent variability opposes wind variability, with this being a consequence of sea-ice and ice-shelf freshwater flux variability. Specifically, periods of fast (more eastward) undercurrent are a result of enhanced brine rejection north of the continental shelf break, which enhances the cross-slope pressure gradient at depth and accelerates the undercurrent baroclinically. Opposite anomalies in the sea-ice freshwater flux decelerate the undercurrent. A positive feedback mechanism between the undercurrent and ice-shelf basal melt strengthens the undercurrent anomalies. Lastly, we show that variability in sea-ice freshwater fluxes, and by extension the Amundsen Sea undercurrent and ice-shelf basal melt, can be attributed to tropical Pacific variability impacting atmospheric conditions over the Amundsen Sea.

How to cite: Haigh, M. and Holland, P.: Freshwater fluxes drive decadal variability of the Amundsen Sea undercurrent and ice-shelf basal melt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1302, https://doi.org/10.5194/egusphere-egu24-1302, 2024.

EGU24-1729 | ECS | Posters on site | OS1.6

Revisiting the iceberg thickness distribution in Southern Ocean simulations. 

Anna Olivé Abelló, Pierre Mathiot, Nicolas Jourdain, Yavor Kostov, and Paul Holland

The acceleration of glaciers in the Antarctic ice sheet amplifies the flow of icebergs into the Southern Ocean. The presence of these icebergs has a significant impact on the penetration of warm water toward ice shelves and can also induce the formation of large polynyas when grounded, thereby promoting dense water formation. The existing implementation of the Lagrangian iceberg module in NEMO does not consider the Antarctic ice-shelf thicknesses from which the icebergs are originated, so the model cannot represent whether icebergs are thin enough to cross the shallow bathymetric ridges. In the present model, the iceberg masses, thicknesses, and size distribution are prescribed a priori as input parameters irrespective of the source ice-shelf characteristics. In addition, the categorization of iceberg classes and the scaling of smaller icebergs are not optimized, which is a strong limitation for climate modelling. Hence, the main aim of this study is to improve the thickness distribution of the calved icebergs based on ice shelf characteristics, decrease the computational cost of the model, and assess how these improvements alter the lifespan of the icebergs and their freshwater flux distribution across Antarctica.

The new approach has been implemented in a 0.25° Southern Ocean configuration of the NEMO ocean–sea-ice model. We used a power-law probability distribution function of iceberg occurrence as a function of iceberg area and a tabular iceberg definition to establish the thickness distribution for the small iceberg categories, imposing that each class exhibits the same total mass. In order to reduce computational costs, we constrained the frequency of icebergs released per class so that the smaller classes of multiple icebergs are gathered into one particle. Our preliminary results show that the iceberg thickness distribution, implemented as a function of areas, is supported by in-situ observations measured from high-resolution SAR-1 satellite images. The released icebergs display a typical thickness per class depending on the ice shelf's source, with a broader distribution when more calving classes are established. Ultimately, the findings reveal that accounting for realistic Antarctic ice-shelf thicknesses leads to thicker icebergs, particularly in larger classes, consequently increasing the mass that each transports westward around Antarctica. Future iceberg simulations, carried out for 25 years, will assess the iceberg's sensitivity to the maximum iceberg area, the number of different sizes and the area bounds used to define each size, among others. It is also expected that these simulations will also unveil high-melting regions and high iceberg densities in regions where icebergs ground, and will determine if fragmentation processes are needed to achieve realistic iceberg lifespans.

How to cite: Olivé Abelló, A., Mathiot, P., Jourdain, N., Kostov, Y., and Holland, P.: Revisiting the iceberg thickness distribution in Southern Ocean simulations., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1729, https://doi.org/10.5194/egusphere-egu24-1729, 2024.

EGU24-2153 | ECS | Orals | OS1.6

Ocean Circulation and Ice Shelf Melting in the Bellingshausen Sea 

Emma White, Adrian Jenkins, Paul Holland, Jan de Rydt, and Miguel Morales-Maqueda

A modified version of warm Circumpolar Deep Water (CDW) is able to flow onto the continental shelf in the Bellingshausen Sea, leading to high melt rates beneath the floating ice shelves. Data are presented from a 2007 research cruise to the Bellingshausen Sea, during which temperature, salinity and dissolved oxygen measurements were made at 253 stations. These observations provide detailed insights into the physical oceanographic regime of the region and its impact on the ice shelves, particularly in the western Bellingshausen Sea where few other ship-based observations exist. The transport of CDW across the shelf break at Marguerite Trough and Belgica Trough is assessed, as well as the modification of CDW properties as it flows onto the continental shelf. The spatial variability seen in water masses across the Bellingshausen Sea and regional circulation patterns are also evaluated. Finally, we present an assessment of the meltwater production and circulation within the ice shelf cavities.

How to cite: White, E., Jenkins, A., Holland, P., de Rydt, J., and Morales-Maqueda, M.: Ocean Circulation and Ice Shelf Melting in the Bellingshausen Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2153, https://doi.org/10.5194/egusphere-egu24-2153, 2024.

EGU24-3500 | Orals | OS1.6

How much can we get from Gade's mixing line? 

Louis-Alexandre Couston

Gade’s meltwater mixing line theory is consistent with numerous under-ice ocean observations. However, it is built on an assumption that is difficult to test with field measurements, especially near the ice boundary, which is that the effective salt and temperature diffusivities are equal. In this presentation, I will discuss the validity of Gade’s mixing line theory and show how it can be used to predict melt rates, using results from direct numerical simulations of a canonical model for externally forced ice-ocean boundary layers. I will first demonstrate that the effective salt and temperature diffusivities are approximately equal across most of the boundary layer in the well mixed regime. Then, I will show how knowledge of one turbulent diffusivity (salt, temperature, or thermal driving) can be combined with knowledge of one vertical profile in the bulk (salt, temperature, or thermal driving) to predict the heat and salt fluxes at the ice-ocean boundary.

How to cite: Couston, L.-A.: How much can we get from Gade's mixing line?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3500, https://doi.org/10.5194/egusphere-egu24-3500, 2024.

EGU24-3579 | ECS | Orals | OS1.6

Turbulent heat exchange between Circumpolar Deep Water and Winter Water on the Amundsen Sea Continental Shelf, Antarctica  

Xingchi Wang, Yvonne Firing, Alberto Naveira Garabato, Bieito Fernández Castro, and Carl Spingys

The intrusion of Circumpolar Deep Water (CDW) onto the Amundsen Sea continental shelf is a primary driver of basal melting and thinning of the West Antarctic ice shelves. The interaction between CDW and the overlying, near-freezing Winter Water (WW) on the continental shelf is thought to limit the heat that ultimately reaches the ice shelves. However, such interaction, and the processes underpinning it, remain little understood. In this study, we analyze over one hundred microstructure and finestructure profiles across the Amundsen Sea continental shelf. Our analysis indicates a strong correlation between microstructure turbulent kinetic energy dissipation and the finescale horizontal kinetic energy (HKE) associated with internal waves, suggesting that wave breaking is key to turbulence production in the region. Exploiting this relationship, we construct a 2-year time series of finescale HKE and turbulent dissipation from a mooring dataset acquired at the Pine Island-Thwaites West (PITW) trough. The time series reveals a distinct seasonal signal, with a range spanning one order of magnitude and diverse drivers. By combining a regional numerical model with the mooring diagnostics, we then estimate the turbulent diapycnal diffusivity and associated vertical heat flux between CDW and WW at the PITW trough. This provides insight into the heat loss experienced by CDW on its pathway toward the ice shelves.

How to cite: Wang, X., Firing, Y., Naveira Garabato, A., Fernández Castro, B., and Spingys, C.: Turbulent heat exchange between Circumpolar Deep Water and Winter Water on the Amundsen Sea Continental Shelf, Antarctica , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3579, https://doi.org/10.5194/egusphere-egu24-3579, 2024.

EGU24-4097 | ECS | Posters on site | OS1.6

A regime change within Amery Ice Shelf cavity by a reversed current in the twenty-first century 

Jing Jin, Antony Payne, and Christopher Bull

The Amery Ice Shelf (AmIS), the third largest ice shelf in Antarctica, has experienced a relatively low basal melting during the past decades. However, it is unclear how AmIS melting will respond to a future warming climate. Here, we use a regional ocean model forced by a low-emission scenario and a high-emission scenario to investigate AIS melting by 2100. The melt rate is projected to increase multiple times in 2100. An abrupt increase in melt rate happens in the 2060s in both scenarios. A mechanism that drives the jump of melting is investigated. A redistribution of local salinity (and then density) in front of AmIS forms a new geostrophic balance, leading to the reversal of local currents. This transforms AmIS from a cold cavity to a warm cavity, and results in a jump of ice shelf melting. This regime change draws our attention to the role of oceanic processes in the basal mass loss of Antarctic ice shelves in climate change.

How to cite: Jin, J., Payne, A., and Bull, C.: A regime change within Amery Ice Shelf cavity by a reversed current in the twenty-first century, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4097, https://doi.org/10.5194/egusphere-egu24-4097, 2024.

EGU24-5201 | ECS | Posters on site | OS1.6

Evolution processes of Ross sea polyny aassociated with modified Circumpolar Deep Water intrusion 

Qing Qin, Zhaomin Wang, Liangjun Yan, Chengyan Liu, and Jan De Rydt

In the central Ross Sea continental shelf, the modified Circumpolar Deep Water could intrude the continental shelf between March and July, and reach about 76 °S. According to seal-CTD data, in March and April, the potential temperature of the modified Circumpolar Deep Water was up to - 0.55 °C, and was widely distributed within the depths of 100 - 300 m. In May and June, the potential temperature of the modified Circumpolar Deep Water was up to - 0.65 °C, and was found to the north of 76 °S above the depth of 350 m, but discontinuous above the depth of 200 m. In July, the modified Circumpolar Deep Water sharply cooled, with the maximum potential temperature being -1.45 °C.

By analysing the seal-CTD observations and the numerical model results, this study found strong mixing in the central part of the shelf, owing to topography-induced upwelling. The resulted mixed layer warming is also attributed to the intrusion of the modified Circumpolar Deep Water. This oceanic process, along with the katabatic wind forcing, contributed to forming unique sea ice distribution characteristics in the Ross Sea Polynya, featured by ‘’less ice, more ice, less ice” from the front of the Ross Ice Shelf to the upwelling zone. This kind of sea ice distribution characteristics can also promote the northward expansion of the Ross Sea Polynya.

How to cite: Qin, Q., Wang, Z., Yan, L., Liu, C., and De Rydt, J.: Evolution processes of Ross sea polyny aassociated with modified Circumpolar Deep Water intrusion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5201, https://doi.org/10.5194/egusphere-egu24-5201, 2024.

EGU24-5406 | ECS | Posters on site | OS1.6

Reconstructing historical ocean changes around the West Antarctic Ice Sheet over the past centuries 

Quentin Dalaiden, Paul Holland, Kaitlin Naughten, Pierre Mathiot, Noé Pirlet, Antoine Barthelemy, and Nicolas Jourdain

Over recent decades, the West Antarctic Ice Sheet (WAIS) has witnessed a large increase in ice shelf melting. This ice loss is recognized to be associated with a response to changes in the ocean state, in particular of the Circumpolar Deep Water (CDW) on the Amundsen Sea continental shelf. It has been shown that the variability of the CDW inflow is strongly related to wind changes. While instrumental-based atmospheric reanalysis products are available since 1979, given the strong natural variability of the West Antarctic climate, this period of a few decades might be too short to identify the mechanisms driving the long-term changes in ice shelf melting, and to distinguish the relative contribution of natural and forced variability to the total changes. Therefore, there is a need to provide long-term historical changes in oceanic conditions to put the recently observed ice shelf melting into a longer context and to ultimately better constrain the future contribution of the WAIS to the global sea-level rise. Over the past few years, atmospheric reanalysis based on paleoclimate records spanning the last centuries have been released. This offers us the opportunity to assess historical changes in oceanic conditions in response to changes in the atmosphere. In this study, we propose a framework to reconstruct past ocean conditions around the WAIS over the last few centuries by using an ocean–sea-ice model (NEMO-SI3) forced by a paleo-based atmospheric reanalysis. Specifically, we use a paleo-reanalysis based on data assimilation that aims at dynamically combining information from paleoclimate records from the Southern Hemisphere (especially ice-core records) and the physics of Earth System Models. This has the advantage of guaranteeing a dynamical consistency between the reconstructed variables. Along with the methodology, we present the first reconstructed oceanic conditions from the NEMO-SI3 simulations.

How to cite: Dalaiden, Q., Holland, P., Naughten, K., Mathiot, P., Pirlet, N., Barthelemy, A., and Jourdain, N.: Reconstructing historical ocean changes around the West Antarctic Ice Sheet over the past centuries, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5406, https://doi.org/10.5194/egusphere-egu24-5406, 2024.

EGU24-6270 | ECS | Orals | OS1.6

Assessing the degree of impact from iceberg activities on penguin colonies of Clarence Island 

Hong Lin, Xiao Cheng, Teng Li, Qian Shi, Qi Liang, Xinyu Meng, Shaoyin Wang, and Lei Zheng

During August and September 2023, three giant icebergs, each bigger than Paris, successively grazed Clarence Island in the northeast of the Antarctic Peninsula, a home to a population of over 100,000 penguins. This incident may serve as a clarion call for the increasing iceberg calving due to global warming and its subsequent impact on the Antarctic ecosystem. Here we investigated this unexpected event using satellite imagery, employing wind speed, ocean currents, and seabed topography data to understand the behavior of the icebergs. During the study period, eastward winds and northward currents favored the drift of icebergs away from the island, and the deeper waters off the east coast reduced the probability of iceberg grounding. Nevertheless, iceberg D30A still left a significant amount of floating ice during its grazing passage. Moreover, we integrated historical records and probabilistic analyses of iceberg grounding to assess the degree of impact on penguin colonies of Clarence Island. Among the eleven colonies, only one in the northern region exhibits low impact, whereas two colonies in the southeastern region experience high impact. In a warming future, with an increase in iceberg calving events, penguin colonies located in iceberg drift hotspots are likely to experience greater impacts from iceberg activities. Therefore, we call upon the public to pay heed to climate warming and implement measures to mitigate anthropogenic greenhouse gas emissions, thereby alleviating the threat to penguin ecosystems.

How to cite: Lin, H., Cheng, X., Li, T., Shi, Q., Liang, Q., Meng, X., Wang, S., and Zheng, L.: Assessing the degree of impact from iceberg activities on penguin colonies of Clarence Island, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6270, https://doi.org/10.5194/egusphere-egu24-6270, 2024.

EGU24-7961 | ECS | Orals | OS1.6

Exploring drivers of change in the Ross Sea with a regional ocean model 

Alethea S. Mountford, Christopher Y. S. Bull, Adrian Jenkins, Nicolas C. Jourdain, and Pierre Mathiot

The Ross Sea and Ross Ice Shelf have remained relatively unchanged over recent decades, despite increasing global anthropogenic influences, and ocean temperatures and ice shelf melt rates increasing nearby. Future atmospheric warming, for example, could lead to a transition of the sub-ice shelf cavity from its current cold state to a warm state, which could in turn result in a dramatic increase to the currently low basal melt rates in the Ross Ice Shelf. We present a regional ocean model configuration (NEMO) at 1/4° resolution, which encompasses the whole of the Ross Sea, Ross Gyre and Ross Ice Shelf, as well as eastwards to include the Amundsen Sea, with a series of perturbation experiments to near-surface air temperature (an increase of 10°C) and precipitation (an increase by a factor of two) and a combination of the two. The model includes static ice shelves, extending from Merz to Venable, and their thermodynamic interaction with the ocean. Perturbations to the air temperature and precipitation alone are not sufficient to significantly alter the circulation or oceanographic conditions within the Ross Sea or within the cavity of the Ross Ice Shelf. However, when both perturbations are applied simultaneously, waters within the Ross Ice Shelf cavity warm to over 1°C, inducing an increase in basal melt of around 1000 Gt/yr within the cavity over the course of the simulated period 2017-2100. We see an increase in the strength of the Ross Gyre, with an eastward extension of the gyre into the Amundsen Sea. Circulation within the cavity is also affected, with a visible reduction in the outflow of waters from the cavity. Oceanographic changes within the cavity and the Ross Sea could have an effect on deep water formation and wider reaching impacts on global circulation.

How to cite: Mountford, A. S., Bull, C. Y. S., Jenkins, A., Jourdain, N. C., and Mathiot, P.: Exploring drivers of change in the Ross Sea with a regional ocean model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7961, https://doi.org/10.5194/egusphere-egu24-7961, 2024.

EGU24-8239 | ECS | Orals | OS1.6

Mid-Holocene ecosystem reorganisation in the Weddell Sea: dynamic sea ice and climate inferred from novel Antarctic snow petrel deposits (Heimefrontfjella Range) 

Mark Stevenson, Dominic Hodgson, Michael Bentley, Darren Gröcke, and Erin McClymont

Sea ice in Antarctica is closely coupled to the climate system, influencing water mass upwelling, albedo and the exchange of heat and gas between the ocean and atmosphere. Sea ice also supports a diverse ecosystem which is sensitive to changes in climate and biogeochemistry. The Heimefrontfjella mountain range in East Antarctica features the nesting sites of the snow petrel (Pagodroma nivea) where finely laminated stomach-oil deposits (regurgitated dietary contents) are deposited. Such deposits can provide valuable information on Holocene dietary changes of the snow petrel that may relate to palaeoclimatic variations. Snow petrel feeding grounds in the Weddell Sea range from neritic (coastal) zones rich in fish, to the productive open ocean where Antarctic krill (Euphausia superba) become increasingly important. Distinct dietary signatures are recorded in the biomarkers of these deposits, providing new evidence of changing sea-ice and climate in the Weddell Sea.

Here we focus on stomach-oil deposits from Heimefrontfjella. A highly resolved radiocarbon-dated (14C-AMS) sequence spanning ~6,500 to 2,000 cal. yr BP has been investigated for organic biomarkers (fatty acids, sterols), stable isotopes of carbon and nitrogen (δ13C, δ15N) and inorganic composition by X-ray fluorescence (XRF).  From ~6,500 to 6,000 cal. yr BP fatty acid markers were generally high in concentration, with particularly high levels of C14:0 mirrored by high δ15N suggesting food sources rich in Antarctic krill and periods of enhanced feeding in the open ocean. Subsequently between 6,000 and 4,500 cal. yr BP there was a marked reduction in C14:0, C18:0 and δ15N, although phytol concentration remained high. This trophic shift suggests a transitional Weddell Sea still rich in productivity with snow petrels feeding in both the open ocean and close to the shore on a mixture of fish, krill and squid. This is consistent with regional mid-Holocene warmth, and also suggests dynamic variable meteorological and oceanographic conditions during this period. Subsequently, between ~4,500 and 2,000 cal. yr BP organic marker concentrations were markedly lower, suggesting a relatively low productivity period, which we anticipate required more coastal feeding by snow petrels. This change is consistent with evidence from regional reconstructions suggesting movement into neoglacial conditions.

Together these findings highlight that the Weddell Sea experienced relatively short-term decadal and centennial-scale changes in sea ice and climate during the Holocene. Our results support existing regional proxies (e.g. offshore sediment records, lake records, ice-core records and palaeo-glacial thinning history) and highlight the importance of snow petrel deposits in recording palaeo-dietary and ecosystem changes in Antarctic marine systems.

How to cite: Stevenson, M., Hodgson, D., Bentley, M., Gröcke, D., and McClymont, E.: Mid-Holocene ecosystem reorganisation in the Weddell Sea: dynamic sea ice and climate inferred from novel Antarctic snow petrel deposits (Heimefrontfjella Range), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8239, https://doi.org/10.5194/egusphere-egu24-8239, 2024.

EGU24-8904 | ECS | Orals | OS1.6

Towards Parameterizing Eddy-Mediated Transport of Circumpolar Deep Water across Antarctic Continental Slopes 

Nicolas Dettling, Martin Losch, Friederike Pollmann, and Torsten Kanzow

The onshore transport of warm Circumpolar Deep Water (CDW) is associated with a heat flux onto the Antarctic continental shelves and strongly contributes to Antarctic ice shelf decline. On the continental shelf of the Weddell Sea, dense water forms through interactions with sea and shelf ice and subsequently propagates down the continental slope. The descent of dense water simultaneously produces an onshore transport of CDW. Here, mesoscale eddies drive a vertical momentum flux that is necessary to overcome the potential vorticity gradient imposed by the continental slope. The resolution of current climate models, however, is too coarse to resolve the Rossby Radius of deformation at high latitudes so that eddies need to be parameterized.

In an idealized model setup (MITgcm) representing the continental slope and shelf of the Weddell Sea, we show that eddy-driven shoreward CDW transport can be parameterized using the classical Gent-McWilliams and Redi (GM/Redi) parameterization for mesoscale eddies. In particular, the coarse resolution model with the GM/Redi parameterization simulates an onshore heat flux that is comparable to a high-resolution reference simulation. In contrast, no shoreward heat flux is observed without the eddy parameterization. When parameterizing eddies, the isopycnal slopes and the hydrographic mean fields also strongly improve compared to the runs without the parameterization. 

We further show that the parameterization works best when the GM transfer coefficient strongly decreases over the continental slope, representing the eddy-suppressive effect of steeply sloped topography. Motivated by this observation, we propose a simple modification to the GM/Redi scheme that reduces the coefficients in the presence of sloping topography. Only this „slope-aware“ version of the GM/Redi parameterization yields coefficients suitable for the continental shelf and slope and the open ocean and produces the best fit to the high-resolution model fields. We expect this addition to also be beneficial for modelling other parts of the ocean where eddy effects are moderated by topographic slopes. We therefore discuss the application of the modified parameterization to a regional model of the Cape Darnley region, East Antarctica, where dense water flows down realistic topography and drives an onshore flow of CDW at high resolution.

How to cite: Dettling, N., Losch, M., Pollmann, F., and Kanzow, T.: Towards Parameterizing Eddy-Mediated Transport of Circumpolar Deep Water across Antarctic Continental Slopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8904, https://doi.org/10.5194/egusphere-egu24-8904, 2024.

EGU24-10184 | ECS | Orals | OS1.6 | Highlight | OS Division Outstanding ECS Award Lecture

The global influence of ice-ocean interactions in Antarctica 

Alessandro Silvano

In this seminar, I will explore the oceanic processes that drive melting of the Antarctic Ice Sheet, and consequent global sea level rise. Different processes lead certain areas of the Antarctic Ice Sheet to be more susceptible to rapid ocean-driven melting, while other areas to be more resilient. I will also show the emergence of a feedback between the ice sheet and Southern Ocean: increased melting leads to warming of the oceanic waters surrounding Antarctica, with consequences for future sea level rise. I will conclude by describing how increased melting of the Antarctic Ice Sheet as well as changes in sea ice affect the global ocean abyss and its ability to store anthropogenic heat and carbon.

How to cite: Silvano, A.: The global influence of ice-ocean interactions in Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10184, https://doi.org/10.5194/egusphere-egu24-10184, 2024.

EGU24-11152 | Posters on site | OS1.6

The effect of grid resolution on sub-ice shelf circulation in the Energy Exascale Earth System Model 

Irena Vaňková, Xylar Asay-Davis, Darin Comeau, Stephen Price, and Jonathan Wolfe

Global ocean models are typically too coarse to explicitly resolve physical processes, such as mesoscale eddies, that transport heat into ice-shelf cavities and contribute to melting. As a result, mesoscale processes around Antarctica in such models need to be parameterized. Here we investigate the performance of these parameterizations in the Energy Exascale Earth System Model (E3SM), specifically focusing on the heat transport into an ice shelf cavity, the strength and direction of sub ice shelf circulation, and the rate of basal melting. Taking advantage of E3SM’s variable-resolution capabilities we set up a sequence of configurations with nominal grid sizes of 12, 8, 4, 2, and 1 km in the southern Weddell Sea, such that with increasing resolution, less eddies are parameterized and more resolved explicitly. The analysis is focused on the Filchner-Ronne Ice Shelf, because it is oceanographically interesting, it is important for sea level projections, and there are relatively abundant datasets from this region available for model validation.

How to cite: Vaňková, I., Asay-Davis, X., Comeau, D., Price, S., and Wolfe, J.: The effect of grid resolution on sub-ice shelf circulation in the Energy Exascale Earth System Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11152, https://doi.org/10.5194/egusphere-egu24-11152, 2024.

Laminated diatomaceous deposits have been documented in some regions of Antarctica, including the Antarctic Peninsula and the Ross Sea. In general, very high sedimentation rates can overwhelm limited bioturbation, thus favoring the varve preservation, for example, in certain glacio-marine environments. The laminated sediments collected in the Edisto Inlet, western Ross Sea, exhibited well-defined dark and light laminae on a mm- to cm-scale. Dark laminae contained relatively high concentrations of a biomarker for fast ice, IPSO25, while low IPSO25 concentrations characterized the light laminae, and the diatom Corethron pennatum became the dominant species. Based on these assumptions, the dynamics of fast ice was reconstructed over the last 2.6 ka for the western Ross Sea. However, the absence of direct observations leaves the paleoclimatic and paleoceanographic interpretation of these laminated sediments with a certain degree of uncertainty.

The project LASAGNE (Laminated Sediments in the Magnificent Edisto Inlet, Victoria Land: What processes control their deposition and preservation?), funded by the Italian Program of Antarctic Research (PNRA), proposes a multidisciplinary study that integrates the characteristics of fast ice, water column, and surface sediment, aiming to obtain information on the factors influencing both formation and preservation of laminated sediment in Edisto Inlet. The project integrates also biological data (phytoplankton, microzooplankton and foraminifera) collected in situ, and time series of satellite images of sea ice. The main goal is to provide new insights into the sub-seasonal formation of laminated sediments, offering a backbone for the interpretation of paleoclimatic sedimentary archives.

Here, we present the results obtained from a comprehensive dataset collected in Edisto Inlet during the XXXVIII Italian PNRA expedition conducted on board the I/B Laura Bassi in February 2023. Collected data include CTD (Conductivity-Temperature-Depth) profiles with additional parameters (Dissolved Oxygen, fluorescence, turbidity) spatially distributed within and at the entrance of the bay, which was still partially covered by seasonal sea ice at the time of the cruise. Additionally, vessel-mounted and lowered ADCPs (Acoustic Doppler Current Profilers) were collected along transects and at each CTD station, respectively.  The synoptic survey conducted during the austral Antarctic summer is used to describe the distribution of water masses and current dynamics in the bay, primarily driven by sea ice formation and melting, as well as atmospheric and tidal forcing. Time series obtained from a mooring deployed 1-year before the cruise (data covers the period February 2022 - February 2024) provide thermohaline variability of the water column even during the winter season, and fluxes and composition of organic debris sinking in the water column through time-series sediment trap samples. Sea ice cores, short sediment cores, and water samples are used to gain insight into the phytoplankton and microzooplankton living in platelet ice in spring and in open water in summer, respectively. Early diagenesis has also been taken into account to define how the original signal is preserved in the sedimentary record.

How to cite: Langone, L. and the LASAGNE team: Environmental factors influencing deposition and preservation of laminated sediments in Edisto Inlet, western Ross Sea (Antarctica), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11258, https://doi.org/10.5194/egusphere-egu24-11258, 2024.

EGU24-11749 | ECS | Orals | OS1.6

Heat and meltwater fluxes across the front of Dotson ice shelf cavity, Amundsen Sea 

Yixi Zheng, Rob Hall, Karen Heywood, Bastien Queste, Peter Sheehan, and Gillian Damerell

Ice shelves terminating in the Amundsen Sea are losing mass rapidly, exporting an increasing amount of meltwater into the ocean. Investigation into the melt rates of ice shelves near in the Amundsen Sea is therefore crucial for predicting the impact of ice shelf processes on future climate. However, observations near ice shelves often lack continuity in either time or space, limiting our knowledge of their melt rates. During a research cruise in austral summer 2022, we undertook a high resolution (horizontal sampling interval: ~ 2 km) CTD/LADCP transect spanning the front of the Dotson ice shelf encompassing the inflow and outflow to the ice shelf cavity.  In addition, we deployed three ocean gliders yielding five fine-resolution (horizontal sampling interval: ~ 650 m) hydrographic transects along the front of the Dotson Ice Shelf over three weeks. With an average of just 4.5 days between occupations, these new observations allow us to comprehensively investigate short-term variability along the ice front. The glider transects revealed considerable temporal variability in the across-ice shelf current speed.

The CTD section reveals that the meltwater content is higher (around 20 g/kg) in the west (outflow) and lower (around 10 g/kg) in the east (inflow), with a meltwater-poor layer centred at about 350 m sandwiched between two meltwater-rich layers along the ice shelf transect (one above about 250 m and the second centred at about 450 m). We reference geostrophic shear to the LADCP velocity profiles and demonstrate that the net volume flux across the ice shelf front is close to zero. We then calculate the net ocean heat flux across the ice shelf front to be  2.9×1011 W. Assuming that this net heat loss all results from basal melting, we estimate the glacial melt rate from this heat flux to be 28.1 Gt yr-1. The net transport of meltwater out of the cavity is 9.8×105 kg s-1, which is equivalent to 31 Gt yr-1, remarkably similar to the heat-flux-derived value. The small difference between the meltwater-flux-derived and heat-flux-derived melt rates might be attributed to subglacial rivers or other uncertainties in the estimates. Finally, we discuss the heat and meltwater fluxes using the glider transects and determine their temporal variability.

How to cite: Zheng, Y., Hall, R., Heywood, K., Queste, B., Sheehan, P., and Damerell, G.: Heat and meltwater fluxes across the front of Dotson ice shelf cavity, Amundsen Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11749, https://doi.org/10.5194/egusphere-egu24-11749, 2024.

EGU24-12570 | Posters on site | OS1.6 | Highlight

Spatial distribution of Antarctic meltwater governs Southern Ocean deep convection and shelf warming feedback 

Torge Martin, Janika Rhein, Malin Ödalen, and Mathias Zeller

Increasing Antarctic ice sheet mass loss is anticipated to become a major player in Southern Ocean and global climate change. Since most climate models are lacking an interactive ice sheet, numerous freshwater-release scenarios have been conducted recently, in which the effect of melting of ice shelves and calving of icebergs on the ocean and climate system is studied by prescribing a freshwater flux to the high latitude Southern Ocean. The Southern Ocean Freshwater Input from Antarctica (SOFIA, https://sofiamip.github.io) initiative is designed to reconcile these studies and quantify model uncertainty.

In the framework of SOFIA we conduct experiments with the Flexible Ocean and Climate Infrastructure (FOCI) model, which consists of NEMO3.6-LIM 0.5˚ ocean-sea ice and ECHAM6.3-JSBACH 1.8˚ atmosphere-land components. We study the effect of freshwater input (0.1 Sv) along the Antarctic coast (antwater) versus a wide-spread, iceberg melt-like input field south of 60˚S (60Swater) under pre-industrial climate control conditions. A small ensemble of eight members each also serves to demonstrate a significant effect by centennial-scale internal variability on the magnitude of the Southern Ocean’s response to the freshwater.

Besides responses like surface cooling, sea ice expansion, deep ocean warming, weakening of the Antarctic Circumpolar Current, which are robust across models and experiments, we find two intriguing differences between antwater and 60Swater experiments: In three of the eight ensemble members of antwater, large-scale open ocean deep convection emerges in the central Weddell Gyre, which is absent from the reference run without freshwater perturbation and the eight ensemble runs of 60Swater. This can be linked to the spin-up of the Weddell Gyre in the experiments, increasing the doming of isotherms, but being counterbalanced by surface freshening in the gyre center in 60Swater. Further, the zonal mean warming of >1˚C at mid depth in the Weddell Sea sector present in all experiments spills onto the continental shelf in antwater whereas it resides below the shelf break in 60Swater. This gives rise to the assumption that the spatial distribution of the freshwater has the potential to drive or limit a positive melt feedback loop associated with warming on the shelf.

How to cite: Martin, T., Rhein, J., Ödalen, M., and Zeller, M.: Spatial distribution of Antarctic meltwater governs Southern Ocean deep convection and shelf warming feedback, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12570, https://doi.org/10.5194/egusphere-egu24-12570, 2024.

EGU24-12592 | ECS | Orals | OS1.6 | Highlight

Sea ice and climate impacts from Antarctic ice-mass loss in the SOFIA multi-model ensemble 

Andrew Pauling, Neil Swart, Torge Martin, Rebecca Beadling, Jia-Jia Chen, Matthew England, Riccardo Farneti, Stephen Griffies, Tore Hattermann, F. Alexander Haumann, Qian Li, John Marshall, Morven Muilwijk, Ariaan Purich, Jeff Ridley, Inga Smith, and Max Thomas

We assess the response of Antarctic sea ice, the Southern Ocean, and global climate to mass loss from the Antarctic continent in a new multi-model ensemble. Antarctic ice-mass loss from ice sheets and ice shelves is increasing and is projected to increase further as the climate warms. The fresh water entering the Southern Ocean due to this ice-mass loss has been proposed as a mechanism responsible for the lack of decline in Antarctic sea ice area between 1979 and 2015, in contrast to the sea-ice loss seen in the Arctic. The fresh water impacts sea ice by increasing the density gradient between the near-surface waters and deeper waters around the Antarctic continent, which inhibits vertical transport of warmer, deeper water to the surface. This results in surface cooling and increased sea ice growth, as has been shown in previous studies. Though this increased Antarctic ice-mass loss is expected to impact climate it is absent from almost all models in the current Coupled Model Intercomparison Project (CMIP6), which typically enforce that the continent remain in perpetual mass balance, with no gain or loss of mass over time. Further, previous non-CMIP6 model experiments that include changing Antarctic ice-mass loss suggest that the climate response depends on the model used, and that the reasons for this model dependence are not clear.

We present results from the Southern Ocean Freshwater Input from Antarctica (SOFIA) Initiative, an international model intercomparison, in which freshwater is added to the ocean surrounding Antarctica to simulate the otherwise missing ice-sheet mass loss. This unique suite of models allows us compare the response to Antarctic mass loss across climate models, identify reasons for model discrepancies, and quantify the potential impact of the absence of increasing Antarctic ice-mass loss on Antarctic sea ice and climate. We will give an overview of the SOFIA initiative including the experiment design and participating models. We will present results from the “antwater” experiment outlined in the SOFIA protocol in which a constant freshwater input of 0.1 Sv is distributed evenly around the Antarctic continent at the ocean surface in an experiment with pre-industrial control forcing. We show that there is a spread of up to a factor of 3 across models in the Antarctic sea ice area response to identical freshwater forcing. There are also substantial differences in the spatial pattern of the sea ice response depending on the model used. We explore the dependence of the response on the mean state of Antarctic sea ice and the Southern Ocean in the pre-industrial control runs, as well as the response of the ocean stratification and oceanic deep convection in the models. We also explore the seasonality of the sea ice and oceanic response.

How to cite: Pauling, A., Swart, N., Martin, T., Beadling, R., Chen, J.-J., England, M., Farneti, R., Griffies, S., Hattermann, T., Haumann, F. A., Li, Q., Marshall, J., Muilwijk, M., Purich, A., Ridley, J., Smith, I., and Thomas, M.: Sea ice and climate impacts from Antarctic ice-mass loss in the SOFIA multi-model ensemble, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12592, https://doi.org/10.5194/egusphere-egu24-12592, 2024.

Antarctica's solar-warmed surface waters subduct beneath the region's ice shelves as a result of Ekman forcing. In December 2022, an ocean glider collected unprecedented observations of such waters beneath the Ross Ice Shelf, during a serendipitous four-day foray into the sub-glacial cavity; the glider surveyed the cavity in high resolution between the ice base and a depth of 200 m. During most of this period, a 50 m-thick layer of water with a high chlorophyll concentration, which must have come from the Ross Sea polynya and which has the same properties as waters immediately beneath adjacent sea ice, was in contact with the ice base. Super-cooled water was also sometimes observed to be in contact with the ice base. When warm water was present, temperature in the uppermost 5 m decreased towards the ice base in near-perfect agreement with an exponential fit. When super-cooled water was present, no such decrease was observed. From these observations, we estimate the heat loss from the ocean to overlying ice sheet. From re-analysis output, we demonstrate that Ekman forcing drives a heat into the sub-glacial cavity sufficient to contribute significantly to near-front melting of the Ross Ice Sheet. We further show that there has been an increase in the Ekman heat flux into the cavity over the last four decades (i.e. since 1979); this is driven by an increase in the heat content of the seasonally ice-free waters of the Ross Sea polynya, immediately in front of the ice shelf. Interannual variability of the Ekman heat flux, however, is driven not by ocean heat content, or indeed by sea ice cover, but by interannual variability of the along-front zonal wind stress.

How to cite: Sheehan, P. and Heywood, K.: Observations and year-on-year increase of warm surface waters entering the Ross Ice Shelf cavity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12628, https://doi.org/10.5194/egusphere-egu24-12628, 2024.

EGU24-13601 | ECS | Posters virtual | OS1.6

Seasonal circulation and volume transport in the Gerlache Strait 

Laia Puyal-Astals, Borja Aguiar-González, Marta Veny, and Francisco Machín

We present the first observational-based assessment of the year-round circulation and volume transport in the Gerlache Strait, a key location for the water mass exchanges occurring along the west Antarctic Peninsula (wAP) between the relatively warm Bellingshausen Sea, flowing northeastward, and the colder Weddell Sea, flowing southwestward. These relatively warm/cold ocean water pathways have been documented to play a major role in the glacier retreat/stabilization of glaciers along the wAP (Cook et al., 2016). Bearing this in mind, we investigate a dataset of direct velocity measurements which were routinely collected along ship tracks from 379 cruises performed by R/V Nathaniel B. Palmer and R/V Laurence M. Gould between 1999 and 2018. A first set of analyses of an earlier version of this dataset was presented in Savidge & Amft (2009), who focused on the summer and winter views of the shelf circulation along the entire wAP. More recently, an updated version of such a dataset addressed the year-round circulation and volume transport of the Bransfield Current in the Bransfield Strait between 1999 and 2014 (Veny et al., 2022).

Preliminary results of this work focus on the ocean current variability displayed between 2008 and 2009, two years known in the literature as featuring remarkably opposite Weddell Sea influences along the central wAP (Wang et al., 2022); the former year with a weaker influence than the later one. Ongoing steps include the construction of a high-resolution (~5km) seasonal climatology of the ocean currents flowing through the Gerlache Strait, where the dataset of study ensures a multi-year spatial coverage of volume transport.

Key words: Gerlache Strait, Direct Velocity Measurements, Dynamic Structure, Volume Transport, Seasonal and Interannual Variability.

References: 

Cook, A. J., Holland, P. R., Meredith, M. P., Murray, T., Luckman, A., & Vaughan, D. G. (2016). Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 353(6296), 283–286. https://doi.org/10.1126/science.aae0017

Savidge, D. K., & Amft, J. A. (2009). Circulation on the West Antarctic Peninsula derived from 6 years of shipboard ADCP transects. Deep Sea Research Part I: Oceanographic Research Papers, 56(10), 1633–1655. https://doi.org/10.1016/j.dsr.2009.05.011

Veny, M., Aguiar-González, B., Marrero-Díaz, Á., & Rodríguez-Santana, Á. (2022). Seasonal circulation and volume transport of the Bransfield Current. Progress in Oceanography, 204, 102795. https://doi.org/10.1016/j.pocean.2022.102795

Wang, X., Moffat, C., Dinniman, M. S., Klinck, J. M., Sutherland, D. A., & Aguiar‐González, B. (2022). Variability and Dynamics of Along‐Shore Exchange on the West Antarctic Peninsula (WAP) Continental Shelf. Journal of Geophysical Research: Oceans, 127(2). https://doi.org/10.1029/2021JC017645

How to cite: Puyal-Astals, L., Aguiar-González, B., Veny, M., and Machín, F.: Seasonal circulation and volume transport in the Gerlache Strait, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13601, https://doi.org/10.5194/egusphere-egu24-13601, 2024.

EGU24-15178 | Orals | OS1.6

Water mass formation and export from the Filchner-Ronne Ice Shelf 

Markus Janout, Mathias van Caspel, Elin Darelius, Peter Davis, Tore Hattermann, Mario Hoppmann, Torsten Kanzow, Svein Østerhus, Jean-Baptiste Sallée, and Nadine Steiger

The Filchner-Ronne-Ice Shelf (FRIS) is the earth’s largest ice shelf by volume and its cavity a crucial part of the southern Weddell Sea ocean circulation. In mid-2017, the Filchner Ice Shelf (FIS) cavity experienced a shift towards a stronger circulation and increased outflow of Ice Shelf Water (ISW) into Filchner Trough. The increase was attributed to enhanced sea ice formation and the associated production of High Salinity Shelf Water (HSSW) in the source region north of Ronne Ice Shelf. The corresponding circulation pattern was termed “Ronne-mode”, which contrasts the “Berkner-mode”, characterized by a more locally-enhanced circulation at the northern FIS edge. Here we employ new time series from two drill hole mooring sites underneath FIS, as well as moorings from the Filchner Trough and Filchner Sill, to highlight the spatial and temporal extent of this recent ISW outflow event. Underneath FIS, the “Ronne-mode” overruled the normally-observed seasonality in currents and hydrography, and resulted in northward ISW transport for about two years. The export led to the subsequent filling of Filchner Trough with ISW from 2018 until mid-2020, which then overflowed across the Sill between late 2018 for nearly one year. Our observations provide new insights into the variability of the southern Weddell Sea shelf and FRIS cavity circulation, which is important for the abyssal water mass export and thus for global ocean circulation.

How to cite: Janout, M., van Caspel, M., Darelius, E., Davis, P., Hattermann, T., Hoppmann, M., Kanzow, T., Østerhus, S., Sallée, J.-B., and Steiger, N.: Water mass formation and export from the Filchner-Ronne Ice Shelf, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15178, https://doi.org/10.5194/egusphere-egu24-15178, 2024.

EGU24-16331 | ECS | Orals | OS1.6

Deciphering the impact of future individual Antarctic freshwater sources on the Southern Ocean properties and ice shelf basal melting 

Christoph Kittel, Nicolas Jourdain, Pierre Mathiot, Violaine Coulon, Clara Burgard, Justine Caillet, Damien Maure, and Clara Lambin

The Antarctic ice sheet is losing mass. This mass loss is primarily due to ice shelf basal melting and the subsequent acceleration of glaciers. The substantial freshwater fluxes resulting from ice shelf and iceberg melting affect the Southern Ocean and beyond. As emphasized by some studies, they slow down the decline of Antarctic sea ice and hinder mixing between surface water and Circumpolar Deep Waters, further intensifying ice shelf basal melting. In this context, most studies so far have neglected the impact of surface meltwater runoff , but recent CMIP6 projections using the SSP5-8.5 scenario challenge this view, suggesting runoff values in 2100 similar to current basal melt rates. This prompts a reassessment of surface meltwater future impact on the ocean.  We use the ocean and sea-ice model NEMO-SI3 resolving the sub-shelf cavities of Antarctica and including an interactive iceberg module. We perform thorough sensitivity experiments to disentangle the effect of changes in the atmospheric forcing, increased ice shelf basal melting, surface freshwater runoff and iceberg calving flux by 2100 in a high-end scenario. Contrary to expectations, the atmosphere alone does not substantially warm ice shelf cavities compared to present temperatures. However, the introduction of additional freshwater sources amplifies warming, leading to escalated melt rates and establishing a positive feedback. The magnitude of this effect correlates with the quantity of released freshwater, with the most substantial impact originating from ice shelf basal melting. Moreover, larger surface freshwater runoff and iceberg calving flux contribute to further cavity warming, resulting in a noteworthy 10% increase in ice shelf basal melt rates. We also describe a potential tipping point for cold ice shelves, such as Filchner-Ronne, before the year 2100.

How to cite: Kittel, C., Jourdain, N., Mathiot, P., Coulon, V., Burgard, C., Caillet, J., Maure, D., and Lambin, C.: Deciphering the impact of future individual Antarctic freshwater sources on the Southern Ocean properties and ice shelf basal melting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16331, https://doi.org/10.5194/egusphere-egu24-16331, 2024.

EGU24-16511 | Orals | OS1.6

Cryospheric Change as a Driver of Antarctic Seep Emergence  

Sarah Seabrook, Andrew Thurber, Yoann Ladroit, Vonda Cummings, Leigh Tait, Alicia Maurice, and Cliff Law

While the climate sensitivity and significance of subsurface fluid and greenhouse gas reservoirs have received attention in the Arctic, the presence of these features in Antarctica and their contribution to global methane and the carbon cycle remains unknown. Here, we report the discovery of extensive and emergent seafloor seeps, some initiated within the last decade, that are releasing climate-reactive fluids and gases in the coastal Ross Sea. Emission of methane in these shallow waters would expedite transfer to the atmosphere, as reported at other shallow global seep systems. While the origin, driving mechanisms, and environmental consequence of these emerging Antarctic seep systems remains unknown, we postulate that the emergent seepage results from cryospheric cap degradation, which initiates new fluid flow pathways and liberates subsurface fluids and gases. This mechanism is inherently climate sensitive with potential for positive feedback, and may be widespread around the Antarctic Continent, yet the magnitude and scale is currently undetermined. 

How to cite: Seabrook, S., Thurber, A., Ladroit, Y., Cummings, V., Tait, L., Maurice, A., and Law, C.: Cryospheric Change as a Driver of Antarctic Seep Emergence , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16511, https://doi.org/10.5194/egusphere-egu24-16511, 2024.

The Southern Ocean (SO) plays a key role in global carbon and nutrient cycles, as the SO overturning circulation feeds into both deep-water formation (lower branch) and Subantarctic intermediate and mode water formation (upper branch). While the air-sea CO2 balance is influenced mainly by deep-water formation, global export production is more sensitive to intermediate and mode water formation, giving rise to the concept of a SO biogeochemical divide [1]. Sea ice formation, transport and melting plays a prominent role in the transformation of buoyancy for both the upper and lower branches of the overturning circulation [2]. Hence, changes in sea ice parameterisation have potential for substantially altering carbon uptake and export production in global Earth System Models (ESMs).

Global ESMs seek to simulate physical, chemical and biological processes that are relevant for the evolution of global climate, including fluxes of greenhouse gasses and aerosols between the atmosphere and ocean. The air-sea gas exchange is determined by the difference in concentration across the air-sea interface, and a gas transfer velocity that is specific for the gas in question. However, the air-sea gas exchange is inhibited by the presence of sea ice. A modified formula proposed by Steiner et al. [3], accounting for cracks and leads in the sea ice, has recently been  implemented in the Norwegian Earth System Model NorESM2 [4]. In this study we investigate how the change in this sea ice parameterisation influences the carbon uptake and export production associated with the Southern Ocean overturning circulation.

REFERENCES

[1] I. Marinov, A. Gnanadesikan, J. R. Toggweiler and J. L. Sarmiento, "The Southern Ocean biogeochemical divide", Nature, Vol. 441, 964-967, 2006. DOI: 10.1038/nature04883

[2] R. P. Abernathey, I. Cerovecki, P. R. Holland, E. Newsom, M. Mazlo and L. D. Talley, "Water-mass transformation by sea ice in the upper branches of
the Southern Ocean overturning", Nature Geoscience, Vol. 9, 596-601, 2016. DOI: 10.1038/ngeo2749

[3] N. S. Steiner, W. G. Lee and J. R. Christian, "Enhanced gas uxes in small sea ice leads and cracks: Efects on CO2 exchange and ocean acidiccation", JGR Oceans, Vol. 118(3), 1195-1205, 2013. DOI: 10.1002/jgrc.20100

[4] Ø. Seland, M. Bentsen, D. Olivié, T. Toniazzo, A. Gjermundsen, L. S. Graff, J. B. Debernard, A. K. Gupta, Y.-C. He, A. Kirkevåg, J. Schwinger, J. Tjiputra, K. S. Aas, I. Bethke, Y. Fan, J. Griesfeller, A. Grini, C. Guo, M. Ilicak, I. H. H. Karset, O. Landgren, J. Liakka, K. O. Moseid, A. Nummelin, C. Spensberger, H. Tang, Z. Zhang, C. Heinze, T. Iversen and M. Schulz, "Overview of the Norwegian Earth System Model (NorESM2) and key climate response of CMIP6 DECK, historical, and scenario simulations", Geoscientifc Model Development, Vol. 13(12), 6165-6200, 2020. DOI: 10.5194/gmd-13-6165-2020

How to cite: Torsvik, T.: Influence of changing sea ice parameterisation on Southern Ocean  carbon uptake and export production, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16957, https://doi.org/10.5194/egusphere-egu24-16957, 2024.

EGU24-17376 | ECS | Posters on site | OS1.6

Present-day ocean simulations of the circumpolar Antarctic 

Birgit Rogalla, Kaitlin Naughten, Paul Holland, Pierre Mathiot, Nicolas Jourdain, and Christoph Kittel

The West Antarctic Ice Sheet (WAIS) is rapidly losing mass due to ocean-driven melt of its ice shelves, contributing to sea level rise. This melt is associated with the intrusion of circumpolar deep water onto the continental shelf which is impacted by winds, the Amundsen undercurrent, thermodynamic processes, and buoyancy forcing. To study the sensitivity of melt to changes in these components, model configurations need to represent key processes while reducing computational cost to allow for large ensemble simulations. Regional ocean simulations have proven useful in this context, however, configurations that allow interactions between Antarctic regions would be beneficial. We will present results from present-day ocean simulations with a  ¼° circumpolar Antarctic NEMO configuration including sea ice, icebergs, and ice shelf cavities, and up-to-date forcing and bathymetry datasets. We will also discuss challenges associated with open boundary conditions and sensitivity to different forcing datasets. This configuration will provide a platform for attribution studies of ocean-driven melt of the WAIS, ocean projections, and form the starting point for coupled ocean-ice sheet simulations. 

How to cite: Rogalla, B., Naughten, K., Holland, P., Mathiot, P., Jourdain, N., and Kittel, C.: Present-day ocean simulations of the circumpolar Antarctic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17376, https://doi.org/10.5194/egusphere-egu24-17376, 2024.

EGU24-17676 | ECS | Orals | OS1.6

Benefits of the Brinkman Volume Penalisation Method for the Ice-Shelf Melt Rates Produced by Z-coordinate Ocean Models 

Antoine-Alexis Nasser, Nicolas C. Jourdain, Pierre Mathiot, and Gurvan Madec

Antarctic ice-shelf basal melting is a major source of uncertainty in sea level rise projections. A persistent challenge in simulating the ice-shelf-ocean interactions in z-coordinate ocean models is the introduction of artificial steps, leading to the generation of noise that impacts both melting and ocean currents. This study explores the potential of the Brinkman Volume Penalisation (BVP) method (Debreu et al. 2020, 2022) to address the recurrent issue of steps in ice-shelf-ocean models. While penalisation methods are typically applied to land topography, here, the method is generalised to ice-shelf interactions with oceans. This approach introduces porous cells that are half-ice, half-ocean, combined with a permeability parameter (friction within porous cells) to model the blocking effect of the ice draft. A unique aspect of this method is its ability to spread the penalisation region, thereby reducing model sensitivity to numerical level changes. We assess the potential benefits of the BVP approach within the idealised ice-shelf configuration ISOMIP+ as presented by Asay-Davis et al. (2016). First, a new calculation of the horizontal pressure gradient is formulated using the BVP approach, which eliminates residual biases in ocean currents down to zero machine precision. Second, the spreading of the penalised interface significantly reduces noise in the melt rates, enabling a smooth response of the ocean beneath the ice-shelf without the need for further mesh refinement. Other simulations are used to investigate the sensitivity of basal melting and freezing in the penalised configuration to changes in numerical parameters (e.g. spatial resolution). These results pave the way for a better numerical treatment of ice-shelves in earth system models.

How to cite: Nasser, A.-A., Jourdain, N. C., Mathiot, P., and Madec, G.: Benefits of the Brinkman Volume Penalisation Method for the Ice-Shelf Melt Rates Produced by Z-coordinate Ocean Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17676, https://doi.org/10.5194/egusphere-egu24-17676, 2024.

EGU24-20002 | ECS | Orals | OS1.6

Weddell Sea subsurface warming revealed by an updated Southern Ocean climatology 

Shenjie Zhou, Pierre Dutrieux, and Andrew Meijers

A new monthly climatology of Southern Ocean hydrography is constructed with updated observational dataset. CTD casts from World Ocean Database, Pangaea Database, CLIVAR and Carbon Hydrographic Data Office, Southern Ocean Database and Korean Polar Data Centre were assembled. All ‘Delayed Mode’ Argo floats profiles and ‘Real-time Mode’ or ‘Real-time Adjusted Mode’ over the within 2000 m isobath near the continental shelves are included. All flagged-good Seal-tag profiles are included. The interpolation scheme employs an elliptical detecting area to select profiles to be averaged into gridded product. The ellipse is designed to align with the dynamic height contour to consider the effect of large-scale circulation. The detecting radius confined by ellipse size varies with the bathymetry and facilitate to resolve local gradient in temperature and salinity field over the continental shelves. A timeseries is constructed by removing the temperature and salinity climatology from the individual profiles in Weddell Sea, and a clear subsurface warming is revealed. An entrainment of warm and saline anomalies from subsurface into the surface layer is captured around 2016 corresponding to the recent sea ice extent decline. A further regional analysis on the temperature and salinity anomaly signal will shed light on the heat delivery pathway and the cause of the subsurface heat entrainment.

How to cite: Zhou, S., Dutrieux, P., and Meijers, A.: Weddell Sea subsurface warming revealed by an updated Southern Ocean climatology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20002, https://doi.org/10.5194/egusphere-egu24-20002, 2024.

EGU24-152 | ECS | Orals | OS1.7

Improving Estimates of Arctic Ocean CO2 Uptake 

Victoria Dutch, Dorothee Bakker, Peter Landschützer, Alizée Roobaert, and Jan Kaiser

The Arctic Ocean covers only 3 % of the Earth’s surface but contributes 5 - 14 % of the global ocean carbon sink. Sparse and unevenly distributed observations of the partial pressure of CO2 (pCO2) hinder our understanding of the magnitude and the controlling mechanisms of this carbon sink. In order to constrain the magnitude of this flux, we adapt the Self-Organising Map – Feed-Forward neural Network (SOM-FFN) method of Landschützer et al. (2016) to interpolate existing observations and construct a monthly 1 x 1 degree pCO2 product for the Arctic Ocean from 1991 - 2022. We first divide the Arctic Ocean (i.e., the region ≥ 55° N) into five biogeochemical provinces; four obtained from using the SOM method and a fifth for all grid cells with greater than 85 % ice cover. For each province, we then derive non-linear relationships between pCO₂ and predictor variables (i.e., biogeochemical drivers) using the FFN method. The monthly reconstructed Arctic pCO2 product is then evaluated against existing observations of surface ocean pCO2, chiefly from SOCATv2023 and from independent timeseries stations. Our study shows that biogeochemical properties previously selected as predictor variables at the global scale are not well suited to the Arctic Ocean. Limiting the spatial domain from which relationships are derived also improves performance, with less biased p(CO2) values predicted when excluding the Baltic Sea. 

How to cite: Dutch, V., Bakker, D., Landschützer, P., Roobaert, A., and Kaiser, J.: Improving Estimates of Arctic Ocean CO2 Uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-152, https://doi.org/10.5194/egusphere-egu24-152, 2024.

EGU24-232 | Orals | OS1.7

Coral Reefs: Sinks of Atmospheric CO2 ? 

Hamish McGowan, Nadav Lensky, Shai Abair, and Mellissa Saunders

Quantification of air-sea CO2 exchange over coral reefs has relied primarily on measurements of the CO2 partial pressure (pCO2) gradient between the water overlying a reef and the lower atmosphere. A gas transfer velocity based on wind speed is then used to estimate the air-sea CO2 mass exchange. While this approach may be suitable over the oceans or where instrumented buoys have been deployed for long-term monitoring, the method overlooks many factors that influence turbulent transport and air-sea CO2 exchange. These include surfactants, bubble exchange, atmospheric turbulence, and wave breaking, which may be particularly important over near shore fringing coral reefs.

 

Using eddy covariance (EC) systems deployed at the shoreline adjacent to coral reefs and on pontoons we show through direct measurements these ecosystems may be net sinks of atmospheric CO2. Results show sequestration of atmospheric CO2 by healthy coral reefs and adjacent lagoons at time scales of several days to several months exceed published CO2 sequestration rates of mature pine plantations measured by EC by an order of magnitude. These findings highlight the importance of coral reefs in carbon budgets in addition to their widely known ecosystem services and societal benefits. Conserving coral reef ecosystems and ensuring they remain healthy and resilient to the threats of climate change, pollution, overfishing, tourism, and mining should be a priority. Future research will aim to track the CO2 influx through coral reef ecosystems.        

How to cite: McGowan, H., Lensky, N., Abair, S., and Saunders, M.: Coral Reefs: Sinks of Atmospheric CO2 ?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-232, https://doi.org/10.5194/egusphere-egu24-232, 2024.

EGU24-290 | ECS | Orals | OS1.7

How the treatment of sea surface temperature affects the water cycle in EURO-CORDEX simulations 

Francis Da Silva Lopes and Michael Schindelegger

Regional climate models (RCMs) over Europe often exhibit wet precipitation biases, primarily attributed to excess oceanic evaporation across time scales. One likely source for such wet biases are therefore imperfections in the models’ lower boundary condition (LBC) over the ocean, as realized by time-evolving sea surface temperature (SST) fields. SST data from atmospheric reanalyses (e.g., ERA5) are commonly adopted in RCMs, but ambiguity exists about the exact SST variable in these products (e.g., foundation or skin temperature) and the manner with which they represent the diurnal cycle and spatial gradients. Here we explore these questions with a ~12-km setup of ICON-CLM (Icosahedral Nonhydrostatic Model in Limited-Area Mode) over the EURO-CORDEX domain, run repeatedly for 6 years with various SST datasets. We use ERA5-based daily SST and skin temperature and hourly upper-layer SST drawn from our own global ocean simulations with FESOM2 (Finite Element Sea-Ice Ocean Model) at ~10-km node spacing in the eastern North Atlantic. Specifically, prescribing the FESOM2 SST fields in ICON-CLM both with and without spatial smoothing allows us to examine the effects of oceanic eddies and fronts on precipitation characteristics onshore. Preliminary results from 7 months of integration with ICON-CLM suggest that the choice of the SST data appreciably impacts latent heat fluxes, moisture transport onto land, and cumulative continental precipitation, generally in areas of pronounced moisture recycling. “Mind your SST” is therefore the advice we can give to ongoing dynamical downscaling efforts aimed at modeling future precipitation changes over land.

How to cite: Da Silva Lopes, F. and Schindelegger, M.: How the treatment of sea surface temperature affects the water cycle in EURO-CORDEX simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-290, https://doi.org/10.5194/egusphere-egu24-290, 2024.

EGU24-730 | ECS | Posters on site | OS1.7

Estimates of Arctic Ocean carbon uptake from atmospheric inverse analyses for the period 2000-2017 

Jayashree Ghosh, Parvadha Suntharalingam, Zhaohui Chen, Jan Kaiser, Dorothee Bakker, and Victoria Dutch

 The Arctic Ocean is responsible for around 5-10% of oceanic CO2 uptake, despite the region only accounting for approximately 4% of the world's oceans (Bates & Mathis, 2009). In this study, we investigate the exchange of CO2 between the atmosphere and the ocean in the Arctic Ocean for the period 2000-2017. Our estimates are obtained using the GEOSChem-LETKF inverse model system (Chen et al. 2021), in combination with data from the NOAA surface CO2 monitoring network (ObsPack, Cooperative Global Atmospheric Data Integration Project, 2018). We evaluate the impact of alternative representations of the prior flux distribution for air-sea CO2 fluxes. These include the following datasets: Landschutzer et al. (2016), Rodenbeck et al. (2014), and Watson et al. (2020). We present estimates of the long-term trend, year-to-year fluctuations, and regional and seasonal variability in air-sea CO2 exchange in the Arctic Ocean, with a focus on the region north of 58˚N. The sea ice extent of the regional seas of the Arctic Ocean has an influence on the magnitude and seasonality of the regional air-sea CO2 flux. We also investigate the potential links between changes in sea-ice extent and changes in air-sea CO2 fluxes.

How to cite: Ghosh, J., Suntharalingam, P., Chen, Z., Kaiser, J., Bakker, D., and Dutch, V.: Estimates of Arctic Ocean carbon uptake from atmospheric inverse analyses for the period 2000-2017, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-730, https://doi.org/10.5194/egusphere-egu24-730, 2024.

EGU24-732 | ECS | Posters on site | OS1.7

Cross-linking laboratory and field measurements to quantify the role of bubbles in air-sea CO2 exchange 

Yuanxu Dong, Bernd Jähne, and Christa Marandino

The global oceans are a major sink of anthropogenic carbon dioxide (CO2), playing a critical role in mitigating climate change. The ocean CO2 uptake estimate contains significant uncertainties due to a lack of mechanistic understanding of the role of bubbles in air-sea CO2 exchange. Bubbles resulting from wave breaking may mediate about 40% of the global air-sea CO2 flux.  However, bubble-mediated transfer is poorly quantified and under-represented in CO2 flux estimates. In this study, we will present a synthesis analysis of the bubble-mediated gas transfer measurements in the last decade. We show contrasting evidence regarding the importance of bubbles in the air-sea CO2 exchange, particularly in the comparison between laboratory and field measurements. This suggests a lack of mechanistic understanding of the air-sea gas exchange processes. Through innovative cross-linking of comprehensive field and laboratory observations using multiple techniques, we aim to make a step change in understanding the mechanisms of bubble-mediated transfer and reconcile field and laboratory measurements.  We also aim to provide novel parameterisations of gas transfer velocity with explicit representation of bubbles, thereby reducing uncertainty in air-sea CO2 flux estimates.

How to cite: Dong, Y., Jähne, B., and Marandino, C.: Cross-linking laboratory and field measurements to quantify the role of bubbles in air-sea CO2 exchange, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-732, https://doi.org/10.5194/egusphere-egu24-732, 2024.

EGU24-1139 | ECS | Orals | OS1.7

Sampling the SML for traces gases: a case study of sampling technique and resulting correction factors 

Lea Lange, Dennis Booge, Josefine Karnatz, Hermann Bange, and Christa Marandino

The sea surface microlayer (SML) is the uppermost thin oceanic surface layer in the range of 100µm with properties that are distinct from the water below. With an ocean coverage of up to 70% it is supposed to have a significant impact on air-sea gas exchange rates. In global studies, the SML is often supposed to be a missing source of trace gases, when oceanic production and the subsequent emissions alone cannot explain observed atmospheric mixing ratios. Despite the attention in the past 20 years, also in the SOLAS science plan, it remains difficult to sample volatile trace gases from the SML with existing sampling techniques. Consequently, an incomplete process understanding of trace gas cycling within the SML inhibits its effect on air-sea gas exchange.

In this study, we focus on existing and common SML sampling methods (glass plate, Garrett screen) in order to ensure that trace gas samples are comparable to other parameters sampled with the same method. A series of laboratory experiments was set up to determine a correction factor which quantifies the loss of trace gases due to the sampling method itself. Dimethyl sulfide, isoprene and carbon disulfide were sampled with a glass plate and with a Garrett screen under varying surfactant concentrations and environmental conditions (salinity, temperature). Based on physiochemical properties of the examined trace gases, we extended the correction factor to nitrous oxide and methane. Losses are high, but not as variable as expected. Around 90% are lost due to sampling with small variations between different gases. The presence of surfactants has a small effect on the losses.

The lab-based correction factors are applied to in-field SML samples from a mesocosm study in May/June 2023 conducted within the DFG research unit BASS. Those results clearly indicate that the composition of the SML highly influences the correction factor for each trace gas individually. Comparing corrected SML concentrations with underlying bulk water concentrations reveal the accumulation of specific traces gases in the SML which highly influence the magnitude of trace gas emissions to the atmosphere.

How to cite: Lange, L., Booge, D., Karnatz, J., Bange, H., and Marandino, C.: Sampling the SML for traces gases: a case study of sampling technique and resulting correction factors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1139, https://doi.org/10.5194/egusphere-egu24-1139, 2024.

EGU24-1372 | ECS | Orals | OS1.7

Carbonate System Changes Within an Evaporating Sea Spray Droplet 

Lucy Hendrickson, Penny Vlahos, and Leonel Romero

Modeling the air-sea flux of CO2 is a key factor in understanding climate change and predicting its effects. The contribution of sea spray to this flux is highly uncertain yet important for reducing error margins in global estimates. In this work, a modified CO2SYS routine is used to quantify the effect of evaporation on aqueous carbonate reactions in sea spray in order to assess this flux. Factors that affect these reactions are the increasing salinity and temperature changes of the droplet as it evaporates. The size of the droplet is also a determining factor as it affects the time aloft and thus the amount of evaporation and gas exchange that can occur. Using these factors and a number of simplifying assumptions, we model the change in DIC, TA, pCO2 and pH in an evaporating sea spray droplet.

How to cite: Hendrickson, L., Vlahos, P., and Romero, L.: Carbonate System Changes Within an Evaporating Sea Spray Droplet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1372, https://doi.org/10.5194/egusphere-egu24-1372, 2024.

EGU24-2187 | ECS | Orals | OS1.7 | Highlight

Impact of the ocean-atmosphere coupling on Mediterranean cyclones 

Marco Chericoni, Giorgia Fosser, and Alessandro Anav

The Mediterranean basin is well recognized as one of the main climate change hotspots; besides, this region is one of the most active cyclogenetic area of the Northern Hemisphere with a large number of intense cyclones occurring every year. Intense Mediterranean cyclones are often responsible for extreme precipitation and strong wind events leading to severe socio-economic and environmental impacts especially over densely populated coastal areas. Complex feedback between the Mediterranean Sea and the atmosphere on various temporal and spatial scales plays a major role in the variability in and extremes of the regional climate system.

This study aims to investigate the impact of the ocean-atmosphere coupling on the regional climate during intense Mediterranean cyclones. To this end, two simulations are performed using the ENEA-REG regional earth system model at 12 km atmospheric horizontal resolution over the Med-CORDEX domain, both driven by ERA5 reanalysis. The first experiment uses the mesoscale WRF model with prescribed ERA5 Sea Surface Temperature (SST), while the second is coupled to the MITgcm ocean model at horizontal resolution of 1/12°. Cyclones are tracked by applying a Lagrangian algorithm to the mean sea level pressure field. The 500 most intense cyclones mainly occur in winter over the Thyrrenian, Adriatic, Ionian and Aegean Sea. They are similarly reproduced between WRFs and ERA5 in terms of seasonal and spatial distribution, due to the same large-scale atmospheric conditions. The coupled simulation is compared with the standalone WRF in terms of sub-daily fields, such as evaporation, precipitation and wind speed, during the mature stage of the cyclones. The different SST distribution between the models appears to be the main controlling factor for the differences in the atmospheric properties affecting not only the surface, but also the entire atmospheric boundary layer (ABL) and its height, due to the mixing of the turbulent processes, enhanced during intense cyclones. A statistically significant higher specific humidity and wind speed are found in the coupled model from the surface to the top of the ABL, as well as higher precipitation over sea and coastal areas. These results are consequences of higher turbulent heat and moisture fluxes in the coupled model that destabilize the ABL and provide higher moisture content available for convection.

We conclude that the use of the coupled model is crucial for a more realistic representation of the energy redistribution in both the ocean mixed layer and the ABL during intense Mediterranean cyclones. This highlights the importance of the coupled model to study the influence of climate change on intense Mediterranean cyclones and associated impacts under different future scenarios.

How to cite: Chericoni, M., Fosser, G., and Anav, A.: Impact of the ocean-atmosphere coupling on Mediterranean cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2187, https://doi.org/10.5194/egusphere-egu24-2187, 2024.

The climatological mean and trend of salinity change evidently due to the acceleration of hydrological cycle under global warming. However, no systematic research has been focused on the decadal and long-term changes of salinity and their contributions to ocean stratification during 1940-2019. In this study, the nonlinear trend of salinity is firstly exacted using the ensemble empirical mode decomposition method, and the corresponding long-term trends and their impacts on stratification in the tropical Pacific are analyzed emphatically. The results confirm that the sea surface water becomes fresher in the tropical western Pacific and southern Pacific convergence zone, while saltier in the southeastern Pacific under global warming. Moreover, the salinity changes are regional- and time-dependent, which the salinity trends in different regions of the tropical Pacific show differences at different periods responding to SST trend. As results, under the combined effects of temperature and salinity, the sea surface density reduces significantly in the tropical Pacific, with the largest reduction centered in the warm pool, while the subsurface density in the tropical western Pacific increases. These opposite changes enhance the contrast for the density between the surface and the subsurface water, leading to more stable ocean stratification. Then, the mixed layer becomes shallower near the equatorial dateline and deeper in the warm pool, mainly due to salinity variations. Salinity and temperature contribute differently to the variations of barrier layer thickness in different regions, where the changes of salinity (temperature) correspond to the thickening of barrier layer located at 160°E east (west). It is suggested that the salinity variations in the Pacific affect the ocean thermodynamic processes under global warming, which then modulate the climate variability.

How to cite: hai, Z.: Salinity Change and Its Implications for Ocean Stratification in the Tropical Pacific under Global Warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2734, https://doi.org/10.5194/egusphere-egu24-2734, 2024.

The cool skin effect, known as the temperature difference (ΔT) across the skin layer of sea surface, is of vital importance for the accurate computation of the latent heat flux (LHF). The observed features of ΔT in the South China Sea are analyzed using in situ data from a buoy platform over an approximately six-week period. Only nighttime data are used to exclude the possible warm layer effect. The positive values of ΔT falling into the range of 0 to 1 K comprise 95% of the data, and the most frequently observed values occur in the range of 0.4 to 0.6 K (38%). The cool skin model in the COARE 3.0 algorithm is then validated against those observations. The cool skin model has an efficient but insufficient ability to reduce the overestimation of the LHF. The overestimation of the LHF is reduced to 9.5% from 18.0%, leaving nearly half of the biases in the LHF unresolved. The Saunders constant (λ) in the cool skin model is markedly underestimated, leading to a much weaker prediction of ΔT. A strong linear relationship exists between the mean values of λ and the LHF with a slope of -0.9 W m-2. With an approximately doubled λ, the biases in ΔT and in the LHF could be eliminated. Considering the possible uncertainties in sensors, the value of λ is estimated as 11.6±6.7 in the current study.

How to cite: Zhang, R.: Cool Skin Effect and its Impact on the Computation of the Latent Heat Flux in the South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3766, https://doi.org/10.5194/egusphere-egu24-3766, 2024.

EGU24-3811 | Posters on site | OS1.7

Impact of ocean vertical mixing parametrization on sea ice properties using NEMO-SI3 model in the Arctic Ocean 

Sofia Allende, Anne Marie Treguier, Camille Lique, Clément de Boyer Montégut, François Massonnet, and Thierry Fichefet

In recent decades, global climate change has strongly affected the Arctic region, leading to a rapid decline in sea ice extent. This decline affects the interactions between sea ice, the atmosphere, and the ocean, driven by complex thermodynamic and dynamical processes. The Arctic mixed layer (ML), located in the upper ocean, plays a key role in regulating the interactions between the deep ocean, sea ice, and the atmosphere. This region is strongly affected by exchanges of mass (such as freshwater and saltwater fluxes) and momentum driven by various forces like ocean currents, tides, waves, and winds. Here, we study the ad-hoc vertical turbulent kinetic energy (TKE) mixing scheme within the NEMO-SI3 model. Specifically, we focus on the influence of surface and internal wave breaking in sea ice-covered regions. The critical parameters are the fraction of surface TKE that penetrates below the ML, the nature of the exponential TKE penetration decrease beneath the ML, and the damping effect on Langmuir and surface wave breaking beneath the ice cover. We aim to assess how these parameters affect the ML and various sea ice properties.

Our findings reveal significant impacts on Arctic sea ice thickness under two scenarios: when ice cover does not affect wave dynamics and when the mixing process weakens. Stronger mixing leads to a deeper ML and reduced sea ice thickness by 30 to 40 centimeters, while weaker mixing results in a shallower ML and a moderate sea ice increase of 10 to 20 centimeters. Results also show that reduced sea ice models exhibit a larger volume of freshwater content in the ocean with consistent spatial patterns. Conversely, increased sea ice simulations reveal reduced freshwater content, although clear spatial patterns are not evident. Differences in upper ocean properties, particularly in ocean stratification, highlight the significant impact of strong sea ice attenuation in the mixing parametrization. These findings underscore the substantial influence of enhanced ocean mixing on the physical properties of ocean and sea ice.

How to cite: Allende, S., Treguier, A. M., Lique, C., de Boyer Montégut, C., Massonnet, F., and Fichefet, T.: Impact of ocean vertical mixing parametrization on sea ice properties using NEMO-SI3 model in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3811, https://doi.org/10.5194/egusphere-egu24-3811, 2024.

EGU24-5785 | ECS | Posters on site | OS1.7 | Highlight

Daily to decadal changes: Insights from a high resolution 10-year record of atmospheric carbon dioxide, observed from coastal Antarctica. 

Freya Squires, Anna Jones, Tony Phillips, James France, Nellie Wullenweber, and Rolf Weller

The Southern Ocean is the dominant marine sink for anthropogenic carbon, absorbing around 40% of carbon emitted since industrialisation, but it is a remote and challenging region to measure. Sparsity of observational data is the main cause of uncertainty in air-sea carbon flux in the Southern Ocean. Year-round observations of CO2 mixing ratios can aid understanding of air-sea flux in this critical region and provide valuable insight into how the carbon sink is changing over time as well as its seasonal and interannual variability.

This work presents ten years of high frequency in situ carbon dioxide mixing ratios measured from two coastal Antarctic research stations; Halley, operated by the British Antarctic Survey, and the German research station, Neumayer. This data set provides a rare long-term measurement of CO2 in the Southern Ocean region, allowing annual growth rates, seasonal changes and interannual variability to be studied. The mean annual growth rate was calculated to be ~2.4 ppm year-1 between 2013 and 2022.

The coastal location of these stations mean they are ideally placed to explore air-sea CO2 exchange in the Southern Ocean. Both the Halley and Neumayer records show short-term fluctuations in CO2 mixing ratios during the summer, with up to ~0.5 ppm decreases in CO2 over the course of a day, about one fifth of the average annual growth rate. Air mass trajectory analysis carried out using Hysplit with ERA5 meteorological data, suggests that these decreases in CO2 correspond to periods where the air sampled has spent time over the Southern Ocean, suggesting CO2 uptake has occurred. This work explores the possible drivers for the short-term variability in CO2 mixing ratios, focusing on the role of ocean uptake in the summer.

How to cite: Squires, F., Jones, A., Phillips, T., France, J., Wullenweber, N., and Weller, R.: Daily to decadal changes: Insights from a high resolution 10-year record of atmospheric carbon dioxide, observed from coastal Antarctica., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5785, https://doi.org/10.5194/egusphere-egu24-5785, 2024.

EGU24-5985 | ECS | Posters on site | OS1.7

Surface density fluxes and water mass transformation over global oceans from reanalysis and climate models 

Vladimir Kukushkin, Sergey Gulev, and Margarita Markina

We analyze interannual and seasonal variability of surface density fluxes and water mass transformation rates over the global oceans for 1979-2018 using data from CFSR reanalysis and historical simulations by climate models. By analyzing density fluxes we quantify the the effect of surface heat and mass fluxes onto the formation of surface waters in the World Ocean. First, by using net fluxes from CFSR reanalysis we derive global climatology of surface density flux and further integrate it for density classes and T,S-classes, providing global and regional view of surface water mass transformation and its variability in space and in time. We precisely looked onto the role of salinity and sea ice formation in the density flux during the winter period. On average, the contribution of salinity to sea ice formation results in the differences of 9% in the density flux with the maximum effect of 12% identified in 1989. Interdecadal variability in surface transformation of the subpolar modal water and Labrador Sea waters shows opposite tendencies for the last decades. Then we analyze historical experiments from CMIP6 model ensemble and compare characteristics of surface water mass transformation with those revealed from reanalysis. We conclude that surface density fluxes and transformation rates derived from INM, MPI and MIROC are stronger compared to those diagnosed by CFSR with the largest differences identified over the Gulf Stream and the North Atlantic Current. For the same models we derive projections of surface density fluxes and surface water mass transformation for 2100 under ssp126, ssp370 and ssp585 scenarios. For all SSP scenarios, computations show a decrease in the magnitude of surface water mass transformation by the end of the century.

How to cite: Kukushkin, V., Gulev, S., and Markina, M.: Surface density fluxes and water mass transformation over global oceans from reanalysis and climate models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5985, https://doi.org/10.5194/egusphere-egu24-5985, 2024.

EGU24-6195 | Orals | OS1.7

Amino acids, carbohydrates and lipids in the tropical oligotrophic Atlantic Ocean: Sea-to-air transfer and atmospheric in situ formation  

Manuela van Pinxteren, Sebastian Zeppenfeld, Khanneh Wadinga Fomba, Nadja Triesch, Sanja Frka, and Hartmut Herrmann

Carbohydrates, amino acids, and lipids are important contributors to organic carbon (OC) in the marine environment. To study their sea-to-air transfer, including their enrichment in the sea surface microlayer (SML), potential atmospheric in situ formation or degradation, and their oceanic contribution to the ambient marine aerosol particles, we provide measurements from the tropical Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO) where the above compounds were investigated in both surface seawater and in ambient submicron aerosol particles.

In bulk seawater and the SML, similar distributions among species were found for the lipids and carbohydrates with moderate SML enrichments (enrichment factor EFSML = 1.3 ± 0.2 and 1.1 ± 0.5 respectively). In contrast, the amino acids exhibited a higher enrichment in the SML with an average EFSML of 2.3 ± 0.4 although they are less surface-active than lipids. The same compounds studied in the seawater were found on the ambient submicron aerosol particles whereas the lipids were more pronounced enriched (EFaer. = 1.6x105) compared to the amino acids and carbohydrates (EFaer. = 1.5x103 and 1.3x103 respectively), likely due to their high surface activity and/or the lipophilic character. Detailed molecular analysis of the seawater and aerosol particles revealed changes in the relative abundance of the individual organic compounds. They were most pronounced for the amino acids and are likely related to an in situ atmospheric processing by biotic and/or abiotic reactions.

On average 49% of the OC on the aerosol particles (≙ 97 ng m-3) could be attributed to the specific components or component groups investigated in this study. The majority (43%) was composed of lipids. Amines, oxalic acid, and carbonyls, comprised an OC fraction of around 6%. Carbohydrates and amino acids made up less than 1% of the OC. This shows that carbohydrates, at least when resolved via molecular measurements of single sugars, do not comprise a very large fraction of OC on marine aerosol particles, in contrast to other studies. However, carbohydrate-like compounds are also present in the high lipid fraction (e.g., as glycolipids), but their chemical composition could not be revealed by the measurements performed here.

Since the identified compounds constituted about 50% of the OC and belong to the rather short-lived biogenic material probably originating from the surface ocean, a pronounced coupling between ocean and atmosphere was indicated for this oligotrophic region. The remaining, non-identified OC fraction might in part contain recalcitrant OC, however, this fraction does not constitute the vast majority of OC in the aerosol particles here investigated.

The study contributes to the international SOLAS program.

 

Ref: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates, and lipids in the tropical oligotrophic Atlantic Ocean: sea-to-air transfer and atmospheric in situ formation, Atmos. Chem. Phys., 23, 6571–6590, https://doi.org/10.5194/acp-23-6571-2023, 2023.

How to cite: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates and lipids in the tropical oligotrophic Atlantic Ocean: Sea-to-air transfer and atmospheric in situ formation , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6195, https://doi.org/10.5194/egusphere-egu24-6195, 2024.

The increasing amount of data in earth-observing systems allows us to move from considering low-order moments (means and variances) of fluctuating observations to their PDFs (Probability Density Functions). For two years of HFR (High Frequency Radar) sea surface current increments in the Gulf of Trieste (Northern Adriatic Sea) we found the analytical fat-tailed PDF form (a combination of a gaussian and a convolution of two exponentials) using superstatistics and the maximum entropy principle twice: on a short and on a longer time scale. The data observed under different wind regimes (Bora, Sirocco and low wind, from the WRF model local forecasts) follow the same analytical PDF, pointing towards a universal behaviour.

We developed an idealised deterministic-stochastic model of the wind-driven sea surface currents in the Gulf of Trieste. The deterministic model consists of a time-dependent Ekman layer system, including the tidal signal, with a quadratic drag. It describes 57% of the variability, missing the fast fluctuations. The stochastic part accounts for the fast fluctuations, reproducing the superstatistical PDFs from the observations. The model, providing a huge amount of data, allows for studying the PDF of the mechanical power-input into the ocean and the associated extreme events.

How to cite: Flora, S., Ursella, L., and Wirth, A.: Superstatistical analysis of HF Radar sea surface currents in the Gulf of Trieste, their idealized wind-driven stochastic modeling and extreme power-input events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6484, https://doi.org/10.5194/egusphere-egu24-6484, 2024.

The Kuroshio Extension (KE) bimodality has important effects on the ocean environment, ecosystem and climate. Previous studies have revealed that the Kuroshio Extension (KE) bimodality is mainly determined by the westward-propagating Rossby wave triggered by the North Pacific decadal variability such as PDO or NPGO: the positive (negative) phase of NPGO corresponds to the stable (unstable) KE state. However, the KE state and the NPGO seem to be decoupled since 2017, during which the NPGO takes a negative phase but the KE is in a stable state. This study employs the Convergent Cross Mapping (CCM) method to investigate the causality between the KE bimodality and NPGO. Simultaneously, we divide the KE region into the upstream (west of 146°E) and downstream regions. It is found that the NPGO has a significant causal impact on the downstream KE state. But the effect on the upstream KE state significantly weakens around 2017. Further analysis indicates that the upstream KE state is mainly caused by eddy activity in the Kuroshio large meander region south of Japan. In particular, the changes in the eddy activity affect the downstream advection of eddies and induce changes in the Kuroshio position over the Izu ridge, which cause different states in the KE upstream region. Therefore, we should not only consider the NPGO change, but also the eddy activity change in the Kuroshio region south of Japan when understanding and predicting the KE low-frequency variability.

How to cite: Wang, Q.: Revisiting the relationship between the North Pacific decadal variability and the Kuroshio Extension bimodality, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7259, https://doi.org/10.5194/egusphere-egu24-7259, 2024.

EGU24-7477 | ECS | Orals | OS1.7 | Highlight

The impact of rain on the global ocean carbon uptake 

Laetitia Parc, Hugo Bellenger, Laurent Bopp, Xavier Perrot, and David Ho

Precipitation alters sea surface physical and biogeochemical properties locally. However, due to its high temporal and spatial variations, it has largely been overlooked in studies assessing global ocean carbon uptake. Air-sea CO2 flux is mainly due to the interfacial exchange of CO2 molecules between the liquid and gaseous phases media. Rain may impact this interfacial air-sea CO2 flux by (i) enhancing the turbulence at the air-sea interface and (ii) diluting the CO2 concentration near the ocean surface. At the same time, rain directly injects into the ocean CO2 absorbed by the raindrops during their fall. This latter component, known as wet deposition, contributes to the CO2 flux into the ocean. This study provides the first comprehensive global estimate of these effects and their combined influence on the global ocean carbon uptake during the period 2008-2018. We use different representations of the ocean surface response to rain and different rain products with different rain rate distributions (ERA5 and IMERG) to quantify the uncertainty of the global impact of rain on CO2 sink. We show that rain increases the global ocean carbon sink by +0.14 to +0.19 PgC yr-1 over 2008-2018, representing an increase of 5 to 7% of the global carbon uptake (2.66 PgC yr-1). Both interfacial flux and wet deposition have comparable orders of magnitude. Rain mainly increases the CO2 sink in the tropics, where strong rain rates and weak winds induce noticeable dilution at the ocean surface, in the storm track regions, and in the Southern ocean.

How to cite: Parc, L., Bellenger, H., Bopp, L., Perrot, X., and Ho, D.: The impact of rain on the global ocean carbon uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7477, https://doi.org/10.5194/egusphere-egu24-7477, 2024.

EGU24-8197 | Orals | OS1.7

SEAS: a simulation system for forecasting the atmosphere-coupled ocean dynamics in the Southern EuropeAn Seas 

Francesco Maicu, Nadia Pinardi, Silvio Guadi, Emanuela Clementi, Francesco Trotta, and Giovanni Coppini

The prototype of a short-term forecasting system of the ocean dynamics of Southern European Seas (SEAS), was developed. It is based on a regional coupled ocean-atmosphere model, with NEMO and WRF codes implemented on the same computational grid, with 1/24° resolution, which encompasses Mediterranean Sea, Marmara Sea and Black Sea. The domain extends also westward and northward in the Atlantic Ocean to downscale properly the mid-latitudes atmospheric perturbations from the parent ECMWF HRES model.

The forecasting uncertainty of the atmospheric regimes in such a complex Euro-Mediterranean region must be considered along with the uncertainties of the parametrizations of the surface processes at the ocean-atmosphere interface. Therefore, the goal of coupling oceanic and atmospheric models is to reduce these uncertainties and exploit the second type predictability to increase the forecast skills of the ocean dynamics.

The uncoupled ocean model has been validated against Sea Surface Temperature (SST) satellite observed data, and the skills compared to those of the Copernicus Mediterranean Forecasting System (MedFS hereafter) both in the short-term forecast over two seasonal periods and in the simulation of the medicane Ianos.

Various physical schemes, domain extensions, boundary, and initial conditions were initially tested using the uncoupled atmospheric model to obtain the best representation of the medicane Ianos. Furthermore, these experiments were also useful to determine the coupling strategy more appropriate to reduce the heat fluxes imbalance between the two components.

The SST differences between coupled and uncoupled experiments are determined by the heat fluxes computation in the atmospheric component rather than using the MedFS bulk formulae implemented in the ocean model. These differences are largely dependent on the surface boundary layer scheme used in WRF, therefore, several coupled experiments were conducted.

In terms of SST, the coupled model replicates the skills of the MedFS in the winter period while in the summer period the skills are worsened due to the larger heat fluxes. Numerical experiments focused on the parametrizations of the atmospheric boundary layer are still ongoing work.

The skill of the coupled model in reproducing the observed SST during the medicane Ianos is comparable with the one of the uncoupled oceanic model in the Ionian Sea. In terms of heat fluxes, the coupling changes significantly the heat budget locally in the Ionian Sea, mainly through the latent heat flux and the shortwave radiation. The coupling is not that relevant for the intensification of the cyclone, whereas it enhances the representation of its path and the time of the landfall on the Ionian Islands.

How to cite: Maicu, F., Pinardi, N., Guadi, S., Clementi, E., Trotta, F., and Coppini, G.: SEAS: a simulation system for forecasting the atmosphere-coupled ocean dynamics in the Southern EuropeAn Seas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8197, https://doi.org/10.5194/egusphere-egu24-8197, 2024.

EGU24-8431 | ECS | Orals | OS1.7 | Highlight

The evolution of turbulence, stratification, and the surface jet in Diurnal Warm Layers 

Mariana Miracca Lage, Claire Ménesguen, Lucas Merckelbach, Julia Dräger-Dietel, Alexa Griesel, and Jeff Carpenter

The ocean's upper layer is inherently turbulent and constantly forced by momentum and buoyancy fluxes, and their interplay operates to mix and/or stratify the first meters of the water column. Incoming solar short-wave radiation acts to stabilize the upper layer, whereas the wind transfers momentum to the ocean and acts to vertically mix the water column. However, if the wind is not strong enough to trigger mixing, stratification in the near-surface is immediately formed in a layer of O(10) m thickness, called the diurnal warm layer (DWL). Above the bottom boundary of the DWL, shear production can be enhanced leading to large dissipation of turbulent kinetic energy (TKE) rates, i.e. high turbulence. Based on observational data from an ocean glider with a mounted microstructure package and drifters, we show the evolution of three DWLs sampled on the rim of a mesoscale eddy in the South Atlantic ocean (32oS, 4oE) with respect to temperature and buoyancy anomalies, potential energy and dissipation of TKE. In the near-surface, temperature and buoyancy anomalies increase with the evolution of the DWL, and the latter has the same magnitude as the time-integrated surface buoyancy flux. We also show the development of a diurnal jet with magnitude of O(10) cm/s that veers with the wind. Late in the afternoon, when the diurnal jet is fully developed, the bulk Richardson number (Rib) indicates that the stratified layer related to the DWL becomes marginally unstable (Rib ~ 0.25). During this period, the potential energy also decays, suggesting that the enhanced turbulence within the DWL acts to destroy stratification through turbulent mixing. We further assess whether a one-dimensional turbulence model is able to reproduce the observed DWL’s characteristics and the change in stability throughout the day.

How to cite: Miracca Lage, M., Ménesguen, C., Merckelbach, L., Dräger-Dietel, J., Griesel, A., and Carpenter, J.: The evolution of turbulence, stratification, and the surface jet in Diurnal Warm Layers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8431, https://doi.org/10.5194/egusphere-egu24-8431, 2024.

EGU24-10253 | ECS | Posters on site | OS1.7

Spatial variation of future trends in Atlantic upwelling cells from CMIP6 models 

Raquel Flügel, Steven Herbette, Anne Marie Treguier, Robin Waldman, and Malcolm Roberts

Eastern Boundary Upwelling Systems (EBUS) are characterised by wind-triggered upwelling of deep waters along the coast. They are hotspots of biological productivity and therefore have a high economic, ecological and social importance. Here we investigate the evolution of the two Atlantic EBUS during the historical period and in a future high-emission scenario in CMIP6 models from two modelling centres, with spatial resolutions ranging from 1° to 1/12° in the ocean. The decomposition of the upwelling systems into subregions reveals differences between the equatorward and poleward parts. Our analysis is focused on the modelled vertical transport, which is shown to be consistent with the wind-derived Ekman index. Integrating the vertical transport provides a synthetic view of the upwelling cells, their strength, depth and distance to the coast. The models show high interannual variability over the 21st century century, which explains why significant trends could only be found in few subregions of the Atlantic EBUS. The results suggest a poleward migration of upwelling systems with climate change and a change of the upwelling cells, rather than the uniform intensification which had been hypothesised by Bakun in 1990.

How to cite: Flügel, R., Herbette, S., Treguier, A. M., Waldman, R., and Roberts, M.: Spatial variation of future trends in Atlantic upwelling cells from CMIP6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10253, https://doi.org/10.5194/egusphere-egu24-10253, 2024.

EGU24-11098 | ECS | Posters on site | OS1.7

Feedbacks between turbulent air-sea fluxes and their role in the adjustment of the Earth Climate System 

Clément Dehondt, Pascale Braconnot, Sébastien Fromang, and Olivier Marti

In state of the art Earth System Models (ESM), the variables at the ocean-atmosphere interface (wind, air temperature, humidity, surface currents and SST) are linked to turbulent surface fluxes (momentum, sensible and latent heat) in a complex manner via bulk closures.

Understanding how turbulent fluxes interact between them and with the ocean-atmosphere interface variables is a major scientific challenge because it connects local interactions with large scale energy and water cycles.

These interactions between the different air-sea turbulent fluxes are difficult to diagnose from fully coupled ocean-atmosphere simulations due to the fact that in most modelling groups coupled and stand alone components do not necessarily use consistent forcing or representation of the air-sea fluxes. Also rigorous protocols between coupled and stand alone atmosphere and ocean simulations need to be implemented to be able to properly disentangle the role of different physical representation at the air-sea interface from global ocean-atmosphere-land adjustment feedbacks that may counteract the direct effects of air-sea fluxes modeling.

Here we use an ensemble of fully coupled and stand alone simulations using a version of the IPSL ESM [1] based on the new DYNAMICO atmospheric dynamical core [2] and the ocean engine NEMO [3]. We analyse an ensemble of experiments differing by the the bulk formulation of the air-sea turbulent fluxes (NCAR, COARE3.6, ECMWF and LMDZng). The analyses will focus on the adjustment of the system in the different cases, especially on the differences in the transport of heat and water, mixed layer depth adjustement, feedback on ocean surface properties, intertropical convergence zone (ITCZ) and mid-latitude storm tracks.


[1] Boucher O., Servonnat, J., Albright, A. L., Aumont, O., Balkanski, Y., Bastrikov, V., et al. (2020). Presentation and evaluation of the IPSL‐CM6A‐LR climate model. Journal of Advances in Modeling Earth Systems, 12, e2019MS002010. https://doi.org/10.1029/2019MS002010

[2] Dubos, T., Dubey, S., Tort, M., Mittal, R., Meurdesoif, Y., and Hourdin, F.: DYNAMICO-1.0, an icosahedral hydrostatic dynamical core designed for consistency and versatility, Geosci. Model Dev., 8, 3131–3150, https://doi.org/10.5194/gmd-8-3131-2015, 2015.
 
[3] “NEMO ocean engine”, Scientific Notes of Climate Modelling Center, 27 — ISSN 1288-1619, Institut PierreSimon Laplace (IPSL), doi:10.5281/zenodo.1464816

How to cite: Dehondt, C., Braconnot, P., Fromang, S., and Marti, O.: Feedbacks between turbulent air-sea fluxes and their role in the adjustment of the Earth Climate System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11098, https://doi.org/10.5194/egusphere-egu24-11098, 2024.

EGU24-11282 | ECS | Orals | OS1.7

Can sea spray aerosol be a source of gas-phase perfluoroalkyl substances (PFAS)? A study in the Eastern North Atlantic Ocean 

Sneha Aggarwal, Olga Garmash, Delaney Kilgour, Christopher Jernigan, Julika Zinke, Xianda Gong, Shengqian Zhou, Jiaoshi Zhang, Jian Wang, Timothy Bertram, Joel Thornton, Matt Salter, Paul Zieger, and Claudia Mohr

Sea spray aerosol (SSA) formed after wave breaking at the ocean surface influences our climate by scattering incoming solar radiation and acting as cloud condensation nuclei. Furthermore, they provide a microenvironment for aqueous phase chemistry, selective uptake of surfactants, and gas-to-particle partitioning of compounds by providing an acidic pH at the air-water interface (Angle et al., 2022). Despite these known effects, a crucial question remains unanswered: which volatile organic compounds (VOCs) are emitted from SSA, and how do they change over time via atmospheric aging?

To address this, we designed a novel experimental setup during the AGENA* Campaign 2022 at Graciosa Island, Portugal. For the first time, we connected a sea spray simulation chamber to a chemical ionization mass spectrometer (CIMS) to measure the freshly emitted gases from both seawater and SSA. Additionally, we aged the samples for an equivalent period of about 3-3.5 days in an oxidation flow reactor to investigate compositional changes after ageing.

Surprisingly, our findings reveal that nearly half of the mass-spectrometer signal from the fresh samples constituted fluorinated compounds, specifically short-chain perfluoroalkyl carboxylic acids - a class of perfluoroalkyl substances (PFAS). While, previous studies have shown that SSA can release and play a key role in the long-range transport of PFAS, these studies have primarily focused on particle-phase emissions (Johansson et al., 2019, Sha et al, et al., 2022). In contrast, our study provides new insights into oceanic PFAS emissions and transport to the atmosphere by examining gas-phase emissions.  

Furthermore, we observed that the gas-phase PFAS almost completely disappears after ageing. Our hypothesis is that these compounds partition into the particle phase. We plan to test this hypothesis by analyzing the particle filters collected during the campaign.

*Aerosol Growth in the Eastern North Atlantic (AGENA) https://www.arm.gov/research/campaigns/ena2022agena

Angle, K. J., Crocker, D. R., Simpson, R. M., Mayer, K. J., Garofalo, L. A., Moore, A. N., ... & Grassian, V. H. (2021). Acidity across the interface from the ocean surface to sea spray aerosol. Proceedings of the National Academy of Sciences118(2), e2018397118.

Johansson, J. H., Salter, M. E., Navarro, J. A., Leck, C., Nilsson, E. D., & Cousins, I. T. (2019). Global transport of perfluoroalkyl acids via sea spray aerosol. Environmental Science: Processes & Impacts21(4), 635-649.

Sha, B., Johansson, J. H., Tunved, P., Bohlin-Nizzetto, P., Cousins, I. T., & Salter, M. E. (2021). Sea spray aerosol (SSA) as a source of perfluoroalkyl acids (PFAAs) to the atmosphere: field evidence from long-term air monitoring. Environmental Science & Technology56(1), 228-238.

How to cite: Aggarwal, S., Garmash, O., Kilgour, D., Jernigan, C., Zinke, J., Gong, X., Zhou, S., Zhang, J., Wang, J., Bertram, T., Thornton, J., Salter, M., Zieger, P., and Mohr, C.: Can sea spray aerosol be a source of gas-phase perfluoroalkyl substances (PFAS)? A study in the Eastern North Atlantic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11282, https://doi.org/10.5194/egusphere-egu24-11282, 2024.

The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model has been employed to simulate the anomalous post-monsoon tropical cyclone (TC) Jawad that originated over the Bay of Bengal (BoB) in December 2021. The atmospheric initial and boundary conditions (IC and BC) have been obtained from the Global Forecasting System (GFS) Analyses and Forecasts and two contrasting ocean IC and BCs, viz., HYCOM (experiment name GFS-HYCOM) and INCOIS (experiment name GFS-INCOIS), are implemented in two separate coupled experiments to evaluate the influence of TC Jawad on the surface and sub-surface characteristics of BoB. The track of the TC, including its recurvature, was well captured by both experiments with significant accuracy. A proper contrast in temperature between the two sides of the TC track was noted in the surface and sub-surface temperatures observed by two buoys, i.e., (1) BD11 (west of the TC track) and (2) BD13 (east of the TC track), and the simulated temperatures were validated with these observations. Contrary to the usual scenario, the higher sub-surface warming on the eastern side of the TC track was captured by GFS-HYCOM, but with a significant overestimation. The lower temperature on the western side of the TC track can be attributed to the weak upwelling associated with the cyclonic circulation caused by the interaction of the TC with the southward coastal currents. An unusually higher downwelling on the eastern side of the TC track was observed in the vertical distribution of the temperature across the longitudes, which suggested the existence of a strong clockwise circulation near the location of BD13. GFS-HYCOM, which simulated a higher current magnitude in the sub-surface than GFS-INCOIS on the eastern side of the TC track, captured the circulation near BD13 more rigorously. From further analysis, it was inferred that the interaction of the cyclonic wind flow of TC Jawad (westerly) near the surface with the easterly flow caused the generation of the clockwise circulation over the ocean surface on the eastern side of the TC track, leading to intense downwelling and warming of the sub-surface temperature. This scenario was further corroborated by the simulated Ekman transport and higher convective activity in the eastern quadrants of the TC. The present study not only emphasizes the capability of the coupled ocean-atmosphere models to simulate TCs but also highlights the necessity of investigating the air-sea interaction processes and their responses to the passage of an anomalous TC like Jawad.

How to cite: Chakraborty, T., Pattnaik, S., and Joseph, S.: Modulation of surface and sub-surface circulation in the Bay of Bengal by the passage of tropical cyclone Jawad: coupled ocean-atmosphere feedback , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11375, https://doi.org/10.5194/egusphere-egu24-11375, 2024.

EGU24-11879 | Posters on site | OS1.7

Estimates of Polar Ocean CO2 Uptake from Atmospheric Inverse Analyses  

Parvadha Suntharalingam, Zhaohui Chen, and Jayashree Ghosh

Estimates of global scale air-sea CO2 fluxes have traditionally been derived from ocean biogeochemistry models and ocean surface pCO2 data products (Friedlingstein et al. 2022). An alternative means of estimating ocean carbon uptake is provided by atmospheric inversions; these use optimization procedures and data assimilation methods to combine atmospheric CO2 measurements with numerical transport model simulations and prior knowledge of air-sea fluxes. 

Here we use the GEOSChem-LETKF (GCLETKF) inverse system (Chen et al. 2021) in conjunction with atmospheric observations from the NOAA-GML surface CO2 measurement network to derive grid-scale air-sea CO2 flux estimates for the period 2000-2017. We focus, in particular, on estimates of CO2 uptake by the polar oceans (Southern and Arctic oceans). These  regions have accounted for a significant component of global oceanic carbon uptake  in recent decades (e.g., more than 20% of global ocean uptake, in comparison to their ocean areal  extent of < 10%).

We present GCLETKF estimates of ocean CO2 uptake at global and regional scales, and assess the robustness of our results with a suite of metrics that include model concentration bias, CO2 flux error reduction, and comparison to independent atmospheric measurements. GCLETKF flux estimates for the 2000-2017 period indicate regional CO2  uptake of 0.1-0.2 PgC/year for the Arctic,  and  0.45-0.55 PgC/yr for the Southern Ocean. We also provide summary estimates of the  interannual variations and  decadal-scale trends of the polar ocean carbon fluxes, and compare the GCLETKF results  to estimates derived from global ocean biogeochemistry models and surface ocean pCO2 data products.  

How to cite: Suntharalingam, P., Chen, Z., and Ghosh, J.: Estimates of Polar Ocean CO2 Uptake from Atmospheric Inverse Analyses , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11879, https://doi.org/10.5194/egusphere-egu24-11879, 2024.

EGU24-12623 | ECS | Posters on site | OS1.7

Evaluation of several meteorological models by comparison with qualified Air-Sea observations 

Saïd Benjeddou, Denis Bourras, and Christopher Luneau

Meteorological models are important simulation tools to improve our understanding of the climate behavior on seasonal, annual, decadal and centennial scales. Their complexity has increased considerably since 1990. The output fieds of several widely available meteorological such as GFS, ECMWF, WRF, ARPEGE and MERRA are evaluated, by comparing the output fields to in situ data performed during six campaigns with the wave-following platform OCARINA (Ocean Coupled with the Atmosphere, Research on the Interface on Annex Ship) developed at MIO. Following a recent comparison for wind and SST by Benjeddou et al. (2024), emphasis will now be laid on the comparison of heat fluxes and associated bulk variables, in open sea conditions, versus close to the shore line.

How to cite: Benjeddou, S., Bourras, D., and Luneau, C.: Evaluation of several meteorological models by comparison with qualified Air-Sea observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12623, https://doi.org/10.5194/egusphere-egu24-12623, 2024.

Estimates of air-sea fluxes rely on the knowledge of the gas transfer velocity. Despite more than half a century of field measurements, starting with the GEOSECS program in the 70ies, there are still many open questions. At low wind speeds, no reliable measurements are available, because all available techniques (dual-tracer, eddy covariance and active thermography) are either not suitable for measurements under these conditions or deliver too uncertain results. At high wind speeds beyond 25 m/s, almost no measurements are available. In the intermediate wind range enough reliable data are available. But the data are partly contradictionary. The effect of the many other parameters influencing the transfer velocity besides the wind speed is still uncertain. This includes the effect of the sea state (wave age), bubbles, and surfactants.

In wind-wave tunnels, it is easy to perform systematic studies. But the conditions deviate significantly from those at the open ocean in traditional linear facilities because of the short interaction length between wind. Therefore, only young wind seas can be generated, far away from a wind sea in equilibrium with the wind (“fetch gap”).

In 2021, we started a laboratory program, funded by a Reinhart Koselleck Project of the German Science Foundation. It includes three innovative key elements, which together overcome most disadvantages of previous wind-wave tunnel experiments. Firstly, a large annular facility is used, the Heidelberg Aeolotron. Because of the infinite fetch, wind waves come into equilibrium with the wind as at the ocean. Secondly, two imaging techniques are used to measure transfer velocities locally and instantaneously. Active thermography is used to measure the heat transfer velocity across the aqueous viscous boundary layer and a novel fluorescence technique to image the concentration fields in the mass boundary layer and to estimate the gas transfer velocity. Thirdly, measurements are performed under non-stationary conditions. In this way the whole fetch range can be investigated, when the wind speed is turned on, and decaying wind seas, when the wind speed is lowered.

In this talk first results of these measurements will be shown:

At low wind speeds, a significant overshoot in the transfer velocity occurs at low-fetch wind-wave fields.

The change in the Schmidt number exponent of the transfer velocity from 2/3 to 1/2 is related to the increasing frequency of microscale wave breaking.

An insoluble monomolecular monolayer of hexadecanol has the same effect as the soluble surfactant TritonX-100 (5 ppm by volume): Wind waves are completely suppressed up to wind speeds of about 8 m/s and the spatial patterns of the concentration field in the boundary layer are the same. In contrast, lowering the surface tension to about 43 mN/m by adding 1-hexanol to the water (2.4 kg/m3) did not suppress wind waves and transfer velocities at all.

How to cite: Jähne, B.: On the Crucial Role of Wind-Wave-Tunnel Studiesto Reveal the Mechanisms of Air-Sea Gas Exchange, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13372, https://doi.org/10.5194/egusphere-egu24-13372, 2024.

EGU24-13779 | ECS | Orals | OS1.7 | Highlight

Two extremes: Investigating the impact of the co-occurrence of medicanes and marine heatwaves in the Mediterranean Sea. 

Kenechukwu Uba, Manal Hamdeno, Alexander Barth, and Aida Alvera-Azcárate

The oceans are steadily warming, which affects global weather and climate and leads to an increase in extreme events such as storms, hurricanes and marine heatwaves (MHWs). Future warming scenarios predict an increase in the frequency and intensity of such events. In the Mediterranean region, both extratropical cyclones and occasional Mediterranean hurricanes (medicanes) occur, causing considerable damage to infrastructure and major socio-economic losses in coastal regions. Using ERA-5 atmospheric reanalysis data and satellite-derived sea surface temperatures (SST), this study looks at medicanes that occurred between 2011 and 2023 and examines their characteristics and impacts on the water column. The interaction with simultaneous MHWs in the Mediterranean is also investigated. A total of 15 medicanes occurred during the study period. Of these, 5 occurred in the western Mediterranean (WMed), mainly in November; 9 in the central Mediterranean and Ionian (CMed) between September and December, two of which terminated in the eastern basin; and 1 event was localised entirely in the eastern Mediterranean (EMed) in October. During the study period, 2014 recorded the highest number of medicanes with three events. One event, Ilona, occurred in January in the WMed, while the other two events, Qendresa and Xandra, occurred in November in the CMed and WMed respectively. Two events took place in both 2020 and 2021. In 2020, both Ianos in September and Elaina in December were in the CMed. In 2021, the CMed and WMed witnessed the passage of Apollo in October and Blas in November respectively. In the 15 medicanes, the mean sea level pressure (MSLP) was between 988 and 1005 hPa, while the wind speeds (Ws) were between 17 and 23 m/s. Among the events, Ilona in January 2014 had the lowest MSLP and highest Ws and the lowest associated MSLP anomaly. Of the 15 events, 11 (73%) were associated with anomalously high sea surface temperatures (SSTA) and five of these SSTAs were defined as MHW events. Moreover, the high SST anomalies were observed three or more days before the onset of these medicanes, which may have contributed to the intensification of the passing storms and amplified their impact through air-sea heat exchange. In turn, the medicanes were also observed to influence the MHWs, as the heat released from the ocean during the medicanes prevented the MHWs from deepening beyond the surface layer, demonstrating a dynamic interplay between these events. In summary, as the oceans warm, medicanes and MHWs in the Mediterranean increase, with complex interactions determining their behavior and impacts. Understanding these dynamics is crucial for predicting and mitigating the impacts of these events on marine ecosystems and coastal regions. 

How to cite: Uba, K., Hamdeno, M., Barth, A., and Alvera-Azcárate, A.: Two extremes: Investigating the impact of the co-occurrence of medicanes and marine heatwaves in the Mediterranean Sea., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13779, https://doi.org/10.5194/egusphere-egu24-13779, 2024.

EGU24-15234 | ECS | Posters on site | OS1.7

A catalogue of wind events for assessing the connectivity among Marine Protected Areas in the German Bight (North Sea) 

Sara Rubinetti, Vera Sidorenko, Enrico Arnone, Alexey Androsov, Kingsly C. Beng, Kerstin Klemm, Anne F. Sell, Anna Akimova, Santiago E. A. Pineda-Metz, Bernadette Pogoda, Sarah Brand, Mathias Wegner, Lisa Shama, Silke Laakmann, Sabine Horn, and Karen H. Wiltshire

Marine protected area (MPA) networks are fundamental for restoring and conserving ecosystem functions like biodiversity and general ecosystem health. Ideally, the effects of local conservation measures are not limited to one particular MPA alone but influence and connect regions beyond, or even other MPAs, through the spreading, replenishment and potential recovery of populations and communities. Connectivity defines, in a probabilistic sense, the functional linkage exchange between individual MPAs or key regions, and it depends on the features of the selected tracers (including the specific biological traits of target organisms), but it is also to a large degree determined by the hydrodynamic circulation patterns in the area. For the German Bight (south-eastern North Sea), we are focusing in particular on potential spillover from a restoration site for the European flat oyster (Ostrea edulis) through the spread of planktonic life stages. 
The circulation regimes are determined mainly by tidal and wind forcings. The prevailing wind-driven surface circulation in the area is cyclonic, influenced by frequent south-westerly to westerly winds. However, winds from other directions, for instance from the North-West, have the potential to modify and even reverse this circulation pattern. Wind intensity and directions have a clear seasonal variability, with higher magnitudes in winter and lower in summer, but also exhibit a significant interannual variability driven by the strength and location of high and low mean sea level atmospheric pressure centres. Moreover, winds from the East are relatively rare compared to the other patterns but can be extremely persistent (up to hundreds of hours) and thus affect the hydrodynamics and, hence, the connectivity between the MPAs. In this study, we catalogued the wind events according to their typical duration and magnitude using 10m eastward and northwards components retrieved from ERA5 reanalysis data and characterized them according to their seasonality and interannual variability. The results can be used to define realistic atmospheric scenarios to numerically simulate the sea dynamics in the southern North Sea and, consequently, assess the connectivity among different sites, including established MPAs. These efforts are crucial for a proper planning of conservation and restoration measures in the German Bight, which is one of the most exploited marine regions in the world. 

How to cite: Rubinetti, S., Sidorenko, V., Arnone, E., Androsov, A., Beng, K. C., Klemm, K., Sell, A. F., Akimova, A., Pineda-Metz, S. E. A., Pogoda, B., Brand, S., Wegner, M., Shama, L., Laakmann, S., Horn, S., and Wiltshire, K. H.: A catalogue of wind events for assessing the connectivity among Marine Protected Areas in the German Bight (North Sea), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15234, https://doi.org/10.5194/egusphere-egu24-15234, 2024.

EGU24-16489 | Orals | OS1.7 | Highlight

Vertical fluxes in subpolar eddies from a high-resolution, multiplatform experiment in the Labrador Sea 

Ahmad Fehmi Dilmahamod, Johannes Karstensen, Jochen Horstmann, and Gerd Krahmann

Mesoscale structures are key dynamical features of the ocean. They are associated with a variety of short lived and small-scale dynamics linked to physical, biological, and chemical processes at the submesoscale, such as cascading energy, impacting ocean stratification, and guiding ocean carbon and oxygen uptake. In the high latitudes, the spatial extent of the mesoscale is only tens of kilometres, making it challenging to observe the submesoscale processes. In August-September 2022, an extensive submesoscale-resolving multiplatform experiment was conducted across an Irminger Ring in the Labrador Sea. The experiment leveraged two underwater electric gliders equipped with nitrate, microstructure shear, chlorophyll fluorescence, oxygen, and turbidity sensors, operated in concert with a variety of ship operated instruments including underway-CTD’s, a moving vessel profiler, Thermosalinograph, ADCPs and a X-band radar system. Observations were acquired both, along the peripheries and within the core of the eddy, and offered insight into submesoscale dynamics of the ring. Making use of nearly concurrent turbulence and nutrients observations, we estimated the vertical flux pattern across the eddy’s frontal and interior regions. From the recorded and expected glider vehicle motion a vertical water velocity could be inferred and compared with the nutrient flux pattern. The stability of the ring was tracked with surface drifters, for weeks after the ship and glider survey ended, and a link between the disintegration of the ring and an atmospheric event was investigated

How to cite: Dilmahamod, A. F., Karstensen, J., Horstmann, J., and Krahmann, G.: Vertical fluxes in subpolar eddies from a high-resolution, multiplatform experiment in the Labrador Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16489, https://doi.org/10.5194/egusphere-egu24-16489, 2024.

EGU24-16669 | ECS | Posters on site | OS1.7 | Highlight

Buoyant gravity currents triggered by a collapsing mid-latitude submesoscale front 

Grete Boskamp, Peter Holtermann, and Lars Umlauf

Sharp fronts with temperature differences of approximately 0.5°C across a remarkably small lateral scale of order 10 m were observed in a subtropical region with strong mesoscale and submesoscale activity in the southeast Atlantic at 34°S, 6.5°E, far away from any coastal freshwater sources. These fronts were formed at the leading edge of a buoyant gravity current of 20-40 m thickness that propagated at a speed of order 0.1 m/s relative to the colder and thus denser surrounding waters. High-resolution turbulence microstructure observations revealed strongly enhanced turbulence inside the nose of the gravity current, while turbulence in the trailing bulk region was mainly wind- and convectively-driven and showed a strong diurnal modulation. Satellite and meteorological data suggest that the gravity current was triggered by the mesoscale strain-induced sharpening and final collapse of a larger-scale front at the edge of a mesoscale eddy during a period with decaying winds. In contrast to previous studies that have identified similar buoyant gravity currents in the equatorial ocean, our data suggest that they can also form at a mid-latitude location where rotational effects are strong. This suggests that even balanced fronts can decay into gravity currents under certain conditions, indicating a potentially important pathway for mesoscale energy dissipation and mixing.

How to cite: Boskamp, G., Holtermann, P., and Umlauf, L.: Buoyant gravity currents triggered by a collapsing mid-latitude submesoscale front, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16669, https://doi.org/10.5194/egusphere-egu24-16669, 2024.

EGU24-16888 | Posters on site | OS1.7

North Atlantic SST variability during strong winter extratropical cyclones 

Margarida L. R. Liberato

Extreme weather and climate events, such as extratropical cyclones and droughts, represent a topic of paramount importance in the Iberian Peninsula and the North Atlantic Ocean plays an important role in shaping their frequency and intensity. Sea surface temperature (SST) variations, which are important indicators of ocean variability, can result in anomalous diabatic heating or cooling of the overlying atmosphere. In this study, the contributions of different physical processes to the development of North Atlantic explosive extratropical cyclones (EC) affecting the Iberian Peninsula are investigated using the ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). Results suggest that the North Atlantic Ocean SST contributed to the formation and intensification of extratropical cyclones, and particularly to the formation and development of intense storms. Furthermore, the combined analysis of SST and net surface heat flux (QN) also shows the cooling of the ocean associated with the EC tracks caused by the heat exchanges between the ocean and the atmosphere.

 

Acknowledgements

This work is supported by national funds by FCT - Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020 (https://doi.org/10.54499/UIDB/04033/2020).

 

How to cite: Liberato, M. L. R.: North Atlantic SST variability during strong winter extratropical cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16888, https://doi.org/10.5194/egusphere-egu24-16888, 2024.

EGU24-18069 | ECS | Posters on site | OS1.7

Seasonal Variation of Mesoscale Horizontal Stirring in the North Pacific Ocean 

Gyuseok Yi, Wonsun Park, and June-Yi Lee

The mesoscale horizontal stirring (MHS) is closely linked to various mesoscale dynamical phenomena, encompassing not only eddies but also meanders, filaments, and fronts. A clear understanding of its seasonality holds the potential to enhance our understanding of horizontal mixing and material dispersion. Here, we analyze the seasonal variation of MHS in the North Pacific surface ocean using ocean reanalysis (GLORYS12) current velocity data with a horizontal resolution of 1/12° from 1993 to 2019. Based on the characteristic of stirring to separate adjacent fluid trajectories, MHS is quantified using the finite-size Lyapunov exponent (FSLE), one of the Lagrangian diagnostics. The FSLE in the North Pacific shows clear seasonality but the phases of its evolution differ regionally. We identify two major modes, which contribute to over 80% of the seasonality of FSLE in the North Pacific, through the application of empirical orthogonal function (EOF) analysis to the climatological monthly mean FSLE. The first mode (57%) exhibits a variation peaking in April within the Kuroshio Extension region and the Subtropical Countercurrent region, where baroclinic instability plays a significant role. The second mode (25%) peaks during the summer season over the Kuroshio area and coastal upwelling areas of western North America. It is found that the strong seasonality in the upwelling area is induced by the North Pacific High.

How to cite: Yi, G., Park, W., and Lee, J.-Y.: Seasonal Variation of Mesoscale Horizontal Stirring in the North Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18069, https://doi.org/10.5194/egusphere-egu24-18069, 2024.

EGU24-19544 | Orals | OS1.7 | Highlight

Plastics Affect the Ocean's Uptake of Atmospheric CO₂ across the Marine Boundary Layer 

Luisa Galgani, Eleni Tzempelikou, Ioanna Kalantzi, Anastasia Tsiola, Manolis Tsapakis, Paraskevi Pitta, Chiara Esposito, Anastasia Tsotskou, Iordanis Magiopoulos, Roberto Benavides, Tobias Steinhoff, Amedeo Boldrini, Alessio Polvani, and Steven A. Loiselle

Microplastics can support biomass production by acting as substrates for microbial activity. This may imply potentially relevant effects for the sea-surface microlayer, the interface mediating air-sea gas exchange and where biological organic compounds can accumulate.

We tested this hypothesis by using six large scale mesocosms to simulate a future “high plastic ocean”. During the course of a 12-days experiment, we explored microbial organic matter dynamics in the sea-surface microlayer in the presence and absence of microplastics in the underlying water. We used as a reference a known number of polystyrene beads of 30 µm diameter and compared the three treatment mesocosms to an equal number of plastic-free control mesocosms.

The presence of microplastics represented a spur for microbial activity, and in the treated mesocosms biomass production was enhanced, leading to an increased concentration of organic compounds accumulating in the sea-surface microlayer. This initial boost in biological productivity led to a ∼3 % reduction of dissolved CO₂ in the underlying water, which we could imagine potentially reversed once the degradation phase took off. Based on our results and on other recent studies, we will discuss potential interference of plastic with the composition of the sea-surface microlayer, with direct and indirect impacts on the uptake of CO₂ and the marine carbon cycle. 

How to cite: Galgani, L., Tzempelikou, E., Kalantzi, I., Tsiola, A., Tsapakis, M., Pitta, P., Esposito, C., Tsotskou, A., Magiopoulos, I., Benavides, R., Steinhoff, T., Boldrini, A., Polvani, A., and Loiselle, S. A.: Plastics Affect the Ocean's Uptake of Atmospheric CO₂ across the Marine Boundary Layer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19544, https://doi.org/10.5194/egusphere-egu24-19544, 2024.

EGU24-20158 | ECS | Orals | OS1.7

On the influence of hydrodynamic and environmental conditions on wave breaking in the nearshore 

Susanne Støle-Hentschel, Patricio Catalán, Michael Streßer, Jochen Horstmann, and Frédéric Dias

An improved understanding of wave breaking is still a hot topic owing to its relevance in the coupling of ocean and atmosphere. Multiple communities are focusing on numerical simulations of the fully coupled two-phase flow, the validation of such models remains challenging. Herein, we demonstrate how coherent marine radars can help to shed light on how different wave and wind parameters influence the evolution of waves towards breaking in the nearshore. The interpretation of the results is undermined by SWASH simulations of shoaling waves for different wave spectra and two beaches and simulations of radvarimages of these waves. Data of three independent measurement campaigns shows that the shoaling characteristics are strongly influenced by the wave steepness, relative depth the Ursell number and the wind. The influence of individual parameters cannot be isolated, but must be understood in its entirety.

How to cite: Støle-Hentschel, S., Catalán, P., Streßer, M., Horstmann, J., and Dias, F.: On the influence of hydrodynamic and environmental conditions on wave breaking in the nearshore, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20158, https://doi.org/10.5194/egusphere-egu24-20158, 2024.

EGU24-20277 | ECS | Posters on site | OS1.7

Impact of marine biogenic VOC emissions on the marine boundary layer of the Eastern Mediterranean 

Elissavet Bossioli, Dimitrios Kourakos, Dionysios E Raitsos, Antonia Kournopoulou, John Karagiorgos, Georgia Methymaki, Panagiotis Portalakis, Stavroula Karatasou, and Sarantis Sofianos

The production by biological and photochemical mechanisms of short-lived Volatile Organic Compounds (VOC) in the surface ocean is regulated by environmental parameters and nutrient abundance, and hence climate change. These gases then enter the atmosphere through the air–sea interface and contribute to photochemical pollution, affect the cloud properties, the radiative forcing and precipitation. Despite the improved understanding of the temporal and spatial distribution of marine trace gases of biogenic origin and their potential effects, further investigation is needed in different geographical regions and especially in polluted marine environments and populated coastal regions (Tinel et al., 2023). In this study we estimate the spatiotemporal distribution of seawater VOC concentrations in the climate sensitive geographical region of Eastern Mediterranean. State-of-the art empirical models linking remotely-sensed data of phytoplankton biomass (EU Copernicus Marine Environment Monitoring Service, CMEMS) and environmental parameters such as sea-surface temperature, and photosynthetically available radiation are used (Gali et al., 2018). Ocean-model data such as mixed layer depth, and euphotic zone are also exploited. The impact of the sea-to-air VOC emission fluxes on photochemistry, marine aerosols and cloud properties are assessed and quantified through advanced atmospheric simulations with the WRF-Chem atmospheric model coupled to chemistry and aerosols during typical conditions but also extreme events.

 

 

References

Gali M., Levasseur, M., Devred, E., Simo, R. and Babin, M., Sea-surface dimethylsulfide (DMS) concentration from satellite data at global and regional scales, Biogeosciences, 15, 2018, pp. 3497-3519, https://bg.copernicus.org/articles/15/3497/2018 , doi:10.5194/bg-15-3497-2018.

Tinel L., J. Abbatt, E. Saltzman, A. Engel, R. Fernandez, et al.. Impacts of ocean biogeochemistry on atmospheric chemistry. Elementa: Science of the Anthropocene, 2023, 11 (1), ff10.1525/elementa.2023.00032ff. ffhal-04221390f

 

How to cite: Bossioli, E., Kourakos, D., Raitsos, D. E., Kournopoulou, A., Karagiorgos, J., Methymaki, G., Portalakis, P., Karatasou, S., and Sofianos, S.: Impact of marine biogenic VOC emissions on the marine boundary layer of the Eastern Mediterranean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20277, https://doi.org/10.5194/egusphere-egu24-20277, 2024.

Current Feedback (CFB) and Thermal Feedback (TFB) strongly influence atmospheric and oceanic dynamics at the oceanic mesoscale (O(10-250) km). At smaller scales, oceanic submesoscale currents (O(0.1-10 km)) play a major role in the ocean's energy budget, variability, and ecosystems. However, air-sea interactions at the submesoscale are not well understood due to observational and modeling limitations related to their scales. 
 
This talk addresses this gap by using submesoscale coupled ocean-atmosphere models.  These models are implemented over diverse regions characterized by distinct physical properties. The findings provide compelling evidence that submesoscale modulation affects both the atmosphere and oceanic dynamics. Both TFB and CFB significantly modulate low-level wind curl and divergence as well as momentum and heat fluxes between the ocean and the atmosphere, with a direct impact on the oceanic submesoscale energy budget.

How to cite: Renault, L.: Submesoscale air-sea interactions: atmospheric response and impacts on the ocean dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20391, https://doi.org/10.5194/egusphere-egu24-20391, 2024.

With the aim of studying the momentum flux in wind-wave modulation situations, a 7 month field experiment was set-up in 2023 from the Belle-Ile-en-Mer island off the West coast of France. The site was selected for its exposure to dominant wind and swell, the proximity of a wave buoy, and the rapidly increasing water depth to allow a focus on deep to intermediate wave dispersion regimes. A scanning wind LiDAR [1,2] installed on the coast of the island was used to measure the vertical profile of the horizontal wind speed and direction from 1 to 3 kilometers from the coast. The configuration allowed for measurements of the wind speed and direction profiles starting at some meter above the water surface and going up to some 150m. This original approach enables to obtain quasi-instantaneous vertical planes of the wind speed as well as 30-min mean profiles simultaneously with a wave parameter.

The wide range of wind and wave combinations observed during the deployment allows statistical analysis. Significant wave heights, wave peak periods, and U10 wind speeds were observed in the range 0-6.5 m, 2-20 s, and 0.5-18 m/s, respectively. From this rich database, the near-surface momentum flux estimated by the wind profile close to the water surface appears to match well with results from COARE 3.5 algorithm. The possibility of the scanning wind LiDAR to measure mean wind profiles allows an original point of view to analyze wind-wave interactions. It was observed that for young seas, the profile can be in equilibrium, following Monin-Obukov similarity theory from close to the water surface up to some 100m. In contrast, for fast-travelling waves, significant deviations of the wind profile are observed compared to the surface fluxes. These deviations are parametrized as function of height and analyzed as function of the wave age.

[1] Paskin, L., Conan, B., Perignon, Y., & Aubrun, S. (2022). Evidence of Ocean Waves Signature in the Space–Time Turbulent Spectra of the Lower Marine Atmosphere Measured by a Scanning LiDAR. Remote Sensing, 14(13), 3007.

[2] Conan, B., & Visich, A. (2023). Measurement and analysis of high altitude wind profiles over the sea in a coastal zone using a scanning wind LiDAR–application to wind energy. Wind Energy Science Discussions, 2023, 1-23.

 

How to cite: Conan, B. and Bruch, W.: Analysis of wind profiles above the water surface in wind-wave interaction thanks to a scanning wind LiDAR, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21643, https://doi.org/10.5194/egusphere-egu24-21643, 2024.

EGU24-162 | ECS | Posters on site | OS1.8

Tropical Pacific Quasi-Decadal Variability Suppressed by Submesoscale Eddies  

Yushan Qu, Shengpeng Wang, Zhao Jing, Yu Zhang, Hong Wang, and Lixin Wu

Tropical Pacific quasi-decadal (TPQD) climate variability is characterized by quasi-decadal sea surface temperature variations in the central Pacific. This low-frequency climate variability is suggested to influence extreme regional weather and substantially impact global climate patterns and associated socio-economy through teleconnections. Previous studies mostly attributed the TPQD climate variability to basin-scale air-sea coupling processes. However, due to the coarse resolution of the majority of the observations and climate models, the role of sub-basin-scale processes in modulating the TPQD climate variability is still unclear. Using a long-term high-resolution global climate model, we find that energetic small-scale motions with horizontal scales from tens to hundreds of kilometers (loosely referred to as equatorial submesoscale eddies) act as an important damping effect to retard the TPQD variability. During the positive TPQD events, compound increasing precipitation and warming SST in the equatorial Pacific intensifies the upper ocean stratification and weakens the temperature fronts along the Pacific cold tongue. This suppresses the growth of submesoscale eddies as well as their associated upward vertical heat transport by inhibiting baroclinic instability and frontogenesis; Conversely, during the negative TPQD events, the opposite is true. Using a series of coupled global climate models that participated in the Coupled Model Intercomparison Project Phase 6 with different oceanic resolutions, we show that the amplitude of the TPQD variability becomes smaller as the oceanic resolution becomes finer, providing evidence for the impacts of submesoscale eddies on damping the TPQD variability. Our study suggests that explicitly simulating equatorial submesoscale eddies is necessary for gaining a more robust understanding of low-frequency tropical climate variability.

How to cite: Qu, Y., Wang, S., Jing, Z., Zhang, Y., Wang, H., and Wu, L.: Tropical Pacific Quasi-Decadal Variability Suppressed by Submesoscale Eddies , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-162, https://doi.org/10.5194/egusphere-egu24-162, 2024.

The sensitivity of the sea surface height anomaly (SSHA) forecasting on the accuracy of mesoscale eddies over the Kuroshio Extension region, which was
determined by the conditional non-linear optimal perturbation (CNOP) method using a two-layer quasigeostrophic model, is evaluated by adopting multiply realistic marine datasets through an advanced particle filter assimilation method. It is shown that, if additional observations are preferentially assimilated to the sensitive area of mesoscale eddies identified by the CNOP, where the eddies present a clear high- to low-velocity gradient along the eddy rotation, the forecasting skill of the SSHA can be more significantly improved. It is also demonstrated that the forecasts of the SSHA in the region where the large-scale mean flow possesses much stronger barotropic and/or baroclinic instability tend to exhibit stronger sensitivity to the accuracy of the initial field in the sensitive area of mesoscale eddies. Therefore, more attention should be preferentially paid to the assimilation of the additional observations of the mesoscale eddies for the SSHA forecast in the region with a strong velocity shear of ocean circulation. The present study verifies the sensitivity on mesoscale eddies of SSHA forecasts derived by the two-layer quasigeostrophic model using multiply sets of realistic oceanic data, especially including observation and reanalysis data, which further additionally demonstrates the importance of targeted observations of mesoscale eddies to the SSHA forecast in the regions of strong velocity shear of ocean circulation and provides a more credible scientific basis for the field campaign of the targeted observations for mesoscale eddies associated with the SSHA forecasting.

How to cite: Jiang, L., Duan, W., and Liu, H.: The Most Sensitive Initial Error of Sea Surface Height Anomaly Forecasts andIts Implication for Target Observations of Mesoscale Eddies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2229, https://doi.org/10.5194/egusphere-egu24-2229, 2024.

EGU24-6196 | ECS | Orals | OS1.8

A model perspective on the drivers of the shallow oxygen minimum zone off the northwestern African coast 

Cláudio Cardoso, Paulo Calil, Rui M. A. Caldeira, and Álvaro Peliz

The Oxygen Minimum Zone (OMZ) off the coast of Mauritania and Senegal is characterized by a shallow and a deep oxygen minimum, each with potentially different formation mechanisms. Although the shallow OMZ has been linked to the en-route degradation of organic matter within highly productive, coastal-generated eddies, less attention has been paid to hypoxic Dissolved Oxygen (DO) concentrations observed along the coastal region, where low-oxygen eddies are formed. This study aims to clarify the spatio-temporal dynamics and underlying mechanisms that lead to the formation of the shallow OMZ along the northwestern African coast.

To achieve this, a Eulerian-Lagrangian numerical framework was employed by combining a coupled physical-biogeochemical model with a Lagrangian particle-tracking simulation. The model domain covers the entire Tropical Atlantic with an horizontal resolution of 3 km, achieving a good representation of the horizontal and vertical structure of the North Atlantic OMZ. To assess the pathways and evolution of the water masses that form the shallow OMZ, lagrangian particles were released in grid cells with DO < 40 μmol.l-1 and traced backwards in time.

Our results reveal distinct seasonal and latitudinal variations of DO concentrations along the coast, with DO concentrations significantly decreasing in the transition from the upwelling to the relaxation season (from May to July). Associated with the transport of more oxygenated South Atlantic Central Waters (SACW), the influence of the Poleward Undercurrent (PUC) on the ventilation of the coastal region is evident, especially when the current loses intensity and becomes a surface-intensified feature in summer. When the PUC reaches its maximum intensity in autumn, its core deepens below the mixed layer and replaces the older, oxygen-poor waters with ventilated waters of southern origin.

The impact of eddies on coastal dynamics was also explored. A quasi-permanent Anticyclonic Modewater Eddy (ACME) formed during the upwelling season by the interaction of the PUC with the Cap-Vert headland is the main mechanism behind the import of offshore waters to the coastal region. Lagrangian particle trajectories suggest that this eddy prevents the direct northward transport of SACW by the PUC. Whilst some of the particles are trapped and subsequently transported offshore inside the eddy, other particles are stirred with an older, less oxygenated SACW variety in the offshore region and re-circulate to the coastal region. Similar particle re-circulation patterns are also observed further north, coinciding with cyclonic and ACME formation hotspots.

Our findings suggest that in addition to their role in the formation and advection of oxygen-depleted waters to offshore, coastal-generated eddies play a crucial role in modulating DO levels along the northwestern African coast.

How to cite: Cardoso, C., Calil, P., Caldeira, R. M. A., and Peliz, Á.: A model perspective on the drivers of the shallow oxygen minimum zone off the northwestern African coast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6196, https://doi.org/10.5194/egusphere-egu24-6196, 2024.

EGU24-6690 | Orals | OS1.8

Parameterizing mesoscale eddy buoyancy transport over sloping topography 

Aleksi Nummelin and Pål Erik Isachsen
Models that do not resolve the mesoscale eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large-scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models (ESMs) rely on an assumption of a flat ocean floor even though observations and high-resolution modelling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. Using a hierarchy of model complexities, we show that (i) the buoyancy transport coefficient diagnosed from idealized eddy-resolving simulations is indeed reduced over bottom slopes (ii) such reduction can be skillfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale (iii) implementing such a modified `GM' parameterization in non-eddying simulations enhances the strength of thermal wind currents over the bottom slopes. 
 
Testing the new parameterization in realistic global coarse-resolution simulations shows that the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean-flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean tracer biases. We find that most of the tracer bias reduction follows from changing the buoyancy transport coefficient (GM), but we also discuss the impact of applying similar changes to the tracer mixing coefficient (Redi).

How to cite: Nummelin, A. and Isachsen, P. E.: Parameterizing mesoscale eddy buoyancy transport over sloping topography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6690, https://doi.org/10.5194/egusphere-egu24-6690, 2024.

The ocean surface mixed layer represents a critical interface linking the ocean and atmosphere. The physical processes determining the surface mixed layer properties and mediate atmosphere-ocean exchange. Submesoscale processes play a key role in cross-scale oceanic energy transformation and the determination of surface mixed-layer properties, including the enhancement of vertical nutrient transport, leading to increased primary productivity. Herein, we presented observations of the spiral chlorophyll-a filament and its influence on turbulence within an anticyclonic eddy in the western South China Sea during August 2021. The filament had a negative Ertel potential vorticity associated with strong upwelled/downward currents (approximately 20-40 m/day). Across-filament sections of the in-situ profiles showed turbulent dissipation rates enhanced in the filament. We suggested this enhancement values can be attributed to submesoscale processes, which accounted for 25% of the total parameterized turbulent dissipation rates. The present parametrized submesoscale turbulent scheme overestimated the in-situ values. The filament transferred kinetic energy upward to anticyclonic eddy via barotropic instability and gained energy from the anticyclonic eddy via baroclinic instability. After kinetic energy budget diagnostic, we suggested besides symmetric instability, centrifugal instability and mixed layer baroclinic instability should also be included in the turbulence scheme to overcome the overestimation. The observed dual energy transfers between the anticyclonic eddy and filament, and the observed high turbulent energy dissipation within the filament, emphasized the need for these processes to be accurately parameterized regional and climate models. 

How to cite: Qiu, C. and Wang, D.: Observational energy transfers of a spiral cold filament within an anticyclonic eddy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6911, https://doi.org/10.5194/egusphere-egu24-6911, 2024.

EGU24-7540 | Posters on site | OS1.8

Spatial variations of eddy vertical structure and energy in the southeastern Indian Ocean 

Yinghui He, Qingyou He, Tongya Liu, and Shuqun Cai

Satellite observations demonstrate that mesoscale eddies are active and show significant spatial variability of surface features in the southeastern Indian Ocean (SEIO). Combining the satellite observation and Argo floats data, this study reveals the spatial variation of eddy vertical structure and volume-integrated energy in the SEIO. The sources of surface-intensified and subsurface-intensified eddies correspond well to the mean current systems. The surface-intensified cyclonic eddies (CEs) mainly originate from the South Indian Countercurrent system, whose density core is at a depth of ≈70 m, and subsurface-intensified CEs mainly originate from the Leeuwin Current system (LCS, ≈10° longitude off the eastern boundary of south Indian Ocean) and the SEIO interior south of 30°S, whose density core and maximum velocity are at depths of ≈750 m and ≈290 m, respectively. The surface-intensified anti-cyclonic eddies (AEs) widely originate from the entire region of SEIO, whose density core is at a depth of ≈110 m, while the sources of subsurface-intensified AEs only scatter in a few regions. The eddy lifespan in the SEIO is significantly correlated with the eddy volume-integrated energy. The most important factor affecting the spatial variability of eddy energy in the SEIO is eddy vertical structure, followed by the eddy amplitude. Finally, by investigating the performance of two reanalysis data in eddy statistical properties, we find that the biases of eddy lifespan and movement distance in the LCS is caused by the bias of eddy vertical structure. This further confirms the impact of the eddy vertical structure on the eddy evolution.

How to cite: He, Y., He, Q., Liu, T., and Cai, S.: Spatial variations of eddy vertical structure and energy in the southeastern Indian Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7540, https://doi.org/10.5194/egusphere-egu24-7540, 2024.

EGU24-9815 | ECS | Posters on site | OS1.8

Characterization, distribution, and evolution of surface fronts in the Amazon Plume region 

Dante Napolitano, Jonathan Gula, Solange Coadou-Chaventon, Xavier Carton, and Sabrina Speich

The Amazon River runoff reigns absolute as the most prominent river discharge to the ocean, with about 0.2 Sv of freshwater entering the Northwest Atlantic. The Amazon River outflow together with the North Brazil Current (NBC), dominates the low sea surface salinity spread into the open ocean. At the edge of the river plume, stirring by the NBC and its eddies generates sharp gradients at scales from Ο(0.1-100) km. These (equatorial to tropical) submesoscale fronts are important, for example, in modulating air-sea interactions and the energy cascade. In the EUREC4A-OA project, we use state-of-the-art Saildrone observations and numerical simulation to assess surface gradients in the northwestern tropical Atlantic. Our objective is to provide a comprehensive picture of surface gradients and associated fronts in the Amazon Plume region. From observations, we find that the plume influences density gradients from scales l < 30 km; sharp gradients skyrocket within the plume at l < 10 km, a scale that has recently been shown to mark a shift from an inverse to a forward energy cascade. Using a Δx ≅1 km CROCO simulation, we assess the spatial distribution of surface fronts and their spatio-temporal variability. Salinity dominates surface gradients even outside the plume due to an almost permanent barrier layer formed by mixing of low salinity water from previous seasons. Near the shelf, the Amazon runoff controls the formation and evolution of fronts. As we move poleward, the NBC dictates the distribution of the surface fronts. The influence of the NBC gradually decreases until the distribution of fronts closely follows the mixed layer dynamics.

How to cite: Napolitano, D., Gula, J., Coadou-Chaventon, S., Carton, X., and Speich, S.: Characterization, distribution, and evolution of surface fronts in the Amazon Plume region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9815, https://doi.org/10.5194/egusphere-egu24-9815, 2024.

EGU24-10572 | ECS | Posters on site | OS1.8

On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic 

Pablo Fernández, Sabrina Speich, Hugo Bellenger, Diego Lange Vega, Johannes Karstensen, Dongxiao Zhang, and Cesar Barbedo Rocha

The Northwest Tropical Atlantic (NWTA) is a region with complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8ºN that leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km2. These two processes affect the spatial variability of the region's surface latent heat flux (LHF). First, the presence of surface freshwater modifies the vertical stratification of the ocean limiting the amount of heat that can be released to the atmosphere. Second, they create a highly heterogeneous mesoscale sea-surface temperature (SST) field that directly influences near-surface atmospheric circulation. These effects are illustrated by observations from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W·m2, of which 100 W·m2 are associated with wind speed changes and 40 W·m2 with SST variations. Surface currents or stratification-change associated heat release remain as second-order contributions with LHF variations of less than 10 W·m2 each. The results highlight the importance of considering these three components to properly characterize LHF variability at different spatial scales.

How to cite: Fernández, P., Speich, S., Bellenger, H., Lange Vega, D., Karstensen, J., Zhang, D., and Barbedo Rocha, C.: On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10572, https://doi.org/10.5194/egusphere-egu24-10572, 2024.

EGU24-11486 | ECS | Posters on site | OS1.8

Unraveling the Eddy-driven Heat Transport in the Agulhas Leakage Region 

Lansu Wei and Chunzai Wang

The Agulhas leakage, which transports warm and salty Indian Ocean water into the Atlantic Ocean, plays a crucial role in global ocean circulation and climate. The mesoscale eddies from the leakage supply the primary source of heat and salt for the Atlantic meridional overturning circulation. This study combines eddy data with Argo profiles from 1993 to 2018 to investigate the three-dimensional structures of eddies, advancing our understanding of eddy-induced transport. Our analysis revealed that both the trapping and stirring processes of eddies influence eddy-induced transport. Anticyclonic eddies are found to transport heat in the meridional direction mainly through propagation (~60%). On the other hand, cyclonic eddies transport heat meridionally to the Atlantic Ocean primarily through the stirring of isotherms in the background field (~25%). These results further confirm that the stirring effect of cyclonic eddies is crucial for evaluating the impact of the Agulhas leakage on the Atlantic Ocean.

How to cite: Wei, L. and Wang, C.: Unraveling the Eddy-driven Heat Transport in the Agulhas Leakage Region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11486, https://doi.org/10.5194/egusphere-egu24-11486, 2024.

EGU24-12815 | Orals | OS1.8

Central structure of a Mozambique Channel mesoscale eddy-ring dipole 

Pierrick Penven, Jean Francois Ternon, Margaux Noyon, and Steven Herbette

Located in the southwest Indian Ocean, between Madagascar and the African continent, the Mozambique Channel is a western boundary current system characterized by an intense eddy activity (Halo et al. 2014). Large anticyclonic rings, reaching up to 300 to 350 km in diameter and 2000 m of vertical extension, are structuring the marine ecosystems from phytoplankton to top predators (de Ruijter et al., 2002; Ternon et al., 2014, Weimerskirch et al., 2004). They impact the environmental conditions on the Mozambican shelves by promoting the upwelling of nutrient rich deeper waters (Lamont et al., 2010; Malauene et al, 2014). Coastal waters, generally rich in plankton and nutrients, can be also be transported offshore along the edges of the rings. The occurrence of an eddy dipole with the anticyclonic ring in the northern side of the cyclonic eddy can enhance the processes (Roberts et al., 2014). Mesoscale eddy flux is supposed to be the dominant source of nutrients for the central Mozambique Channel (José et al., 2016). The first leg of the RESILIENCE (fRonts, EddieS and marIne LIfe in the wEstern iNdian oCEan) multidisciplinary oceanographic cruise on board R/V Marion Dufresne II in April-May 2022 was focusing on the central structure of a dipole composed by a Mozambique Channel Ring and a cyclonic spiral eddy. The goals were here to observe at high resolution the mesoscale and submesoscale structures in the core of the dipole, their origins and evolution, and their potential implications for biogeochemical and ecological processes in the Mozambique Channel. To do so we crossed several times the eastern side of the dipole, towing a moving vessel profiler in addition to SADCP continuous observations and multidisciplinary stations and trawls at regular intervals. The dipole event commenced on 24 April 2022 and endured for 24 days. Existence of strong currents, reaching speeds of 150 cm/s, leads to the prevalence of horizontal stirring as the dominant process. This results in an efficient and fast transport of material from the shelf to the central Mozambique Channel. The Omega equation was used to show the dominance of a smaller scale meander for the vertical velocities. Layering is evident in the frontal structure. This first documentation of the in-situ central structure of a dipole, formed by the convergence of a Mozambique Channel Ring and a spiral eddy, lays the foundation for subsequent ecological investigations.

How to cite: Penven, P., Ternon, J. F., Noyon, M., and Herbette, S.: Central structure of a Mozambique Channel mesoscale eddy-ring dipole, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12815, https://doi.org/10.5194/egusphere-egu24-12815, 2024.

EGU24-12927 | ECS | Orals | OS1.8

Surface circulation and marine debris: exploring the impact of northwestern African upwelling on offshore transport 

Luuk Rader, Borja Aguiar-González, Timothy Price, Eugenio Fraile-Nuez, Daura Vega-Moreno, and Francisco Machín

Amidst the global challenge of plastic pollution, the marine environment surrounding the Canary Islands is not immune to this pressing issue. Besides, the northwestern African upwelling system is an ideal environment for fisheries, which eventually become potential contributors to marine floating debris. Entanglement in large marine floating debris of fisheries origin represents a prevalent cause of stranding incidents for sea turtles. However, connecting the fisheries activity with the offshore flow of this debris towards the open ocean and the Canary Islands proves challenging due to the high mesoscale variability in the region, which hampers a straightforward visualization of clear patterns of distribution.

This study aims to investigate the offshore transport of marine floating debris originating from the upwelling zone and elucidate the underlying driving mechanisms. Additionally, the study also aims to uncover the upwelling-related origins of marine debris observed in proximity to the Canary Islands.

To analyse the oceanward transport of marine debris, OceanParcels is used, a Lagrangian tool to estimate the trajectories of virtual particles released into the ocean. These particles are released along the African coast, and their trajectories are computed following two different approaches. Firstly, seasonally averaged surface velocities are used to account for the mean seasonal fields leading to the marine debris distribution. Secondly, daily-varying surface velocities are used to simulate real ocean conditions as closely as possible. Jointly, these views provide insights into the key features responsible for transporting particles offshore. Lastly, Stokes drift is incorporated to account for its impact on particle trajectories.

The results using seasonally-averaged surface velocities reveal the formation of offshore-orientated corridors through which particles, representing marine debris, are advected oceanward. This is confirmed following the daily-varying simulations. These corridors are hypothesized to be formed by the recurrent detachment of the coastal jet stream at certain key locations of the African coastline, then leading the transport of marine debris offshore. Furthermore, virtual particles are observed that are advected offshore via upwelling filaments, i.e. cold-water tongues that extend oceanward from the inner continental shelf. Importantly, Stokes drift appears to counterwork the offshore transport of marine debris likely due to a prevailing strong southward and coastward surface advection. However, it is noted that accounting for the Stokes drift is an ongoing field of research and its effect may be overestimated as currently implemented.

On the one hand, the upwelling zone north of Cape Ghir seems to be responsible for the largest amount of upwelling-related marine debris of a northern origin, reaching the Canary Islands through a northeast-to-southwest orientated corridor. On the other hand, the upwelling zone between Cape Ghir and Cape Bojador appears to be mostly responsible for the marine debris reaching the Canary Islands with a southern origin.

How to cite: Rader, L., Aguiar-González, B., Price, T., Fraile-Nuez, E., Vega-Moreno, D., and Machín, F.: Surface circulation and marine debris: exploring the impact of northwestern African upwelling on offshore transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12927, https://doi.org/10.5194/egusphere-egu24-12927, 2024.

EGU24-14170 | Posters on site | OS1.8

Eddy covariance measurements of air-sea heat and momentum fluxes under tropical cyclones and hurricanes in the northwest Tropical Atlantic 

Dongxiao Zhang, Gregory Foltz, Chidong Zhang, Chris Fairall, Jun Zhang, Hyun-Sook Kim, Avichal Mehra, Andrew Chiodi, Meghan Cronin, Elizabeth Thompson, Jim Thomson, Lev Looney, Nan-Hsun Chi, Hauke Schulz, Ajda Savarin, and Edoardo Mazza

Tropical cyclones (TCs) and hurricanes are among the strongest Mesoscale Convection Systems originating from the tropical oceans and can cause significant loss of lives and properties when landing. Prediction of TCs, especially their rapid intensification, remains challenging for numerical forecasts. Theoretical and modeling studies have shown that the surface turbulence heat flux fuels hurricane intensification, while the momentum flux or wind stress transfers the kinetic energy from the storm to the ocean to regulate the ocean mixing and stratification which in turn affect the Sea Surface Temperature and heat flux. The balance between the surface enthalpy flux (sum of sensible and latent heat flux) and drag plays a critical role in the TC and hurricane intensification. Due to the lack of direct observations inside the TCs and hurricanes, studies largely based on numerical models, lab experiments, air-deployed dropsondes, and indirectly from momentum budget analysis, have suggested a large deviation of wind stress and drag coefficients at high wind speed of > 20 m/s in TC and hurricane conditions. During the 2021-2023 hurricane seasons, a fleet of 5-12 Saildrone Uncrewed Surface Vehicles (USVs) have been deployed each year to intercept the TCs and hurricanes to make direct observations of the extreme air-sea interaction process. They provided real-time 1-minute averages of near-surface meteorology and ocean variables (5-minute for ocean currents) to hurricane forecast centers. This study utilizes the high-resolution 20-Hz data made available once the Saildrone USVs returned from their cruises after the hurricane season to investigate direct eddy covariance (EC) measurements of wind stress for a better understanding of the drag coefficients under TC and hurricanes. The directly observed drag coefficient, as well as the EC heat transfer coefficient (for sensible heat flux), will be compared to those used in the bulk flux algorithm (COARE) and in forecast models. Particular attention will be paid to the variations in different wind and wave conditions within the mesoscale system.

How to cite: Zhang, D., Foltz, G., Zhang, C., Fairall, C., Zhang, J., Kim, H.-S., Mehra, A., Chiodi, A., Cronin, M., Thompson, E., Thomson, J., Looney, L., Chi, N.-H., Schulz, H., Savarin, A., and Mazza, E.: Eddy covariance measurements of air-sea heat and momentum fluxes under tropical cyclones and hurricanes in the northwest Tropical Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14170, https://doi.org/10.5194/egusphere-egu24-14170, 2024.

Mesoscale SST perturbations induced wind stress field perturbations have feedback effect on the ocean through influencing air-sea heat and momentum fluxes. Unlike the thermal feedback mechanism that is well understood, momentum feedback still needs to be studied, especially about the respective roles of divergent and rotational wind components. In this study, momentum feedback was examined using an ocean model and an empirical equation, which solved wind stress field perturbations from their divergence and curl that were estimated from time-evolving downwind and crosswind SST gradients. Through several numerical experiments, it was found that the divergent wind can induce positive and negative SST changes at varying regions and depths. On the contrary, the rotational wind can cool the upper ocean and reduce SST by 0.1°C on average.

How to cite: Wei, Y.: Cooling effect of mesoscale SST perturbations induced rotational wind in the Kuroshio Extension, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14267, https://doi.org/10.5194/egusphere-egu24-14267, 2024.

EGU24-16202 | ECS | Orals | OS1.8

What happens when an inertially unstable jet approaches a lateral boundary? 

Matheus Ferreira Azevedo, Francis Poulin, and Kevin Lamb

Much of our understanding of inertial instability in geophysical flows comes from atmospheric physics, and these studies have neglected the impact of lateral boundaries. To address this shortcoming, we performed a series of high-resolution 3D numerical simulations in Oceananigans in the context of the nonhydrostatic Boussinesq equations assuming a rigid-lid approximation. An inertially unstable baroclinic jet was investigated both far away and adjacent to a vertical boundary. The jet was chosen to be in thermal-wind balance and the buoyancy field was perturbed to instigate the instability.

We found that when the unstable jet is sufficiently close to the vertical boundary, the wavenumber of the fastest-growing unstable mode nearly doubled when compared to the jet far away from the boundary. We have not observed this shift to smaller scales in the context of a barotropic jet. The growth rates of the instability, measured by taking the l2 norm of the velocity components, showed an initial linear growth phase in the first few days with no significant differences regarding the positioning of the jet. After this period, non-linear saturation stabilized the jet to inertial instability, and a secondary baroclinic instability developed. These findings suggest a previously unaccounted factor that can influence the bio-physicochemical properties of the ocean in proximity to coastal boundaries, contributing to the current understanding of the importance of submesoscale phenomena.

How to cite: Ferreira Azevedo, M., Poulin, F., and Lamb, K.: What happens when an inertially unstable jet approaches a lateral boundary?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16202, https://doi.org/10.5194/egusphere-egu24-16202, 2024.

EGU24-19188 | Orals | OS1.8

Multi-platform high resolution in situ observations for understanding mesoscale and sub-mesoscale processes and their role in the air-sea exchanges: Experiences and prospects from the EUREC4A-OA/ATOMIC field experiment 

Sabrina Speich, Johannes Karstensen, Xavier Carton, César Barbedo Rocha, Hugo Bellenger, Claudia Pasquero, Antonio Parodi, Jonathan Gula, Denis Bourras, Richard Davy, Lionel renault, Anna del Moral-Méndez, Dongxiao Zhang, Chris Fairall, David Farrell, Jin-Song von Storch, Hervé Giordani, Gilles Reverdin, Jochen Horstmann, and Noel Keenlyside and the EUREC4A-OA/ATOMIC Ocean-atmosphere processes

In January-February 2020, the EUREC4A-OA/ATOMIC experiment took place in the Northwest Tropical Atlantic with the overall goal of understanding the role of fine-scale processes in internal ocean dynamics and air-sea interaction. Four oceanographic ships, the French Atalante, the German Maria S. Merian and Meteor, and the US Ron Brown, were closely coordinated with airborne observations and autonomous ocean platforms (gliders, ©Saildrones, Argo floats, and drifters) to simultaneously measure the ocean and atmosphere from east of Barbados to the northern border of French Guyana. The multiple observations of the ocean, atmosphere, and their interface have revealed more complex ocean dynamics than expected, in particular a strong interaction between the Amazon River outflow (despite its reduced winter discharge), the North Brazil Current (NBC), and several mesoscale eddies (including the highly energetic NBC rings). This leads to even richer submesoscale dynamics that shape an important fraction of the air-sea exchange of heat, momentum, and CO2, and efficiently isolates the NBC northward flow waters from intense and continuous interactions with the atmosphere. Owing to the many complementary observations from ships and autonomous platforms, we have been able to quantify some of these processes, including the diurnal cycle and the 3D dynamics of different mesoscale eddies, as well as to map and quantify different terms of the air-sea fluxes and their impacts on the marine atmospheric boundary-layer water budget. The results have been widely used not only to validate numerical simulations of the region, but also to guide their analyses and to improve various numerical parameterizations.

The collection of these observations was the result of an important international coordination between many different groups of ocean and atmospheric scientists. In addition, the special strategy for targeted data collection of meso- and submesoscale processes relied on daily planning of the field experiment and on detailed analysis of the near-real-time satellite data and the observations already obtained during the experiment, which was essential for providing the right snapshots of the ocean and atmosphere for the quantification of many processes. The lessons learned from this experiment will be implemented and extended in the upcoming major high-resolution oceanographic endeavor, the WHIRLS experiment, which will take place in June-July 2025, southwest of Africa.

How to cite: Speich, S., Karstensen, J., Carton, X., Barbedo Rocha, C., Bellenger, H., Pasquero, C., Parodi, A., Gula, J., Bourras, D., Davy, R., renault, L., del Moral-Méndez, A., Zhang, D., Fairall, C., Farrell, D., von Storch, J.-S., Giordani, H., Reverdin, G., Horstmann, J., and Keenlyside, N. and the EUREC4A-OA/ATOMIC Ocean-atmosphere processes: Multi-platform high resolution in situ observations for understanding mesoscale and sub-mesoscale processes and their role in the air-sea exchanges: Experiences and prospects from the EUREC4A-OA/ATOMIC field experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19188, https://doi.org/10.5194/egusphere-egu24-19188, 2024.

EGU24-19696 | ECS | Orals | OS1.8

Coupled climate effects of eddy rich model resolution in and south of the Agulhas 

Malin Ödalen, Abhishek Savita, Joakim Kjellsson, Sebastian Wahl, David Ferreira, Holly Ayres, Fabien Roquet, and Wonsun Park

In this study, we compare global coupled climate simulations (1950’s and abrupt 4xCO2) with different ocean resolution in the Atlantic sector of the Southern Ocean (ASO), including the Agulhas, with parameterised and explicitly simulated eddies respectively. We find that the eddy-rich 1950’s simulation has a reduced South Atlantic warm bias, because of a more defined Agulhas retroflection, and coupled climate effects are observed outside the region with increased ocean resolution where e.g. equatorial precipitation changes markedly.

The Agulhas leakage plays a key role in connecting the Indian and the Atlantic oceans, with mesoscale eddies carrying heat and salt into the South Atlantic. In most state-of-the-art coupled climate models, the ocean resolution is insufficient to explicitly simulate those eddies, and they are instead represented through a parameterisation of the eddy induced flow. We use the coupled climate model FOCI, which combines a NEMO3.6 ocean with an ECHAM6 atmosphere, LIM2 sea ice, and a JSBACH land module, via an OASIS coupler. Through AGRIF nesting, we increase the ocean resolution from 1/2° to 1/10° in the Atlantic sector of the Southern Ocean.

The eddy-rich 1950’s simulation exhibits a reduced warm bias in the South Atlantic compared to the simulation without it. The bias reduction is a result of a more defined Agulhas retroflection which reduces ocean heat transport into the South Atlantic while increasing heat transport poleward. This change in ocean temperature distribution is anticipated from previous studies with ocean-only models. However, we also see coupled climate effects extending to the equatorial region, well outside the region with increased ocean resolution. We observe changes in precipitation and surface wind fields over both the tropical Atlantic and tropical/South Pacific. The changes over the tropical Atlantic are likely linked to a direct response to changes in sea surface temperature that extend across the South Atlantic. The eddy-rich 1950’s simulation also shows significant reduction of surface air temperature (SAT) biases, mostly in the Northern Hemisphere, and winds in the Southern Hemisphere, w.r.t. observationally based reanalysis products. In the strong warming scenario (abrupt 4xCO2), the eddy-rich simulation shows less SAT increase over the Atlantic and a larger seasonality in the response of the westerly wind fields over the Southern Ocean. In conclusion, increased resolution of the ASO, allowing for explicit simulation of mesoscale eddies e.g. in the Agulhas, leads to reduction of model biases and coupled climate effects.

How to cite: Ödalen, M., Savita, A., Kjellsson, J., Wahl, S., Ferreira, D., Ayres, H., Roquet, F., and Park, W.: Coupled climate effects of eddy rich model resolution in and south of the Agulhas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19696, https://doi.org/10.5194/egusphere-egu24-19696, 2024.

EGU24-19719 | ECS | Orals | OS1.8

An Offline Biogeochemical Model within the Regional Ocean Modelling System (ROMS): application to the Northwestern Mediterranean Sea 

Júlia Crespin Esteve, Jordi Solé Ollé, and Miquel Canals Artigas

Modelling the distribution of biogeochemical components in the ocean is essential for further understanding climate change impacts and assessing the functioning of marine ecosystems. This requires robust and efficient physical-biological simulations of coupled ocean-ecosystem models, which are often hindered by limited data availability and computational resources. The option of running biological tracer fields offline, independently from the physical ocean simulation, is appealing due to increased computational efficiency. Here, we present an assessment and implementation of an offline biogeochemical model — the Offline Fennel model — within the Regional Ocean Modeling System (ROMS). Our methodology employs ROMS hydrodynamic outputs to run the biogeochemical model offline. This work also includes the first evaluation exercise of the referred offline biogeochemical model. We used a variety of skill metrics to compare the simulated surface chlorophyll to an ocean colour dataset (CMEMS-Mediterranean Ocean Colour) and BGC-ARGO floats for the 2015-2020 period. The model is able to reproduce the temporal and spatial structures of the main chlorophyll fluctuation patterns in the study area, the Northwestern Mediterranean Sea, as well as the vertical distribution of chlorophyll and nitrate. This area is of particular interest as it is one of the most productive regions in the entire Mediterranean Basin, with open-ocean upwellings and deep winter convection events occurring seasonally. The typical behaviour of the region is likewise effectively represented in the implementation, including offshore primary production, nutrient supplies from the Rhone and Ebro rivers, and mesoscale hydrographic structures. This study provides a baseline for ROMS users in need of executing more biogeochemical simulations independently from more computationally demanding physical simulations.

How to cite: Crespin Esteve, J., Solé Ollé, J., and Canals Artigas, M.: An Offline Biogeochemical Model within the Regional Ocean Modelling System (ROMS): application to the Northwestern Mediterranean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19719, https://doi.org/10.5194/egusphere-egu24-19719, 2024.

EGU24-1830 | ECS | Posters on site | OS1.9

A Detailed Analysis of the Diahaline Overturning Circulation in a Marginal Sea 

Erika Henell, Hans Burchard, Ulf Gräwe, and Knut Klingbeil

We apply a local water mass transformation framework to quantify and decompose the exchange flow associated with diahaline mixing. As a realistic example we analyze two years of numerical model results for the Baltic Sea, which serves as a natural laboratory for processes relevant on the global scale. Despite this regional focus, the diagnostic methods of this study are applicable to diverse regions, as well as for other tracers than salinity, e.g. temperature. We verify relations between local diahaline volume and diffusive salt fluxes, and local diahaline mixing, and present them as maps on chosen isohaline surfaces. In this way, hot spots for mixing and the diahaline circulation are visualized. Two dominant types of diahaline exchange flow are analyzed. First of all there is a large scale overturning circulation with inflow at places where the isohaline surface is close to the bottom and with outflow at places where the isohaline is surfacing. Secondly, there is the well-known small-scale overturning circulation localized inside the bottom boundary layer over sloping bathymetry, driven by boundary mixing. One major result is that about 50% of the simulated diahaline exchange flow is generated by numerical mixing caused by the truncation error of the advection scheme, despite the fact that an anti-diffusive advection scheme and vertically-adaptive coordinates are used. We also demonstrate how model ensembles can be used to study short-term episodic and local events.

How to cite: Henell, E., Burchard, H., Gräwe, U., and Klingbeil, K.: A Detailed Analysis of the Diahaline Overturning Circulation in a Marginal Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1830, https://doi.org/10.5194/egusphere-egu24-1830, 2024.

The atmospheric circulation response to global warming is an important problem that is theoretically still not well understood. This is a particular issue since climate model simulations provide uncertain, and at times contradic- tory, projections of future climate. In particular, it is still unclear how a warmer and moister atmosphere will affect midlati- tude eddies and their associated poleward transport of heat and moisture. Here we perform a trend analysis of three main components of the global circulation}the zonal-mean state, eddies, and the net energy input into the atmosphere}and examine how they relate in terms of a moist static energy budget for the JRA-55 reanalysis data. A particular emphasis is made on understanding the contribution of moisture to circulation trends. The observed trends are very different between the hemispheres. In the Southern Hemisphere there is an overall strengthening and during boreal summer, also a poleward shifting, of the jet stream, the eddies, and the meridional diabatic heating gradients. Correspondingly, we find an overall strengthening of the meridional gradients of the net atmospheric energy input. In the Northern Hemisphere, the trend pat- terns are more complex, with the dominant signal being a clear boreal winter Arctic amplification of positive trends in lower-tropospheric temperature and moisture, as well as a significant weakening of both bandpass and low-pass eddy heat and moisture fluxes. Consistently, surface latent and sensible heat fluxes, upward and downward longwave radiation, and longwave cloud radiative fluxes at high latitudes show significant trends. However, radiative fluxes and eddy fluxes are in- consistent, suggesting data assimilation procedures need to be improved.

How to cite: Franzke, C. and Harnik, N.: Long-Term Trends of the Atmospheric Circulation and Moist Static Energy Budget in the JRA-55 Reanalysis , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3002, https://doi.org/10.5194/egusphere-egu24-3002, 2024.

It has previously been shown that trends in sensible heat from climate models have had a substantial contribution to global precipitation changes. We illustrate that this is the case also in the most recent Coupled Model Intercomparison Project Phase 6 (CMIP6). However, we find that over the period since 1980 reanalysis do not support the reduction in sensible heat from the CMIP6 models and rather estimate a global increase in sensible heat which would contribute to a precipitation reduction. Satellite data over a period of 2 decades over global ocean similarly to reanalysis show an opposite sign of the sensible heat trend to the CMIP6 models.

How to cite: Myhre, G. and Jouan, C.: Strong contribution from sensible heat to global precipitation increase by climate models is not supported by observational based data. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3902, https://doi.org/10.5194/egusphere-egu24-3902, 2024.

EGU24-4138 | ECS | Posters on site | OS1.9

Robust predictions of changes in evenness of global precipitation under global warming 

Hsin Hsu and Stephan Fueglistaler

Global mean precipitation is anticipated to increase by 2-4% per degree Kelvin, with intense events scaling at 7%, driven by boundary layer humidity. The understanding of the change in daily-to-annual precipitation probability density function remains rather incomplete. To address this knowledge gap, we employ Gini index to evaluate spatial unevenness and temporal inequality of precipitation under global warming in CMIP6 models. We observe heightened spatial unevenness of daily precipitation in tropics and extratropics over land and ocean. While the tropics maintain this unevenness over time, indicating large-scale convection aggregation, extratropical precipitation evens out with increasing timescales. This disparity suggests distinct processes governing daily and annual mean precipitation, underscoring the intensification of stronger storms over weaker events.

 

Globally, temporal inequality is on the rise, with more pronounced intensification in regions where projected precipitation deviates significantly from Clausius–Clapeyron scaling. Our hypothesis posits that the shift in precipitation distribution under warming projections stems from an increase in no-rain days coupled with rainfall events scaled by a constant. To assess this proposition, we construct a toy model predicting projected temporal inequality based on local hydroclimate conditions pre-warming, the projected mean precipitation, and a theorem-derived stretching parameter. The toy model demonstrates robust performance overall, except in regions notably influenced by the Hadley cell. Additionally, the model suggests that local precipitation events are scaled by a constant of approximately 1.07. Our analysis establishes meaningful connections among changes in mean precipitation, precipitation distribution, and dry-day number, offering comprehensive insights into hydroclimate transformations under global warming.

How to cite: Hsu, H. and Fueglistaler, S.: Robust predictions of changes in evenness of global precipitation under global warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4138, https://doi.org/10.5194/egusphere-egu24-4138, 2024.

EGU24-6543 | Orals | OS1.9

Relative role of land and ocean in shaping tropical hydroclimate after large volcanic eruptions 

Claudia Timmreck, Roberta D'Agostino, Shih-Wei Fang, Andrew Ballinger, Gabriele Hegerl, Sarah Kang, Dirk Olonscheck, and Andrew Schurer

Volcanic eruptions substantially impact tropical precipitation over the historical period but they differ in their emission strength, geographical latitude and season of the eruption, which makes it difficult to draw general conclusions. Sufficient large ensembles simulations with the same model and radiative forcing scenario but varying initial conditions have become a great tool in recent years to disentangle forced and internal variability).  Here we use a suite of 100-member ensembles of the MPI-ESM-LR for idealized equatorial and extratropical eruptions of different eruption strengths and an additional 100-member ensemble without forcing. We find that precipitation reduction is primarily energetically constrained by less atmospheric net energy input (NEI).  NEI decreases rapidly in the first months after the eruption due to reduced incoming solar radiation and then the circulation weaken as a consequence of less moist static energy (MSE) exported away from the intertropical convergence zone. Only afterwards, when the overturning has already weakened, the MSE, and then the gross moist stability (GMS) contribute stronger to the precipitation reduction. Tropical precipitation over land reacts immediately to forcing changes, while the precipitation response over the ocean and the temperature response have much longer response times. Altered dry-wet pattern (“wet gets drier”) and the decreased monsoon precipitation are strongly tied to the weakening of the regional tropical overturning. Differences related to the geographical locations of the volcanic eruptions will be highlighted.

How to cite: Timmreck, C., D'Agostino, R., Fang, S.-W., Ballinger, A., Hegerl, G., Kang, S., Olonscheck, D., and Schurer, A.: Relative role of land and ocean in shaping tropical hydroclimate after large volcanic eruptions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6543, https://doi.org/10.5194/egusphere-egu24-6543, 2024.

The ocean temperature response to tropical cyclones (TCs) is important for TC development, local air–sea interactions, and the global air–sea heat budget and transport. As TCs and ocean temperature structures are changing in the recent decades, it is worthy to study their contribution on ocean heat uptake. The modulation of the upper ocean temperature structure after TCs were studied at the observation stations in the northern South China Sea. The upper ocean temperature and heat response to the TCs mainly depend on the combined effect of mixing and vertical advection. Mixing cooled the sea surface and warmed the subsurface, while upwelling (downwelling) reduced (increased) the subsurface warm anomaly and cooled (warmed) the deeper ocean. An ideal parameterization that depends on only the nondimensional mixing depth (HE), non-dimensional transition layer thickness (HT), and nondimensional upwelling depth (HU) was able to roughly reproduce sea surface temperature (SST) and upper ocean heat change. After TCs, the subsurface heat anomalies moved into the deeper ocean. The air–sea surface heat flux contributed little to the upper ocean temperature anomaly during the TC forcing stage and did not recover the surface ocean back to pre-TC conditions more than one and a half months after the TC. This work shows how upper ocean temperature and heat content varies by a TC, indicating that TC-induced mixing modulates the warm surface water into the subsurface, and TC-induced advection further modulates the warm water into the deeper ocean and influences the local and global ocean heat budget.

How to cite: Zhang, H.: Modulation of Ocean Temperature Structure and Heat Content by Tropical Cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6938, https://doi.org/10.5194/egusphere-egu24-6938, 2024.

EGU24-7000 | ECS | Orals | OS1.9

Ocean heat uptake and interbasin redistribution driven by anthropogenic aerosols and greenhouse gases 

Shouwei Li, Wei Liu, Robert J. Allen, Jia-Rui Shi, and Laifang Li

Anthropogenic aerosols and greenhouse gases have played important roles in modulating the storage and distribution of heat in oceans since the industrial age. Here we isolate and quantify the effects of both using coupled climate model simulations. We show that, relative to the pre-industrial ocean, the Southern Ocean imports heat from the Indo-Pacific Ocean but exports heat into the Atlantic Ocean in response to anthropogenic aerosols. Ocean heat uptake diminishes in the subpolar Atlantic. Alterations in ocean circulation and temperature have a weak compensation in contributing to interbasin heat exchange. Consequently, interbasin heat exchange contributes comparably to ocean heat uptake changes to modifying the stored heat in the Atlantic and Indo-Pacific. The greenhouse-gas-associated changes are the opposite of the aerosol-associated changes. Anthropogenic greenhouse gases promote the ocean heat uptake in the subpolar Atlantic and allow the Southern Ocean to import heat from the Atlantic but export heat to the Indo-Pacific. The cause of this ocean heat redistribution is distinct from the aerosol-forcing scenario, seeing that ocean circulation effects are strongly offset by temperature shifts. Accordingly, interbasin heat exchange is much less important than ocean heat uptake changes for greenhouse-gas-associated ocean heat storage. Our results suggest that the aerosol-driven changes in ocean circulations and associated interbasin heat transports are more effective in altering oceanic heat distribution than those driven by globally increasing greenhouse gases.

How to cite: Li, S., Liu, W., Allen, R. J., Shi, J.-R., and Li, L.: Ocean heat uptake and interbasin redistribution driven by anthropogenic aerosols and greenhouse gases, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7000, https://doi.org/10.5194/egusphere-egu24-7000, 2024.

EGU24-7842 | Orals | OS1.9

Tracking sea salt instead of saline water 

Kristofer Döös, Inga Koszalka, and Lars Axell

Lagrangian (parcel following) approach is a powerful method to diagnose the modelled flow and associated property changes in atmosphere and ocean and is used to investigate causal links between the property changes between the different regions. The salt in the saline sea water has traditionally been tracked as tracer property or a marker of sea water despite the seawater is constituted of both water and salt molecules. In the present study, we propose an new approach relying on tracking  separately the mass of fresh water and salt in the ocean. As a study region we have chosen the Baltic Sea, a semi-enclosed sea characterised by a distinct estuarine circulation due to river runoff and deep salt water inflow from the North Sea. The salt was tracked by summing over both the advective and diffusive salt fluxes simulated by the circulation model NEMO. Salt and water trajectories were computed with the mass conserving TRACMASS model, where each trajectory tube is in units of m3/s of water flux or kg/s of salt flux. 
The preliminary results show a clear difference between salt and water trajectories, where e.g. the salt trajectories (red in the attached Figure) do not reach as far into the Baltic Sea as the (blue) water trajectories. Many diagnostics such as the residence time and age also differ, which opens up a completely new vision of the ocean circulation

.

Figure: Water mass (blue) and salt mass (red) trajectories entering the Baltic Sea through the Danish straits.

How to cite: Döös, K., Koszalka, I., and Axell, L.: Tracking sea salt instead of saline water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7842, https://doi.org/10.5194/egusphere-egu24-7842, 2024.

EGU24-7960 | Orals | OS1.9

Zonal Wave Three: Trends and links to extreme events 

James Arthur Renwick

In the Southern Hemisphere atmospheric circulation, one of the most prominent wave patterns is zonal wave three (ZW3), which exhibits three positive and three negative anomalies in the zonal eddy field around the Southern Hemisphere, with maximum amplitude over the Southern Oceans. Using ERA5 data, this presentation will describe the form of ZW3 and trends in its behaviour. Over the past 60 years, the amplitude of ZW3 exhibits significant upward trends throughout the year but most prominently in summer (Dec-Feb). Such trends are related to increasing meridional temperature gradients and to trends in eddy activity in general and to trends in poleward energy fluxes. Implications for surface climate temperature and precipitation extremes will be outlined.

How to cite: Renwick, J. A.: Zonal Wave Three: Trends and links to extreme events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7960, https://doi.org/10.5194/egusphere-egu24-7960, 2024.

EGU24-8252 | ECS | Orals | OS1.9

Surface forcing controls on the volume and heat content of subtropical and subpolar mode waters over the global ocean 

Ciara Pimm, Richard Williams, Dani Jones, and Andrew Meijers

Mode waters provide an important role within the climate system, sequestering large amounts of heat and anthropogenic carbon and play a key role in the transport of these properties around the globe. Our aim is to assess the roles of local versus remote surface forcing in controlling the properties of mode waters over the northern Atlantic and Pacific basins and the Southern Ocean. A set of adjoint sensitivity experiments are conducted using the ECCOv4r4 state estimate to assess the impacts of surface heat flux, freshwater flux, and wind stresses on the volume and heat content of mode waters in density space. Mode waters are identified using areas of deep winter mixed layers and their characteristic temperature, stratification, and neutral density properties. The adjoint modelling approach calculates time-evolving sensitivity maps that identify where and when specific surface forcing impacts properties in the mode water formation sites. The sensitivity analysis reveals the dominance of local forcing from surface heat fluxes with surface cooling initially increasing volume. On longer time scales, the sensitivities have differing responses to surface forcing including surface heat loss leading to a delayed restratification due to a haline contribution after a thermal contribution is effectively damped. The responses of the mode waters to surface forcing are then compared across their formation sites, in the northern basins involving western boundary currents and gyre interiors and in the Southern Ocean involving the Antarctic Circumpolar Current.

How to cite: Pimm, C., Williams, R., Jones, D., and Meijers, A.: Surface forcing controls on the volume and heat content of subtropical and subpolar mode waters over the global ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8252, https://doi.org/10.5194/egusphere-egu24-8252, 2024.

EGU24-8399 | ECS | Posters on site | OS1.9

Future AMOC recovery modulated by atmospheric water vapor shortwave absorption 

Doseok Lee, Hanjun Kim, and Sarah Kang

The amount of shortwave radiation absorbed by atmospheric water vapor is highly model dependent. This study examines how differences in the atmospheric water vapor shortwave radiation absorption affect the CO2-induced climate response pattern. We control the atmospheric water vapor shortwave radiation absorption in Community Earth System Model 1.2.2 (CESM1-CAM4-POP2) by altering the water vapor shortwave absorptivity parameter k by 60% to 120% of the default value. The pre-industrial control simulations with different k values are integrated for 150 years and additional 150 years are integrated after abruptly quadrupling CO2 concentrations. Regardless of the k value, the Atlantic meridional overturning circulation (AMOC) weakens in response to the quadrupling of CO2. However, the simulation with a higher k value exhibits a faster AMOC recovery approximately 30 years after the quadrupled CO2, with the lowest k simulation exhibiting a persistent AMOC weakening with no sign of recovery for the entire 300-year integration period. The faster AMOC restoration with a larger k value is attributed to the climatologically colder and saltier subpolar North Atlantic sea surface condition arising from the larger Arctic sea ice fraction due to colder temperature associated with stronger atmospheric shortwave absorption. The colder and more saline subpolar North Atlantic sea surface facilitates a more rapid destratification of surface density, establishing a favorable condition for the AMOC restoration. The faster restoration of the AMOC with the higher k value leads to a larger inter-hemispheric energy asymmetry followed by a more northward ITCZ shift as well as a stronger equilibrium climate sensitivity. This study demonstrates the complex interaction among different elements within the Earth system, encompassing radiation, sea ice, AMOC, and large-scale atmospheric circulation, suggesting a way to reduce uncertainties in future climate projections by improving the parameterization of shortwave radiation absorption by atmospheric water vapor.

How to cite: Lee, D., Kim, H., and Kang, S.: Future AMOC recovery modulated by atmospheric water vapor shortwave absorption, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8399, https://doi.org/10.5194/egusphere-egu24-8399, 2024.

EGU24-10237 | ECS | Posters on site | OS1.9

New insights into seasonal to interannual salinity variability on the Northeast U.S. continental shelf and slope 

Svenja Ryan, Caroline C. Ummenhofer, and Glen G. Gawarkiewicz

The Northeast U.S. continental shelf is a highly productive and economically important region that has experienced robust changes in upper-ocean properties in recent decades. Warming rates exceed the global and North Atlantic average and in particular several episodes of anomalously warm temperatures, so called marine heatwaves, have had devastating impacts on regional fisheries over the past decade. There are also indicators of a salinification of the region, which might be linked to large-scale changes in the North Atlantic circulation as well as changes in regional processes, such as the number of Warm Core Rings shedding of the Gulf Stream, driving an increased salinity flux into the continental slope and shelf region. With now more than a decade of remote-sensing sea surface salinity data, we revisit seasonal to interannual salinity variability and assess the role of salinity for modulating stratification on the continental shelf. We provide important regional context for the interpretation of data from the OOI Coastal Pioneer array, a local shelf-break observatory. We find that the local seasonal cycle is an interplay of seasonal freshwater input via local river discharge, driving decreasing salinities in spring and summer not just on the shelf but also in the Slope Sea. An observed salinification in the fall is likely linked to offshore forcing over the slope associated with the presence of Warm Core Rings. A coherent low-frequency salinity variability is found over the slope and shelf region in the Mid-Atlantic Bight (MAB) and Gulf of Maine, highlighting that shelf conditions in particular in the MAB are not solely dominated by upstream shelf conditions but are significantly impacted by local offshore variability. Furthermore, we synthesise hydrographic data from the NOAA ECOsystem MONitoring (ECOMON) program to construct mean cross-shelf sections along the MAB to investigate the relative contributions of thermal and haline components to the seasonal stratification. Overall, salinity serves as a valuable tracer, in addition to temperature, of these multi-variate processes and with now more than a decade of satellite surface salinity can shed new light on the spatio-temporal variability on the Northeast U.S. continental shelf. 

How to cite: Ryan, S., Ummenhofer, C. C., and Gawarkiewicz, G. G.: New insights into seasonal to interannual salinity variability on the Northeast U.S. continental shelf and slope, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10237, https://doi.org/10.5194/egusphere-egu24-10237, 2024.

EGU24-10440 | ECS | Posters on site | OS1.9

Linking midlatitude transient eddy moist static energy transport and extratropical cyclones 

Jan Zibell, Sebastian Schemm, and Alejandro Hermoso Verger

Earth's equator-to-pole net radiation gradient is counteracted by poleward atmospheric energy transport. In the extratropics, the largest contribution to this poleward flux can be attributed to variability on the timescale of weather systems. Even though the radiative imbalance has been argued not to strongly differ in a warmer climate, the partitioning of heat flux into moist and dry components is expected to change due to a moister atmosphere. On the synoptic scale, an increase in moisture and associated latent heat release enhances the intensification of cyclones, prolongs cyclone lifetimes, and also strengthens downstream anticyclones. Conversely, latent heating locally alters static stability and thereby affects projected trends in baroclinicity, which in turn vary across height due to different trends in temperature. Given that these drivers of cyclones and thereby storm tracks are subject to change and the resulting interplay is complex, isolating the influence of changes in latent heating on cyclone number and storm track intensity is not straight-forward. By combining the global moist static energy (MSE) budget perspective with cyclone numbers and other feature-based characteristics such as intensity and intensification, we aim to better understand the role of latent heat transport and release on midlatitude storm tracks. In particular, we ask: How are changes in zonal and time mean poleward transient eddy MSE flux and its divergence related to changes in cyclone number and intensities?

We start investigating the linkage between MSE fluxes and surface cyclones in reanalysis data by calculating cyclone composites. These analyses reveal that in general, poleward flux in the vicinity of low-pressure systems reaches its maximum during the intensification phase and drops after cyclones reaching mature stage. Furthermore, MSE flux peaks slightly equatorward and downstream of the cyclone center. In the mean picture, this signal can be related to warm-sector flux along the cold front, also indicating that the footprint of cold-sector flux is not as dominant. Further separating dry and moist flux components is expected to reveal additional insight into how heat transport is distributed across cyclones. These diagnostics can readily be applied to climate model data and idealized aquaplanet simulations, which we make use of to reduce the complexity and single out the effect of individual drivers of storm track changes.

How to cite: Zibell, J., Schemm, S., and Hermoso Verger, A.: Linking midlatitude transient eddy moist static energy transport and extratropical cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10440, https://doi.org/10.5194/egusphere-egu24-10440, 2024.

The Atlantic Multidecadal variability (AMV) is a multivariate climate phenomenon with wide societal impacts in the North Atlantic region and beyond. In order to gain insight into the circulation dynamics controlling the AMV, we calculate Atlantic upper ocean heat and salt budgets at the basin and sub basin scale, focussing on multi-year to multidecadal timescales, for the upper ocean using output from a subset of CMIP6 models which have the same ocean component (the NEMO model) at nominal horizontal resolutions of 1 degree, ¼ degree and 1/12 degree and corresponding atmosphere-forced ocean-only models. We decompose the advection term into geostrophic and ageostrophic components and further use a Reynolds type decomposition to understand contributions from time-mean versus transient components of the flow. We use a novel decomposition of the large-area heat budget which highlights contributions due to spatial covariance between the large scale circulation and temperature/salinity gradients. Finally we relate the spatial pattern of the heat and salt advection to the meridional overturning (zonal) and horizontal gyre (azonal) components of the flow.

How to cite: Sinha, B.: Simulation of historical ocean heat and salt content changes in the Atlantic basin in CMIP6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10628, https://doi.org/10.5194/egusphere-egu24-10628, 2024.

EGU24-11362 | ECS | Posters on site | OS1.9

Deep ocean hydrographic heterogeneity inferred from offshore geodetic experiments 

Anna Jegen, Dietrich Lange, Johannes Karstensen, Oscar Pizarro, and Heidrun Kopp

Observational evidence, supported by high resolution numerical model simulations, indicate that meso- and submesoscale dynamics exists in the deep ocean (>2000m). However, over most parts, observing the deep ocean is restricted to address either spatial but not temporal (ship surveys) or temporal but not spatial (moored sensors) scales of variability. The advent of a growing number of offshore geodesy experiments, conducted with networks of distributed sensor arrays, aiming to evaluate tectonic deformation through strain measurements can potentially provide new ways to observe deep sea hydrographic variability. Despite the different observing objectives of offshore geodetic and oceanographic experiments, a great overlap in the measured parameter space exists, which has motivated analyses exploring possible cross-benefits. Here we present the evaluation of temperature, pressure, and sound speed observations from a 2.5-year offshore geodesy experiment centered along the northern Chilean subduction zone (~21.5°S and ~71.5°W to ~70.5°W). Our analysis confirms multi-year warming trends that previous studies have reported for the deep ocean but shows an additional regionalization of warming trends. Superimposed onto the multi-year warming trend are temperature fluctuations that show multi-hourly to multi-weekly periods and amplitudes that show both spatial and depth/regional dependencies. Aside from a general decrease in energy levels of the fluctuations with depth, we see evidence of ocean-topography interactions through barotropic topography waves. Taken together, the observations reveal de-coupled dynamical regimes seaward and landward of the deep-sea trench that mark the extent of the abyssal part of the eastern boundary current off Chile and demonstrate the potential of time series from offshore geodetic surveys for hydrographic analyses.

How to cite: Jegen, A., Lange, D., Karstensen, J., Pizarro, O., and Kopp, H.: Deep ocean hydrographic heterogeneity inferred from offshore geodetic experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11362, https://doi.org/10.5194/egusphere-egu24-11362, 2024.

EGU24-11856 | ECS | Posters on site | OS1.9

Seasonal Salinification of the US Northeast Continental Shelf Driven by an Imbalance Between Along-Shelf Advection and Cross-Shelf Eddy-Covariance Fluxes 

Lukas Taenzer, Ke Chen, Albert Plueddemann, and Glen Gawarkiewicz

The US Northeast continental shelf “cold pool” defines the body of winter-cooled Shelf Water that decouples from the surface layer during the stratified season. The cold pool canonically preserves fresh Shelf Water properties throughout the summer, which fulfills vital needs for the regional benthic ecosystem in the economically most productive fisheries region across the United States. However, recent warming trends significantly above the global average have put the ecosystem under environmental stress. While the cold pool’s heat content has been studied in detail, data limitations and large interannual variability in salinity have hampered an assessment of the cold pool’s salt budget. Here, we provide first evidence that the cold pool’s salt content increases significantly during the stratified season and investigate dynamical drivers of this trend, using a combination of multi-year mooring and glider observations and high-resolution regional model output. Cold pool salinification rates of 6 mPSU/day remain steady throughout the stratified season, leading to salinity differences of 1 PSU between April and October. The annual cold pool salinification is caused by an imbalance between eddy-covariance salt fluxes across the US Northeast shelfbreak front and advection of freshwater from upstream. While eddy-fluxes deposit salt onto the continental shelf at all times of year, the US Northeast shelfbreak jet is weakest during the summer, which reduces along-shelf advection. A seasonal reduction in the along-shelf salinity gradient is likely caused by processes in the Gulf of Maine/on Georges Bank. The observed interannual variability of the salinification signal is shaped by the intermittency of strong cross-shelfbreak eddy-covariance fluxes that are concentrated within 3-4 episodic events per year. Capturing the hydrographic trends in coastal water mass budgets and identifying their underlying dynamical mechanisms will lead to a better understanding of ecosystem responses and support sustainable fisheries management in a rapidly changing coastal ocean region. 

How to cite: Taenzer, L., Chen, K., Plueddemann, A., and Gawarkiewicz, G.: Seasonal Salinification of the US Northeast Continental Shelf Driven by an Imbalance Between Along-Shelf Advection and Cross-Shelf Eddy-Covariance Fluxes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11856, https://doi.org/10.5194/egusphere-egu24-11856, 2024.

EGU24-11925 | Posters virtual | OS1.9

Spontaneous equatorial flow reversals at the equator in  moist shallow water turbulence 

Nili Harnik, Josef Schröttle, Dl Suhas, and Jai Sukhatme

Equatorial superrotation is a striking feature in planetary circulations, also found in atmospheric circulation models. Geological evidence shows that Earth was in a state of super-rotation during the Eocene and Pliocene. On Earth, such a time period of super-rotation is sometimes referred to as permanent El Niño. While it is well established that a tropical wave source is needed for superrotation, the mechanism that provides this wave source, and what conditions allow it to be maintained are still not understood, and vary between different models. Specifically, in shallow water models with Earth like parameters, superrotation has only been found when relatively strong thermal damping was added. In this study we examine the spontaneous evolution of super-rotation in fully developed isotropically forced two-dimensional moist shallow-water turbulence, and examine the role of moisture by varying the strength of moisture coupling, and performing large ensembles of simulations. 

We find that while the dry runs exhibit both superrotation and sub-rotation, with spontaneous transitions between the two states, moisture results in all runs eventually reaching a stable superrotating state. We further find that a stable superrotation develops in the dry runs when we strengthen the thermal damping. We find that a meridional mass flux from the equator to the subtropics, develops in the runs with stable superrotation, and examine the role of this mass flux, which is enabled by the latent heating and the thermal damping, for the maintenance of the stable superrotation. 

How to cite: Harnik, N., Schröttle, J., Suhas, D., and Sukhatme, J.: Spontaneous equatorial flow reversals at the equator in  moist shallow water turbulence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11925, https://doi.org/10.5194/egusphere-egu24-11925, 2024.

EGU24-13947 | ECS | Posters on site | OS1.9

An Argo Float Study of Temperature and Salinity in the Subpolar region of the Cambpell Plateau 

Ana Amaral Wasielesky, Milena Menna, Angelo Rubino, Riccardo Martellucci, Yuri Cotroneo, Giuseppe Aulicino, Antonino Ian Ferola, and Elena Mauri

The Subantarctic region of New Zealand is marked by a unique and complex bathymetry that includes an ocean ridge and a substantial submarine plateau known as the Campbell Plateau. This plateau is located near the Pacific sector of the Southern Ocean, and plays a vital role in the export of heat, salt, and nutrients into the lower thermocline, primarily through the formation of mode waters. In the present study, Argo floats data from 2003 to 2023 are used to identify the main water masses along the eastern margin of the Campbell Plateau. This region, located at the boundary between subtropical and subantarctic fronts, is characterized by the formation of Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), which make an important contribution to the broader oceanic circulation patterns. First results reveal the presence of eight distinct water masses in the study region and emphasize their peculiar seasonal variability. A decadal analysis describes colder waters in the period 2003-2013 compared to 2014-2023, while significant changes in salinity are observed in 2017-2018. Water mass identification, depicted through Temperature-Salinity plots, is consistent with existing literature, but can also provide new insights on the interaction between subantarctic and subtropical waters. This research contributes to describe the ocean dynamic of Subantarctic New Zealand. The use of Argo float data provides an unprecedented level of detail in examining the spatial and temporal resolution of an area located between two different current systems, whose changes potentially influence the global and Southern Ocean circulation patterns, with consequent implication on the climate.

How to cite: Amaral Wasielesky, A., Menna, M., Rubino, A., Martellucci, R., Cotroneo, Y., Aulicino, G., Ferola, A. I., and Mauri, E.: An Argo Float Study of Temperature and Salinity in the Subpolar region of the Cambpell Plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13947, https://doi.org/10.5194/egusphere-egu24-13947, 2024.

EGU24-14534 | Posters on site | OS1.9

A new spice/heave decomposition of thermohaline variability 

Remi Tailleux

The temporal variability of temperature and salinity in the oceans is strongly impacted by the ocean stratification, which tends to constrain lateral advection and mixing to preferentially take place along approximately neutral surfaces. As a result, it is natural to seek a decomposition of thermohaline variability into heave and spice components, which splits temperature and salinity into a component contributing to density and one that is density-compensated. In this talk, I will outline the theoretical foundations for such an approach, based on a recent redefinition of spiciness, and illustrate its usefulness for understanding the variability of the ocean heat and salt contents in the EN4 dataset over the past century.

How to cite: Tailleux, R.: A new spice/heave decomposition of thermohaline variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14534, https://doi.org/10.5194/egusphere-egu24-14534, 2024.

EGU24-14622 | ECS | Posters on site | OS1.9

Multidecadal meridional dipole mode in the Indian Ocean subsurface ocean heat content 

Anand Babu Amere, Mihir Kumar Dash, and Balaji Senapati

Multidecadal changes in the background state of the Indian Ocean, such as variations in ocean circulation patterns, sea level and heat storage, can act as a carrier wave for the climate change and other variabilities. The long-term (~60 years since 1958) analysis of subsurface ocean heat content (sub-OHC) in the Indian Ocean exhibits the presence of a dominant multidecadal meridional dipole mode in the region. The analysis shows that until the late 1980s, a basin-wide meridional dipole mode is present, followed by the mode embedded in uniform basin-wide patterns. The trends of thermocline depth and sea surface height also exhibit the similar patterns. It is found that two distinct mechanisms are account for the observed patters in the Indian Ocean. Firstly, Local wind forcing is responsible for the meridional dipole patterns. In the off-equatorial southern Indian Ocean region, wind stress and Ekman pumping velocity trends favor downwelling (upwelling), resulting in thermocline depth deepening (shallowing) during 1958-1975 and 1976-1987, respectively. Secondly, the observed basin-wide warming and cooling trends during 1988-2000 and 2001-2014 are explained by the combined effect of local wind forcing and heat transport from the western Pacific through the Indonesian throughflow.

How to cite: Amere, A. B., Dash, M. K., and Senapati, B.: Multidecadal meridional dipole mode in the Indian Ocean subsurface ocean heat content, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14622, https://doi.org/10.5194/egusphere-egu24-14622, 2024.

EGU24-15419 | ECS | Posters on site | OS1.9

Multi-centennial evolution of the climate response and deep ocean heat uptake in a set of abrupt stabilization scenarios 

Federico Fabiano, Paolo Davini, Virna L. Meccia, Giuseppe Zappa, Alessio Bellucci, Valerio Lembo, Katinka Bellomo, and Susanna Corti

A set of 1000-year long abrupt stabilization simulations have been performed with the EC-Earth3 climate model. Each simulation follows a sudden stabilization of the external forcing, starting at different years of the CMIP6 historical and SSP5-8.5 scenario. The final global mean temperature increases range between 1.4 and 9.6 K with respect to the pre-industrial baseline.

We first explore here the evolution of the climate response at multi-centennial timescales and its dependence on the level of forcing, with regards to the climate feedback parameter and to patterns of surface warming. We then focus on the rate of heat storage in the global ocean, which is the main driver of the climate response at multi-centennial timescales. We find that the rate of warming of the deep ocean is almost independent from the amplitude of the forcing, so that most of the additional heat remains in the upper layers at high forcing. We hypothesize that this is due - at least partly - to a decreased ventilation of the deep ocean, caused by a general reorganization of the Meridional Overturning Circulation (MOC).

 

How to cite: Fabiano, F., Davini, P., Meccia, V. L., Zappa, G., Bellucci, A., Lembo, V., Bellomo, K., and Corti, S.: Multi-centennial evolution of the climate response and deep ocean heat uptake in a set of abrupt stabilization scenarios, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15419, https://doi.org/10.5194/egusphere-egu24-15419, 2024.

EGU24-16423 | ECS | Posters on site | OS1.9

Estimating Atlantic meridional heat transport through Bayesian modelling of altimetry, Argo and GRACE data 

Parvathi Vallivattathillam and Francisco M Calafat

The Atlantic Meridional Overturning Circulation (AMOC) plays a pivotal role in the meridional transport of heat, freshwater and major dissolved gases such as carbon, and oxygen, making it a crucial component of earth’s climate system and the biosphere. On millennial timescales, the AMOC is believed to act as a conveyor belt of ocean currents wherein the flow varies coherently across latitudes. Past studies have drawn on this conveyor-belt idea to establish links between the AMOC and the Earth's climate tipping points. However, recent research and observations suggest that, on shorter timescales (days to decades), the AMOC may not operate as coherent flow of water. Understanding AMOC variability and coherence on such timescales and how these might respond to anthropogenic influences is crucial to predicting the climate of the next decades. This is, however, challenging due to the sparseness of the observational data in both time and space. Here, we present a Bayesian Hierarchical modelling framework that combines observations from altimetry, gravimetry, and Argo floats to estimate meridional heat transport across the Atlantic. Our approach considers error structures jointly and accounts for spatiotemporal dependencies between processes (thermosteric, halosteric and ocean mass), providing a coherent way to propagate uncertainty and overcoming the limitations of hydrography-only based analyses. Our estimate of heat transport is in very good agreement (correlation of ~0.8 for 3-month means) with that from RAPID observations at 26°N. A meticulous comparison of mean and variance further underscores the precision of our estimates compared to those derived from heat budgets. Our method can be extended to gain further insights into the dynamics and meridional coherence of AMOC at shorter timescales.

How to cite: Vallivattathillam, P. and Calafat, F. M.: Estimating Atlantic meridional heat transport through Bayesian modelling of altimetry, Argo and GRACE data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16423, https://doi.org/10.5194/egusphere-egu24-16423, 2024.

EGU24-16722 | ECS | Posters on site | OS1.9

On critical dependence of atmospheric circulation response to regional SST biases on background SST 

Yuan-Bing Zhao, Nedjeljka Zagar, and Frank Lunkeit

This study examines how the geographic location of sea surface temperature (SST) biases influences global atmospheric responses. Utilizing an intermediate-complexity atmospheric model, 106 century-long simulations with idealized SST perturbations—emulating biases in coupled climate models—were performed. The intensity of the global atmospheric response to SST biases is evaluated by quantifying changes in global wave energy and interannual variance. The findings underscore the response's dependency on local background SST. Notably, with an imposed SST bias of +1.5 K, a significant global response is triggered once background SST surpasses approximately 25°C. This geographic dependency is related to the critical SST threshold for intense convection. Consequently, these results highlight the need for heightened focus on tropical oceans, especially the Indo-West Pacific, where SST biases can significantly impact the accuracy of global climate simulations.

How to cite: Zhao, Y.-B., Zagar, N., and Lunkeit, F.: On critical dependence of atmospheric circulation response to regional SST biases on background SST, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16722, https://doi.org/10.5194/egusphere-egu24-16722, 2024.

EGU24-17118 | Posters on site | OS1.9

Heating rate and energy gradient from the tropics to the North Pole 

Luca Ferrero, Niccolò Losi, Martin Rigler, Asta Gregorič, Griša Močnik, Piotr Markuszewski, Przemysław Makuch, Tymon Zielinski, Paulina Pakszys, Matteo Rinaldi, Marco Paglione, Angelo Lupi, and Ezio Bolzacchini

Absorbing aerosol species, such as Black (BC) and Brown (BrC) Carbon, are able to warm the atmosphere. The role of aerosols is one of the least clear aspects in the so called “Arctic Amplification” (AA) and up to now this was mostly modelled [1,2]. For this reason, we took part in four scientific cruises (AREX, Arctic-Expedition, summer 2018, 2019, 2021 and EUREC4A, 2020) in the North Atlantic, eastward and south-eastward of Barbados, aiming at the determination of the aerosol chemical composition and properties from the Tropics to the North Pole.

The Heating Rate (HR) was experimentally determined at 1 minute time-resolution along different latitudes by means of an innovative methodology [3], obtained by cumulatively taking into account the aerosol optical properties, i.e. the absorption coefficients (measured by AE33 Aethalometer) and incident radiation (direct, diffuse and reflected) across the entire solar spectrum. The HR computed along AREX and in Milan (in the same period) were used to determine the energy gradient, due to the LAA induced heat storage at mid-latitudes, which contributes to AA through the atmospheric heat transport northward.

Moreover, aerosol chemical composition was achieved by means of sampling via high volume sampler (ECHO-PUF Tecora) and analysis via ion chromatography, TCA08 for Total Carbon content, Aethalometer AE33 (for BC), ICP-OES for elements.

A clear latitudinal behaviour in Black Carbon concentrations, with the highest values at low latitudes (e.g. average BC concentration in Gdansk up to 1507±75 ng/m3) and a progressive decrease moving northwards and away from the big Arctic settlements (Black Carbon concentrations within the 81st parallel: 5±1 ng/m3).

According to the latitudinal behaviour of BC concentrations and solar radiation (decreases towards the north while the diffuse component increases), HR decreases noticeably towards the Arctic: e.g. higher in the harbor of Gdansk (0.290±0.010 K/day) followed by the Baltic Sea (0.04±0.01 K/day), the Norvegian Sea (0.010±0.010 K/day) and finally with the lowest values in the pure Arctic Ocean (0.003±0.001 K/day). Accordingly, the energy density added to the system by the aerosol, a positive forcing that differs by 2 orders of magnitude between mid-latitudes and North Pole was found: 347.3 ± 11.8 J/m3 (Milan), 244.8 ± 12.2 J/m3 (Gdansk) and 2.6 ± 0.2 J/m3 (80°N). These results highlight the presence of a great energy gradient between mid-latitudes and Arctic that can trigger a heat transport towards the Arctic. Moreover this was strengthen by the HR value for EUREC4A in Barbados that was 0.175±0.003 K/day. Finally, preliminary results from Antarctica collected onboard the Italian RV Laura Bassi cruising the Southern Ocean and the Ross Sea will be shown.

 

 

Acknoledgements: GEMMA Center, Project TECLA MIUR – Dipartimenti di Eccellenza 2023–2027. JPI EUREC4A-OA project. CAIAC (oCean Atmosphere Interactions in the Antarctic regions and Convergence latitude) PNRA project

 

References

[1] Navarro, J. C. A. et al. (2016) Nat. Geosci. 9, 277–281.

[2] Shindell, D. and Faluvegi, G. (2009) Nat. Geosci. 2, 294–300.

[3] Ferrero, L. et al. (2018) Environ. Sci. Technol. 52, 3546 3555.

How to cite: Ferrero, L., Losi, N., Rigler, M., Gregorič, A., Močnik, G., Markuszewski, P., Makuch, P., Zielinski, T., Pakszys, P., Rinaldi, M., Paglione, M., Lupi, A., and Bolzacchini, E.: Heating rate and energy gradient from the tropics to the North Pole, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17118, https://doi.org/10.5194/egusphere-egu24-17118, 2024.

After many decades of relative stability, the heat content of the oceans has been increasing at a relatively steady rate since the late 70’s, as confirmed by the analysis of many observational datasets. How global warming affects the available potential energy (APE) of the oceans, however, has received comparatively much less attention, yet is important to assess changes in the strength of wind-driven gyres and Antarctic Circumpolar Current for instance. In this talk, I will contrast the temporal changes over the past century of the APE and background potential energy (BPE) of the oceans based on the analysis of the EN4 dataset, by making use of the most recent local theory of APE. Results show that temporal changes in the BPE mimic that of the ocean heat content estimated in terms of potential temperature or Conservative Temperature. In contrast to the ocean heat content, the total APE of the oceans does not exhibit any marked trend. In this talk, I will discuss how regional APE estimates differ from the global APE estimate, to identify whether global warming has any detectable impact the large-scale ocean circulation features.

How to cite: Tailleux, R.: A complete energy analysis of ocean background and available potential energy over the past century, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17583, https://doi.org/10.5194/egusphere-egu24-17583, 2024.

EGU24-19893 | Posters on site | OS1.9

Analysis of water mass transformations and the spurious thermohaline overturning circulation in numerical ocean models 

Knut Klingbeil, Erika Henell, Tridib Banerjee, Hans Burchard, and Sergey Danilov

Numerical models have become an important tool for investigating the oceans heat and salt contents. The large-scale thermohaline overturning circulation in the world ocean is directly linked to the transformation of water masses caused by small-scale diapycnal mixing, which is parameterized in models. However, in addition to this physically justified "physical mixing", numerical transport schemes rely on additional "numerical mixing" for stability reasons. Thus, the simulated overturning circulation in ocean models is strongly affected by this spurious mixing.
Diagnostics of spurious mixing in terms of local tracer variance decay offer a detailed analysis of water mass transformations (WMT). Vice versa, analysis methods for WMT can be used to deduce information about the effects of mixing. In contrast to direct mixing diagnostics based on discrete variance decay (DVD) in geographical space, the WMT analysis framework is based on a mapping to tracer space, where diatracer fluxes that quantify the WMT can directly be diagnosed. Recently, a new local framework was derived, which combines the classical WMT framework with the local DVD analysis (Klingbeil & Henell, 2023). The derived analytical relations between dia-surface fluxes and mixing were demonstrated in an isohaline framework by Henell et al. (2023) [see corresponding submission to this session].
We will present how this methodology can be transferred to the world ocean in order to diagnose local diapycnal mixing and to quantify the spurious contribution to the simulated thermohaline overturning circulation in ocean models. In particular, the extension to density space requires the consistent quantification of density DVD, which is challenging in numerical models with prognostic equations for salinity and temperature and a non-linear equation of state.

 

Henell, E., H. Burchard, U. Gräwe, K. Klingbeil (2023) Spatial composition of the diahaline overturning circulation in a fjord-type, non-tidal estuarine system. Journal of Geophysical Research (Oceans). https://dx.doi.org/10.1029/2023JC019862.

Klingbeil, K. and E. Henell (2023) A Rigorous Derivation of the Water Mass Transformation Framework, the Relation between Mixing and Diasurface Exchange Flow, and Links to Recent Theories in Estuarine Research. Journal of Physical Oceanography. https://doi.org/10.1175/JPO-D-23-0130.1.

How to cite: Klingbeil, K., Henell, E., Banerjee, T., Burchard, H., and Danilov, S.: Analysis of water mass transformations and the spurious thermohaline overturning circulation in numerical ocean models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19893, https://doi.org/10.5194/egusphere-egu24-19893, 2024.

EGU24-20262 | ECS | Posters on site | OS1.9

Multi-scale temperature variability in the Arctic Mediterranean between 1958 to 2023 – from surface to abyss  

Birgit Rinde, Shengping He, and Camille Li

The temperature of the Arctic Ocean and adjacent areas has increased over recent decades and is expected to continue to increase through this century. Discrepancies between previous studies based on different periods, domains, and datasets suggest that the warming of the Arctic Ocean is nonlinear and characterized by large temporal and spatial variability. The behavior of this warming signal has numerous local effects on aspects from sea ice cover and surface fluxes to ecosystems, all of which react differently to linear warming compared to episodic warming events. The spatiotemporal warming pattern will also play a role in defining the product of dense-water formation in the region, which feeds into the lower limb of the Atlantic Meridional Overturning Circulation. Utilizing the ORAS5 reanalysis, the Arctic Subpolar gyre sTate Estimates, as well as in-situ observations, we present an overview of the warming of the Arctic Ocean and the Nordic Seas between 1958 and 2023. We shed light on the variability of trends and seasonal signals across the Arctic Ocean, from surface to abyss. This analysis provides a solid baseline for detecting regional changes in the mean state and variability of the Arctic Ocean with global warming and exploring the physical mechanisms causing the warming trend. As such, it will be key for grounding investigations of future changes in heat budgets for the Arctic Ocean and the Nordic Seas. 

How to cite: Rinde, B., He, S., and Li, C.: Multi-scale temperature variability in the Arctic Mediterranean between 1958 to 2023 – from surface to abyss , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20262, https://doi.org/10.5194/egusphere-egu24-20262, 2024.

EGU24-508 | ECS | Posters on site | OS1.10

Interactions Between a Marine Heatwave and Tropical Cyclone Amphan in the Bay of Bengal in 2020 

Saurabh Rathore, Rishav Goyal, Babita Jangir, Caroline Ummenhofer, Ming Feng, and Mayank Mishra

Interactions are diagnosed between a marine heatwave (MHW) event and tropical super cyclone Amphan in the Bay of Bengal. In May 2020, an MHW developed in the Bay of Bengal driven by coupled ocean-atmosphere processes which included shoaling of the mixed layer depth due to reduced wind speed, increased net surface shortwave radiation flux into the ocean, increased upper ocean stratification, and increased sub-surface warming. Ocean temperature, rather than salinity, dominated the stratification that contributed to the MHW development and the subsurface ocean warming that also increased tropical cyclone heat potential. The presence of this strong MHW with sea surface temperature anomalies >2.5°C in the western Bay of Bengal coincided with the cyclone track and facilitated the rapid intensification of tropical cyclone Amphan to a super cyclone in just 24 h. This rapid intensification of a short-lived tropical cyclone, with a lifespan of 5 days over the ocean, is unprecedented in the Bay of Bengal during the pre-monsoon period (March-May). As the cyclone approached landfall in northern India, the wind-induced mixing deepened the mixed layer, cooled the ocean's surface, and reduced sub-surface warming in the bay, resulting in the demise of the MHW. This study provides new perspectives on the interactions between MHWs and tropical cyclones that could aid in improving the current understanding of compound extreme events that have severe socio-economic consequences in affected countries.

How to cite: Rathore, S., Goyal, R., Jangir, B., Ummenhofer, C., Feng, M., and Mishra, M.: Interactions Between a Marine Heatwave and Tropical Cyclone Amphan in the Bay of Bengal in 2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-508, https://doi.org/10.5194/egusphere-egu24-508, 2024.

EGU24-1252 | ECS | Posters on site | OS1.10

Future Projections of Marine Heatwaves in the Indian Ocean under Different Socioeconomic Pathways 

Dushantha Sandaruwan Jayarathna Wijendra Naidhelage, Wen Zhou, Matthew Collins, Oluwafemi E. Adeyeri, Xuan Wang, Erandani Lakshani Widana Arachchige, and Ni Zekai

Marine heatwaves (MHWs) are extended periods of abnormal warm sea surface temperature (SST) events that can have considerable impact on the marine ecosystems and associated services. Despite recent developments in studying MHWs in the Indian Ocean, our understanding of their future occurrence remains limited. Hence, this study is crucial to expanding our understanding of future MHWs in the region. We use observational data from the Optimal Interpolated Sea Surface Temperature analysis (OISSTv2) and daily SST data from 14 models obtained from Coupled Model Intercomparison Project Phase 6 (CMIP6) to investigate the spatial and temporal characteristics of MHWs in the historical period (1982-2014) and future (2015-2100) under three shared socioeconomic pathways (SSPs, e.g., SSP126, SSP245, SSP585). During the historical period, more intense MHWs concentrated near the northern Arabian and Bay of Bengal region, with total MHW days of 20 ~ 25 days per year and mean intensity of 2 ~ 3 oC per year. The CMIP6 models overestimate the duration of MHWs while underestimating their intensity. Nevertheless, we employ the quantile delta mapping bias correction method to minimize these uncertainties in the CMIP6 multi model ensemble mean for a robust and reliable depiction of the future MHWs characteristics. We note accelerated positive trend in MHW metrics, including total days, and cumulative intensity, in the future compared to the historical period, resulting from global warming. Moreover, different emission scenarios exhibit different future MHWs characteristics. Specifically, the duration and mean intensity of MHWs are distinctly higher under SSP585 compare to other two scenarios, except for MHW frequency. Considering that we focused on a fixed baseline for MHW detection, we attribute the increase in MHWs duration to anthropogenic greenhouse gas emissions. Therefore, we emphasize the need for proactive measures to mitigate the impacts on future MHWs on marine ecosystems and associated services in the face of climate change.

 

How to cite: Wijendra Naidhelage, D. S. J., Zhou, W., Collins, M., E. Adeyeri, O., Wang, X., Widana Arachchige, E. L., and Zekai, N.: Future Projections of Marine Heatwaves in the Indian Ocean under Different Socioeconomic Pathways, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1252, https://doi.org/10.5194/egusphere-egu24-1252, 2024.

Heading into a potential El Niño in 2023/24, concern was high amongst Australian marine stakeholders regarding potential marine heatwave impacts on marine industries and systems in the coming summer. Targeted climate outlook briefings for the Great Barrier Reef and Western Australian coral reefs have been provided prior to and throughout the summer months by the Australian Bureau of Meteorology for the past 10-15 years, however in 2023 these were requested much earlier than usual. Also in 2023, national level seafood-focused briefings were requested by the fisheries sector for the first time, with various state and regional level meetings and information requests also occurring.

Subseasonal to seasonal forecast information played a critical role in these briefings, providing both the big picture in terms of climate drivers impacting Australian waters as well as regional information regarding sea surface temperatures around Australia. These forecast products are operationally produced by the Australian Bureau of Meteorology using the seasonal prediction system ACCESS-S. Clear communication of forecast probabilities and model skill was essential. New prototype marine heatwave forecasts were also presented to marine stakeholders, indicating where there was a high likelihood of marine heatwaves occurring in the upcoming season, together with likely severity. Demand for this new information on temperatures extremes was high and provided impetus for setting up coordinated briefings and response plans across sectors.

Forecasts can provide a 'preparation window' for marine stakeholders to implement proactive management strategies prior to high-risk conditions, noting however that not all industries have the same level of agility to respond. Subseasonal to seasonal forecast tools, that are useful, usable and used, provide valuable information to assist marine stakeholders in managing climate risk and vulnerability in a warming climate.

How to cite: Spillman, C., Hobday, A., Smith, G., and Hartog, J.: Building industry resilience through seasonal forecast briefings to Australian marine stakeholders heading into the 2023/24 summer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1558, https://doi.org/10.5194/egusphere-egu24-1558, 2024.

EGU24-1779 | ECS | Posters on site | OS1.10

Assessing Marine Heatwave Variability in the Luzon Strait 

Rose Angeli Macagga and Po-Chun Hsu

The Luzon Strait, a 350-km wide channel located between Taiwan and the Philippines, connects the West Philippine Sea and the north Pacific Ocean. Multiple factors affect the circulation in the Luzon Strait, such as the Kuroshio Current, monsoon, and the West Philippine Sea circulation. Discrete periods of extreme ocean warming events, also known as marine heatwaves (MHWs), have been occurring longer and more frequently across the globe. Anomalous temperature events can cause drastic changes in the biogeochemical processes and trigger adverse effects on marine ecology in the surrounding areas. This study aims to understand the variation in MHWs in the study area (16-24°N, 115-126°E), focusing on the Luzon Strait, using a daily global 5-km sea surface temperature (SST) product from 1985 to 2022. Four points of known coral reef areas were also chosen to further assess the MHWs and their possible effects on marine ecology.  Six MHW indices were utilized to describe the frequency, duration, and intensity of MHW events. The highest frequency of 17 MHWs in a year occurred in 1998, while the longest duration per event of 144 days and the total duration in a year of 308 days were recorded in 2020 and 2021, respectively. The highest values for all three intensity parameters were recorded in 2021, with mean, maximum, and cumulative intensities reaching 2.62°C, 3.86°C, and 227.42°C-days, respectively. The spatial distribution of monthly SST and ocean current profile showed thermal areas and helped identify high-risk areas. Climate variations, such as El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), were also explored as physical drivers of MHW in the study area. It has been observed that most of the years featuring MHW events at the four coral reef points occurred during the La Niña phase of ENSO, in conjunction with the negative phase of PDO, including 1998, 2010, and from 2020 onwards. Additionally, from 2016 to 2019, MHWs were observed at the same points during the positive phase of PDO, in conjunction with El Niño, La Niña, or Neutral phases of ENSO.

How to cite: Macagga, R. A. and Hsu, P.-C.: Assessing Marine Heatwave Variability in the Luzon Strait, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1779, https://doi.org/10.5194/egusphere-egu24-1779, 2024.

EGU24-1883 | ECS | Orals | OS1.10

Extreme and compound ocean events are key drivers of projected low pelagic fish biomass  

Natacha Le Grix, William Cheung, Gabriel Reygondeau, Jakob Zscheischler, and Thomas Frölicher Frölicher

Ocean extreme events, such as marine heatwaves, can have harmful impacts on marine ecosystems. Understanding the risks posed by such extreme events is key to develop strategies to predict and mitigate their effects. However, the underlying ocean conditions driving severe impacts on marine ecosystems are complex and often unknown as risks to marine ecosystems arise not only from hazards but also from the interactions between hazards, exposure and vulnerability. Marine ecosystems may not be impacted by extreme events in single drivers but rather by the compounding effects of moderate ocean anomalies. Here, we employ an ensemble climate-impact modeling approach that combines a global marine fish model with output from a large ensemble simulation of an Earth system model, to identify the key ocean ecosystem drivers associated with the most severe impacts on the total biomass of 326 pelagic fish species. We show that low net primary productivity is the most influential driver of extremely low fish biomass over 68% of the ocean area considered by the model, especially in the subtropics and the mid-latitudes, followed by high temperature and low oxygen in the eastern equatorial Pacific and the high latitudes. Severe biomass loss is generally driven by extreme anomalies in at least one ocean ecosystem driver, except in the tropics, where a combination of moderate ocean anomalies is sufficient to drive extreme impacts. Single moderate anomalies never drive extremely low fish biomass. Compound events with either moderate or extreme ocean conditions are a necessary condition for extremely low fish biomass over 78% of the global ocean, and compound events with at least one extreme variable are a necessary condition over 61% of the global ocean. Overall, our model results highlight the crucial role of ex-treme and compound events in driving severe impacts on pelagic marine ecosystems.

How to cite: Le Grix, N., Cheung, W., Reygondeau, G., Zscheischler, J., and Frölicher, T. F.: Extreme and compound ocean events are key drivers of projected low pelagic fish biomass , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1883, https://doi.org/10.5194/egusphere-egu24-1883, 2024.

EGU24-1925 * | ECS | Posters on site | OS1.10 | Highlight

Drivers of Global Marine Heatwaves in a Warming World 

Ce Bian, Zhao Jing, and Lixin Wu

Global warming has exacerbated occurrence of extreme events, threatening the environment of human living. Marine heatwaves (MHWs) are prolonged extreme warm water events in the ocean, exerting devastating impacts on marine ecosystems. Comprehensive knowledge of physical processes controlling MHW life cycles is pivotal to improving MHW forecast capacity, yet it is still lacking. Here, we use a historical simulation from a global eddy-resolving climate model with an improved representation of MHWs, and innovatively show that heat flux convergence by oceanic mesoscale eddies acts as a dominant driver of MHW life cycles over most parts of the global ocean. In particular, the mesoscale eddies make an important contribution to growth and decay of MHWs, whose characteristic spatial scale is comparable or even larger than that of mesoscale eddies. Moreover, our results proved that features of global MHWs are scale-dependent. The primary drivers of MHWs shift from oceanic advection to atmospheric forcing as their spatial scale becomes larger. There is evident geographic heterogeneity in the transition scale between these oceanic and atmospheric-process dominated regimes. Our study reveals the crucial role of mesoscale eddies in controlling the global MHW life cycles and highlights that using eddy-resolving ocean models is essential for accurate MHW forecasts. Another contribution is we clarified the transition scale of global MHWs, which is essential for parameterization of MHWs forecasting in a warmer future. 

How to cite: Bian, C., Jing, Z., and Wu, L.: Drivers of Global Marine Heatwaves in a Warming World, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1925, https://doi.org/10.5194/egusphere-egu24-1925, 2024.

Under global warming, the frequency and intensity of marine heatwaves are increasing. However, the inhibition of atmospheric forcing marine heatwaves (AMHW) on upwelling and its impact on marine ecosystems remain poorly understood. To address this issue, the satellite sea surface temperature and reanalysis data during 1998-2021 were analyzed in three distinct upwelling systems, northwestern South China Sea. The results showed that the coastal tide-induced upwelling in the west (W) of Hainan Island is primarily suppressed by enhanced stratification during the AMHW events, since the coastal tide-induced upwelling is insensitive to wind weakening. Contrarily, the wind-driven upwelling in the east (E) and northeast (NE) of Hainan Island are jointly regulated by wind and stratification during the AMHW. Specifically, the AMHW events have a stronger inhibitory effect in the upwelling and phytoplankton growth in the NE than that in the E. The causes could be the followings: (1) the background upwelling in the NE region is stronger than in the E, thus the NE region has a higher susceptibility to the wind weakening; (2) the wind-driven upwelling begins to be suppressed by AMHW when the high-pressure system is aligned with the coastline of the upwelling. In the NE region, the location of the high-pressure center during the occurrence of AMHW is positioned in closer proximity to the upwelling area. Moreover, the inhibitory effect of wind weakening and stratification enhancing on upwelling changes with the development of the AMHW. Before and during the mature phase of AMHW, stratification and wind jointly inhibit upwelling and phytoplankton growth, while it shifts to stratification dominated (>85%) during the decline phase. This study suggests that MHW has a great impact on the upwelling ecosystem, especially the wind-driven upwelling, which should be given high attention under global warming (with increasing MHW events in the future).

How to cite: Liu, S., Lao, Q., and Chen, F.: Impacts of Marine Heatwave Events on Three Distinct Upwelling Systems and its Implication for Marine Ecosystems in the Northern South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1942, https://doi.org/10.5194/egusphere-egu24-1942, 2024.

EGU24-1948 | ECS | Posters on site | OS1.10

Frequent marine heatwaves hidden below the surface of the global ocean 

Di Sun, Furong Li, Zhao Jing, Shijian Hu, and Bohai Zhang

Marine heatwaves are extreme warm water events that can cause devastating impacts on ecosystems and have complex socio-economic ramifcations. Surface signals and drivers of marine heatwaves have been extensively investigated based on satellite observations, whereas their vertical structure in the global ocean remains unclear. In this study, we identify marine heatwave events in the epipelagic zone (0–200 m) using a four-dimensional spatio-temporal framework based on three ocean reanalysis datasets. We find that only about half of the marine heatwave events have continuous surface signals during their life cycles and around one-third always reside in the subsurface ocean without any imprint on sea surface temperature. The annual number of these subsurface marine heatwave events shows a signifcant increase in response to subsurface mean-state warming during the past three decades. Our findings reveal the limitation of identifying marine heatwaves solely based on the sea surface temperature and underscore the necessity of subsurface observations for monitoring marine heatwaves.

How to cite: Sun, D., Li, F., Jing, Z., Hu, S., and Zhang, B.: Frequent marine heatwaves hidden below the surface of the global ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1948, https://doi.org/10.5194/egusphere-egu24-1948, 2024.

EGU24-2617 | ECS | Orals | OS1.10

Underestimated Arctic warming and potential ecosystem impact due to unresolved marine heatwaves 

Ruijian Gou, Yaocheng Deng, Klara Wolf, Yingzhe Cui, Clara Hoppe, Lixin Wu, Qi Shu, and Gerrit Lohmann

The Arctic is warming faster than any other regions, a phenomenon known as Arctic amplification, which has far-reaching effects for global climate. Modelled historical simulations show a significant underestimation of the amplification and the future projection exhibits non-negligible model spread. Here we show that in a future warming scenario, the warming in the Arctic is generally larger when comparing high-resolution climate models with low-resolution versions. We attribute the different extent of Arctic warming to Arctic marine heatwaves (MHWs), known as episodes of extreme ocean surface warming. The resolution of the MHWs, which are stronger and more realistic in the high-resolution model versions, increases the melting of sea ice and thus the absorption of solar radiation by the ocean in the short term, thereby reinforcing the long-term trend of Arctic warming. We point out that the amplification of Arctic warming is underestimated by the current generation of climate models, which generally have low resolution, thereby underestimating Arctic marine heat waves. In addition, Arctic heatwaves cause extreme temperature fluctuations associated with increased stratification. This poses major challenges to Arctic ecosystems and has a negative impact through direct physiological temperature effects and indirectly through nutrient supply and taxonomic shifts. We conclude that the eddy- and storm-resolving models provide a new perspective on how the Earth system responds to past and future climate and environmental extremes.

How to cite: Gou, R., Deng, Y., Wolf, K., Cui, Y., Hoppe, C., Wu, L., Shu, Q., and Lohmann, G.: Underestimated Arctic warming and potential ecosystem impact due to unresolved marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2617, https://doi.org/10.5194/egusphere-egu24-2617, 2024.

EGU24-2720 | ECS | Orals | OS1.10

Significant reduction of potential exposure to extreme marine heatwaves by achieving carbon neutrality 

Seok-Geun Oh, Seok-Woo Son, Sujong Jeong, and Yang-Ki Cho

Marine heatwave (MHW), a prolonged period of anomalously warm seawater, has a catastrophic repercussion on marine ecosystems. With global warming, MHWs have become increasingly frequent, intense, and prolonged. To avoid irreversible damages from such extreme events, net-zero human-caused carbon emissions by 2050s, called carbon neutrality, were proposed. Here, we evaluate the impact of carbon neutrality on MHWs in the late 21st century using multi-model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) Shared Socioeconomic Pathway (SSP)1-1.9 and SSP3-7.0 scenarios. It is found that if the current “regional rivalry” over carbon emissions policy continues into this century (i.e., SSP3-7.0), the MHWs in the late 21st century will become stronger over 1°C and longer lasting over 365 days than historical ones, especially in the western boundary current and equatorial current regions. Approximately 68% of the global ocean will be exposed to permanent MHWs, regionally 93% in the Indian Ocean, 76% in the Pacific Ocean, 68% in the Atlantic Ocean, 65% in the Coastal Ocean, and 48% in the Southern Ocean. Such extreme MHWs can be significantly reduced by achieving carbon neutrality (i.e., SSP1-1.9). In particular, the proportion of exposure to permanent MHWs can be reduced to as low as 0.02 to 0.07%, depending on the region. This result underscores the critical importance of ongoing efforts to achieve net-zero carbon emissions to reduce the potential ecological risks induced by extreme MHW exposure.

How to cite: Oh, S.-G., Son, S.-W., Jeong, S., and Cho, Y.-K.: Significant reduction of potential exposure to extreme marine heatwaves by achieving carbon neutrality, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2720, https://doi.org/10.5194/egusphere-egu24-2720, 2024.

Extreme and persistent marine heatwaves (MHWs) occur frequently in the Northeast Pacific, with huge impacts on climate, ecosystem and socio-economic. This study investigates the atmospheric circulations associated with the 33 MHWs since 1951 in observations. The composite results reveal that the MHWs in the Northeast Pacific can be triggered by a couple of anticyclonic and cyclonic anomalies, i.e., the anticyclonic anomaly to the northeast of the MHW region and cyclonic anomaly to the southwest.  This atmospheric circulation pattern can be detected as the dominant mode through EOF analysis on 500-hPa geopotential height anomalies over the Northeast Pacific-North America region, following the Pacific–North American teleconnection. These observational results are verified by using the outputs of 34 models in the historical simulation from phase 6 of the Coupled Model Intercomparison Project (CMIP6). Further diagnosis of the heat budget is performed, in attempt to illustrate the processes of MHW formation and maintenance.

How to cite: Tang, C. and Lu, R.: The atmospheric circulation anomalies associated with the formation of marine heatwaves in the Northeast Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3459, https://doi.org/10.5194/egusphere-egu24-3459, 2024.

EGU24-4034 | ECS | Orals | OS1.10

Projected amplification of summer marine heatwaves in a warming Northeast Pacific Ocean 

Marylou Athanase, Antonio Sánchez-Benítez, Helge Goessling, Felix Pithan, and Thomas Jung

Marine heatwaves are expected to become more frequent, intense, and longer-lasting in a warming world. However, it remains unclear whether feedback processes could amplify or dampen extreme ocean temperatures. Here we impose the observed atmospheric flow in coupled climate simulations to determine how the record-breaking 2019 Northeast Pacific marine heatwave would have unfolded in preindustrial times, and how it could unravel in a +4°C warmer world compared to present-day conditions. We find that air-sea interactions, involving reductions in clouds and ocean mixed-layer depth and air advection from fast-warming subpolar regions, modulate warming rates within the marine heatwave. In a +4°C warmer climate, global oceans are +1.9°C warmer than present levels, and regional mean warming in the Northeast Pacific can reach +2.3–2.7 ± 0.25°C. Our identified feedback processes are projected to further amplify the intensity and spatial extent of analogous Northeast Pacific summer marine heatwaves beyond those thresholds, with a warming reaching +2.9 ± 0.15°C above present levels. Such an event-specific amplification would place even greater stress on marine ecosystems and fisheries.

How to cite: Athanase, M., Sánchez-Benítez, A., Goessling, H., Pithan, F., and Jung, T.: Projected amplification of summer marine heatwaves in a warming Northeast Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4034, https://doi.org/10.5194/egusphere-egu24-4034, 2024.

EGU24-4862 | ECS | Posters on site | OS1.10

Depth-dependent coastal Marine Heatwaves: a case study in Shark Bay, Western Australia 

Yuwei Hu and Chunzai Wang

Marine Heatwaves (MHWs) are commonly defined as extreme warm weather or climate events and typically have large impacts on the local ecosystems and economy. Coastal seas that connect the open ocean and land are highly impacted by local terrestrial weather and climate systems. Distinct geographical features (e.g. water depth and bottom topography) of each coastal sea may locally contribute to the spatiotemporal pattern and associated drivers of coastal MHWs. To unravel this undetermined contribution, we choose the Shark Bay (Western Australia) as a case study domain. It is a semi-enclosed bay adjacent to the warm Leeuwin Current with in-bay water depth varying around 0 to 25m and out-bay depth from 25m down to 200m in the selected study area. Thus, the contribution of air-sea heat flux, advection, and other oceanic processes can be quantitatively evaluated by applying the mixed layer heat budget analysis based on a 0.1-degree model reanalysis dataset, Bluelink ReANalysis (BRAN) 2020. Additionally, three high-resolution satellite sea surface temperature (SST) products are used to identify, visualize, and analyze the spatiotemporal patterns of MHWs in Shark Bay. The spatial maps of MHW mean duration, mean cumulative intensity and event frequency exhibit a highly consistent pattern with large differences between metrics in shallow and deep areas. Mixed layer heat budget analysis within a month before each corresponding peak day of three selected major events, to some extent, confirms that this distinct spatial pattern is partially due to the constrained contribution of the entrainment processes below the mixed layer in shallow areas. The entrainment processes that are closely related to the mixed layer depth change may warm the surface layer during mixed layer shoaling by excluding less warm water below the mixed layer. This is not the case in very shallow regions. Interestingly, slightly different from what was previously assumed, the in-bay areas, instead of being warmed by the horizontal advection when the out-bay areas are warmed by the anomalous warming Leeuwin current, are slightly cooled by a constrained net cooling effect. We found that coastal MHW events in shallow areas are typically frequent but less intense if they occur independently under the typical net cooling effects of horizontal advection. Whereas coastal MHWs in deep areas are less frequent, but more intense and prolonged when concurrent with anomalous warm water advection. The shallowest in-bay areas that are not included in the heat budget analysis are outside the influence area of the net cooling effects. Thus, these areas may be intrinsically embedded with frequently fast warming effects of the net heat flux. By using the 90th percentile definition, these frequent warming are defined as MHWs, but the regularity of historical events may not lead to catastrophic impacts regarding the shorter duration and smaller cumulative intensity of an individual event. We then suggest that a global assessment of the net cooling effects of horizontal advection is necessary, to identify qualified coastal areas associated with higher resistance to sudden and prolonged ocean warming. 

How to cite: Hu, Y. and Wang, C.: Depth-dependent coastal Marine Heatwaves: a case study in Shark Bay, Western Australia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4862, https://doi.org/10.5194/egusphere-egu24-4862, 2024.

EGU24-5083 | ECS | Orals | OS1.10

Long-term warming and interannual variability contributions’ to marine heatwaves in the Mediterranean 

Amelie Simon, Carlos Pires, Thomas L. Frölicher, and Ana Russo

In the past 40 years, marine heatwaves (MHWs) have experienced a worldwide increase in duration, intensity, frequency and spatial extent. This trend has been particularly evident in the Mediterranean, where exceptional events were observed during the summers of 2022, 2018 and 2003. This study proposes a twofold analysis of MHWs in the Mediterranean, focusing on their statistical characteristics and physical causes. A satellite dataset is utilized to analyze MHWs via an index, called activity, which aggregates the occurrence, duration, intensity and spatial extent of events. Our results show that the trend toward more active summers for MHWs is strongest in the western Mediterranean basin and long-term warming is the main driver in the whole Mediterranean basin. We also show that in the western and Adriatic Mediterranean region, the increase of SST variability contributes about a third to the MHW activity long-term trend whereas in the central, eastern and Aegean basins, the variability of SST mostly acts to diminish this trend. Through principal component analysis (PCA) of MHW activity, we found that the three most severe summer MHW events in the Mediterranean occur at the same location where the overall trend is highest. Interannual variability increased MHW activity in 2022 around the Balearic Sea, in 2018 in the eastern basins and in 2003 in the central basins. A joint PCA revealed that the long-term trend in MHW activity co-varies with a positive geopotential height anomaly over the Mediterranean, which is consistent with the generation of atmospheric-driven MHWs and which, at the North Atlantic scale, resembles the positive phase of the summer East Atlantic. The additional interannual variability contribution to these three severe summers was associated with western warming and projected onto the positive phase of the summer North Atlantic Oscillation. The increase in MHW over the last 40 years is also associated in the western, central and Adriatic regions with increased downward short-wave radiation and in the eastern Mediterranean with decreased upward long-wave radiation. Increased upward latent heat flux partly compensated for the MHW long-term increase over the whole Mediterranean basin. The interannual variability of MHW activity is related in the western, central and Adriatic basins to increased downward sensible and decreased upward latent heat flux possibly due to warm and humid air intrusion.

 

A.S., A.R. and C.P. thank Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES http://doi.org/10.54499/JPIOCEANS/0001/2019 (ROADMAP), T.L.F. thank the Swiss National Science Foundation (Grant P00P2_198897), A.R and C.P thanks the national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). A.S. was supported by ANR and France 2030 through the project CLIMArcTIC (grant ANR-22-POCE-0005). A.R. was supported by FCT through https://doi.org/10.54499/2022.01167.CEECIND/CP1722/CT0006.

How to cite: Simon, A., Pires, C., Frölicher, T. L., and Russo, A.: Long-term warming and interannual variability contributions’ to marine heatwaves in the Mediterranean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5083, https://doi.org/10.5194/egusphere-egu24-5083, 2024.

EGU24-5286 | ECS | Posters virtual | OS1.10

Marine Heat Waves long-term trend assessment in the Northeast Atlantic region.  

Lluís Castrillo-Acuña, Silvia Martínez-Martínez, and Álvaro de Pascual-Collar

Marine heat waves (MHWs) may be understood as prolonged periods of anomalously high sea surface temperatures (SST). These events are associated to impacts on marine ecosystems such as coral bleaching, mass mortality of marine invertebrates due to heat stress, rapid species’ migrations, fishery closures or quota changes, among others.

The Iberia-Biscay-Ireland (IBI) region, covering from the Canary basin to the Celtic Sea, demonstrated for the year 2022 peak anomaly values of 15 MHWs events, 128 days of mean durations, and 261 total days of MHW according to a reference period from 1982 to 2022 (Castrillo-Acuña et al. 2024). The result of almost 300 days of MHW in some areas for the 2022 suggest that the current methodology may not be complete, as MHW are expected to be an extreme phenomenon. Global assessments such us Oliver et al. (2018) and Schlegel et al. (2019) had demonstrated the correlation between long term mean SST trends and some MHWs properties, but may this influence be strong enough to invalidate the results?

In this study we present a sensitive experiment of the affection of long term mean trends of SST and MHW detection by using different detrending methods. Also considering different refence periods.  It is performed in the IBI domain which covers upwellings, straits, bays, continental shelfs, open waters, etc. The study aims to investigate how the presence of medium to long-term trends may condition the MHW properties in different key oceanographic areas. In this way, we can differentiate regions where the variability of MHW is not conditioned by SST trends from those where it is and its magnitude.

 

 

 

 

 

 

 

 

Castrillo-Acuña, L., Alonso-Valle, A., de Pascual-Collar, A.: Characterization of Marine Heat Waves in the IBI Region in 2022. Manuscript submitted to the 8th edition of the Copernicus Ocean State Report (OSR8), Copernicus Publications, State Planet, 2024.

 

Oliver, E. C. J., Donat, M. G., Burrows, M. T., Moore, P. J., Smale, D. A., Alexander, L. V., Benthuysen, A., Feng, M., Sen Gupta, A., Hobday, A. J., Holbrook, N. J., Perkins-Kirkpatrick, S. E., Scannell, H. A., Straub, S. C., and Wernberg, T.: Longer and more frequent marine heatwaves over the past century. Nature Communications, 9(1), Article 1. https://doi.org/10.1038/s41467-018-03732-9, 2018.

 

Schlegel, R. W., Oliver, E. C. J., Hobday, A. J., & Smit, A. J. : Detecting Marine Heatwaves With Sub Optimal Data. Frontiers in Marine Science, 6.    https://www.frontiersin.org/articles/10.3389/fmars.2019.00737, 2019.

How to cite: Castrillo-Acuña, L., Martínez-Martínez, S., and de Pascual-Collar, Á.: Marine Heat Waves long-term trend assessment in the Northeast Atlantic region. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5286, https://doi.org/10.5194/egusphere-egu24-5286, 2024.

EGU24-5667 | Orals | OS1.10

Marine heatwaves: Can we predict them in the Barents Sea? 

Helene R. Langehaug, Anne Britt Sandø, Robinson Hordoir, Francois Counillon, Ping-Gin Chiu, and Roshin Raj

Marine heatwaves (MHW) can have large negative impacts on life in the ocean, such as kelp forest and corals. These environments are vital for protecting a range of different species in the ocean. With global warming, the occurrence and intensity of MHW are expected to increase, also in the polar regions. The Barents Sea has experienced large climate changes, becoming less influenced by sea ice during the last decades. Being able to predict the likelihood of MHW to occur in the Barents Sea could be highly beneficial to fisheries, aquaculture, and other relevant stakeholders. Such information could be useful in long-term risk assessment. In this study, we assess for the first time the skill of the Norwegian Climate Prediction Model (NorCPM) in predicting the likelihood of MHW. For this analysis, we focus on intense MHW in July 2016 taking place in the Barents Sea, and previously documented by satellite data. We find promising results in the seasonal predictions from NorCPM, where the predictions show increased probability for MHW to occur in July 2016 compared to July 2015 (when the MHW activity was lower than in 2016). The increased probability was already seen six months prior to the event. Furthermore, we downscale the results from the global NorCPM to a more refined grid with a horizontal resolution of 10km. This test case shows that downscaling can provide valuable information on the subsurface signature of MHW. We found the event in July 2016 to be shallow (down to about 50m) compared to another MHW event in July 2013, where warm anomalies occupied the whole water column. These results suggest that the event in July 2016 was atmospheric-driven, consistent with a previous study, whereas the event in 2013 is more likely to be ocean-driven. The results from this case study are promising for future seasonal prediction of MHW using NorCPM, and more in-depth studies are needed to quantify the predictive skill for different cases and different regions.

How to cite: Langehaug, H. R., Sandø, A. B., Hordoir, R., Counillon, F., Chiu, P.-G., and Raj, R.: Marine heatwaves: Can we predict them in the Barents Sea?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5667, https://doi.org/10.5194/egusphere-egu24-5667, 2024.

EGU24-5681 | Posters on site | OS1.10

Global surface ocean temperature anomalies in 2023 and their climate context 

Matthew Menary and Leon Hermanson

Around 3 billion people rely on the ocean for their livelihoods, with around 10% of the world’s population directly relying on fishing. As human-driven climate change causes the world to warm, the ocean and the ecosystems within are increasingly susceptible to heatwave events that can have severe consequences. Such marine heatwaves (MHWs) can last from several days to a year and result in the destruction of ocean habitats and the diminution or relocation of fish species, with knock-on effects for coastal communities. The frequency of MHWs has doubled since 1982 and they are likely to continue to increase in frequency, intensity, and duration. However, the link between MHWs and modes of climate variability remains uncertain. Here, we investigate to what extent maps of temperature anomalies in 2023 can be attributed to large-scale climate modes with centres of action in the Atlantic, North Pacific, and tropical Pacific. Specifically, we regress global sea surface temperatures on to indices of Atlantic Multidecadal Variability (AMV), the 2nd EOF of North Pacific variability (commonly linked to MHWs), and El Nino/Southern Oscillation (ENSO, which strongly correlates with the 1st EOF of North Pacific variability). We find that around 30% of the variance in global, annual sea surface temperature anomalies can be explained by a linear combination of these indices. Since 2012, the combination of these indices has been unprecedented, associated with anomalous warming (on top of the global trend) throughout the northern hemisphere. As such, climate variability (which may include a forced component) is currently providing an unusually high baseline for further MHW events. Further work will aim to use decadal prediction models to investigate the predicted evolution of these indices over the coming years.

How to cite: Menary, M. and Hermanson, L.: Global surface ocean temperature anomalies in 2023 and their climate context, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5681, https://doi.org/10.5194/egusphere-egu24-5681, 2024.

EGU24-6542 | Orals | OS1.10

Large-scale drivers of Northeast Pacific MHWs in a changing climate 

Antonietta Capotondi, Matthew Newman, Tongtong Xu, and Emanuele Di Lorenzo

The Northeast Pacific Ocean has experienced episodes of intense and persistent warm conditions, also known as marine heatwaves, with devastating ecological impacts. Being able to predict these extreme events a few seasons in advance is therefore very important, but has proven elusive in many cases. While the intensity of Northeast Pacific marine heatwaves has been related to local stochastic atmospheric forcing with limited predictability, their evolution and persistence may be controlled by large-scale climate influences. Here we use a multi-variate statistical approach to identify these large-scale drivers, as well as the initial states that optimally develop into a marine heatwave at a later time in this region. Results indicate that a decadal mode of variability related to the Pacific Decadal Oscillation plays a key role in creating conditions favorable to the development of Northeast Pacific marine heatwaves. This mode is also implicated in the development of Central Pacific El Niño events, which may contribute to the persistence of the Northeast Pacific warm anomalies. In addition, this mode of variability appears to be responsible for the increased Northeast Pacific sea surface temperature variance in recent decades, suggesting that changes in internal climate variability may be responsible for the enhanced MHW activity in this region during this recent period.

How to cite: Capotondi, A., Newman, M., Xu, T., and Di Lorenzo, E.: Large-scale drivers of Northeast Pacific MHWs in a changing climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6542, https://doi.org/10.5194/egusphere-egu24-6542, 2024.

EGU24-6880 | Posters on site | OS1.10

Just another Tasman Sea marine heatwave? 

Neil Holbrook

Through late November and early December 2023, a severe category marine heatwave (MHW) was detected moving southwards off the east coast of Tasmania, Australia. The MHW was characterised by offshore sea surface temperature anomalies ~4oC above climatological values embedded within and around large anticyclonic eddies with warm anomalies to >1000m depth. Given the deleterious impacts from previous MHWs on marine ecosystems, fisheries, and aquaculture in the region, serious concerns were raised. To advise and prepare stakeholders, a series of online briefings was given by physical, biogeochemical, fisheries, and social scientists on the current and likely evolving environmental conditions associated with the MHW. So, how unusual was this event? Was it successfully forecast? Was it expected from our knowledge of large-scale modes of climate variability and their teleconnections? This presentation will discuss the characteristics, evolution – both forecast and projected – and emerging impacts of the November-December 2023 Tasman Sea MHW. It will be argued that the characteristics of this event mirror expectations from anthropogenic climate change, and that initialised seasonal SST forecasts were little different from expectations under climate change projections and trend persistence.

How to cite: Holbrook, N.: Just another Tasman Sea marine heatwave?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6880, https://doi.org/10.5194/egusphere-egu24-6880, 2024.

EGU24-7402 | ECS | Orals | OS1.10 | Highlight

Future Intensification of Marine Heatwaves in Marine Protected Areas 

Eun Byeol Cho, Eun Young Kwon, and Axel Timmermann

Marine Protected Areas (MPAs) are designated areas aimed at preserving marine ecosystems. However, they encounter the persistent obstacle of increasing ocean temperature. The occurrence of extreme warming events, known as Marine Heatwaves (MHWs), poses a significant threat to the delicate balance of marine ecosystems within MPAs. To understand the future changes in marine heatwaves (MHWs) in these regions, it is crucial to utilize advanced climate modeling capable of accurately capturing regional bathymetric features in MPAs, like coastlines, continental shelves, or islands. In this study, we utilized the SSP585 greenhouse warming simulations conducted with the OpenIFS-FESOM2 coupled model (AWI-CM3, 31 km atmosphere resolution, 4-15 km ocean resolution) to explore future changes in MHWs in the epipelagic to the upper mesopelagic zones (0-500m depth) of the global MPAs. In the current climate, MHWs in the MPAs exhibit greater maximum intensity and higher frequency than the global averages. However, MHWs in MPAs have shorter durations, leading to a lower cumulative intensity. The average warming rate within the MPAs is similar to or slightly lower than the average warming rate of the global ocean. Nevertheless, the MPAs are expected to see a 20% greater increase in the cumulative intensities of MHWs compared to the global ocean, from the past to the future. The findings suggest that marine protected areas (MPAs) are more susceptible to extreme temperature events compared to open ocean zones. Our findings underscore the significance of addressing anthropogenic warming to safeguard MPAs, emphasizing the need for prompt measures to mitigate these impacts and protect these vital marine ecosystems. 

How to cite: Cho, E. B., Kwon, E. Y., and Timmermann, A.: Future Intensification of Marine Heatwaves in Marine Protected Areas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7402, https://doi.org/10.5194/egusphere-egu24-7402, 2024.

EGU24-7671 | ECS | Orals | OS1.10

The 2023 marine heatwave in the North Atlantic and the Mediterranean Sea: ocean response to atmospheric circulation 

Lorine Behr, Elena Xoplaki, Niklas Luther, Elina Tragou, Jürg Luterbacher, and Vassilis Zervakis

The year 2023 was characterized by record-breaking global surface air and sea surface temperatures (SSTs), the latter reaching a record 21° C in April (excluding the polar regions; Copernicus 2023). As June to October were the warmest on record globally (WMO 2023), extreme and long-lasting marine heatwave (MHW) events were observed, especially in the North Atlantic and the Mediterranean Sea. In general, the occurrence of MHWs in the subtropics and western boundary current regions is predominantly driven by atmospherically induced processes such as the net ocean heat uptake from the atmosphere, associated with a reduction in latent heat loss and increased shortwave radiation (Schlegel et al. 2021; Vogt et al. 2022). The atmospheric circulation with persistent high‑pressure systems and anomalously weak wind speeds associated with increased insolation is the dominant driver of the above processes. We focus on the state of the atmosphere at the surface and in the mid-troposphere during 2023 and identify specific atmospheric patterns and SST anomaly structures. To detect MHWs and calculate their characteristics we use the daily gridded NOAA OI SST version 2.1 dataset (Huang et al. 2021, updated), derived from the AVHRR satellite, in-situ ship and buoy SST data. For the atmospheric component, we used the mean sea level pressure (SLP), the horizontal wind at 10 m, the geopotential height at 500 hPa (zg500) and the 2 m maximum temperature (Tmax) from the ECMWF ERA5 reanalysis (Hersbach et al. 2020, updated). Atmospheric and ocean datasets are provided globally with a high resolution (0.25°). We use daily anomalies with 1983 to 2012 as the reference period (as recommended by Hobday et al. 2018). The evaluation of MHW metrics such as frequency, duration, mean and cumulative intensity in different subregions of the North Atlantic and Mediterranean revealed that the most frequent MHWs were observed in the western Mediterranean (WMED), the longest MHWs in the central northeast Atlantic and the cumulatively most intense MHWs in the northwest Atlantic and central northeast Atlantic. The most intense MHWs are found in the WMED and off Newfoundland. During summer we detect asynchronous, above normal SLP, zg500 and Tmax over the northwest Atlantic, the WMED and the Black Sea, representing a type of blocking condition. A weakened Azores High, associated with reduced wind speed, mixing and upwelling, allows SSTs to rise substantially in the central northeast Atlantic during summer (Copernicus 2023). The first Empirical Orthogonal Function shows an antiphase dipole of SST and zg500 anomalies (explained variances of 43.9 % and 34.3 %, respectively) between the Mediterranean and West of the British Isles as well as monopol SST and zg500 anomalies (explained variances of 57.7 % and 41.9 %, respectively) over the northwestern Atlantic and the Labrador Sea.

How to cite: Behr, L., Xoplaki, E., Luther, N., Tragou, E., Luterbacher, J., and Zervakis, V.: The 2023 marine heatwave in the North Atlantic and the Mediterranean Sea: ocean response to atmospheric circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7671, https://doi.org/10.5194/egusphere-egu24-7671, 2024.

EGU24-8423 | ECS | Orals | OS1.10

Mechanism and Forecast Potential of North Pacific Marine Heatwaves inferred from Adjoint Sensitivities 

Xiaoxue Wang, Armin Köhl, and Detlef Stammer

The increasing frequency and intensity of heatwave events have led to a significant rise in heat-related threads on land and in the ocean during recent years. A classic example of a marine heat wave (MHW) is the 2014 – 2016 warm event that spread across the northeastern Pacific (NEP) Ocean—an event that researchers coined “the blob”. Here we use an adjoint sensitivity approach to shed new light on potential causes for reoccurring NEP marine heatwaves events in the region of the NEP. The study is based on the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint, for which the mean sea surface temperature (SST) of different target regions (region 1: 145°~ 160°W, 48°~ 56°N; region 2: 130°~ 145°W, 40°~ 48°N) and different target years (e.g. year 2014) was set as objective function. The adjoint sensitivities show that during the year of emergence, air-sea turbulent surface heat flux is the dominant atmospheric driver. The horizontal temperature advection, i.e., the impact of the basin-wide ocean circulation, is found to be less important, but might act as a preconditioning of MHW through climate oscillations (e.g. NPGO). Because atmospheric forcing anomalies occurring within the 18 months prior to the MHW event play a particularly critical role in driving the overall response locally through air-sea interactions, the leading 18 month atmospheric conditions in the central North Pacific can be considered as predictive signals for later marine heatwave events. Based on our preliminary findings, it can be concluded that 2024 may not be a heatwave year for NEP region. 

How to cite: Wang, X., Köhl, A., and Stammer, D.: Mechanism and Forecast Potential of North Pacific Marine Heatwaves inferred from Adjoint Sensitivities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8423, https://doi.org/10.5194/egusphere-egu24-8423, 2024.

EGU24-10707 | ECS | Orals | OS1.10

Investigating the role of air-sea heat flux for marine heatwaves in the Mediterranean Sea 

Dimitra Denaxa, Gerasimos Korres, Giulia Bonino, Simona Masina, and Maria Hatzaki

The Mediterranean Sea (MS) has been experiencing progressively intensified Marine heatwave (MHW) conditions over the past decades, associated with severe environmental and socioeconomic impacts. Building upon prior research on physical mechanisms underlying the occurrence of MHWs, here we assess the relative role of air-sea heat exchange in driving the onset and decline phases of surface MHWs in the basin, utilizing remote sensing and reanalysis data for the period 1993-2022. Although contributing positively to the SST evolution during most MHWs, surface heat flux is identified as the primary driver in less than half of the onset/decline MHW phases. This finding suggests that oceanic processes play a crucial role in driving SST anomalies during MHWs in the basin. The role of surface heat flux becomes more pronounced during onset periods and warmer seasons, with the latent heat being the most significant heat flux component in modulating SST anomalies during both MHW phases and across all seasons. Heat flux emerges as the major driver of most onset phases in the Adriatic and the Aegean Seas. Onset/decline phases shorter than 5 days exhibit a weaker heat flux contribution compared to longer phases. Moreover, an inverse relationship between event severity and heat flux contribution is observed. At the subsurface, mixed layer shoaling is observed over the entire duration of most events, particularly for those of shorter duration. Therefore, the surface cooling right after the peak intensity day is likely not associated with vertical mixing in such cases. After the MHW end day, a significant mixed layer deepening in most cases suggests that further dissipation of heat is commonly driven by vertical mixing. This study emphasizes the need for considering subsurface information for MHW studies and accounting for limitations associated with the definitions employed for MHW phases. Clearly articulating such choices, tailored to the specific contexts of individual studies, is vital for precise interpretation and meaningful comparisons across different studies on MHW drivers.

How to cite: Denaxa, D., Korres, G., Bonino, G., Masina, S., and Hatzaki, M.: Investigating the role of air-sea heat flux for marine heatwaves in the Mediterranean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10707, https://doi.org/10.5194/egusphere-egu24-10707, 2024.

EGU24-12034 | ECS | Orals | OS1.10 | Highlight

The increased likelihood of plankton community changes following marine heatwaves 

Ryan Deeley, Tobias Grafke, and Ulrike Feudel

When modelling any climatic system, it is important to carefully consider the relation between the many timescales that govern its evolution, since a certain change in their interplay can drastically affect the likelihood of observing critical transitions to distinct environmental regimes. In this study, we present how the onset of marine heatwaves - that are responsible for inducing prolonged periods of positive temperature fluctuations - can weaken state-based resilience leading to noise-induced shifts between species’ concentration levels in plankton communities. This is shown in a modified Truscott-Brindley model, a stochastically forced fast-slow system that encapsulates the interaction between phytoplankton and zooplankton species during red tide events in marine environments. Deterministically, the system can be bistable, possessing stable states with high and low phytoplankton biomass, or in an excitable monostable regime, where phytoplankton blooms form following perturbations. Environmental perturbations to the (temperature-dependent) species’ growth rates are modelled using multiplicative noise terms, namely Ornstein-Uhlenbeck processes with a correlation time parameter τ. During marine heatwaves, the correlation time τ of the external perturbations will increase. With ensemble Monte Carlo simulations of phytoplankton collapses, we demonstrate how mean first-exit times from the domain of attraction scale as the noise intensity weakens, across different prescribed values for the correlation time τ. These results yield numerical approximations for the systems’ quasipotential barrier heights - a concept from Freidlin and Wentzell’s theory of large deviations that quantifies resistance to noise-induced escape from a given domain - which elucidates a non-monotonic relation between the system vulnerability to critical transitions and the correlation time τ of the external perturbations. Indeed, initially there is a notable drop in system resilience as the correlation time τ grows from zero, although as τ increases further beyond a critical value, the system resilience begins to then increase. This non-monotonic relation is also reflected in the action values of most probable transition paths for escaping the domain of attraction, found using an augmented Lagrangian method to overcome the degenerate noise present in the system. These findings are compared and contrasted with results from other studies exploring how climate tipping points, or stochastic escapes from a domain of attraction, depend on the correlation time of the external perturbations. Finally, we consider candidate time-series for correlation times constructed from temperature records for the North Sea across periods including anomalously high values, and discuss whether - subject to these - varying system vulnerability to critical transitions is more sensitive to the rate of emergence or duration of the marine heatwaves.

How to cite: Deeley, R., Grafke, T., and Feudel, U.: The increased likelihood of plankton community changes following marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12034, https://doi.org/10.5194/egusphere-egu24-12034, 2024.

EGU24-13423 | Posters on site | OS1.10

Marine heatwaves in the Red Sea: a study of their spatial characteristics, trends and relationships to climate modes 

Manal Hamdeno, Aida Alvera-Azcárate, George Krokos, and Ibrahim Hoteit

Episodes of very warm sea surface temperatures (SST), known as marine heatwaves (MHWs), can potentially alter ocean ecosystems with far-reaching ecological and socio-economic consequences. In this work, we focused on the Red Sea (RS), a region of outstanding socio-economic importance, and investigated its spatio-temporal MHW variability between 1982 and 2021. In addition, the relationship between MHWs and different climate teleconnection patterns was investigated. Our results show that during the study period (1982-2021), the highest frequencies of MHWs were in the southern Red Sea (SRS), while the prolonged and more intense ones were in the northern Red Sea (NRS). By analyzing satellite-derived sea surface temperatures (SST), we identified a warming trend in the RS that began from the mid-1990s, and has intensified since 2016. This temperature increase was accompanied by an increase in the MHW frequency and total days. 78 MHW events with a total of 1016 heat days occurred in the RS between 1982 and 2021, of which 36 events (46%) and 590 days (58%) were recorded in the last decade. In the NRS, the annual MHW frequency was highest in 2010, 2018, 2019 and 2021, while in the SRS it was highest in 1998 and from 2017 to 2021. In cold years, characterized by a negative average SST anomaly, MHWs were mainly found in the NRS. In contrast, in warm years characterized by a positive average SST anomaly, MHWs mainly affected the SRS. However, an exception was observed in 2010, which is considered one of the warmest years in the last four decades. In this year, MHWs were predominantly localized in the NRS, deviating from the typical pattern observed in warm years. The MHW frequency showed a strong positive correlation (> 0.7) with the Atlantic Multidecadal Oscillation (AMO) over the entire RS and a positive correlation (> 0.4) with the Indian Ocean Dipole Index (IOD), which was more pronounced in the SRS, whereas it had a negative correlation (< -0.5) with the East Atlantic/Western Russia (EATL/WRUS) pattern, particularly in the NRS. It was noted that 2010 was also an exceptional year for the climate modes as the AMO and IOD were in strong positive phases, and  the EATL/WRUS was in its highest negative phase, both of which may have contributed to the increased MHWs in that year. This study highlights the link between climate patterns and the occurrence of marine heatwaves in the Red Sea and provides valuable insights into this important aspect of climate change.

How to cite: Hamdeno, M., Alvera-Azcárate, A., Krokos, G., and Hoteit, I.: Marine heatwaves in the Red Sea: a study of their spatial characteristics, trends and relationships to climate modes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13423, https://doi.org/10.5194/egusphere-egu24-13423, 2024.

EGU24-14495 | Posters on site | OS1.10

Unraveling the Indian Monsoon's Role in Fueling the Unprecedented 2022 Marine Heatwave in the Western North Pacific 

Qianghua Song, Chunzai Wang, Yulong Yao, and Hanjie Fan

An unprecedented marine heatwave (MHW) event occurred in the middle-high latitude of the western North Pacific in the summer of 2022. We demonstrate that enhanced precipitation thousands of kilometers away fueled this extreme MHW event in July 2022. In the upper atmosphere of the MHW region, a persistent atmospheric blocking system is formed, which reduces convection and cloud cover and increases shortwave radiation at the ocean surface, leading to higher sea surface temperatures. Atmospheric perturbations induced by latent heat release from the extreme precipitation in the Indian summer monsoon region enhance this atmospheric blocking through the propagation of quasi-stationary Rossby waves. Our hypothesis is verified by using a numerical model that is forced with the observed atmospheric anomalous diabatic heating. This study sheds light on how a subtropical extreme event can fuel another middle-high latitude extreme event through an atmospheric bridge.

How to cite: Song, Q., Wang, C., Yao, Y., and Fan, H.: Unraveling the Indian Monsoon's Role in Fueling the Unprecedented 2022 Marine Heatwave in the Western North Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14495, https://doi.org/10.5194/egusphere-egu24-14495, 2024.

EGU24-15499 * | ECS | Orals | OS1.10 | Highlight

Modelling marine heatwaves impact on shallow and upper mesophotic tropical coral reefs  

Nicolas Colombi, Chahan M. Kropf, Friedrich A. Burger, David N. Bresch, and Thomas L. Frölicher

Coral reefs ecosystems, often compared to rain forests for their high biodiversity, are threatened by coral bleaching. Coral bleaching occurs when the symbiotic relationship between dinoflagellates and corals breaks under environmental stresses, notably high ocean temperatures. Thermal stress on coral reefs predominantly occurs during marine heatwaves, which can take place synchronously at the surface and subsurface, or asynchronously in either one of the two levels. Subsurface marine heatwaves tend to last longer with potentially higher cumulative intensities compared to their surface counterpart. However, to the best of our knowledge, no global coral bleaching model takes into account the variability between the thermal stress measured at the surface and the one experienced by coral reefs at their specific depth. Here we show that developing a marine heatwave impact model for shallow and upper mesophotic coral reefs, increased coral bleaching modelling accuracy by 4.7 ± 1.3% compared to a model using surface marine heatwaves. To define marine heatwaves at coral reef depth, we used trilinear interpolation using the GLORYS12 reanalysis temperature product. Our model provides coral bleaching values at times and locations where no record was taken, providing a global reconstructed dataset of coral bleaching with daily resolution from January 1st 1993 to December 31st 2020 in 9944 locations. Furthermore, our model indicates that since 1993 over 40% of coral reefs bleached. We anticipate this study to be a starting point for more accurate coral bleaching modelling. Observing that upper mesophotic coral reefs (30-50m) might be more threatened than shallow coral reefs, provides additional evidence to reshape our perception of upper mesophotic coral reefs as potential refugees from climate change.

How to cite: Colombi, N., Kropf, C. M., Burger, F. A., Bresch, D. N., and Frölicher, T. L.: Modelling marine heatwaves impact on shallow and upper mesophotic tropical coral reefs , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15499, https://doi.org/10.5194/egusphere-egu24-15499, 2024.

EGU24-16606 | ECS | Orals | OS1.10 | Highlight

Vertical structures of global marine heatwaves 

Ying Zhang, Yan Du, Ming Feng, and Alistair J. Hobday

A marine heatwave (MHW) is typically defined as an anomalous warm event in the surface ocean, with wide-ranging impacts on marine and socio-economic systems. The surface warming associated with MHWs can penetrate into the deep ocean; however, the vertical structure of MHWs is poorly known in the global ocean. Here, we identify four main types of MHWs with different vertical structures using Argo profiles: shallow, subsurface-reversed, subsurface-intensified, and deep MHWs. These MHW types are characterized by different spatial distributions with hotspots of subsurface-reversed and subsurface-intensified MHWs at low latitudes and shallow and deep MHWs at middle-high latitudes. These vertical structures are influenced by ocean dynamical processes, including oceanic planetary waves, boundary currents, eddies, and mixing. The area and depth of all types of MHWs exhibit significant increasing trends over the past two decades. These results contribute to a better understanding of the physical drivers and ecological impacts of MHWs in a warming climate. 

How to cite: Zhang, Y., Du, Y., Feng, M., and Hobday, A. J.: Vertical structures of global marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16606, https://doi.org/10.5194/egusphere-egu24-16606, 2024.

An extreme event, Marine heatwave, has become a pressing concern in recent years. It is defined as a discrete event where the sea surface temperature remains above a specific threshold value of climatology for several consecutive days, and the intervals between two consecutive abnormal high-temperature events are less than two days. Due to climate change, there is an increasing trend in both the intensity and duration of marine heat waves. Marine heatwaves may not directly affect human society; however, they can pose significant threats to marine ecosystems, coastal communities, and the ocean carbon sink, thereby impacting human well-being. The ocean carbon sink is the most significant carbon sink among the world's three major carbon sinks. It absorbs around 25% of anthropogenic carbon dioxide emissions annually. Dissolved inorganic carbon within the ocean carbon sink relies on the carbon sequestration of biological pumps such as coral, seagrasses, and kelps to store it in the deep water. Influenced by the El Niño-Southern Oscillation and currents, the northeastern Pacific Ocean is a hotspot for marine heatwaves, typically beginning from the North Pacific offshore regions in the spring and impacting the U.S. West Coast in the fall. Consequently, the coastal area of California is selected as the study area and divided into three regions.

Previous studies have shown that the escalating severity of marine heatwaves may result in these biological pumps losing their functions or habitats. However, regarding ocean carbon sequestration, whether the incapacities of these biological pumps due to marine heatwaves will have a short-term impact on the carbon sequestration capacity in the ocean remains to be verified. This study aims to analyze the time series of marine heatwaves and ocean carbon sink capacity with the time series analysis and determine the impacts on ocean carbon sink. We categorize marine heatwave extreme events in California into three indicators and the ocean carbon sequestration capacity into physical and biological indicators. Improved Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (ICEEMDAN) is employed to extract the trends and interannual variations. Meanwhile, to identify the correlations between the marine heatwave and the ocean carbon sink at different time points and different time scales, we apply Time-Dependent Intrinsic Correlation (TDIC). Due to the longer temporal scales in changes in the ocean, the impact of marine heatwaves on the ocean carbon sink may have a potential delay. Therefore, we employ Time-Dependent Intrinsic Cross-Correlation (TDICC), a method based on TDIC that could be utilized to analyze the time-lag effects in the interaction between marine heatwaves and the ocean carbon sink.

How to cite: Fu, C.-H. and Tsai, C. W.: Impact of Marine Heatwaves on Ocean Carbon Sink: A Case Study of Coastal Areas in California, USA, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16628, https://doi.org/10.5194/egusphere-egu24-16628, 2024.

The oceanographic and climate communities are putting significant effort into reaching a consensus on a common definition for Marine Heatwaves (MHW). The absence of such a unified definition poses a substantial obstacle when it comes to making retrospective comparisons between various MHW studies. This hindrance is critical for achieving a mechanistic understanding of the role of MHWs in marine ecosystems.

However, why is it so challenging to characterize and define MHWs? The answer is straightforward: there isn't a single, distinct dynamical mechanism responsible for the persistence of heat anomalies in the ocean, which we refer to as MHWs. Unlike variability associated with phenomena such as large oceanic eddies, oceanic fronts, upwelling systems, tropical cyclones, or climate modes, prolonged heat anomalies do not exhibit characteristic time or spatial scales. As a result, common MHW definitions group together prolonged temperature anomalies lasting from days to years and spanning from a few kilometers to thousands of kilometers in scale.

Analyzing sea surface temperature anomalies through power spectra reveals a "red" power spectrum with no discernible time scales. A similar analysis in spatial dimensions similarly shows a lack of any specific scale. Given this absence of emergent scales, we suggest adopting a process-based definition for MHWs. Such an approach would classify all events into a smaller number of categories, each linked to a specific driver or dynamical process operating on certain spatiotemporal scales. This shift could significantly reduce the subjectivity involved in selecting the temporal and spatial scales required for current MHW definitions, ultimately advancing our understanding of these events.

How to cite: Liguori, G.: The need to adopt process-based or impact-based definitions for marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17696, https://doi.org/10.5194/egusphere-egu24-17696, 2024.

EGU24-18946 | Orals | OS1.10

Towards monitoring subsurface marine heatwaves based on sea surface properties in the Eastern Pacific 

Eike E. Köhn, Matthias Münnich, Meike Vogt, and Nicolas Gruber

As marine heatwaves (MHWs) become a growing concern for marine ecosystems, an effective ecosystem management necessitates precise monitoring of such periods with exceptionally high water temperatures. As satellite-based temperature measurements do not reach beyond the sea surface, identifying subsurface MHWs has so far relied on lower-resolution data obtained from (autonomous) in-situ measurements. In this study, we assess to which extent subsurface MHWs, defined statically by a seasonally varying 90th percentile, can be deduced from surface properties that can be remotely-sensed at a high spatio-temporal resolution. To this end, we build a Random Forest (RF) classification model with daily data from a high-resolution numerical hindcast simulation focused on the Eastern Pacific (1979-2019). The RF is trained to distinguish between extreme and non-extreme temperatures at the depth of the climatologically maximum mixed layer depth (MLD), i.e. a depth that is decoupled from the sea surface throughout most parts of the year. We train the RF on the first 80% of the hindcast simulation data (i.e., 1979-2011) and use a range of predictor variables, such as anomalies of sea surface temperature (SST), height (SSH) and salinity (SSS) as well as derivatives of these physical variables. Testing the model on the last 20% of the hindcast simulation (2012-2019), the RF correctly identifies more than two thirds of all subsurface extreme states, leaving only about 30% of subsurface extremes unidentified. Yet, of all RF-based subsurface extreme classifications, about 40% of subsurface temperatures are false positives. Nevertheless, the RF model outperforms a simple SST based extrapolation of extreme states into the ocean interior. The RF-based classification is mostly guided by SSH and SST anomalies (together reduce impurity by about 50%), followed by climate indices like the Oceanic Niño Index (ONI) and the Pacific Decadal Oscillation (combined impurity reduction by 20%). This simulation-based study emphasizes the potential of exploring remote sensing data, particularly SST and SSH, to extend the monitoring of MHWs beneath the sea surface. Integrating this high-resolution statistical estimate with lower-resolution in-situ hydrographic information has the potential to make subsurface MHW monitoring a feasible and valuable tool for marine ecosystem management.

How to cite: Köhn, E. E., Münnich, M., Vogt, M., and Gruber, N.: Towards monitoring subsurface marine heatwaves based on sea surface properties in the Eastern Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18946, https://doi.org/10.5194/egusphere-egu24-18946, 2024.

EGU24-20695 | Orals | OS1.10

Marine Heatwaves in the Chesapeake Bay: Characteristics, Subsurface Structure and Impact on Hypoxia 

Piero Mazzini, Nathan Shunk, Cassia Pianca, and Ryan Walter

Marine Heatwaves (MHWs) are prolonged events of anomalously warm sea water temperature, and have major detrimental effects to marine ecosystems and the world's economy. Thanks to satellite remote sensing of sea surface temperature, significant advances have been made regarding the characterization and impact of MHWs on global scales, however, these data are typically inadequate to resolve most estuarine environments with complex shorelines and reduced spatial scales. In our work we analyzed a novel data set with over three decades of in situ surface and subsurface temperature records to investigate MHWs in the largest estuary in the US: the Chesapeake Bay. Our major findings will be presented in detail, including MHW characteristics in the Bay, their trends, subsurface structure and impact on Bay hypoxia. Projections of trends found in our work suggest that by the end of the century the Chesapeake Bay will reach a semi-permanent MHW state, when extreme temperatures will be present over half of the year, and thus could have devastating impacts to the bay ecosystem and regional economy. Improving our basic understanding of MHWs, their trends and impact on hypoxia in the Chesapeake Bay is necessary to guide management decisions in this valuable environment.

How to cite: Mazzini, P., Shunk, N., Pianca, C., and Walter, R.: Marine Heatwaves in the Chesapeake Bay: Characteristics, Subsurface Structure and Impact on Hypoxia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20695, https://doi.org/10.5194/egusphere-egu24-20695, 2024.

EGU24-446 | ECS | Posters on site | OS1.11

Untangling the Multi-model Spread in 21st Century AMOC Projections 

Harry Ashton-Key, Jennifer Mecking, and Sybren Drijfhout

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in the global climate by transporting heat northward. According to the latest IPCC report (AR6) the strength of the AMOC is very likely to weaken by 2100 (Fox-Kemperer et al. 2021). A weaker AMOC would significantly impact local and global climate. However, there is large model spread in the magnitude of the projected reduction in AMOC strength (Weijer et al. 2020) so it is unclear to what extent the AMOC will weaken by the end of the 21st century.

This study investigates the spread in AMOC response among CMIP6 models. As an initial step we investigated the model correlations of AMOC weakening across different ScenarioMIP experiments. Preliminary results show that the decline for similarly forced scenarios, such as ssp370 and ssp585, have stronger correlations than for scenarios with significantly different forcing, such as ssp126 and ssp585.

Further analyses into the relationship between the projected weakening and model biases in ocean temperature,  salinity and meridional density gradients are performed. In addition, we investigate how the weakening correlates with possible drivers. A better understanding of how model biases influence AMOC changes will allow for more accurate projections of future AMOC changes and their impacts, as well as improved understanding of what the driving processes of the weakening are in various models.

How to cite: Ashton-Key, H., Mecking, J., and Drijfhout, S.: Untangling the Multi-model Spread in 21st Century AMOC Projections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-446, https://doi.org/10.5194/egusphere-egu24-446, 2024.

EGU24-1969 | ECS | Orals | OS1.11

Crucial role of ocean dynamics for the CMIP models equatorial Pacific warming pattern diversity 

Vincent Danielli, Matthieu Lengaigne, Sadhvi Kwatra, Gopika Suresh, and Jérome Vialard

Coupled Model Intercomparison Project (CMIP) projections indicate a distinct future warming pattern in the tropical Pacific, with enhanced warming in the equatorial Pacific (resembling El Niño warming) and subdued warming in the southeast tropical Pacific. There is currently no consensus on the mechanisms shaping this pattern and its inter-model diversity.

Here, we employ the Sea Surface Temperature (SST) heat budget proposed by Zhang and Li (2014, ZL14), adapted to Relative SST (SST minus its tropical average), a proxy for atmospheric stability and circulation changes. This approach helps uncover the mechanisms that shape the tropical Pacific Multi-Model Mean (MMM) warming pattern and its diversity across historical and unmitigated scenario (RCP85 and SSP585) simulations from 53 CMIP5 and CMIP6 models.

We find that the MMM southeast Pacific relative cooling arises from locally intensified winds, leading to increased latent heat flux cooling. This process also explains the inter-model diversity in this region, alongside the diversity of cloud feedbacks.

Consistent with ZL14 conclusions, our results underscore that the MMM equatorial Pacific relative warming results from a less efficient evaporative cooling feedback over the climatologically cooler central and eastern Pacific. However, our study highlights a pivotal role of ocean dynamics in driving the equatorial Pacific relative warming inter-model diversity. In the eastern Pacific, this diversity is related to the cold tongue bias, with a stronger cold tongue bias leading to a more efficient thermostat mechanism that dampens the MMM relative warming. In the western Pacific, diversity is related to the intensity of the equatorial trade winds relaxation, with stronger westerly anomalies leading to enhanced warming, suggesting a strong role of the Bjerknes feedback.

These results advocate for more comprehensive studies using dynamical approaches to better understand the respective roles of the Bjerknes feedback and cold tongue bias in the equatorial Pacific warming pattern and, ultimately, in the Walker Circulation changes.

How to cite: Danielli, V., Lengaigne, M., Kwatra, S., Suresh, G., and Vialard, J.: Crucial role of ocean dynamics for the CMIP models equatorial Pacific warming pattern diversity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1969, https://doi.org/10.5194/egusphere-egu24-1969, 2024.

Examining the wave climate under climate change scenarios requires a concurrent analysis of both historical and predicted future wave data. This involves using historical wave data to understand seasonal fluctuations and long-term trends, while also utilizing future wave data to predict waves under diverse climate change scenarios. This information is pivotal for evaluating forthcoming risks and formulating strategies for climate change adaptation. This study employs historical wind field data, including ERA5 reanalysis data and CWB/WRF analysis field data, as well as wind field data from the CMIP6 dataset under the SSP5-8.5 extremely high emission scenario. These data are used to drive the WAVEWATCH III wave model for simulating waves. This study initially compared the simulated wave data from the WAVEWATCH III wave model with one year of observed wave data from met-ocean buoys. The results confirmed the high credibility of the simulated waves. Subsequently, extensive data simulations are conducted, encompassing historical wave data (1975-2022) and projected wave data for the future (2025-2100).

This study delves into the long-term temporal variations in wave height in Taiwanese waters and the differential regional trends in spatial changes. Regarding temporal changes, the wave heights are averaged year by year, and then linear regression is performed in units of years. The slope of the regression equation indicates the long-term linear trend of wave height in Taiwanese waters over the years, revealing an increasing trend from the past to the future. Regarding spatial changes, the average wave height at each grid point is calculated, and linear regression is applied to determine the long-term trends in wave height at each grid point from the past to the future. The findings unveil a positive growth trend in Taiwanese waters. Furthermore, Taiwanese watersexperience distinct weather patterns in each season, such as the influence of the northeasterly monsoon in winter and typhoons or southwestern winds in summer. This study further explores the differences and variations of wave during spring (March to May), summer (June to August), autumn (September to November), and winter (December to February of the following year). The analysis results indicate negative growth trends in spring and summer, and positive growth trends are observed in autumn and winter, indicating a noticeable increase in wave height in Taiwanese waters during autumn and winter under the influence of climate change.

How to cite: Fan, Y.-M.: Temporal and spatial varieties of future wave climate under the scenario of climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3018, https://doi.org/10.5194/egusphere-egu24-3018, 2024.

EGU24-3137 | Posters on site | OS1.11

Contrasting future changes in the North Atlantic and Nordic Seas overturning circulations 

Marius Årthun, Helene Asbjørnsen, Leon Chafik, Helen L. Johnson, and Kjetil Våge

The Atlantic meridional overturning circulation (AMOC) carries warm and saline waters northwards near the surface and cold, dense waters southwards at depth. The northward branch of the AMOC terminates north of the Greenland-Scotland Ridge that separates the North Atlantic Ocean from the Nordic Seas and Arctic Ocean. Here, we use large ensemble simulations and CMIP6 models to show that future circulation changes in the subtropical North Atlantic (26.5°N) and in the Nordic Seas show contrasting behavior.

In a high emission scenario (SSP585), CMIP6 models show a gradual weakening of the subtropical AMOC. This weakening can be deconstructed by quantifying changes in the Gulf Stream, Deep Western Boundary Current (DWBC), and gyre recirculation (Asbjørnsen & Årthun 2023). By the end of the century, the Gulf Stream weakens by 29% and the DWBC weakens by 47%. The gyre recirculation component shows a weakening of 12%, indicative of a weakened subtropical gyre. 33% of the Gulf Stream weakening is due to changes in winds.

In contrast to the North Atlantic, the overturning circulation in the Nordic Seas increases throughout most of the 21st century as a result of changes in water mass transformation and horizontal circulation (Årthun et al. 2023). The increased Nordic Seas overturning is furthermore manifested in the overturning circulation in the eastern subpolar North Atlantic (OSNAP-East). A strengthened Nordic Seas overturning circulation could therefore be a stabilizing factor in the future AMOC.

 

Årthun, M., Asbjørnsen, H., Chafik, L.Johnson, H. L., Våge, K. Future strengthening of the Nordic Seas overturning circulation. Nature Communications, 14, 2065 (2023). https://doi.org/10.1038/s41467-023-37846-6

Asbjørnsen, H., & Årthun, M. (2023). Deconstructing future AMOC decline at 26.5°N. Geophysical Research Letters, 50, e2023GL103515. https://doi.org/10.1029/2023GL103515

How to cite: Årthun, M., Asbjørnsen, H., Chafik, L., Johnson, H. L., and Våge, K.: Contrasting future changes in the North Atlantic and Nordic Seas overturning circulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3137, https://doi.org/10.5194/egusphere-egu24-3137, 2024.

EGU24-3144 | ECS | Orals | OS1.11

Overturning Pathways Control AMOC Weakening in CMIP6 Models 

Jonathan Baker, Michael Bell, Laura Jackson, Richard Renshaw, Geoffrey Vallis, Andrew Watson, and Richard Wood

Future projections indicate the Atlantic Meridional Overturning Circulation (AMOC) will weaken and shoal in response to global warming, but models disagree widely over the amount of weakening. We analyse projected AMOC weakening in 34 CMIP6 climate models, in terms of changes in three return pathways of the AMOC. The branch of the AMOC that returns through diffusive upwelling in the Indo-Pacific, but does not later upwell in the Southern Ocean (SO), is particularly sensitive to warming, in part, because shallowing of the deep flow of the AMOC prevents it from entering the Indo-Pacific via the SO. In most models, this Indo-Pacific pathway declines to zero by 2100. Thus, the present-day strength of this pathway provides a strong constraint on the projected AMOC weakening. However, estimates of this pathway using four observationally based methods imply a wide range of AMOC weakening under the SSP5-8.5 scenario of 29%–61% by 2100. Our results suggest that improved observational constraints on this pathway would substantially reduce uncertainty in 21st century AMOC decline. We also present new findings that compare the AMOC response in realistic warming scenarios with those found under more extreme climate forcings, including quadrupled CO2 concentrations and large North Atlantic freshwater forcing.

How to cite: Baker, J., Bell, M., Jackson, L., Renshaw, R., Vallis, G., Watson, A., and Wood, R.: Overturning Pathways Control AMOC Weakening in CMIP6 Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3144, https://doi.org/10.5194/egusphere-egu24-3144, 2024.

EGU24-3561 | Orals | OS1.11

AMOC thresholds in CMIP6 models: NAHosMIP 

Laura Jackson, Alastrue de Asenjo Eduardo, Bellomo Katinka, Danabasoglu Gokhan, Haak Helmuth, Hu Aixue, Jungclaus Johann, Lee Warren, Meccia Virna, Saenko Oleg, Shao Andrew, and Swingedouw Didier

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system, which keeps the North Atlantic relatively warm. It is predicted to weaken under climate change. The AMOC may have a threshold beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models.

We present results from the North Atlantic hosing model intercomparison project, where we applied an idealised forcing of a freshwater flux over the North Atlantic in 8 CMIP6 models to explore this threshold. The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in some models, and in others it stays in a weakened state. We will discuss mechanisms behind the different behaviour in the different models. 

 

How to cite: Jackson, L., Eduardo, A. D. A., Katinka, B., Gokhan, D., Helmuth, H., Aixue, H., Johann, J., Warren, L., Virna, M., Oleg, S., Andrew, S., and Didier, S.: AMOC thresholds in CMIP6 models: NAHosMIP, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3561, https://doi.org/10.5194/egusphere-egu24-3561, 2024.

EGU24-4017 | ECS | Orals | OS1.11

The weakening AMOC under extreme climate change 

Gaurav Madan, Ada Gjermunsen, Silje C. Iversen, and Joseph H. LaCasce

Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO2 experiments conducted underthe sixth Coupled Model Intercomparison Project (CMIP6) are examined. Increased CO2 triggers extensive Arctic warming,causing widespread melting of sea ice. The resulting freshwater spreads southward, first from the Labrador Sea and then theNordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of theseparated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the currentweakens in response, reducing the inflow to the deepwater production regions. The AMOC cell weakens in tandem, firstnear the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perceptionthat freshwater caps the convection regions, stifling deepwater production; rather, it is the inflow to the subpolar gyre thatis suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local orremote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is theprecursor to AMOC weakening in these simulations.

How to cite: Madan, G., Gjermunsen, A., Iversen, S. C., and LaCasce, J. H.: The weakening AMOC under extreme climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4017, https://doi.org/10.5194/egusphere-egu24-4017, 2024.

EGU24-4310 | ECS | Orals | OS1.11

New Arctic quality metrics based on oceanic transports for CMIP6 

Susanna Winkelbauer, Michael Mayer, and Leopold Haimberger

Oceanic transports of heat, volume and salinity are an integral part of the Earth's energy and mass budgets and play a key role in regulating the Earth's climate. Changes in the ocean’s transport patterns may affect regional as well as global climates. Accurate monitoring is critical and there are several regional measuring lines like the RAPID 26N and OSNAP (Overturning in the Subpolar North Atlantic Program) array, as well as measuring lines across Arctic water straits, which are equipped with moorings and other advanced measuring systems. It is desirable to compare the transports calculated by these instruments with ocean reanalyses and climate models. However, this is challenging because the moorings are not aligned with the model grids, and the ocean model grids get complicated especially towards more northern latitudes.

To address this challenge, we introduce StraitFlux (https://pypi.org/project/straitflux/), a versatile tool enabling precise and mass-consistent calculation of volume, heat, and salinity transports across any oceanic section. We have used StraitFlux to calculate transports from reanalyses and climate models (CMIP6) in the Arctic region and to compare them to available observations. While we find some biases, especially in straits that are narrow and bathymetrically complicated, the results generally show that reanalyses capture the main current patterns quite well. Climate models on the other hand exhibit larger and often systematic deviations from the mooring and reanalysis output. The spread among climate models is 3-5 times larger than the spread between observation-based transports and reanalyses or among reanalyses, and it cannot be explained by natural variability. The large spread in flux quantities is related to mean-state biases in relevant state quantities. It helps to quantify and understand the strong connections between lateral OHT and the mean state as well as changes in the Arctic Ocean and sea ice.

Expanding on our methodology, we develop physically based metrics tailored to the Arctic, to detect outliers from the CMIP6 model ensemble and constrain model projections using a weighting approach incorporating the models’ performance and independence. This effectively reduces the spread of future projections of Arctic change. Further, using StraitFlux, we investigate constrained changes in Arctic volume, heat, and salinity transports for the main SSP scenarios. We examine cross-sections of the main Arctic gateways to assess future changes in the structures and strengths of the main currents and their effects on the Arctic system.

How to cite: Winkelbauer, S., Mayer, M., and Haimberger, L.: New Arctic quality metrics based on oceanic transports for CMIP6, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4310, https://doi.org/10.5194/egusphere-egu24-4310, 2024.

The ability of a coarse-resolution ocean model to simulate the response of the Southern Ocean Meridional Overturning Circulation (MOC) to enhanced westerlies is evaluated as a function of the eddy transfer coefficient (κ), which is commonly used to parameterize the bolus velocities induced by unresolved eddies. The strongest eddy-induced MOC response, accounting for 82% of the reference eddy-resolving simulation, is achieved using a stratification-dependent κ with spatiotemporal variability. By decomposing the eddy-induced velocity into its vertical variation (VV) and spatial structure (SS) components, we find that the intensified eddy compensation response is primarily driven by the enhanced SS term, while the introduced VV term weakens the response. Additionally, the temporal variation of the stratification-dependent κ plays a key role in strengthening the eddy compensation response to intensified westerlies. The stronger eddy compensation response in the experiment with stratification-dependent κ than the constant κ can be attributed to the structure of κ and the vertical variation of the density slope. These findings highlight the significance of accurately representing κ for capturing the response of the Southern Ocean MOC and emphasize the role of the isopycnal slope in modulating the eddy compensation mechanism.

How to cite: Li, Y., Liu, H., Lin, P., Chassignet, E., Yu, Z., and Wu, F.: Quantifying the role of the eddy transfer coefficient in simulating the response of the Southern Ocean Meridional Overturning Circulation to enhanced westerlies in a coarse-resolution model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4971, https://doi.org/10.5194/egusphere-egu24-4971, 2024.

EGU24-5263 | ECS | Posters on site | OS1.11 | Highlight

AMOC tipping under Climate Change in the Community Earth System Model 

René van Westen and Henk Dijkstra

Recent quasi-equilibrium simulations with the Community Earth System Model (CESM) have shown that the Atlantic Meridional Overturning Circulation (AMOC) in a pre-industrial climate is a multi-stable system (van Westen & Dijkstra, 2023). By slowly increasing the surface freshwater forcing strength over the North Atlantic Ocean, the AMOC tips from a northward overturning state (strength of 17 Sv) to a fully  collapsed state (strength of 0 Sv). When reversing the freshwater forcing, the AMOC recovers at  smaller values of this forcing compared to the collapse, giving rise to hysteresis behaviour. Here we analyse AMOC tipping under climate change using the same CESM version. From the hysteresis experiment, we branch off simulations under fixed freshwater forcing values to find the statistical steady states. We follow these states under climate change up to 2100 (historical forcing followed by SSP5-8.5) and then run the simulation into equilibrium under constant year 2100 conditions. We find an AMOC tipping event during the 21st century and we compare this event to the one from the pre-industrial quasi-equilibrium simulation. The rate of AMOC changes and the AMOC-related impacts are comparable to the quasi-equilibrium simulation. However, the initial AMOC weakening and the collapsed AMOC state are very different under climate change. Temperature changes primarily drive the initial AMOC weakening and the collapsed state has a very weak (strength of 1 Sv) and shallow (< 1000 m) northward overturning circulation in the Atlantic Ocean. The results indicate that the strong northward overturning statistical steady states disappear under climate change and that only the collapsed AMOC state exists under a high-end emission scenario.

How to cite: van Westen, R. and Dijkstra, H.: AMOC tipping under Climate Change in the Community Earth System Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5263, https://doi.org/10.5194/egusphere-egu24-5263, 2024.

EGU24-6654 | ECS | Orals | OS1.11

Understanding CMIP6 Inter-model Spread of Projected Change in Tropical Sea Surface Salinity 

Shanshan Pang, Jérôme Vialard, Matthieu Lengaigne, and Xidong Wang

Here, we analyze projected tropical sea surface salinity (SSS) changes in 32 Coupled Model Intercomparison Projects phase 6 (CMIP6) global climate models historical simulations and representative concentration pathway 8.5 (SSP5-8.5) scenario. A robust “fresh gets fresher” pattern emerges by the end of the twenty-first century, with fresher tropical Indian and Pacific Oceans and saltier tropical Atlantic Ocean. We examine the inter-model diversity in this pattern using Empirical Orthogonal Function (EOF) analysis. The first two EOFs explain 45% of the total variance. EOF2 (22%) is a modulation of the multi-model mean SSS change, associated with the tropical-average warming intensity (r=0.61). Higher climate sensitivity leads to a more pronounced El Niño-like (positive IOD-like) warming pattern and stronger rainfall in the equatorial and north subtropical Pacific (west Indian) Ocean, leading to local freshening. In the equatorial Atlantic, an enhanced warming leads to more evaporation through the Clausius–Clapeyron relation, and a stronger SSS saltening. The “fresh gets fresher” SSS pattern inter-model diversity is thus more a response to the SST pattern diversity through the “warmer gets wetter” mechanism than an evidence of the “wet gets wetter” intensification of the hydrological cycle. EOF1 (25%) is characterized by saltening in the Indian Ocean and freshening in the Pacific Ocean, associated with changes in the inter-hemispheric relative SST gradient (r=0.55). Enhanced warming in the south hemisphere shifts the precipitation south, reducing total rainfall and saltening the Indian Ocean, while increasing rainfall and freshening the south Pacific Ocean. Overall, we find a strong influence of SST changes on the rainfall distribution, which influences SSS with some effects related to transport by the oceanic circulation.

How to cite: Pang, S., Vialard, J., Lengaigne, M., and Wang, X.: Understanding CMIP6 Inter-model Spread of Projected Change in Tropical Sea Surface Salinity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6654, https://doi.org/10.5194/egusphere-egu24-6654, 2024.

EGU24-11017 | Orals | OS1.11

Constraining CMIP6 model ensemble spread to reduce uncertainty in the representation of the Atlantic water layer temperature in the Arctic Ocean 

Marion Devilliers, Steffen M. Olsen, Shuting Yang, Helene R. Langehaug, Tian Tian, Chuncheng Guo, and Rashed Mahmood

We aim at reducing the uncertainties in the climate predictions of the Arctic region which is going under rapid changes with global repercussions. We analyse the spread in the Atlantic water core temperature across multi member CMIP6 historical simulations, focusing on different regions of the Arctic Ocean. While the redistribution of heat plays a critical role in the dynamics of the Arctic Ocean basins, it is usually not well represented in climate models, leading to divergent projections of future changes in the Arctic. To address this limitation, we compare CMIP6 model outputs with available reanalysis and observational products, in order to identify the biases within the model simulations and develop new metrics to constrain the model ensemble spread. Such metrics can be used to select the multi model ensemble members and construct a subsample with improved representation of the core temperature evolution over the historical period resulting in a reduced uncertainty in near-term future projections of the Arctic climate.

How to cite: Devilliers, M., Olsen, S. M., Yang, S., Langehaug, H. R., Tian, T., Guo, C., and Mahmood, R.: Constraining CMIP6 model ensemble spread to reduce uncertainty in the representation of the Atlantic water layer temperature in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11017, https://doi.org/10.5194/egusphere-egu24-11017, 2024.

EGU24-11172 | ECS | Posters on site | OS1.11

The thermohaline stream function in a changing climate 

Verena Jung and Kristofer Döös

The thermohaline stream function is a powerful tool to analyse water mass transformation (WMT). Traditionally, the meridional overturning circulation is visualised in geographical coordinates with stream functions as a function of latitude whereby the meridional velocity is zonally integrated. Conversely, in the thermohaline framework the entire global ocean is represented in oceanographic well-established coordinates namely absolute salinity and potential temperature. This allows to analyse WMT between cold and warm as well as saline and fresh waters in one single graph. It is generally constituted of a tropical cell, a conveyor belt and a polar cell. Here, we present stream functions from various CMIP6 climate scenarios computed by the EC-Earth model and compare pre-industrial, present-day and climate scenario simulations to study changes in WMT. We further provide background information on how the thermohaline stream function (left panel of the attached Figure) is motivated physically and computed mathematically using Helmholtz decomposition. This allows us to identify sources and sinks of mass in the corresponding thermohaline tendency potential, as shown in the right panel of the attached Figure. The position in the temperature and salinity space of the overturning cells reveal significant differences in the climate scenarios, as well as  differences in the mass sources and sinks revealed by the tendency potentials. These sources are due to the fresh water fluxes through the sea surface  and for the data assimilation data sets, they are also due to mass, heat and salt sources and sinks withing the ocean subsurface domain.


Fig: The thermohaline stream function (left panel) and tendency potential (right panel) computed using data from the ocean component of an EC-Earth model (present-day simulation coloured in red and blue, SSP585-simulation in grey contour lines). They capture the entire ocean circulation in two figures describing the water mass transformation in temperature and salinity.

How to cite: Jung, V. and Döös, K.: The thermohaline stream function in a changing climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11172, https://doi.org/10.5194/egusphere-egu24-11172, 2024.

EGU24-11529 | ECS | Orals | OS1.11

The impact of Greenland ice sheet melt on the future North Atlantic ocean circulation 

Oliver Mehling, Katinka Bellomo, Federico Fabiano, Marion Devilliers, Jost von Hardenberg, and Susanna Corti

Changes in surface freshwater fluxes are a main factor governing the response of the ocean circulation to future climate change. However, they are not well-represented in the most recent generation of Earth System Models (CMIP6), as most CMIP6 models do not include an interactive ice sheet component. Instead, most of them use a very idealized representation of ice sheets. While this approach may yield the correct order of magnitude for present-day meltwater runoff, it might not accurately extrapolate the increasing ice melt under future global warming.

Here, we address this deficiency by prescribing physically plausible meltwater fluxes from the Greenland ice sheet in a CMIP6 model, EC-Earth3, under a strong global warming scenario (SSP5-8.5) until the 23rd century. The meltwater fields were obtained from a CESM2-CISM simulation in which the Greenland ice sheet was fully coupled. The corresponding meltwater flux reaches about 0.4 Sv by the year 2300, comparable to what is often used in water hosing experiments. Using two EC-Earth ensembles of four members each (with and without Greenland meltwater flux), we compare the impact of this previously underestimated runoff on long-term projections of deep-water formation in the North Atlantic and on the evolution of the Atlantic Meridional Overturning Circulation. Our results allow us to quantify the importance of Greenland meltwater on AMOC weakening under strong global warming.

How to cite: Mehling, O., Bellomo, K., Fabiano, F., Devilliers, M., von Hardenberg, J., and Corti, S.: The impact of Greenland ice sheet melt on the future North Atlantic ocean circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11529, https://doi.org/10.5194/egusphere-egu24-11529, 2024.

EGU24-789 | ECS | Orals | CR2.3

Spatio-temporal variable drag for the sub-ice-shelf melt parameterisation in NEMO, ocean model 

Dorothée Vallot, Nicolas Jourdain, and Pierre Mathiot
Ice-shelf basal melting in NEMO, as in most ocean models, is parameterised based on a friction velocity calculated from a drag coefficient that is constant in space and time, usually tuned to approach observed melt. But the drag between the ice and the ocean should depend on the roughness at different scale. This means that roughness evolution in space and time is not taken into account in today's model. In recent decades, some ice shelves, particularly in the Amundsen Sea Embayement (ASE), have experienced an increase of their damage, associated with more surface and basal crevasses so their sub-shelf environment is rougher. There is good chances that this phenomenon is to happen more in the future and in an extended number of ice shelves. Here we present a study using a spatially variable coefficient of drag, which depends on the topography and is applied on the first wet cell height. We use the ice shelf parameterisation of NEMO4.2 on a configuration of ASE at 12th of a degree.

How to cite: Vallot, D., Jourdain, N., and Mathiot, P.: Spatio-temporal variable drag for the sub-ice-shelf melt parameterisation in NEMO, ocean model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-789, https://doi.org/10.5194/egusphere-egu24-789, 2024.

EGU24-797 | ECS | Orals | CR2.3

Annual Terminus Prediction Errors for Greenland Glaciers from Calving Laws and Melt Parameterizations 

Benjamin Reynolds, Sophie Nowicki, Kristin Poinar, and Sophie Goliber

Many calving laws have been proposed leading to a need to characterize the ability of these laws to predict terminus movement across years. The influence of terminus change on glacier discharge makes this an important source of uncertainty for multi-decadal sea level rise prediction from ice sheet models. Here, we develop a workflow to tune calving laws and then calculate error in predicted terminus positions based on Greenland Ice Sheet Mapping Project (GrIMP) surface velocity data sets compiled from Sentinel, Landsat, TerraSAR-X, TanDEM-X, and COSMO-SkyMed satellites as well as digital elevation models (DEMs) from ASTER mission and ArcticDEM data.  Greenland glaciers with available data are used to test the height above flotation, fraction above flotation, crevasse depth criterion, von Mises criterion, and surface stress maximum calving laws over a multi-year period. Several versions of the crevasse depth law based on stress input are tested providing insight into the law’s dependence on stress calculation. This dependence is important as the crevasse depth law has been recommended by calving law comparison but has been implemented with various stress calculations to work with three-dimensional stress fields. The terminus melt parameterization used in the Ice Sheet Model Intercomparison Project for CMIP6 standard experiments is included as reference to show the degree to which calving laws are needed to accurately model retreat for future model intercomparison efforts. While testing calving laws independent of an ice sheet model will not provide insight into all the challenges of calving implementation for ice-sheet-wide studies, this remote-sensing based workflow can rapidly test calving laws’ terminus prediction errors. With the availability of monthly-averaged velocity data sets and frequent instantaneous DEMs, this method will allow for analysis of calving law success on many regimes of multi-year glacier movement.  

How to cite: Reynolds, B., Nowicki, S., Poinar, K., and Goliber, S.: Annual Terminus Prediction Errors for Greenland Glaciers from Calving Laws and Melt Parameterizations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-797, https://doi.org/10.5194/egusphere-egu24-797, 2024.

EGU24-1569 | ECS | Posters on site | CR2.3

A framework for observing and modelling ice-ocean interactions building on a community workshop organised by the Joint Commission on Ice-Ocean Interactions 

Isabel Nias, Felicity McCormack, Sue Cook, Susheel Adusumilli, Lu An, Daniel Goldberg, Tore Hattermann, Yoshihiro Nakayama, Hélène Seroussi, and Donald Slater

Mass loss from the Antarctic and Greenland Ice Sheets could lead to a rise in global mean sea level of 0.25 m by 2100 and several metres by 2300 if greenhouse gas emissions remain unmitigated. Uncertainties in these estimates are strongly related to ocean-driven ice melt, which can lead to grounding line retreat, thinning and acceleration of the fast-flowing regions of both Antarctica and Greenland. The processes of ocean-driven ice melt on large spatial and temporal scales are imperfectly known, and measurements are sparse, impacting the accuracy of ice sheet and ocean model projection studies. The Joint Commission on Ice-Ocean Interactions (JCIOI) hosted the first community workshop in October 2022 with the aims to: (1) identify critical knowledge gaps surrounding processes that govern ocean-driven melt of ice sheets across a range of spatio-temporal scales; and (2) identify options to address the knowledge gaps through observing, parameterising, and modelling ice-ocean interactions, and their impacts on ice mass loss and ocean dynamics. Community discussions from the workshop highlighted the need for concurrent and sustained measurements of ice, ocean and atmosphere properties at the ice sheet-ocean interface, and making best use of existing observations to improve models, capture observed changes, better understand physical mechanisms and improve future projections. Building on the workshop outputs, we propose to develop a framework for ice-ocean observations that details the essential measurements that need to be collected, and the temporal and spatial scales on which to measure. This framework will require widespread community engagement on key scientific questions, agreement and coordination, including protocols for data collection, processing, and sharing.

How to cite: Nias, I., McCormack, F., Cook, S., Adusumilli, S., An, L., Goldberg, D., Hattermann, T., Nakayama, Y., Seroussi, H., and Slater, D.: A framework for observing and modelling ice-ocean interactions building on a community workshop organised by the Joint Commission on Ice-Ocean Interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1569, https://doi.org/10.5194/egusphere-egu24-1569, 2024.

EGU24-3027 | Orals | CR2.3

Strong ice-ocean interaction drives and enhances calving of Antarctic ice shelves 

Yan Liu, Xiao Cheng, Jiping Liu, John Moore, Xichen Li, and Sue Cook

Since 2015, there has been a significant increase in iceberg calving rates from Antarctic ice shelves. It is crucial to comprehend the climate-related reasons for this enhanced iceberg calving to improve coupled simulations with the ice sheet and predict their future effects on sea-level rise. Based on continuous observations of iceberg calving around Antarctica over 15 years, we demonstrate that sea ice extent is the primary control on iceberg calving rates in Antarctica, regardless of ice shelf size, location, or ocean regime. The recent increase in calving rates coincides precisely with a significant reduction in sea ice area in most sectors around the continent. We propose a calving model, where iceberg calving is dominated by ocean-wave induced flexure and basal shear and enhanced by ice-shelf basal melt. We also find links between iceberg calving rate and El Niño/Southern Oscillation (ENSO), which are particularly strong in East Antarctica. Given that further decreases in sea ice extent and increases in extreme ENSO events are predicted in future, we raise concern that previously stable East Antarctic ice shelves may soon begin to retreat, with potential to trigger significant mass loss from this massive ice sheet.

How to cite: Liu, Y., Cheng, X., Liu, J., Moore, J., Li, X., and Cook, S.: Strong ice-ocean interaction drives and enhances calving of Antarctic ice shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3027, https://doi.org/10.5194/egusphere-egu24-3027, 2024.

EGU24-3138 | ECS | Posters on site | CR2.3

Variability of calving and ice flow during a two-week period using terrestrial radar interferometry 

Armin Dachauer, Andrea Kneib-Walter, and Andreas Vieli

Frontal ablation at tidewater outlet glaciers is responsible for a major part of mass loss of the Greenland Ice Sheet. This underscores the need to understand the underlying processes, such as calving and ice flow, with regard to global sea level rise. In this study we explore the temporal and spatial variability of calving activity and ice flow at the major tidewater outlet glacier Eqalorutsit Kangilliit Sermiat (also referred to as Qajuuttap Sermia) in South Greenland and thereby try to get insights into the forcing and relationships between these two processes. This requires high-resolution data which we achieve by using a terrestrial radar interferometer. The instrument provides a temporal resolution of 1 minute and a spatial resolution of a few meters and was running continuously for a two-week field period in August 2023. The data shows considerable spatial and temporal variability of both calving activity and ice flow. Parts of the flow variability can be attributed to a diurnal cycle that is forced by surface melt, whereas enhanced calving activity seems to be tightly linked to locations of major subglacial discharge plumes.

How to cite: Dachauer, A., Kneib-Walter, A., and Vieli, A.: Variability of calving and ice flow during a two-week period using terrestrial radar interferometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3138, https://doi.org/10.5194/egusphere-egu24-3138, 2024.

EGU24-3442 | Orals | CR2.3

Tidewater Glaciers and Ice Shelves as Self-Organising Systems 

Douglas Benn, Jan Åström, Iain Wheel, Adrian Luckman, and Faezeh Nick

Marine-terminating glaciers and ice shelves are notoriously complex, with a wide range of ice-dynamic and calving processes occuring in response to oceanographic, atmospheric and glaciological influences. Within this complexity, however, we can recognise order on at least two scales. First, marine ice fronts typically form vertical cliffs, reflecting competition between oversteepening (ice flow and melt-undercutting) and failure. Calving magnitude-frequency distributions have power-law form with an exponent of -1.2, characteristic of self-organising criticality (SOC). Such systems have a critical point as an attractor, such that the system converges on the failure threshold.

The second scale is that of the whole ice tongue. Tidewater glaciers and ice shelves typically oscillate around stable positions for multiple years, punctuated by transitions to new quasi-stable positions. Stability is encouraged by pinning points which function as attractors at thresholds between stable and metastable states. Ice tongues may exist in metastable states for variable amounts of time, from days to decades. Factors encouraging rapid relaxation to the threshold include large stress gradients and rapid basal melt, and factors encouraging long relaxation times include low stress gradients, low melt rates, and buttressing from mélange or sea ice. Calving magnitude-frequency distributions have exponential form, reflecting the stochastic nature of calving in the metastable zone.

Both scales of self-organisation emerge spontaneously from physically-based calving models such as the Helsinki Discrete Element Model (HiDEM) and the crevasse-depth (CD) calving law implemented in Elmer/Ice. Purely deterministic models, however, are not optimal for long-term simulations, especially in Antarctic contexts. We present results of preliminary simulations using a stochastic CD calving law, which opens up the possibility of a universal calving model applicable to both the Greenland and Antarctic ice sheets.

How to cite: Benn, D., Åström, J., Wheel, I., Luckman, A., and Nick, F.: Tidewater Glaciers and Ice Shelves as Self-Organising Systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3442, https://doi.org/10.5194/egusphere-egu24-3442, 2024.

EGU24-3542 | ECS | Orals | CR2.3

Ice base slope effects on the turbulent ice shelf-ocean boundary current 

Josephine Anselin, Paul Holland, John Taylor, and Adrian Jenkins

The majority of Antarctica’s contribution to sea level rise can be attributed to changes in ocean-driven melting at the base of ice shelves. Turbulent ocean currents and melting are strongest where the ice base is steeply sloped, but few studies have systematically examined this effect. Here we use 3-D, turbulence-permitting large-eddy simulations (LES) of an idealised ice shelf-ocean boundary current to examine how variations in ice base slope influence ocean mixing and ice melting. The range of simulated slope angles is appropriate to the grounding zone of small Antarctic ice shelves and to the flanks of relatively wide ice base channels, with far-field ocean conditions representative of warm-water ice shelf cavities. Within this parameter space, we derive formulations for the friction velocity, thermal forcing, and melt rate in terms of total melt-induced buoyancy input and ice base slope. This theory predicts that melt rate varies like the square root of slope, which is consistent with the LES results and differs from a previously proposed linear trend. With the caveat that further simulations with an expanded range of basal slope angles and ocean conditions would be necessary to evaluate the validity of our conclusions across the full Antarctic ice base slope parameter space, the derived scalings provide a potential framework for incorporating slope-dependence into parameterisations of mixing and melting at the base of ice shelves.

How to cite: Anselin, J., Holland, P., Taylor, J., and Jenkins, A.: Ice base slope effects on the turbulent ice shelf-ocean boundary current, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3542, https://doi.org/10.5194/egusphere-egu24-3542, 2024.

EGU24-5087 | ECS | Orals | CR2.3

The integrated ice sheet response to stochastic iceberg calving 

Aminat Ambelorun and Alexander Robel

Iceberg calving is one of the dominant sources of ice loss from the Antarctic and Greenland Ice sheets. Iceberg calving is still one of the most poorly understood aspects of ice sheet dynamics due to its variability at a wide range of spatial and temporal scales. Despite this variability, current large-scale ice sheet models assume that calving can be represented as a deterministic flux. Failure to parameterize calving accurately in predictive models could lead to large errors in warming-induced sea-level rise. In this study, we introduce stochastic calving within a one-dimensional depth-integrated tidewater glacier and ice shelf models to determine how changes in the calving style and size distribution of calving events cause changes in glacier state. We apply stochastic variability in the calving rate by drawing the calving rate from two different probability distributions.e also quantify the time scale on which individual calving events need to be resolved within a stochastic calving model to accurately simulate the probabilistic distribution of glacier state. We find that incorporating stochastic calving with a glacier model with or without buttressing ice shelves changes the simulated mean glacier state, due to nonlinearities in glacier terminus dynamics. This has important implications for the intrinsic biases in current ice sheet models, none of which include stochastic processes. Additionally, changes in calving frequency, without changes in total calving flux, lead to substantial changes in the distribution of glacier state. This new approach to modeling calving provides a framework for ongoing work to implement stochastic calving capabilities in large-scale ice sheet models, which should improve our capability to make well-constrained predictions of future ice sheet change.

How to cite: Ambelorun, A. and Robel, A.: The integrated ice sheet response to stochastic iceberg calving, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5087, https://doi.org/10.5194/egusphere-egu24-5087, 2024.

EGU24-5666 | ECS | Orals | CR2.3

Circulation, mixing and heat transport in a Greenland fjord 

Anneke Vries, Lorenz Meire, John Mortensen, Kirstin Schulz, Willem Jan van de Berg, and Michiel van den Broeke

Greenland's glacial fjords transport heat and freshwater between the shelf and the outlet glaciers of the Greenland Ice Sheet. Therefore they are crucial to understand ice-ocean interaction in the Norhern Hemisphere. Despite increasing attention from the research community, much of the seasonal variability of fjord circulation remains unknown, especially in the non-summer months. This study presents current velocity and water mass data for a full year in Nuup Kangerlua. We provide insights into the dynamics of this South West Greenland fjord, focusing on winter and the upper layer currents. We show that in winter fjord circulation remains active, including a large cross fjord component that has not been observed before. There is a disconnect between the mouth and the inner part of the fjord, causing heat to be stored in the inner fjord. The stored heat could potentially act as reservoir of melt energy for glaciers in winter.

How to cite: Vries, A., Meire, L., Mortensen, J., Schulz, K., van de Berg, W. J., and van den Broeke, M.: Circulation, mixing and heat transport in a Greenland fjord, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5666, https://doi.org/10.5194/egusphere-egu24-5666, 2024.

EGU24-5804 | ECS | Orals | CR2.3

Sub-shelf melt patterns… does detail matter? 

Franka Jesse, Erwin Lambert, and Roderik van de Wal

Observations show that some of the ice shelves surrounding Antarctica are thinning, driven by warming of the underlying ocean. These ice shelves play an important role in moderating the rate of mass loss from the ice sheet by buttressing the ice flow from the grounded parts of the ice sheet. The increased ocean-induced sub-shelf melt is therefore an important process for the stability of the ice sheet and representing it in ice sheet models is essential to study the evolution of the Antarctic Ice Sheet. Here, we present a coupled ice-ocean setup, applied to an idealised ice shelf.

The sub-shelf melting occurs in highly heterogeneous patterns, typically exhibiting higher melt rates near the grounding line. Currently, most ice sheet models rely on parameterisations which derive sub-shelf melt rates from far-field ocean hydrography. Despite their computational advantage and ease in handling grounding line migration, these parameterisations fall short of accurately representing the right details in the melt patterns. To capture more physically consistent melt patterns, we implemented an online coupling between the ice sheet model IMAU-ICE and the sub-shelf melt model LADDIE. The latter resolves the necessary physics governing the melt, including the Coriolis deflection and topographic steering of meltwater, and provides sub-shelf melt fields at sub-kilometre spatial resolution.

We will show the impact of detailed sub-shelf melt fields in an idealised set-up. We compare IMAU-ICE simulations using existing sub-shelf melt parameterisations with simulations in the coupled set-up with IMAU-ICE and LADDIE. Three parameterisations are considered for this comparison: the quadratic scaling with temperature, the box model PICO, and the plume model. All simulations are performed in the idealised MISMIP+ domain. We consider a range of oceanic temperature forcings similar to present-day temperatures in warmer and colder basins surrounding Antarctica. We present and discuss the results, primarily focusing on the evolution of three key indicators for ice sheet stability: grounding line position, ice shelf extent, and grounding zone shape. These results demonstrate the importance of accounting for realistic melt patterns in ice sheet models.

How to cite: Jesse, F., Lambert, E., and van de Wal, R.: Sub-shelf melt patterns… does detail matter?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5804, https://doi.org/10.5194/egusphere-egu24-5804, 2024.

EGU24-6423 | ECS | Orals | CR2.3

Re-evaluating Rapid Glacier Retreats: Hektoria Glacier’s Unprecedented Tidewater Collapse 

Naomi E. Ochwat, Ted A. Scambos, Robert S. Anderson, Catherine C. Walker, and Bailey L. Fluegel

Hektoria Glacier on the Eastern Antarctic Peninsula underwent a heretofore unseen rate of tidewater-style glacier retreat from 2022 to 2023 after the loss of decade-old fast ice in the Larsen B embayment. The glacier has retreated 25 km between February 2022 and January 2024, of which at least 8-13 km was grounded ice. Remote sensing data in the months following the fast ice break-out reveals an ice flow speed increase of up to 4-fold, and rapid elevation loss up to 20-30 m, representing an 8-fold increase in the glacier thinning rate. Hektoria and Green Glaciers underwent three phases of retreat displaying differing calving styles. During the first two months after the loss of the fast ice in January 2022 the Hektoria-Green ice tongue calved large tabular bergs. In March 2022, an abrupt change in Hektoria’s calving style was observed, changing from large tabular icebergs to buoyantly rotated smaller bergs. Following this transition, Hektoria underwent several short periods of rapid retreat. In December 2022, 2.5 km of grounded ice were lost over 2.5 days. These retreat rates for grounded tidewater ice are greater than any reported in the modern glaciological record. Here we examine the evidence for locating the pre-fast ice break-out grounding zone as well as the drivers that could cause such a rapid retreat. We link these observations to known causes of glacier instability, such as Marine Ice Sheet Instability and Marine Ice Cliff Instability, as well as the classical tidewater glacier retreat cycle.

How to cite: Ochwat, N. E., Scambos, T. A., Anderson, R. S., Walker, C. C., and Fluegel, B. L.: Re-evaluating Rapid Glacier Retreats: Hektoria Glacier’s Unprecedented Tidewater Collapse, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6423, https://doi.org/10.5194/egusphere-egu24-6423, 2024.

EGU24-6639 | ECS | Orals | CR2.3

Multi-decadal evolution of Crary Ice Rise region, West Antarctica, amidst modern ice stream deceleration 

Hannah Verboncoeur, Matthew Siegfried, J. Paul Winberry, Nicholas Holschuh, Duncan Byrne, Wilson Sauthoff, Tyler Sutterley, and Brooke Medley

The ongoing deceleration of Whillans Ice Stream, West Antarctica, provides an opportunity to investigate the role of grounded ice flux in downstream pinning point evolution on decadal time scales. Here, we construct and analyze a 20-year, multi-mission satellite altimetry record of dynamic ice surface-elevation change (dh/dt) in the grounded region between lower Whillans Ice Stream and Crary Ice Rise, a major Ross Ice Shelf pinning point. We developed a new method for generating multi-mission time series that reduces spatial bias and implemented this method with altimetry data from the Ice, Cloud, and land Elevation Satellite (ICESat; 2003–09), CryoSat-2 (2010–present), and ICESat-2 (2018–present) altimetry missions. We then used the 20-year dh/dt time series to identify persistent patterns of surface elevation change and to evaluate regional mass balance. Our results suggest that changes in ice flux associated with Whillans Ice Stream stagnation drive non-linear mass change responses isolated to the Crary Ice Rise region, producing persistent, spatially heterogeneous thickness changes. The resulting mass redistribution modifies the grounding zone and mass balance of the Crary Ice Rise region, in turn adjusting the buttressing regime of the southern Ross Ice Shelf embayment.

How to cite: Verboncoeur, H., Siegfried, M., Winberry, J. P., Holschuh, N., Byrne, D., Sauthoff, W., Sutterley, T., and Medley, B.: Multi-decadal evolution of Crary Ice Rise region, West Antarctica, amidst modern ice stream deceleration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6639, https://doi.org/10.5194/egusphere-egu24-6639, 2024.

EGU24-6749 | ECS | Orals | CR2.3

Two Decades of Satellite Observations: Sensible-Heat Polynya Variability at Pine Island Glacier, West Antarctica 

Elena Savidge, Tasha Snow, and Matthew R. Siegfried

Thermodynamically maintained open ocean areas surrounded by sea ice, or sensible-heat polynyas, are linked to key ice-sheet processes, such as ice-shelf basal melt and ice-shelf fracture, when they occur near ice-shelf fronts. However, the lack of detailed multi-year records of polynya variability pose a barrier to assessing the potential interconnectivity between polynya and frontal dynamics. Here, we present the first multi-decadal record (2000–2022) of polynya area at Pine Island Glacier (PIG) from thermal and optical satellite imagery. We found that although polynya area was highly variable, there were consistencies in the timing of polynya maximal extent, and opening and closing. Furthermore, we found that the largest polynya (269 km2) in our record occurred at PIG’s western margin just 68 days before iceberg B-27 calved, suggesting that polynya size and position may influence rifting dynamics. We suspect that large sensible-heat polynyas have the potential to reduce both ice-shelf buttressing (via reduced landfast ice) and shear margin dynamics (via reduced contact with slower marginal ice), which may lead to structural instability and eventually contribute to calving. Our new dataset provides a pathway to assess coevolving polynya and frontal dynamics, demonstrating the importance of building long-term records of polynya variability across the continent.

How to cite: Savidge, E., Snow, T., and Siegfried, M. R.: Two Decades of Satellite Observations: Sensible-Heat Polynya Variability at Pine Island Glacier, West Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6749, https://doi.org/10.5194/egusphere-egu24-6749, 2024.

EGU24-7829 | ECS | Orals | CR2.3

High fidelity modelling of iceberg capsize 

Nicolas De Pinho Dias, Alban Leroyer, Anne Mangeney, Olivier Castelnau, and Jean-Baptiste Thiebot

One of the major questions in climate science is to improve the accuracy of sea-level rise prediction, for which mass loss of the polar ice caps has a significant contribution. In this work, the focus is on buoyancy-dominated capsize of large icebergs. The capsizes generate specific seismic signals, which in turn can be analysed and used as a unique tool to study the long term evolution of such large icebergs capsize and the glacier response.

To better quantify ice mass loss due to iceberg calving at marine terminating glaciers, coupling iceberg calving simulation and inversion of the seismic waves generated by these events and recorded at teleseismic distances is necessary. To achieve our task, a complex fluid/structure model of the iceberg capsize is required to obtain accurate forces history acting on the glacier terminus. The simulated forces can then be compared to the force inverted from the seismic signal. Therefore, based on our recent work, we implement a Computation Fluid Dynamics (CFD) approach to reach a high fidelity modelling of the iceberg capsize. First work using the experimental data of an iceberg capsize showed the need and ability of CFD computations to precisely reproduce the iceberg kinematics for different cases. We will present more advanced CFD configurations, including the contact between the capsizing iceberg and a rigid glacier front. Computation results are compared and validated against lab scale experiments, where we outline that some 3D effects cannot be neglected. We will also present full scale capsize simulations, in which the mixing of ocean layers occurs. In particular, we will quantify the transport of particles within the ocean to illustrate the potential change of nutriments distribution or of pressure experienced by local fauna due to iceberg calving.

How to cite: De Pinho Dias, N., Leroyer, A., Mangeney, A., Castelnau, O., and Thiebot, J.-B.: High fidelity modelling of iceberg capsize, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7829, https://doi.org/10.5194/egusphere-egu24-7829, 2024.

EGU24-8352 | ECS | Orals | CR2.3

A reassessment of the role of atmospheric and oceanic forcing on ice dynamics at Jakobshavn Isbræ, Ilulissat Icefjord 

Hannah Picton, Peter Nienow, Donald Slater, and Thomas Chudley

Jakobshavn Isbræ (Sermeq Kujalleq) has been the largest single contributor to mass loss from the Greenland Ice Sheet over recent decades. Previous work has emphasised the dominant role of oceanic forcing on ice dynamics, with the short-lived (2016-2018) advance, deceleration and thickening of Jakobshavn attributed to decreased ocean temperatures within Disko Bay. Here, we use satellite imagery to extend observations of ice dynamics at Jakobshavn Isbræ between 2018 and 2023. We then employ hydrographic measurements, weather station data, and modelled estimates of surface runoff, to explore the role of climatic forcing on ice dynamics over this most recent five-year period. 

Between 2018 and 2022, Jakobshavn Isbræ accelerated significantly, with peak summer terminus velocity increasing by 79%, from 9.4 to 16.8 km/yr. Despite sustained surface lowering, peak solid ice discharge also increased, rising from 39.4 Gt/yr in 2018 to 54.7 Gt/yr in 2021. Whilst the initial onset of re-acceleration occurred in 2019, a dramatic speedup occurred between May and August 2020, with ice velocity increasing from 7.6 to 13.8 km/yr. In contrast to previous years, ice velocity remained high throughout the subsequent winter, thereby facilitating a peak velocity of 16.8 km/yr in July 2021.

Jakobshavn Isbræ exhibited a typical seasonal calving cycle of winter advance and summer retreat throughout 2018 and 2019. However, a clear switch in dynamics was observed in 2020, with the terminus undergoing minimal readvance over the winter months. This shift coincided with a clear reduction in the extent of rigid mélange within Ilulissat Icefjord, in contrast to preceding years. Although sparse, hydrographic measurements indicate that the mean water temperature within Disko Bay was ~ 0.75⁰C higher in 2020, relative to 2019.

We argue that the initial onset of reacceleration and thinning at Jakobshavn Isbræ was driven primarily by atmospheric forcing, with annual runoff in 2019 approximately double that observed in the other years. Furthermore, we emphasise that at glaciers close to floatation, such as Jakobshavn, surface thinning can significantly impact buoyant flexure, and hence rates of calving. However, we also provide evidence of oceanic forcing, postulating that increased water temperatures reduced the formation of rigid mélange in 2020, thereby facilitating sustained calving and elevated ice velocities throughout the winter months. Our study therefore highlights the critical importance of considering both atmospheric and oceanic forcing when investigating and predicting the future behaviour of ice dynamics at marine-terminating outlet glaciers.

How to cite: Picton, H., Nienow, P., Slater, D., and Chudley, T.: A reassessment of the role of atmospheric and oceanic forcing on ice dynamics at Jakobshavn Isbræ, Ilulissat Icefjord, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8352, https://doi.org/10.5194/egusphere-egu24-8352, 2024.

EGU24-8616 | ECS | Orals | CR2.3

Calving dynamics and mélange buttressing conditions at the Thwaites Glacier calving face 

Anna Crawford, Jan Åström, Doug Benn, Adrian Luckman, Rupert Gladstone, Thomas Zwinger, Fredrik Robertsén, and Suzanne Bevan

Thwaites Glacier, a large outlet glacier of the West Antarctic Ice Sheet, holds over a half meter of sea level rise equivalent. The large potential contribution to sea level is concerning given that the glacier may be vulnerable to self-sustaining processes of rapid retreat due to the retrograde bed slope that characterises much of the glacier’s bed. Such a reverse-sloping bed exists behind the relatively high ridge on which the western calving front (WCF) of the Thwaites Glacier terminus currently rests. Our study focuses on the factors that control the calving dynamics of the WCF and the ability of mélange to influence these dynamics. Employing the 3D Helsinki Discrete Element Model (HiDEM), we find that calving at this location currently occurs as rifts form and widen due to longitudinal tensile stresses associated with ice flow across the grounding line. Calving is restricted in HiDEM simulations that include a constricted mélange field that is confined within the bounds of the model domain. A thicker, constricted mélange field fully suppresses calving. These simulations show the development of robust force chains that transmit resistive forces to the Thwaites WCF. In the future, the ability for mélange to influence the calving dynamics at the WCF will depend on the degree to which it is constrained in the wide Amundsen Sea Embayment, either through binding in land-fast sea ice or jamming behind large, grounded icebergs. As such, sea-ice conditions and iceberg characteristics will need to be considered along with the presence of mélange in investigations of the future retreat of the prominently recognised Thwaites Glacier.

How to cite: Crawford, A., Åström, J., Benn, D., Luckman, A., Gladstone, R., Zwinger, T., Robertsén, F., and Bevan, S.: Calving dynamics and mélange buttressing conditions at the Thwaites Glacier calving face, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8616, https://doi.org/10.5194/egusphere-egu24-8616, 2024.

EGU24-9102 | ECS | Posters on site | CR2.3

Calving of floating ice shelves and icebergs in Antarctica triggered by internal ocean waves driven by marine ice-cliff 

Zhenfu Guan, Yan Liu, Teng Li, and Xiao Cheng

Ice calving around Antarctica has a significant impact on glacier dynamics, sea ice, and marine productivity, which in turn affect global sea level and climate.  However, there is limited documented knowledge of the causes of ice calving triggered by internal ocean processes throughout Antarctica, especially during the austral winter.  A total of 3708 iceberg calving events were observed along the circum-Antarctic coastline over a three-month winter period.  These events included the calving of ice cliffs, ice shelves, and icebergs, spanning seven orders of magnitude in spatial scale.  The results suggest that ice cliff calving is primarily driven by internal glacier stresses and is widespread along the Antarctic coast.  The frequency of calving is primarily controlled by glacier ice velocity.  About 70% of the calving in Antarctica occurs on the Antarctic Peninsula.  Internal waves generated by ice cliff calving cascade to small enough scales to induce shear that leads to near-field (~40 km) calving of floating ice shelves and icebergs in regions of high topographic relief.  This study presents a newly discovered mechanism for ice shelf and iceberg calving driven by oceanic forces.  The mechanism has broad applicability and can serve as a catalyst for calving modeling and the study of oceanic internal waves.

How to cite: Guan, Z., Liu, Y., Li, T., and Cheng, X.: Calving of floating ice shelves and icebergs in Antarctica triggered by internal ocean waves driven by marine ice-cliff, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9102, https://doi.org/10.5194/egusphere-egu24-9102, 2024.

EGU24-9429 | Posters on site | CR2.3

Distributed and time-series estimates of basal melt from Kamb Ice Stream’s grounding zone ocean cavity 

Huw Horgan, Natalie Robinson, Craig Stevens, Craig Stewart, Christina Hulbe, Justin Lawrence, Britney Schmidt, and Peter Washam

Melt beneath Antarctica’s large cold-cavity ice shelves remains a major source of uncertainty in ice sheet projections. Beneath these ice shelves melt is typically greatest both at the ice shelf front and at the grounding zone where ice first goes afloat. Grounding zone melt is thought to have a significant influence on ice flow across the grounding line, but can be difficult to estimate using remote sensing methods due to flexure of the overriding ice shelf. Added complexity in the grounding zone is caused by the thin water column, abundant basal crevassing, and the possible addition of subglacial fresh water draining from beneath the ice sheets. Here we present two independent estimates of basal melt from the ocean cavity of Kamb Ice Stream’s grounding zone, Ross Ice Shelf, West Antarctica. The first method uses repeat phase-sensitive radar observations to estimate melt in profiles from approximately 5 km seaward of the grounding line to approximately 3 km upstream of the grounding line. The second method uses an approximately 10-month long time series of oceanographic observations from a site 3.5 km seaward of the grounding line. Both methods are complemented by the high resolution observations provided by the Remotely Operated Vehicle (ROV) Icefin. The spatially distributed estimates show a more than tripling of melt rate within 5 km of the grounding line. The mooring derived melt rates demonstrate a melt-rate dependence on diurnal and spring-neap tidal currents. The average mooring melt rate more closely matches the radar-based estimates when a drag coefficient previously estimated using Icefin observations is used. Lastly we demonstrate an interesting correlation between mooring derived melt rates and ice shelf surface velocities obtained from Global Navigation Satellite System (GNSS) observations.

How to cite: Horgan, H., Robinson, N., Stevens, C., Stewart, C., Hulbe, C., Lawrence, J., Schmidt, B., and Washam, P.: Distributed and time-series estimates of basal melt from Kamb Ice Stream’s grounding zone ocean cavity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9429, https://doi.org/10.5194/egusphere-egu24-9429, 2024.

EGU24-9500 | Posters on site | CR2.3

A viscoelastic phase-field model for iceberg calving 

Robert Arthern, Jakub Stocek, and Oliver Marsh

Iceberg calving accounts for around half of the ice lost annually from Antarctica, but realistic representation of fracture and calving in large-scale ice sheet models remains a major unsolved problem in glaciology. We present a new phase-field viscoelastic model for fracture that simulates the slow deformation of ice and the distribution and evolution of cracks. Cracks nucleate and propagate in response to the evolving stress field, and are influenced by water pressure below sea level. The model incorporates nonlinear-viscous rheology, linear-elastic rheology, and a phase-field variational formulation, which allows simulation of complex fracture phenomena. We show that this approach is capable of simulating the physical process of calving. Numerical experiments supported by a simplified model suggest that calving rate will scale with the fourth power of ice thickness for a floating ice front that has no variation across flow. The equations make no assumptions about the style of calving, so they would also simulate numerous more realistic settings in Antarctica for which material parameters and three-dimensional effects can be expected to influence the calving rate.

How to cite: Arthern, R., Stocek, J., and Marsh, O.: A viscoelastic phase-field model for iceberg calving, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9500, https://doi.org/10.5194/egusphere-egu24-9500, 2024.

EGU24-9876 | ECS | Posters on site | CR2.3

Is climate change responsible for recent retreat of the Pine Island Glacier in West Antarctica? 

Alex Bradley, David Bett, Paul Holland, C. Rosie Williams, and Robert Arthern

Pine Island Glacier is a fast flowing ice stream in West Antarctica. At present, it is rapidly thinning and retreating, and has been since at least the 1970s, when satellite records began. Sediment records indicate that this retreat was initiated in the 1940s, but the influence of climate change on key forcing components only became significant in the 1960s, i.e. the trigger for retreat occurred naturally. However, current ice loss remains responsive to fluctuations in forcing, indicating that Pine Island Glacier is not undergoing a purely unstable retreat after this trigger. This begs the question: to what extent is climate change responsible for the recent retreat of the Pine Island Glacier?

Adopting a recently published framework, we assess this question. One major challenge is the computational expense associated with the large ensemble of simulations required to account for significant uncertainties in ice sheet model parameters; to overcome this, we use a two stage Ensemble Kalman Inversion and Model Emulation approach. Ultimately, this procedure yields posterior distributions of parameters, including the trend in forcing resulting from climate change; essentially, this allows us to address the question: given the observed Pine Island Glacier retreat, how large does the trend in forcing have to have been?

How to cite: Bradley, A., Bett, D., Holland, P., Williams, C. R., and Arthern, R.: Is climate change responsible for recent retreat of the Pine Island Glacier in West Antarctica?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9876, https://doi.org/10.5194/egusphere-egu24-9876, 2024.

EGU24-10186 | ECS | Orals | CR2.3

Exposing Underestimated Channelized Basal Melt Rates in Antarctic Ice Shelves 

Ann-Sofie Priergaard Zinck, Stef Lhermitte, and Bert Wouters

Ice shelves play a pivotal role in stabilizing the Antarctic ice sheet by providing crucial buttressing support. However, their vulnerability to basal melting poses significant concerns for ice sheet and shelf stability. Our study focuses on assessing basal melt rates at a 50 m posting of 12 ice shelves where earlier studies have identified high melt rates. We make use of the Reference Elevation Model of Antarctica (REMA) strips to generate surface elevation- and melt rates using the Basal melt rates Using Rema and Google Earth Engine (BURGEE) methodology.

BURGEE reveals higher melt rates in areas with thinner ice than existing remote sensing basal melt products. This is for instance the case for basal channels on both Dotson, Totten and Pine Island ice shelves. Modelling studies have already shown that remote sensing inferred basal melt rates are underestimated at the thinnest part of basal channels, and that this underestimation scales with resolution coarsening. Since the thinner parts of an ice shelf also represent its weakest part, it is crucial that we capture its melting well to fully grasp the vulnerability of the ice shelf.

Our work, therefore, represents a crucial step in uncovering the vulnerability of Antarctic ice shelves. By exposing detailed melting patterns, particularly in areas like basal channels, we highlight not just extensive melting but also potential weak points, significantly contributing to our understanding of ice shelf stability. These findings bear substantial importance in comprehending the broader implications of ongoing climate changes on Antarctica's ice sheet integrity and, consequently, global sea levels.

How to cite: Zinck, A.-S. P., Lhermitte, S., and Wouters, B.: Exposing Underestimated Channelized Basal Melt Rates in Antarctic Ice Shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10186, https://doi.org/10.5194/egusphere-egu24-10186, 2024.

EGU24-10287 | ECS | Posters on site | CR2.3

Ocean-induced glacier retreat drives mass loss in Svalbard  

Tian Li, Konrad Heidler, Adam Igneczi, Stefan Hofer, Xiao Xiang Zhu, and Jonathan Bamber

Arctic Amplification is making Svalbard one of the most climatically sensitive regions in the world and it has been undergoing accelerated mass loss over the past several decades. A major uncertainty in predicting the future sea-level rise contribution from marine-terminating glaciers is ice dynamics, which can be driven by non-linear calving processes. However, the relationship between calving and ice dynamics is not well understood in Svalbard, in part due to the lack of high-resolution calving front observations. To improve our understanding of the glacier calving dynamics and its relation to dynamic mass loss, here we use a novel fully automated deep learning framework to produce a new calving front dataset of 149 marine-terminating glaciers in Svalbard. This dataset, which includes 124919 glacier calving front positions from 1985 to 2023, has high spatial and temporal resolutions and is derived from multiple optical and SAR satellite images. We then use this new calving front dataset to systematically quantify the calving front change variabilities at different temporal scales, and identify the key climate drivers controlling the calving dynamics. We show that ocean forcing plays a central role in controlling the glacier calving front changes and mass imbalance. Our study highlights the importance of including ice-ocean interaction in projecting future glacier mass loss from Svalbard.  

How to cite: Li, T., Heidler, K., Igneczi, A., Hofer, S., Zhu, X. X., and Bamber, J.: Ocean-induced glacier retreat drives mass loss in Svalbard , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10287, https://doi.org/10.5194/egusphere-egu24-10287, 2024.

EGU24-10402 | ECS | Posters on site | CR2.3

Idealized, High Resolution, 3D Modelling of Ice-Sheet Ocean interactions in long and narrow fjords 

Jonathan Wiskandt, Inga Monika Koszalka, and Johan Nilsson

Ocean forcing of basal melt at the Greenland and Antarctic ice sheets remains a major source of uncertainty in climate ice sheet modelling. Previous efforts to represent these effects focused mainly on the properties of the ocean waters reaching the marine terminating glaciers as well as the near-ice boundary layer flows and processes at the ice-ocean interface. We use high resolution, three dimensional modelling to show the influence that rotational effects have on the fjords circulation and the melt rate distribution and compare the total melt to earlier estimates from two dimensional simulations. Furthermore we investigate the influence that the along and across fjord bathymetry of Greenlandic glacial fjords has on the exchange flow of the warm ocean waters towards the ice sheets and the glacially modified water toward the open ocean. We find that the circulation pattern produced by rotational effects has a profound effect on the distribution of the melt rate at the ice base, producing a concentrated outflow and a melt maximum at the eastern side of a fjord that opens to the open ocean in the north even in narrow fjords (width of the order of the local Rossby Radius). The bathymetry in the fjord has a restricting effect on the inflow of warm Atlantic water and hence on the temperature forcing at the ice base. We compare the inflow strengths for different fjord bathymetries to theoretical estimateion using hydraulic theory (Whitehead, 1998).

How to cite: Wiskandt, J., Koszalka, I. M., and Nilsson, J.: Idealized, High Resolution, 3D Modelling of Ice-Sheet Ocean interactions in long and narrow fjords, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10402, https://doi.org/10.5194/egusphere-egu24-10402, 2024.

EGU24-10590 | ECS | Orals | CR2.3

Weathering crust and cryoconite holes on the Hells Gate and Nansen Ice Shelves (East Antarctica) 

Giacomo Traversa and Biagio Di Mauro

The penetration of shortwave radiation at the surface of an ice shelf has the potential to induce internal melting, resulting in the formation of a porous layer close to the surface commonly known as the weathering crust. This dynamic hydrological system is known to host light-absorbing impurities and microbes, forming a highly porous layer at the ice sheet's surface. The presence of the weathering crust significantly impacts the overall volume of generated meltwater by modulating the extent to which shortwave radiation is absorbed or reflected by the ice. Beyond external meteorological forcing, local conditions leading to weathering crust formation can be influenced by biological impurities on ice surfaces. This interplay between surface ice structures, cryoconite holes (CHs) and weathering crust contributes to the spatial and temporal variability of albedo and surface melt. In this study, we analysed uncrewed aerial vehicle (UAV) data and ground-based field spectroscopy data collected during the 2022/23 austral summer in Antarctica. The aim is to map CHs spatial distribution and to evaluate their radiative impact on blue ice fields at the Hells Gate Ice Shelf in Northern Victoria Land (East Antarctica). Furthermore, we documented the formation of the weathering crust and supraglacial ponds at Hells Gate and Nansen Ice Shelves across the summer solstice. By analysing Sentinel-2 satellite data, we were able to determine the spatial variability in surface albedo before and after the formation of the weathering crust. In detail, at the Hells Gate Ice Shelf, we estimated < 1% of area covered by CHs. Over frozen ponds and ice bands the area covered in CHs reached almost 10%. The corresponding spatially integrated-radiative forcing resulted to be about 1 Wm-2 in average, but locally it reached values of over 200 Wm-2, thus sustaining liquid water inside the CHs. As for the weathering crust, the delta albedo (Δα) was found to be about +0.10 and +0.40 respectively where weathering crust covered blue and marine ice. On the other hand, the supraglacial pond and stream formation provided an opposite Δα of about -0.30 over blue ice and -0.50 over areas previously characterised by snow cover. However, the fractional area interested by positive Δα resulted to be significantly higher than positive Δα areas over the two ice shelves.

How to cite: Traversa, G. and Di Mauro, B.: Weathering crust and cryoconite holes on the Hells Gate and Nansen Ice Shelves (East Antarctica), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10590, https://doi.org/10.5194/egusphere-egu24-10590, 2024.

EGU24-10983 | ECS | Posters on site | CR2.3

Ice-ocean coupled modelling for Nioghalvfjerdsbræ (79NG), Greenland 

Joanna Zanker and Jan De Rydt

The Northeast Greenland Ice Stream (NEGIS) drains approximately 12 % of the Greenland Ice Sheet’s surface area, containing an ice volume of 1.1 m sea-level equivalent. Nioghalvfjerdsbræ (79NG) is one of two main outlet glaciers of NEGIS, extending into a large floating ice tongue, one of few remaining in Greenland. It is currently not well understood how 79NG will respond to the changing atmosphere and warming oceans, with possible implications for the catchment’s surface mass balance (SMB) and ocean-induced ablation. This research aims to assess the importance of feedbacks between ice-sheet geometry, SMB and ocean-driven melt by having a mutually evolving dynamical ice sheet with evolving SMB parameterization and a 3D ocean circulation model utilising the ice-ocean coupled model Úa-MITgcm. The potential feedbacks between changes in ice-sheet surface geometry, ice-tongue cavity geometry and the atmosphere/ocean mass balance are as-of-yet poorly understood, especially in the context of Greenland. Of particular interest for NEGIS is the potential for geometry induced changes in melting of the ice tongue, as found for some Antarctic ice shelves. Development of the Úa ice-flow model will begin with a Greenland-wide setup and experiments based on the ISMIP6 protocol, before focussing on a regional setup of the NEGIS catchment and coupling to a regional configuration of the MITgcm ocean model of the adjacent fjord and continental shelf. The coupled approach of this project aims to improve the representation of the feedbacks between different climate components at a regional scale and draw conclusions about the fidelity of projections of ice sheet-wide mass loss and sea-level rise from ISMIP. 

How to cite: Zanker, J. and De Rydt, J.: Ice-ocean coupled modelling for Nioghalvfjerdsbræ (79NG), Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10983, https://doi.org/10.5194/egusphere-egu24-10983, 2024.

EGU24-11023 | ECS | Posters on site | CR2.3

Spatiotemporal evolution of subaerial ice cliff heights at marine-terminating outlet glaciers in Northwestern Greenland 

Emma Carr, Rachel Carr, Chris Stokes, Emily Hill, Hilmar Gudmundsson, and Neil Ross

Many tidewater glaciers in Greenland terminate in near-vertical ice cliffs from which icebergs calve. Marine Ice Cliff Instability (MICI) is the hypothesis that above a subaerial ice cliff height limit, the tensile or shear stresses at the glacier terminus surpass the ice yield strength, causing catastrophic cliff failure and self-sustaining ice frontal retreat as sequentially taller subaerial cliffs are exposed. Previous modelling studies have proposed this threshold subaerial cliff height is at least 100 m, with estimated thresholds including 100 m and 110 m for damaged ice, and up to 540 m when ice is treated as undamaged. However, modern-day observations to test MICI are limited because few marine-terminating outlet glaciers without a buttressing ice shelf are known to terminate in subaerial ice cliffs greater than 100 m high. Here, we expand the observations of subaerial ice cliff heights at ten marine-terminating outlet glaciers in northwest Greenland using 2 m spatial resolution Arctic DEM strips. Our results identify three marine-terminating outlet glaciers that currently terminate in exposed subaerial ice cliffs approaching or exceeding the stability thresholds estimated for MICI. During at least two years between 2016 and 2021, subaerial ice cliffs at Nuussuup Sermia (NuS), Nunnatakassaap Sermia (NkS) and Sermeq North (SqN) exceeded heights of 100 m and 110 m. Despite being above these postulated thresholds thought conducive for cliff failure, SqN underwent relatively limited net retreat (0.25 km), and NuS and NkS exhibited distinct seasonal cycles of terminus advance (up to 0.92 km) from March to June/July each year prior to the disintegration and removal of proglacial ice mélange. Consequently, none of the glaciers identified as potentially susceptible to MICI underwent rapid, unforced retreat. We hypothesise that MICI processes were mitigated by dynamic thinning lowering the ice surface elevation immediately up-glacier of the ice cliff so that progressively taller subaerial cliffs were not exposed after retreat. Further research is required to monitor and model the evolution of subaerial ice cliffs to better understand the potential for unstable retreat in West Antarctica due to MICI.

How to cite: Carr, E., Carr, R., Stokes, C., Hill, E., Gudmundsson, H., and Ross, N.: Spatiotemporal evolution of subaerial ice cliff heights at marine-terminating outlet glaciers in Northwestern Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11023, https://doi.org/10.5194/egusphere-egu24-11023, 2024.

EGU24-11297 | Posters on site | CR2.3

Temporal evolution of basal terraces at Ekström Ice Shelf, East Antarctica  

Reinhard Drews, Falk Oraschewski, M. Reza Ershadi, Jonathan Hawkins, Christian Wild, Rebecca Schlegel, Inka Koch, Ole Zeising, and Olaf Eisen

Ekström Ice Shelf is a representative ice shelf for the ice-shelf belt of the Dronning Maud Land Coast in East Antarctica. It has cold ocean-cavity with moderate basal melt rates averaging a few meters per year across the ice shelf. In spite of the comparatively small average basal melt rates, we find basal terraces in a ground-penetrating radar dataset revealing near-vertical walls of more than 30 meters height. Such features have also been observed  elsewhere and linked to large localized basal melt rates which is in parts oriented in the horizontal direction. Here we use a ground-penetrating radar dataset with a profile spacing of <100 m which was revisited in an Eulerian sense in two consecutive field seasons 2021 and 2022. This dataset images the 3D extent of basal terracing and shows that these are remarkably stable and can be clearly identified in both seasons. They are  laterally offset  by along-flow advection and possibly also horizontal basal melting oriented perpendicular to the vertical walls. There is very little vertical difference between both datasets which is consistent with the small sub-daily melt rates derived from a continuously measuring ApRES located above a horizontal plateau linking two basal terraces at the ice base. These two 3D time slices are a unique dataset to better understand how such basal terraces initially form, how they are maintained over time and whether or not ocean-induced melting in the horizontal direction (which is typically not picked up by the ApRES data) is relevant on larger spatial scales.

How to cite: Drews, R., Oraschewski, F., Ershadi, M. R., Hawkins, J., Wild, C., Schlegel, R., Koch, I., Zeising, O., and Eisen, O.: Temporal evolution of basal terraces at Ekström Ice Shelf, East Antarctica , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11297, https://doi.org/10.5194/egusphere-egu24-11297, 2024.

EGU24-11490 | ECS | Orals | CR2.3

The buttressing capacity of Antarctic ice shelves 

Tom Mitcham, G. Hilmar Gudmundsson, and Jonathan L. Bamber

Ice shelves can control the flux of ice across the grounding line of the Antarctic Ice Sheet (AIS), and hence the rate of mass loss, through the process of ice-shelf buttressing. Recent, increased mass loss from the AIS, particularly in the Amundsen Sea Embayment and the Antarctic Peninsula, has been attributed to a reduction in buttressing due to ice-shelf thinning, calving or ice-shelf collapse events. To determine how further changes in ice-shelf geometry might affect the contribution of the AIS to global sea levels, it is therefore important to quantify the total amount of buttressing that the ice shelves currently provide and to determine where within the ice shelves that buttressing is generated.

Previous work has sought to characterise the buttressing of Antarctic ice shelves by, for example, calculating the sensitivity of grounding line flux (GLF) to small perturbations in ice-shelf thickness, or defining regions of passive shelf ice. In this work, we calculate the total buttressing capacity of all Antarctic ice shelves for the first time and then explore the spatial distribution of that total buttressing capacity within each ice shelf.

We use the ice-flow model Úa to conduct a series of diagnostic, idealised calving experiments on a present-day, Antarctic-wide model domain, with high spatial resolution over ice shelves and grounding lines. We calculate the total buttressing capacity of each ice shelf as the relative change in GLF in response to the complete removal of the shelf and find that the total buttressing capacity varies by over two orders of magnitude around the ice sheet.

We then conduct a series of idealised calving perturbations, using a range of procedures for generating new calving front locations, and explore the spatial distribution of the total buttressing capacity within each ice shelf. We find that the vast majority of the buttressing is typically generated in ice shelf regions within a few kilometres of the grounding line. Thus, we suggest that a greater area of Antarctica’s ice shelves could be considered passive than previously proposed.

 
 
 

How to cite: Mitcham, T., Gudmundsson, G. H., and Bamber, J. L.: The buttressing capacity of Antarctic ice shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11490, https://doi.org/10.5194/egusphere-egu24-11490, 2024.

EGU24-11541 | ECS | Posters on site | CR2.3

Regime-shifts in ice-shelf melt could trigger irreversible ice loss from the Antarctic Ice Sheet 

Emily Hill, G. Hilmar Gudmundsson, and David Chandler

Changes in ocean conditions surrounding the Antarctic ice sheet, and the impact on melt rates beneath buttressing ice shelves, is one of the largest sources of uncertainty in future ice loss projections. If conditions were to suddenly undergo a regime-shift from cold to warm, melt rates could increase drastically and trigger large and potentially irreversible changes in the interior of the ice sheet. Here, we take an ensemble of ocean-circulation model melt rates as input to an ice-sheet model, to quantify ice loss and the potential for irreversible retreat under such warm conditions. We find that the currently cold-cavity basins of the Filchner-Ronne and Ross ice shelves, in contrast to present-day, could become large contributors to future sea level relevant ice loss. In major basins in West Antarctica, we find high-melt rates can trigger instances of irreversible grounding line retreat, which could only be recovered if arguably unattainable melt rate conditions prevailed over timescales of 100s of years.

How to cite: Hill, E., Gudmundsson, G. H., and Chandler, D.: Regime-shifts in ice-shelf melt could trigger irreversible ice loss from the Antarctic Ice Sheet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11541, https://doi.org/10.5194/egusphere-egu24-11541, 2024.

EGU24-12071 | ECS | Orals | CR2.3

Monitoring Shear-Zone Weakening in East Antarctic Outlet Glaciers through Differential InSAR Measurements 

Christian Wild, Reinhard Drews, Niklas Neckel, Joohan Lee, Kim Sihyung, Hyangsun Han, Won Sang Lee, Veit Helm, Oliver Marsh, and Wolfgang Rack

The stability of polar ice sheets is governed by the seaward movement of ice streams which is decelerated by resistance originating from lateral shear zones. We explore the impact of crystal-scale anisotropy on effective ice stiffness, with regional-scale consequences on ice dynamics. Using the flexural response of Priestley Glacier to tidal forcing as an experimental framework, we constrain isotropic and anisotropic elastic models of vertical tidal ice-shelf flexure. We find that a five-fold reduction of local ice stiffness within narrow lateral shear-zone best fits DInSAR measurements from Sentinel-1. Our modeling not only reproduces 31 double-differential interferograms but also resolves them into 56 individual maps of vertical displacement during SAR image acquisition. Validated with GPS measurements, the inclusion of effective shear-zone weakening significantly reduces the root-mean-square-error of predicted and observed vertical displacement by 84%, from 0.182 m to 0.03 m. These results highlight the untapped potential of DInSAR imagery for mapping ice anisotropy along the feature-rich Antarctic grounding zone, an essential parameter for advancing current ice-sheet flow models.

How to cite: Wild, C., Drews, R., Neckel, N., Lee, J., Sihyung, K., Han, H., Lee, W. S., Helm, V., Marsh, O., and Rack, W.: Monitoring Shear-Zone Weakening in East Antarctic Outlet Glaciers through Differential InSAR Measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12071, https://doi.org/10.5194/egusphere-egu24-12071, 2024.

EGU24-12332 | Posters on site | CR2.3

Summer speedup at Zachariæ Isstrøm, northeast Greenland 

Shfaqat Abbas Khan, Mathieu Morlighem, Youngmin Choi, Shivani Ehrenfeucht, Eric Rignot, Angelika Humbert, and Javed Hassan

The dynamics of The North East Greenland Ice Stream (NEGIS) are influenced by various factors such as ice thickness, topography, basal conditions, and surface meltwater inputs. The presence of basal lubrication significantly affects NEGIS ice flow by reducing friction at the ice-bed interface. Consequently, alterations in subglacial hydrology and the prevalence of meltwater can result in significant variations in ice stream velocity and mass discharge. In this study, we utilize GPS data from six stations along the main trunk to identify the inland propagation of summer speed-ups, peaking between June and August. Complementing the GPS data, we incorporate ice speed information from mosaics based on ESA Sentinel-1 SAR offset tracking, covering the entire NEGIS. These velocity maps, derived from intensity-tracking of ESA Sentinel-1 data with a 12-day repeat and utilizing the operational interferometric post-processing chain IPP for analysis, reveal substantial acceleration in surface speed from June onwards, followed by a deceleration in August. To simulate the observed summer speed-up, we employ the Ice-sheet and Sea-level System Model (ISSM). Our model results indicate that hydrology is the primary driver of the summer speed-up, leading to changes in speed that extend deep into the interior, reaching over 150 km inland. Understanding the dynamics of NEGIS is essential for predicting its future behavior and potential contributions to sea level rise in a warmer climate with increased meltwater.

How to cite: Khan, S. A., Morlighem, M., Choi, Y., Ehrenfeucht, S., Rignot, E., Humbert, A., and Hassan, J.: Summer speedup at Zachariæ Isstrøm, northeast Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12332, https://doi.org/10.5194/egusphere-egu24-12332, 2024.

EGU24-12334 | Orals | CR2.3

Observed and modelled meltwater-induced flexure and fracture at a doline on north George VI Ice Shelf, Antarctica 

Alison Banwell, Ian Willis, Laura Stevens, Rebecca Dell, and Douglas MacAyeal

Hundreds of surface lakes are known to form each summer on north George VI Ice Shelf, Antarctic Peninsula. To investigate surface-meltwater induced ice-shelf flexure and fracture, we obtained Global Navigation Satellite System (GNSS) observations and ground-based timelapse photography over north George VI for three melt seasons from November 2019 to November 2022.

In particular, we used these field observations to characterize the flexure and fracture behaviour of a mature doline (i.e. drained lake basin formed in a prior melt season) on north George VI Ice Shelf. The GNSS displacement timeseries shows a downward vertical displacement of the doline centre with respect to the doline rim of ~60 cm in response to loading from the development of a central meltwater lake. Viscous flexure modelling indicates that this vertical displacement generates flexure tensile surface stresses of ~>75 kPa. The GNSS data also show a tens-of-days episode of rapid-onset, exponentially decaying horizontal displacement, where the horizontal distance from the rim of the doline with respect to its centre increases by ~70 cm. We interpret this event as the initiation and/or widening of a single fracture, possibly aided by stress perturbations associated with meltwater loading in the doline basin. This observation, together with our observations of circular fractures around the doline basin in timelapse imagery, suggests the first such documentation of “ring fracture” formation on an ice shelf, equivalent to the type of fracture proposed to be part of the chain reaction lake drainage process involved in the 2002 breakup of Larsen B Ice Shelf.

How to cite: Banwell, A., Willis, I., Stevens, L., Dell, R., and MacAyeal, D.: Observed and modelled meltwater-induced flexure and fracture at a doline on north George VI Ice Shelf, Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12334, https://doi.org/10.5194/egusphere-egu24-12334, 2024.

EGU24-12347 | ECS | Posters on site | CR2.3

The effects of including Antarctic subglacial meltwater flux to the ocean in the Energy Exascale Earth System Model 

Carolyn Branecky Begeman, Irena Vaňková, Xylar Asay-Davis, Darin Comeau, Alex Hager, Matthew Hoffman, Matthew Maltrud, Courtney Shafer, and Jonathan Wolfe

Subglacial runoff beneath ice shelves is a source of freshwater, and therefore buoyancy, at the grounding line. Being released at depth, it accelerates an ascending plume along the ice-shelf base, enhancing entrainment of ambient waters, and increasing melt rates. By now it is understood that subglacial runoff is a key control on melt rate variability at the majority of Greenland's glaciers. However, its importance in present-day and future Antarctic melt rates is less clear.

To address this point, we use the Energy Exascale Earth System Model (E3SM) and investigate the effects of Antarctic freshwater volume flux addition in both idealized setups and realistic, global, sea-ice ocean coupled configurations. For realistic Antarctic configurations, we use the subglacial hydrology model from the MALI ice-sheet model with both distributed and channelized drainage run at 4-20 km resolution to calculate steady state subglacial discharge across the grounding line under historical ice-sheet conditions.  This meltwater discharge is implemented as a freshwater flux in MPAS-Ocean, the ocean component of E3SM.

How to cite: Branecky Begeman, C., Vaňková, I., Asay-Davis, X., Comeau, D., Hager, A., Hoffman, M., Maltrud, M., Shafer, C., and Wolfe, J.: The effects of including Antarctic subglacial meltwater flux to the ocean in the Energy Exascale Earth System Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12347, https://doi.org/10.5194/egusphere-egu24-12347, 2024.

EGU24-12499 | ECS | Posters on site | CR2.3

Role of buttressing in the dynamic response to Western Antarctic Peninsula ice shelf collapse 

Luisa Wagner, Martin Rückamp, and Johannes Fürst

Ice shelves on the western Antarctic Peninsula have partially or completely disappeared due to widespread thinning and retreat. The loss of floating ice results in a reduction of the buttressing on the upstream grounded ice body. As a consequence, tributary glaciers are accelerating and retreating further, leading to increased ice discharge and, in turn, an increased contribution to sea-level rise. Improving projections of the rate of sea-level rise from the area demands an in-depth understanding of the current mechanisms at play.

In order to gain this, we aim to quantify and characterise the buttressing effect of the ice shelves. To achieve this, we model hypothetical upper-end scenarios by either an immediate complete collapse of all floating ice or a sustained extreme melting. The main focus here is on the stability of the tributary glaciers and the ability of the ice shelf to rebuild itself.

To run the scenarios, we operate ISSM based on surface and basal topography from BedMachine and MEaSURE velocities. A Shallow-Shelf-Approximation with Budd and Weertman sliding laws, Beckmann and Goosse basal forcing parameterisation and von Mises calving is used. To initialise the retreat scenarios, we determine the basal friction coefficient of the grounded area and the ice shelf rheology using a joint inversion technique with regularisation.

How to cite: Wagner, L., Rückamp, M., and Fürst, J.: Role of buttressing in the dynamic response to Western Antarctic Peninsula ice shelf collapse, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12499, https://doi.org/10.5194/egusphere-egu24-12499, 2024.

EGU24-12886 | Posters on site | CR2.3

The Crevasse Depth Calving Law Applied to Ice Shelves: Insights from a 1D Flowline Model  

Faezeh M. Nick and Doug Benn
The crevasse depth (CD) calving law predicts the position of glacier termini from the penetration of surface and basal crevasses computed from stresses in the ice. When applied to Greenland tidewater glaciers, it has high skill when implemented in a full-Stokes 3D model, although its performance in 2D and 1D models is still subject to debate, especially its ability to induce ice shelf calving without the addition of unrealistic amounts of  water in surface crevasses.  This study re-evaluates the CD law within a 1D flowline model of an ice shelf.
 
We show that the model predicts deep crevasse penetration at locations where drag at the shelf boundaries diminishes,such as the grounding line or embayment mouths. Crevasse depth depends on the rate at which these resistance sources decrease along-flow, influencing the longitudinal stress gradient. While full-depth penetration may occur in thinned shelves (due to extensive basal melt), full-depth calving is generally not predicted for unconfined ice shelves. Observations of Antarctic ice shelves and floating ice tongues well beyond embayments or basal pinning points suggest that additional triggers, like slow rift growth, basal melting, or oceanographic stresses, are essential for calving.
 
The addition of water to surface crevasses can greatly facilitate calving. In some cases, reflecting real-world conditions, such as the hydrofracturing-induced collapse of vulnerable ice shelves. However, the need for water-depth tuning in other situations has raised concerns about the physical fidelity of the model. We propose a modified stochastic CD calving criterion in which the probability of calving ramps from zero for a threshold crevasse depth to one for full-depth penetration. This non-deterministic approach captures the statistical structure of calving events, and allows a range of observed behaviours to emerge, such as long Antarctic ice shelf calving cycles (ice-tongue advance punctuated by rare calving events), and short-term fluctuations of tidewater glaciers (frequent calving retreat back to pinning points). We argue that a probabilistic approach represents an important step towards a universal calving law.  

How to cite: M. Nick, F. and Benn, D.: The Crevasse Depth Calving Law Applied to Ice Shelves: Insights from a 1D Flowline Model , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12886, https://doi.org/10.5194/egusphere-egu24-12886, 2024.

EGU24-13188 | ECS | Orals | CR2.3

Circum-Antarctic seasonality in grounded ice flow 

Karla Boxall, Ian Willis, Jan Wuite, Thomas Nagler, Stefan Scheiblauer, and Frazer Christie

Recent advances in high-temporal-resolution satellite imaging has revealed the occurrence of seasonal ice-flow variability in the Antarctic Peninsula for the first time. This newly documented phenomenon provides motivation for identifying the as-yet-unknown ice, ocean and climate interactions responsible for driving the seasonal signals observed across the Antarctic Peninsula, and raises important questions about the possible presence and drivers of seasonality elsewhere in Antarctica. Knowledge of such mechanisms and the extent of seasonality around Antarctica will be important for refining discharge-based ice-sheet mass balance estimations, and for improving predictions of Antarctica’s future response to climate change.

Here, we identify the likely drivers of the recently observed ice-flow seasonality in the western Antarctic Peninsula by carrying out statistical time series analysis using our published Sentinel-1-derived velocity observations (Boxall et al., 2022; doi:10.5194/tc-16-3907-2022) and an array of environmental variables. Our results reveal that both surface and oceanic forcing are statistically significant controls upon ice-flow seasonality in the western Antarctic Peninsula, although each mechanism elicits a unique lag between forcing and the ice-velocity response.

By upscaling our Sentinel-1-derived velocity observations, we also report upon the nature of ice-flow seasonality along Antarctica’s entire coastal margin for the first time and, through additional time series analysis, assess the glacier- to regional-scale importance of surface and ocean forcing upon circum-Antarctic rates of flow.

How to cite: Boxall, K., Willis, I., Wuite, J., Nagler, T., Scheiblauer, S., and Christie, F.: Circum-Antarctic seasonality in grounded ice flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13188, https://doi.org/10.5194/egusphere-egu24-13188, 2024.

EGU24-14126 | ECS | Orals | CR2.3

Improved Parameterizations of Ice-Ocean Boundary Layers 

Ken Zhao, Tomas Chor, Eric Skyllingstad, and Jonathan Nash

Glacial melt rates at ice-ocean interfaces are critical to understanding ice-ocean interactions in polar regions and are commonly parameterized as a turbulent shear boundary with a time-invariant drag coefficient. This assumes the exchange of heat and freshwater across the mm-scale diffusive thermal and salinity boundary layers varies proportionally with the strength of external momentum. However, this is only appropriate when melt/buoyancy-driven turbulence and the suppression of turbulence by stratification is weak.

Guided by GPU-accelerated Direct Numerical Simulations (10 micron resolution) of the ice-ocean boundary layer for varying geometric and ocean forcing parameters, I will present an updated understanding of the basic principles of ice-ocean boundary layers as a complex interplay between diffusive freshwater/thermal and viscous shear layers nested within different types of turbulent boundary layers. I will present numerical simulation results that seek to merge the different turbulent ice-ocean boundary layer regimes: (1) meltwater-driven buoyancy, (2) meltwater-driven shear, and (3) externally-driven shear from both horizontal and vertical sources of momentum.

This updated understanding allows us to develop more accurate predictions for the turbulently-constrained momentum, thermal, and freshwater boundary layer thicknesses, which is required to predict the ocean-driven melt rate of ice in polar regions.

How to cite: Zhao, K., Chor, T., Skyllingstad, E., and Nash, J.: Improved Parameterizations of Ice-Ocean Boundary Layers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14126, https://doi.org/10.5194/egusphere-egu24-14126, 2024.

EGU24-14713 | Posters on site | CR2.3

Multi-sensor approach of monitoring ice-ocean interaction at high resolution at a major ocean-terminating glacier in South Greenland 

Andreas Vieli, Armin Dachauer, Dominik Gräff, Andrea Walter, Brad Lipovsky, Fabian Walter, and Ethan Welty

About half of the current rapid mass loss of the Greenland ice sheet (GIS) is through dynamic processes driven by calving and frontal ablation. However, related insitu observations in such dynamic environments are challenging and our process understanding is therefore still limited. Within the wider context of the GreenFjord-project on Greenland Fjord ecosystem we introduce here a multi-sensor approach for observing process interactions at high spatial and temporal resolution at the ice-ocean boundary of the major ocean-terminating outlet glacier Eqalorutsit Kangillit Sermiat (EKaS) in South Greenland.

Besides multiple all year-round time lapse cameras, broadband seismometers and tidegauges distributed around the glacier terminus and running since summer 2022, we deployed for the first time in summer 2023 a fibre optic cable at the fjord-bed along the calving front and performed continuous distributed acoustic and temperature sensing measurements (DAS and DTS) during more than two weeks. In parallel, we run a terrestrial radar interferometer (TRI) at 1min repeat intervals that recorded high resolution flow-fields as well as calving events (time, size and location). Our comprehensive observational approach is further complemented by local meteo-station data and more than 20 CTD profiles in the fjord near the calving front. In addition, two ocean bottom seismometers together with a simple CTD mooring have been deployed in summer 2023 and are planned to be recovered in the coming summer.

Besides our observational approach, we present here a broad overview and preliminary analysis of this unique observational dataset. We are not only able to record and cross-validate the same processes or events (e. g. calving and ice flow) from multiple sensors, but also clearly extend our observational ability (e. g. detection sensitivity, calving type and size, fjord circulation, spatial and temporal resolution).  We further get more insights into related subglacial and submarine processes such as fjord temperature variations, plume discharge and internal waves in the fjord. Our results thereby contribute to improve our understanding of ice-ocean interaction at a calving front and helps to develop sustainable observational systems of related processes.

How to cite: Vieli, A., Dachauer, A., Gräff, D., Walter, A., Lipovsky, B., Walter, F., and Welty, E.: Multi-sensor approach of monitoring ice-ocean interaction at high resolution at a major ocean-terminating glacier in South Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14713, https://doi.org/10.5194/egusphere-egu24-14713, 2024.

EGU24-15935 | ECS | Posters on site | CR2.3

Modelling Ryder Glacier in Northern Greenland until 2100 under various emissions scenarios; Under which circumstances is the ice tongue lost? 

Felicity Holmes, Jamie Barnett, Henning Åkesson, Johan Nilsson, Nina Kirchner, and Martin Jakobsson

The Greenland Ice Sheet is currently the largest single contributor to global sea level rise, with recent decades having been characterised by an acceleration of mass loss. The Northern sector of the Greenland Ice Sheet has been relatively understudied, but is also the sector containing several of the last remaining ice tongues in Greenland. If these floating ice tongues are lost, the associated reduction in buttressing has the potential to lead to large increases in velocities and mass loss. One such glacier is Ryder glacier which, in contrast to the nearby Petermann glacier, has been reasonably stable in recent decades. As such, this glacier was targeted during the Ryder 2019 expedition with Swedish Icebreaker Oden, leading to a wealth of data on its present-day setting and Holocene history. In conjunction with this observational data, the numerical Ice Sheet and Sea Level System Model (ISSM) is used to investigate both the controls on glacier behaviour since 1900 and the likely trajectory of Ryder glacier towards 2100 under different emissions scenarios. The key focus is on understanding under which circumstances Ryder glacier may lose its ice tongue and what the impacts of this are likely to be in terms of glacier dynamics and sea level rise contribution.

How to cite: Holmes, F., Barnett, J., Åkesson, H., Nilsson, J., Kirchner, N., and Jakobsson, M.: Modelling Ryder Glacier in Northern Greenland until 2100 under various emissions scenarios; Under which circumstances is the ice tongue lost?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15935, https://doi.org/10.5194/egusphere-egu24-15935, 2024.

EGU24-15939 | ECS | Posters on site | CR2.3

Modelling the evolution of Ryder Glacier, Greenland, through the Holocene to investigate its responses to marine and atmospheric forcings. 

Jamie Barnett, Felicity Holmes, Henning Åkesson, Johan Nilsson, Nina Kirchner, and Martin Jakobsson

Coupling paleo numerical simulations of the Greenland Ice Sheet with physical geological evidence of past ice sheet extent can greatly improve our understanding of the factors driving ice loss. Geological observations can be used to reconstruct the state of the Greenland Ice Sheet at snap shots in time, thus acting as constraints to test the fidelity of ice sheet models that can tell a continuous story of retreat over the same geologic timescales. Swedish Ice Breaker Oden’s visit to Sherard Osborn Fjord and Ryder Glacier in 2019 collected a plethora of marine-geological data that describes the glacier’s behaviour and retreat during the Holocene. Here we use a 3D thermo-coupled Higher-Order ice flow module incorporated in the Ice-sheet and Sea-level System Model (ISSM) to simulate the dynamics of Ryder Glacier from 12500 ka to present day. By focusing on a specific individual glacier, we can run the model at resolutions <1km near the grounding line to shed light on the marine (calving and submarine melt) and atmospheric factors that potentially drove Ryder’s retreat from its Younger Dryas position. Of particular interest is understanding whether the glacier withdrew from its marine setting during the Holocene Thermal Maximum and what conditions were required for Ryder to regrow its modern-day ice tongue during the neoglacial cooling at the end of the Holocene.

How to cite: Barnett, J., Holmes, F., Åkesson, H., Nilsson, J., Kirchner, N., and Jakobsson, M.: Modelling the evolution of Ryder Glacier, Greenland, through the Holocene to investigate its responses to marine and atmospheric forcings., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15939, https://doi.org/10.5194/egusphere-egu24-15939, 2024.

EGU24-16868 | ECS | Posters on site | CR2.3

Large-scale and High-resolution Frontal Ablation Estimates in the Arctic through a Machine Learning Approach 

Dakota Pyles, Nora Gourmelon, Vincent Christlein, and Thorsten Seehaus

Frontal ablation is an important component of tidewater glacier mass loss, however, high temporal resolution estimates have remained elusive due to difficulty in reliably capturing terminus position changes with satellite imagery. Recent developments in machine learning-based radar image segmentation to automatically delineate glacier fronts has opened an opportunity to calculate frontal ablation over fine timescales. Through segmentation of Sentinel-1 synthetic aperture radar image sequences, we aim to quantify seasonal and annual frontal ablation across several Arctic regions, using a deep learning-based terminus segmentation algorithm. Svalbard, an Arctic region characterized by variable and complex glacier and fjord geometries, will serve as a methodological test site before expanding the scope to the Canadian Arctic, Greenland periphery, and Russian Arctic, or ~1400-1500 marine-terminating glaciers in the Northern Hemisphere. The derived frontal ablation information is valuable to climate and glacier models, which could benefit from high-resolution reference data, resulting in improved calibrations and parameterizations. Future project efforts will include quantifying total mass budget for all glaciers in the study by integrating frontal changes, ice discharge calculations from ice thickness and surface velocity products, and climatic mass balance data. To identify and evaluate external drivers of glacier change, the frontal ablation and mass balance products will be combined with modeled and observational atmospheric, oceanic, and sea ice data. Through multivariate statistical analyses between these Earth system datasets and mass balance components, we look to provide an improved understanding of dynamic tidewater glacier processes, their spatio-temporal variability, and the influence of glacier geometry on observed changes throughout the Arctic.

How to cite: Pyles, D., Gourmelon, N., Christlein, V., and Seehaus, T.: Large-scale and High-resolution Frontal Ablation Estimates in the Arctic through a Machine Learning Approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16868, https://doi.org/10.5194/egusphere-egu24-16868, 2024.

EGU24-17109 | ECS | Posters on site | CR2.3

Ice- Ocean- Atmosphere Interactions in the Arctic: Glaciers and Ice Caps 

Morag Fotheringham, Noel Gourmelen, Michel Tsamados, and Donald Slater

Arctic glaciers and ice caps are currently major contributors to global sea level rise, with future projections showing a sustained input. The monitoring of these smaller land-ice masses is challenging due to the high temporal and spatial resolution required.

These glaciers and ice caps are losing mass in response to climate forcings, both atmospheric and oceanic. The relative significance of these forcings is currently unknown with most recent catagorisation focusing on separating loss caused by internal dynamics vs surface mass balance changes.

This leaves the specific roles of the atmosphere and the ocean unconstrained; this understanding is key to improving the accuracy of future loss of ice from these smaller land-ice masses and future sea level rise projections.

This study uses CryoSAT-2 swath interferometric radar altimetry to provide high spatial and temporal observations to produce elevation timeseries in order to evaluate the trends of mass loss. It also utilises an ocean thermal model, previously used to study Greenland's outlet glaciers, to gain a better understanding of the relative contributions of atmospheric and ocean forcings to this mass loss.

How to cite: Fotheringham, M., Gourmelen, N., Tsamados, M., and Slater, D.: Ice- Ocean- Atmosphere Interactions in the Arctic: Glaciers and Ice Caps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17109, https://doi.org/10.5194/egusphere-egu24-17109, 2024.

EGU24-17177 | ECS | Posters on site | CR2.3

Testing a PICO quadratic sub-shelf basal melt module in the GRISLI ice sheet model 

Maxence Menthon, Pepijn Bakker, Aurélien Quiquet, and Didier Roche

The Antarctic ice sheet dynamics is primarily driven by basal melting under the ice shelves. The limitation of computational resources forces the usage of simplified parametrizations in ice-sheet models. Multiple parametrizations have been developed and implemented over the last years (Reese et al. 2018, Lazeroms et al. 2018, Pelle et al. 2019, Jourdain et al. 2020, etc.). The PICO module (Reese et al. 2018) demonstrates to be a good trade-off between complexity and computational resources for paleo ice-sheet reconstructions. Lately, Burgard et al. 2022 suggested that the implementation of a quadratic version of the PICO module could improve it significantly.

Here we test the implementation of the PICO module with a quadratic relationship between the thermal forcing and the melt in the GRISLI ice sheet model. We test a wide range of parameter values to calibrate the module, we compare the quadratic version of the module with the original version, under 2 different resolutions. Eventually, we show the results of simulations on paleo and future applications.

How to cite: Menthon, M., Bakker, P., Quiquet, A., and Roche, D.: Testing a PICO quadratic sub-shelf basal melt module in the GRISLI ice sheet model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17177, https://doi.org/10.5194/egusphere-egu24-17177, 2024.

EGU24-17297 | Orals | CR2.3

Geometric amplification and suppression of ice-shelf basal melt in West Antarctica 

Jan De Rydt and Kaitlin Naughten

Ice shelves along the Amundsen Sea coastline in West Antarctica are continuing to thin, albeit at a decelerating rate, whilst ice discharge across the grounding lines has been observed to increase by up to 100% since the early 1990s. Here, the ongoing and future evolution of ice-shelf mass balance components (basal melt, grounding line flux, calving flux) is assessed in a high-resolution coupled ice-ocean model that includes the Pine Island, Thwaites, Crosson and Dotson ice shelves. For a range of idealized ocean-forcing scenarios, the combined evolution of ice-shelf geometry and basal melt rates is simulated over a 200-year period. For all ice-shelf cavities, a reconfiguration of the 3D ocean circulation in response to changes in cavity geometry is found to cause significant and sustained changes in basal melt rate, ranging from a 75% decrease up to a 75% increase near the grounding lines, irrespective of the far-field ocean conditions. These poorly explored feedbacks between changes in ice-shelf geometry, ocean circulation and basal melting have a demonstrable impact on the net ice-shelf mass balance, including grounding line discharge, at multidecadal timescales. They should be considered in future projections of Antarctic mass loss, alongside changes in ice-shelf melt due to anthropogenic trends in the ocean temperature and salinity.

How to cite: De Rydt, J. and Naughten, K.: Geometric amplification and suppression of ice-shelf basal melt in West Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17297, https://doi.org/10.5194/egusphere-egu24-17297, 2024.

EGU24-17302 | ECS | Posters on site | CR2.3

Modelling ocean melt of ice mélange at Greenland's marine-terminating glaciers 

Lokesh Jain, Donald Slater, and Peter Nienow

Greenland’s marine-terminating glaciers have retreated and accelerated in recent decades, contributing significantly to sea level rise. An increase in ocean temperatures, and in particular the increased submarine melting of calving fronts, is often cited as the dominant driver of this retreat. However, the presence of ice mélange and its associated buttressing force on a glacier terminus also has a substantial impact on glacier advance and retreat. The buttressing force theoretically depends on the mélange thickness, and thickness will be modulated by ocean melt rate, but our understanding of mélange melting remains limited, and it is not yet known how melt rates vary across a range of glacial and environmental conditions.

Here, we perform high-resolution numerical simulations using MITgcm to model the melting of ice mélange. In order to map out the parameter space for mélange melting at Greenland’s marine-terminating glaciers, we vary each of the ocean temperature, ocean stratification, the flux of freshwater emerging from beneath the glacier (subglacial discharge) and the mélange geometry. We study how each factor affects the magnitude and distribution of ocean melt of the ice mélange and seek a parameterisation that would allow us to simply predict mélange melt rate. Furthermore, this work is also a step towards including iceberg melting in larger climate and ice sheet models which is important because of the need to improve the characterisation of freshwater fluxes into fjord systems.

How to cite: Jain, L., Slater, D., and Nienow, P.: Modelling ocean melt of ice mélange at Greenland's marine-terminating glaciers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17302, https://doi.org/10.5194/egusphere-egu24-17302, 2024.

EGU24-17430 | Posters on site | CR2.3

The changes of basal conditions on Fleming Glacier, Antarctic Peninsula, between 2008 and 2021 

Yuting Dong, Huimin Liu, Angelika Humbert, Ji Zhao, Dana Floricioiu, Lukas Krieger, Michael Wolovick, Thomas Kleiner, and Lea-Sophie Höyns

The Wordie Ice Shelf (WIS) in the Antarctic Peninsula (AP) has continued to retreat since 1966, and it almost completely disintegrated in the late 1990s. Although the main supply glacier of the WIS, the Fleming Glacier (FG), did not respond immediately, increases in the glacier velocity and dynamic thinning have been observed over the past two decades, especially after 2008 when only a small ice shelf remained at the Fleming Glacier front. As FG is now the fastest flowing outlet glaciers in the west Antarctic Peninsula, ice dynamics is the primary cause of mass loss. Basal sliding is the key mechanism for glacier acceleration and as it responds to thinning and changes in basal conditions. Furthermore, changes in ice-ocean interaction, such as changes in buttressing of ice streams and outlet glaciers like Fleming Glacier, are also leading to acceleration.

Here, we use the Shallow Shelf Approximation (SSA) implementation of the Ice-sheet and Sea-level System Model (ISSM) simulating the basal shear stress distribution of FG in the years 2008, 2011, 2014, 2017, 2019 and 2021 using inverse modelling. To better regularize the glaciological inverse problem, we adopt the latest published L-curve analysis to select the optimal regularization level. Considering Fleming Glacier has a relatively small drainage basin, high resolution geometric data is necessary to obtain better constrained information of the basal conditions. We use TanDEM-X DEMs acquired in austral winter of 2011, 2014, 2017, 2019, and 2021 to provide accurate glacier surface elevations. These DEMs were generated from bi-static InSAR data acquired by the TanDEM-X mission and are with the most complete time series and the best quality that can be obtained in this area at present.  We evaluate the existing ice velocity products and performed a spatio-temporal interpolation to obtain the average velocity of the year corresponding to the elevation data. We use the higher Antarctic ice sheet surface mass balance data RACM2.3p2 at 2 km resolution as a boundary condition. Regarding the bedrock topography, one of the main factors restricting the inversion accuracy, we evaluated all the existing subglacial topography data products within our inversions. To more accurately represent friction at the bed, we also tested Budd’s, Weertman’s and Schoof’s sliding laws, with different friction exponents and variable geometric data.

Comparison of simulated basal shear stresses for 2008 and 2021 suggests the migration of the grounding line 8~9 km upstream by 2021 from the 2008 ice front/grounding line positions. This migration is consistent with the change in floating areas deduced from the calculated height above buoyancy. Our results indicate that the reducing basal shear stress may be directly related to the subglacial hydrologic system and lead to rapid increases in basal sliding and ongoing ungrounding. It will further promote the dynamic loss of glaciers when coupled with ocean forcing and retrograde bedrock. 

How to cite: Dong, Y., Liu, H., Humbert, A., Zhao, J., Floricioiu, D., Krieger, L., Wolovick, M., Kleiner, T., and Höyns, L.-S.: The changes of basal conditions on Fleming Glacier, Antarctic Peninsula, between 2008 and 2021, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17430, https://doi.org/10.5194/egusphere-egu24-17430, 2024.

EGU24-17929 | ECS | Orals | CR2.3

The relative importance of subglacial discharge and iceberg melt forcing in Greenlandic glacial fjord circulation 

Eleanor Johnstone, Donald Slater, Tom Cowton, Neil Fraser, Mark Inall, and Martim Mas e Braga

Glacial fjords form a crucial coupling between the Greenland ice sheet and the surrounding ocean, but observational data is scarce and their complex multi-scale physics can be difficult to model. Thus, glacial fjord processes are often excluded from large-scale ice sheet models that project  sea level contribution and ocean models that are forced by ice sheet freshwater. A key driver of fjord dynamics is the input of ice sheet freshwater, primarily from subglacial discharge rising in a buoyant plume and from iceberg melt. These freshwater sources set up a density gradient between the fjord and shelf, driving fjord circulation and exporting freshwater to the ocean. Observational evidence from a few fjords suggests that fjords can store this freshwater, leading to an export to the shelf that is modified in properties and lagged in time compared to the input of the freshwater to the fjord. Yet little is known about how this freshwater modification varies across Greenland’s diverse fjords, and the relative importance of the sources of freshwater in this process has not been quantified.  

Here, we use a two-layer box model to simulate fjord dynamics in a simple yet realistic way. We isolate the circulation driven by freshwater input from each of subglacial discharge and iceberg melting to assess the relative impact of each process on (i) strength of circulation and (ii) modification and export of freshwater. The model suggests that fjord geometry and the strength of the fjord-shelf exchange are the key controllers of the lag time for freshwater export, with strong fjord-shelf exchange and smaller fjords promoting nearly instant freshwater export, and weak fjord-shelf exchange and large fjords giving long lags in freshwater export. The wider aims of the project are to quantify freshwater export and heat import at glacial fjords on a Greenland-wide scale.

How to cite: Johnstone, E., Slater, D., Cowton, T., Fraser, N., Inall, M., and Mas e Braga, M.: The relative importance of subglacial discharge and iceberg melt forcing in Greenlandic glacial fjord circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17929, https://doi.org/10.5194/egusphere-egu24-17929, 2024.

EGU24-18382 | ECS | Posters on site | CR2.3

An attempt to capture diverse tidewater glacier calving styles within a single framework 

Donald Slater, Doug Benn, and Till Wagner

The complexity of the processes and the difficulty of collecting observations mean that the treatment of the ice-ocean boundary remains one of the most challenging aspects of running models of the Greenland ice sheet. With geometry, climate forcing, ice properties and feedbacks between these factors all playing a role, tidewater glaciers display a range of calving styles that are hard to capture within the simple parameterisations that are necessary for large-scale ice sheet modeling.

Here we attempt to place some dominant calving styles within a single framework. We study submarine melt undercut-driven calving using linear elastic fracture mechanics within 2D elastic simulations, together with analytical approaches to calving driven by the intersection of basal and surface crevasses and to ice cliff failure. Taken together, these approaches give a prediction of calving style as a function of the calving front ice thickness, ocean depth and submarine melt undercut length, or equivalently as a function of the frontal tension, bending moment and shear. We discuss possible implementations in ice sheet models.

How to cite: Slater, D., Benn, D., and Wagner, T.: An attempt to capture diverse tidewater glacier calving styles within a single framework, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18382, https://doi.org/10.5194/egusphere-egu24-18382, 2024.

The marine-terminating glaciers in Svalbard are retreating and losing mass at an alarming rate due to the rapidly warming climate. And glacier calving is one of the most important process contributing to the glacier mass loss. Hence, it is very important to observe the glacier termini to understand calving variability and the influence of local environmental conditions on them.

Here, high frequency time-lapse images have been used to observe the calving front of Hansbreen (a tidewater glacier in the Honrsund fjord, Svalbard) at a 15-minute interval. The time-lapse images have been visually analyzed from April 2016 to October 2016, to observe the calving variability. The calving events are identified and then classified based on several parameters. The environmental parameters like air temperature, sea surface temperature, tidal cycle, water salinity, etc, for the same region have been understood to see if they have any influence spatial and temporal distribution of the observed calving events. [This research has been supported by the National Science Centre, Poland (grant no. 2021/43/D/ST10/00616) and the Ministry of Education and Science, Poland (subsidy for the Institute of Geophysics, Polish Academy of Sciences).]

How to cite: Maniktala, D. and Glowacki, O.: Studying the influence of environmental parameters on calving variability at Hansbreen, in Svalbard, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18558, https://doi.org/10.5194/egusphere-egu24-18558, 2024.

EGU24-18561 | ECS | Orals | CR2.3

Simulating cracks in glacier ice by means of the phase field method 

Rabea Sondershaus, Angelika Humbert, and Ralf Müller

Calving is still a poorly understood process, hence a physically based calving law has not yet been found. Large ice sheet models are using simplified parameterisations to describe calving, which are tuned by observational data. Therefore the demands for the development of physically based models for calving are large.

Calving is facilitated by fracture formation and propagation, which description is the objective of fracture mechanics. Based on the fundamental theory for fracture proposed by Griffith a numerical approach has been developed to describe cracks: the so-called phase field method. This method represents the state of a material, whether it is intact or broken, by means of an additional continuous scalar field. The advantage of the phase field method is its simple numerical implementation and the avoidance of explicit representation of crack faces as well as costly remeshing.

This work adjust the phase field method for fracture to simulate fracture in glacier ice. Thereby the ice rheology is considered by using a viscoelastic material description where a nonlinear viscosity, based on Glen’s flow law, is taken into account. Furthermore finite strain theory is used to capture the large deformations occurring in ice shelves and floating glacier tongues.

The developed theoretical framework is utilized to simulate crack initiation and propagation at ice rises. Here the calving front geometry of the 79N Glacier in Greenland is used to validate the proposed model by comparing the simulated crack paths to satellite imagery.

How to cite: Sondershaus, R., Humbert, A., and Müller, R.: Simulating cracks in glacier ice by means of the phase field method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18561, https://doi.org/10.5194/egusphere-egu24-18561, 2024.

EGU24-19514 | ECS | Orals | CR2.3

Stability of radially spreading extensional flows and ice shelves 

Lielle Stern and Roiy Sayag

Ice shelves that spread into the ocean can develop rifts, which can trigger ice-berg calving and enhance ocean-induced melting. Fluid mechanically, this system is analogous to the radial propagation of a non-Newtonian, strain-rate-softening fluid representing ice that displaces a relatively inviscid and denser fluid that represents an ocean. Laboratory experiments showed that rift patterns can emerge in such systems and that the number of rifts declines in time. Such a dynamics was confirmed theoretically, but only for the earlier stage of the flow and for a fluid layer of uniform thickness. We investigate numerically the stability and late-time evolution of radially spreading, axisymmetric fluid layer of non-uniform thickness. We validate the two dimensional finite-element Úa model using similarity solutions of radially spreading layers of Newtonian fluid that were found consistent with laboratory experiments. We then explore the stability of the flow by introducing geometric perturbations to the initial front and tracing their evolution. Our simulations show that the front of Newtonian fluids is stable, though memory of the perturbation spectral form persists.

How to cite: Stern, L. and Sayag, R.: Stability of radially spreading extensional flows and ice shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19514, https://doi.org/10.5194/egusphere-egu24-19514, 2024.

EGU24-22433 | Orals | CR2.3

Grounding line migration at Orville Coast, Ronne Ice Shelf, West Antarctica, based on long interferometric Sentinel-1 time series 

Michał Tympalski, Marek Sompolski, Anna Kopeć, and Wojciech Milczarek

Determining the grounding lines of ice shelf glaciers (the border at which the ice begins to float in the ocean) is obligatory in precise measuring and understanding of ice sheet mass balance and glacier dynamics. Awareness of its migration range (grounding zone) is also crucial when estimating the impact of glacier/ice sheet waters on the ocean water level. Currently, the most precise large-scale method is based on the viscoelastic tidal movement of the ice shelf identified on a 4-pass DInSAR results. In some places, however, the measurements are impossible or significantly difficult due to the decorrelation between scenes. According to our preliminary results, it may be possible to use unwrapped phase interferograms as a new/supportive method for detecting ground lines. Combined with algorithms for automatic delineation, it can become a powerful solution for obtaining results with unprecedented frequency.


The latest results revealed that for many glaciers the grounding zone width is two orders of magnitude larger than expected. This contradicts existing physical models, which are based on zero ice melt and fixed grounding line position. Irregular interactions between ice and seawater might have a strong impact on glacier evolution and projections if implemented in physical models. We employed a long-time series of Sentinel-1 differential radar interferometry from 2017 to 2021 to detect the variability in grounding line position on Orville Coast, the region of the western Ronne Ice Shelf. The research carried out over a long period and with high frequency allowed a more detailed study of changes occurring in the grounding zone. Observation from a broader perspective gave us the opportunity to detect seasonality and a persistent trend. We compared changes in grounding line migration with external factors e.g. ocean tides. This might provide a better understanding of the behavior of the ice sheet and glaciers, which are currently undergoing such rapid changes.

How to cite: Tympalski, M., Sompolski, M., Kopeć, A., and Milczarek, W.: Grounding line migration at Orville Coast, Ronne Ice Shelf, West Antarctica, based on long interferometric Sentinel-1 time series, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22433, https://doi.org/10.5194/egusphere-egu24-22433, 2024.

EGU24-926 | ECS | Posters on site | CR2.2

Simulating the impact of an AMOC weakening on the Antarctic Ice Sheet using a coupled climate and ice sheet model 

Anna Höse, Moritz Kreuzer, Willem Huiskamp, Torsten Albrecht, Stefan Petri, Ricarda Winkelmann, and Georg Feulner

Many model studies show that a shutdown of the Atlantic meridional overturning circulation (AMOC) causes reduced northward heat transport into the North Atlantic and a warming Southern Ocean in addition to shifts in large-scale atmospheric circulations. How these changing climate conditions could influence the present-day state of the Antarctic Ice Sheet is little studied even though observational data of AMOC strength show a slowdown trend over the last decades. The ocean current as well as the Antarctic Ice Sheet might reach climate tipping points triggering irreversible processes with consequences already on human time-scales. It's unclear whether increasing Southern Ocean temperatures due to a AMOC shutdown could accelerate basal melting rates, the critical parameter which in turn may induce tipping of the West Antarctic Ice Sheet.

Here, a freshwater hosing that forces the shutdown of the AMOC is applied to the North Atlantic in a global climate model with an interactive ice sheet model for Antarctica. This model framework consists of the Parallel Ice Sheet Model (PISM) that is coupled to the CM2Mc global Earth system model via the ice shelf cavity model PICO (Potsdam Ice-shelf Cavity mOdel). PISM is interactively coupled to the ocean module in order to investigate feedbacks at the ice-ocean boundary, while the atmospheric forcing is prescribed. Preliminary results show that an AMOC shutdown results in warming sea surface temperatures in the southern hemisphere along with a small shift in the mid-latitude westerlies due to reduced northward heat transport, which is in line with previous studies. Antarctic marginal temperatures decrease, however, resulting in a reduction of Antarctic mass through increased calving and decreased basal melting.

How to cite: Höse, A., Kreuzer, M., Huiskamp, W., Albrecht, T., Petri, S., Winkelmann, R., and Feulner, G.: Simulating the impact of an AMOC weakening on the Antarctic Ice Sheet using a coupled climate and ice sheet model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-926, https://doi.org/10.5194/egusphere-egu24-926, 2024.

EGU24-966 | ECS | Orals | CR2.2

Greenland Ice Sheet evolution during the Last Interglacial with an improved surface mass balance modeling approach  

Thi Khanh Dieu Hoang, Aurélien Quiquet, Christophe Dumas, Andreas Born, and Didier M. Roche

The Last Interglacial period (LIG) (130 - 116 kaBP), characterized by higher global mean temperature and sea levels compared to the present-day due to the Earth’s orbit configuration, has been well-studied as a recent example of a climate period warmer than today. There is particular interest in studying the ice sheet-climate interactions in view of our current climate change. However, the extent of the ice sheet and its contribution to the rise of sea levels during the LIG remain debatable as different approaches suggest a wide range of estimations. In order to cover such a long period, some processes are simplified in the modeling approach by using prescribed forcings, simple surface mass balance (SMB) schemes, or equilibrium simulations, which all affect the numerical estimation of ice sheet evolution. 

In our work, to perform transient simulations, we use an Earth system model of intermediate complexity (iLOVECLIM), which has been widely used to study various long-timescale periods. Additionally, we use a physically-based energy and mass balance model with 15 vertical snow layers BESSI (BErgen Snow Simulator) to account for the effect of insolation changes as well as snow-albedo feedback. The climate forcings of the snow model are obtained by running iLOVECLIM transiently from 135 to 115 kaBP, downscaled over the Northern Polar region. Using the SMB computed by BESSI, we then simulate the ice sheet evolution during the LIG with GRISLI - the ice sheet model in the iLOVECLIM framework. 

To assess the benefits of using a physically-based SMB model in the ice sheets simulation, the outputs of GRISLI-BESSI are compared to the current SMB scheme of iLOVECLIM, a simple parametrization called ITM (Insolation Temperature Melt). The Greenland ice sheet volume simulated by the two SMB models reaches the minimum value at 127.5 kaBP, around 500 years after the peak of global mean temperature. The magnitude of ice sheet retreat and its contribution to the sea level in ITM simulations are significantly higher than in BESSI due to an overestimation of the zones of ablation. 

The findings suggest that, compared to a parameterization, we have more confidence in the ice sheet estimation with a physically-based SMB model. Further works with fully interactive ice sheet modeling that takes into account the melt-elevation feedback can improve the simulation of the ice sheet-climate interactions of long-time scales. 

How to cite: Hoang, T. K. D., Quiquet, A., Dumas, C., Born, A., and Roche, D. M.: Greenland Ice Sheet evolution during the Last Interglacial with an improved surface mass balance modeling approach , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-966, https://doi.org/10.5194/egusphere-egu24-966, 2024.

EGU24-1991 | ECS | Orals | CR2.2 | Highlight

When will the Antarctic ice shelves not be viable anymore? 

Clara Burgard, Nicolas C. Jourdain, Christoph Kittel, Cyrille Mosbeux, Justine Caillet, and Pierre Mathiot

The Antarctic contribution to sea-level rise in the coming centuries remains very uncertain, due to the possible triggering of instabilities such as the Marine Ice Sheet Instability (MISI) and Marine Ice Cliff Instability (MICI). These instabilities are mainly linked to the evolution of the floating ice shelves, which usually buttress the ice flow from the ice-sheet to the ocean. However, these are currently thinning. Better understanding the evolution of ice shelves in the next decades to centuries is therefore important and crucial to better anticipate the evolution of sea-level rise.

In this study, we investigate the viability of ice shelves for a number of climate models and scenarios. This is estimated from the emulation of the surface and basal mass balance of MAR and NEMO respectively, and from high-end dynamical ice flows obtained through Elmer/Ice. We then use a Bayesian calibration to give weight to members closer to observations. We find that large uncertainties remain, mainly because of the uncertainty in basal melt, and that viability limits vary largely depending on the ice-shelf location.

How to cite: Burgard, C., Jourdain, N. C., Kittel, C., Mosbeux, C., Caillet, J., and Mathiot, P.: When will the Antarctic ice shelves not be viable anymore?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1991, https://doi.org/10.5194/egusphere-egu24-1991, 2024.

EGU24-3666 | Orals | CR2.2

Deciphering Antarctic Ice Sheet Mass Loss: A Modeling Approach to Distinguish Climate Change from Natural Variability 

Johanna Beckmann, Hélène Seroussi, Lawrence Bird, Justine Caillet, Nicolas Jourdain, Felcity McCormack, and Andrew Mackintosh

The Antarctic Ice Sheet (AIS) is currently undergoing accelerated mass loss, significantly contributing to rising sea levels (SLR). Despite numerous observations, uncertainties persist in understanding the drivers and dynamic responses of AIS mass loss. Climate variability strongly influences AIS dynamics, but limited observational data hinders precise attribution to climate change or natural variability. This study addresses this gap by employing advanced modeling techniques to assess the extent to which observed and future AIS mass loss can be attributed to climate change versus variability. Utilizing a unique "initialization method" with the ISSM model, we approximate the AIS state circa 1850, a period minimally affected by anthropogenic forces. From this baseline, we project AIS development using UKESM1 forcing, comparing scenarios with and without anthropogenic influence. This investigation aims to enhance our understanding of the impact of climate change on the AIS and its implications for future SLR.

How to cite: Beckmann, J., Seroussi, H., Bird, L., Caillet, J., Jourdain, N., McCormack, F., and Mackintosh, A.: Deciphering Antarctic Ice Sheet Mass Loss: A Modeling Approach to Distinguish Climate Change from Natural Variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3666, https://doi.org/10.5194/egusphere-egu24-3666, 2024.

EGU24-4093 | ECS | Posters on site | CR2.2

Interactions between ocean circulation and the Northern Hemisphere ice sheets at 40 ky B.P. in an Earth System Model (iLOVECLIM-GRISLI) 

Louise Abot, Claire Waelbroeck, Aurélien Quiquet, Casimir Delavergne, and Nathaelle Bouttes

During the last glacial period, the climate went through rapid fluctuations together with changes in ocean circulation and ice sheets volume accompanied by iceberg discharges. These rapid climate variations, namely Dansgaard-Oeschger events, are still not fully explained. This study’s aim is to contribute to their better understanding, focusing on interactions between ice sheets and ocean circulation. To this end, we use the iLOVECLIM-GRISLI coupled climate-ice sheet model and run two different perturbation experiments related to the ice sheet and ocean components. Starting from a quasi equilibrium corresponding to 40 ky B.P. greenhouse gas concentration, incoming solar radiation and ice sheet volume, the first experiment consists in imposing either constant or amplified sub-shelf melt rates in comparison with the control simulation. In the second experiment, we focus on the interface between the ice sheets and the bedrock. The basal friction coefficient values are imposed following the same procedure. These two experiments are similar to freshwater hosing experiments but here the water comes directly from the interactively computed ice sheets change. For each experiment, the perturbation is imposed for 500 years before returning to the unperturbed conditions for one thousand years and its impacts on the climate system are investigated. Our results highlight feedbacks that may help to explain the abrupt nature of the climate transitions observed during the last glacial period. 

How to cite: Abot, L., Waelbroeck, C., Quiquet, A., Delavergne, C., and Bouttes, N.: Interactions between ocean circulation and the Northern Hemisphere ice sheets at 40 ky B.P. in an Earth System Model (iLOVECLIM-GRISLI), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4093, https://doi.org/10.5194/egusphere-egu24-4093, 2024.

EGU24-4802 | Orals | CR2.2

A synchronously coupled global model iOM4: a new modeling tool for simulations of the ocean-cryosphere interactions  

Olga Sergienko, Matthew Harrison, Alexander Huth, and Nicole Schlegel

How to cite: Sergienko, O., Harrison, M., Huth, A., and Schlegel, N.: A synchronously coupled global model iOM4: a new modeling tool for simulations of the ocean-cryosphere interactions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4802, https://doi.org/10.5194/egusphere-egu24-4802, 2024.

EGU24-5104 | ECS | Posters on site | CR2.2

Simulating Antarctic Ice Sheet evolution through the mid-Pleistocene transition 

Christian Wirths, Antoine Hermant, Christian Stepanek, Johannes Sutter, and Thomas Stocker

Unravelling the main drivers of the mid-Pleistocene transition (MPT; around 1.2–0.8 million years ago) remains a significant challenge in paleoclimate research. Noteworthy changes that occurred in the climate system during that time include a pronounced shift from 41-kyr to 100-kyr periodicity of glacial cycles and the emergence of much larger ice sheets. While a number of studies have focused on the interplay between the climate system and northern hemispheric ice sheets during the MPT, the role of Antarctica in driving and responding to climate change at that time remains largely unknown. This is particularly relevant as, consequently, the response of Antarctica’s vast ice sheets to a major transition in Quaternary climate, and their potential role in shaping the transition, remain uncertain. 

Here, we use the Parallel Ice Sheet Model (PISM) to simulate the transient evolution of the Antarctic Ice Sheet through the MPT. Computation of the evolution of ice sheets in PISM is enabled by means of a climate index approach that is based on snapshots of climatic conditions at key periods. The climate index approach interpolates between individual climate snapshots based on various paleo-proxy records. Further, we test Antarctica's response to different pre-MPT GCM snapshots of different CO2, orbital, and land-sea mask configurations. Climate snapshots are derived from the Community Earth System Models (COSMOS), a general circulation model that simulates atmosphere, ocean, sea ice and land vegetation in dependence of reconstructions of paleogeography, orbital configuration, and greenhouse gas concentrations.  

Our study aims to better understand the evolution of the Antarctic Ice Sheets during the MPT and to constrain potential dynamical transitions in the climate-cryosphere system. Furthermore, we seek to clarify the influence of different pre-MPT ice sheet configurations on simulated characteristics of this transition.  

The findings from this study will contribute to an improved understanding of cryospheric changes that occurred during the Quaternary. Furthermore, we aim to provide insights into potential future Antarctic trajectories under anthropogenic climate change. 

How to cite: Wirths, C., Hermant, A., Stepanek, C., Sutter, J., and Stocker, T.: Simulating Antarctic Ice Sheet evolution through the mid-Pleistocene transition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5104, https://doi.org/10.5194/egusphere-egu24-5104, 2024.

EGU24-5525 | Orals | CR2.2

Modeling the Antarctic Surface Mass Balance with a coarse temporal resolution 

Enrico Maiero, Florence Colleoni, Cécile Agosta, Carlo Barbante, and Barbara Stenni

Sublimation is the most important ablation term in the Antarctic Surface Mass Balance (SMB) (Agosta et al., 2019), while it is currently negligible for both Greenland and mountain glaciers (prevailing surface melt). Since simple parameterized SMB models are usually developed for Greenland and Alpine glaciers, they mostly misrepresent sublimation. To face this problem, we developed EBAL, a new parameterized Energy SMB model for Antarctica based on SEMIC (Krapp et al., 2017), which is an Energy SMB model developed for Greenland whose main innovations are a sinusoidal parameterization for the diurnal cycle to assess melt and refreezing and an albedo dependence on snow depth. EBAL was calibrated with both MAR (Kittel et al., 2022) and RACMO (Wessem et al., 2018) outputs for the period 1979-2000 and for the period 2075-2099 under the SSP5-8.5. EBAL can reproduce the statistical properties of MAR and RACMO sublimation time series and spatial distribution even if it uses a coarse time step (1 day). However, our final aim is to use EBAL for paleoclimate simulations, for which the temporal resolution of the inputs is even coarser, as often only monthly data is available. Thus, we have tested the idea of superimposing the present day-to-day variability on the MAR monthly atmospheric forcing of SSP5-8.5. Simulated SMB with EBAL forced with MAR original daily SSP5-8.5 inputs leads to a 210 Gt/yr sublimation, and to a 1425 Gt/yr melt. When forcing EBAL with monthly means only (linearly interpolated), we obtain a 113 Gt/yr sublimation and a 620 Gt/yr melt. When adding present-day variability to linearly interpolated monthly inputs, EBAL computes a 175 Gt/yr sublimation and a 1386 Gt/yr melt. Those latter numbers are very similar to those obtained when forcing with daily inputs. We propose to use this method to test EBAL for paleoclimate applications.

References

  • Agosta, C. et al., (2019). “Estimation of the Antarctic surface mass balance using the regional climate model MAR (1979–2015) and identification of dominant processes”. The Cryosphere. 13,  pp. 281-296. 10.5194/tc-13-281-2019. 
  • Kittel, C. et al., (2022). “Clouds drive differences in future surface melt over the Antarctic ice shelves”. The Cryosphere. 16, pp. 2655-2669. 10.5194/tc-16-2655-2022.
  • Krapp, M et al., (July 2017). “SEMIC: an efficient surface energy and mass balance model applied to the Greenland ice sheet”. The Cryosphere 11.4, pp. 1519–1535. 10.5194/tc-11-1519-2017
  • Wessem, J. M. et al., (Apr. 2018). “Modelling the climate and surface mass balance of polar ice sheets using RACMO2 – Part 2: Antarctica (1979–2016)”. The Cryosphere 12, pp. 1479–1498. 10.5194/tc-12-1479-2018

How to cite: Maiero, E., Colleoni, F., Agosta, C., Barbante, C., and Stenni, B.: Modeling the Antarctic Surface Mass Balance with a coarse temporal resolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5525, https://doi.org/10.5194/egusphere-egu24-5525, 2024.

EGU24-5584 | Orals | CR2.2

Future Greenland melt in coupled ice sheet-climate CESM simulations: feedbacks, thresholds, reversibility 

Miren Vizcaino, Thirza Feenstra, Michele Petrini, Raymond Sellevold, Georgiou Sotiria, Katherine Thayer-Calder, William Lipscomb, and Julia Rudlang

Estimates of future Greenland ice sheet (GrIS) melt are mostly based on regional climate modelling for a fixed GrIS topography or on ice sheet modelling with forcing from climate models. This prevents the modelling of climate and GrIS feedbacks and other types of interaction. Here we examine a set of multi-century simulations with the Community Earth System Model featuring an interactive GrIS to explore future relationship between global climate change and ice sheet change. To this end, we compare a set of coupled CESM-CISM 1% CO2 increase simulations until stabilization at two, two and a half, three and four times pre-industrial CO2 levels to examine the sensitivity of the GrIS to emission mitigation. Here we find a large role of ocean circulation weakening and associated regional climate changes on GrIS melt for moderate emission scenarios and large melt differences between the three times and four times CO2 stabilization scenarios. In addition, we examine the role of feedbacks on ice sheet evolution by comparing a 1% to 4xCO2 coupled simulation with a simulation where the GrIS topography and meltwater fluxes to the ocean are prescribed as pre-industrial. Finally, we explore the effects on GrIS melt rates of a fast 5% CO2 reduction from four times to pre-industrial levels, with a focus on restoration of high latitude climate, GrIS albedo, surface energy fluxes and refreezing capacity.  

How to cite: Vizcaino, M., Feenstra, T., Petrini, M., Sellevold, R., Sotiria, G., Thayer-Calder, K., Lipscomb, W., and Rudlang, J.: Future Greenland melt in coupled ice sheet-climate CESM simulations: feedbacks, thresholds, reversibility, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5584, https://doi.org/10.5194/egusphere-egu24-5584, 2024.

EGU24-5698 | ECS | Posters on site | CR2.2

Geoengineering's role in reducing future Antarctic mass loss is unclear 

Mira Adhikari, Daniel Martin, Tamsin Edwards, Antony Payne, James O'Neill, and Peter Irvine

Using the BISICLES ice sheet model, we compare the Antarctic ice sheet’s response over the 22nd century in a scenario where idealised large scale, instantaneous geoengineering is implemented in 2100 or 2050 (geoengineering), with scenarios where the climate forcing is held constant in the same year (stabilisation). Results are highly climate model dependent, with larger differences between models than between geoengineering and stabilisation scenarios, but show that geoengineering cannot prevent significant losses from Antarctica over the next two centuries. If implemented in 2050, sea level contributions under geoengineering are lower than under stabilisation scenarios. If implemented in 2100, under high emissions, geoengineering produces higher sea level than stabilisation scenarios, as increased surface mass balance in the warmer stabilisation scenarios offsets some of the dynamic losses. Despite this, dynamic losses appear to accelerate and may eventually negate this initial offset, indicating that beyond 2200, geoengineering could eventually be more effective.

How to cite: Adhikari, M., Martin, D., Edwards, T., Payne, A., O'Neill, J., and Irvine, P.: Geoengineering's role in reducing future Antarctic mass loss is unclear, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5698, https://doi.org/10.5194/egusphere-egu24-5698, 2024.

EGU24-6140 | ECS | Orals | CR2.2

Long term ice-sheet albedo feedback constrained by most recent deglaciation 

Alice Booth, Philip Goodwin, and Bb Cael

Slow climate feedbacks that operate on timescales of more than a century are currently underrepresented in model assessments of climate sensitivity, and this continues to hinder efforts to accurately predict future climate change beyond the end of the 21st Century. As such, the magnitude of multi-centennial and millennial climate feedbacks are still poorly constrained. We utilise recent reconstructions of Earth’s Energy Imbalance (EEI) to estimate both the total climate feedback parameter and the ice-sheet albedo feedback since the Last Glacial Maximum. This new proxy-based record of EEI facilitates the first opportunity to simultaneously calculate both the magnitude and timescale of Earth’s climate feedback over the most recent deglaciation using a purely proxy data-driven approach, and without the need for simulated reconstructions. We find the ice-sheet albedo feedback to have been an amplifying feedback reaching an equilibrium magnitude of 0.55 Wm-2K-1, with a 66% confidence interval of 0.45 Wm-2K-1 to 0.63 Wm-2K-1. The timescale for the ice-sheet albedo feedback to reach equilibrium is estimated as 3.61Kyrs, with a 66% confidence interval of 1.88Kyrs to 5.48Kyrs. These results provide new evidence for the timescale and magnitude of the amplifying ice-sheet albedo feedback that will continue to drive anthropogenic warming for millennia to come, further increasing the urgency for an effective climate change mitigation strategy to avoid serious long-term consequences for our planet and its ecosystems.

How to cite: Booth, A., Goodwin, P., and Cael, B.: Long term ice-sheet albedo feedback constrained by most recent deglaciation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6140, https://doi.org/10.5194/egusphere-egu24-6140, 2024.

EGU24-7415 | ECS | Orals | CR2.2 | Highlight

Stability regimes and safe overshoots in West and East Antarctica 

Ann Kristin Klose and Ricarda Winkelmann

Earth's climate will likely exceed a warming of 1.5°C in the coming decades. Maintaining such warming levels for a longer period of time may pose a considerable risk of crossing critical thresholds in Antarctica and, thereby, triggering self-sustained, potentially irreversible ice loss, even if the forcing is reduced in a temperature overshoot. Due to the complex interplay of several amplifying and dampening feedbacks at play in Antarctica, the duration and amplitude of such warming overshoots as well as their eventual 'landing' climate will determine the long-term evolution of the ice sheet.

Using the Parallel Ice Sheet Model, we systematically test for the reversibility of committed large-scale ice-sheet changes triggered by warming projected over the next centuries, and thereby explore (1) the stability regimes of the Antarctic Ice Sheet and (2) the potential for safe overshoots of critical thresholds in Antarctica.

We demonstrate crucial features of the Antarctic Ice Sheet's stability landscape for its long-term trajectory in response to future human actions: Given ice-sheet inertia, an early reversal of climate may allow for avoiding self-sustained ice loss that would otherwise be irreversible (for the same reduction in warming) due to multistability of the ice sheet at the basin- and continental scale. While we show that such safe overshoots of critical thresholds in Antarctica may be possible, it is also clear that limiting global warming is the only viable option to evade the risk of widespread ice loss in the long term.

How to cite: Klose, A. K. and Winkelmann, R.: Stability regimes and safe overshoots in West and East Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7415, https://doi.org/10.5194/egusphere-egu24-7415, 2024.

EGU24-8333 | ECS | Orals | CR2.2

Coupled ensemble simulations of the Northern Hemisphere ice sheets at last two glacial maxima  

Violet Patterson, Lauren Gregoire, Ruza Ivanovic, Niall Gandy, Stephen Cornford, and Sam Sherriff-Tadano

Coupled climate-ice sheet models can capture important interactions between the ice sheets and the climate that can help us better understand an ice sheet's response to changes in forcings. In this respect, they are a useful tool for simulating future ice sheet and sea level changes as a result of climate change. However, such models have large uncertainties related to the choice of climate and ice sheet parameters used. The same processes that operate today, also occurred in glacial times, and previous work has shown that simulating the North American ice sheet at the Last Glacial Maximum (LGM; ~21 ka BP) provides a strong benchmark for testing coupled climate-ice sheet models and recalibrating uncertain parameters that control surface mass balance and ice flow (Gandy et al., 2023).

Here, we build on this work by performing the first coupled FAMOUS-BISICLES simulations of the last two glacial maxima, including all Northern Hemisphere ice sheets interactively. The ice sheet component of this model is capable of efficiently simulating marine ice sheets, such as the Eurasian ice sheet, despite the high computational cost of higher order physics. We simulate and compare both the LGM and the Penultimate Glacial Maximum (PGM; ~140 ka BP), since both periods displayed major differences in the distribution of ice between Eurasia and North America. Uncertainty is explored by running ensembles of 120 simulations, randomly varying the uncertain parameters controlling ice sheet dynamics and climate through Latin Hypercube Sampling. We also work on improving the representation of ice streams in the model through performing internal ice temperature spin ups and sensitivity tests varying till water drainage properties. The ensemble members are evaluated against empirical data on ice sheet extent and ice stream location to find combinations of parameters that produce reasonable simulations of the North American and Eurasian ice sheets for both periods. We determine the impact of the uncertainty in these parameters on the result and whether both ice sheets show similar sensitivities to the model parameters. These simulations will provide a starting point for analysing some of the interactions between the climate and the ice sheets during glacial periods and how they may have led to different ice sheet evolutions.

How to cite: Patterson, V., Gregoire, L., Ivanovic, R., Gandy, N., Cornford, S., and Sherriff-Tadano, S.: Coupled ensemble simulations of the Northern Hemisphere ice sheets at last two glacial maxima , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8333, https://doi.org/10.5194/egusphere-egu24-8333, 2024.

The dynamics of the ice sheets on glacial-interglacial time scales are highly controlled by interactions with the solid Earth, i.e., glacial isostatic adjustment (GIA). Particularly at marine ice sheets, competing feedback mechanisms govern the migration of the ice sheet’s grounding line and hence the ice sheet stability.

In this study, we run coupled ice sheet–solid Earth simulations over the last two glacial cycles. For the ice sheet dynamics we apply the Parallel Ice Sheet Model PISM and for the load response of the solid Earth we use the three-dimensional viscoelastic Earth in view of sea-level and vertical displacement changes we apply the Viscoelastic Lithosphere and Mantle Model VILMA.

With our coupling setup we evaluate the relevance of feedback mechanisms for the glaciation anddeglaciation phases in Antarctica considering different 3D Earth structures resulting in a range of load-response time scales. For rather long time scales, in a glacial climate associated with far-field sea level low stand, we find grounding line advance up to the edge of the continental shelf mainly in West Antarctica, dominated by a self-amplifying GIA feedback, which we call the ‘forebulge feedback’. For the much shorter time scale of deglaciation, dominated by the Marine Ice Sheet Instability, our simulations suggest that the stabilizing GIA feedback can significantly slow-down grounding line retreat in the Ross sector, which is dominated by a very weak Earth structure (i.e. low mantle viscosity and thin lithosphere).

The described coupled framework, PISM-VILMA, allows for defining restart states to which to run multiple sensitivity simulations. It can be easily implemented in Earth System Models (ESMs) and provides the tools to gain a better understanding of ice sheet stability on glacial time scales as wellas in a warmer future climate.

How to cite: Albrecht, T., Bagge, M., and Klemann, V.: Feedback mechanisms controlling Antarctic glacial cycle dynamics simulated with a coupled ice sheet–solid Earth model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9032, https://doi.org/10.5194/egusphere-egu24-9032, 2024.

EGU24-10162 | ECS | Orals | CR2.2

A new climate and surface mass balance product for the Antarctic and Greenland ice sheet using RACMO2.4.1 

Christiaan van Dalum, Willem Jan van de Berg, Srinidhi Nagarada Gadde, and Michiel van den Broeke

Recent progress in parameterizations of surface and atmospheric processes have led to the development of a major update of the polar version of the Regional Atmospheric Climate Model (RACMO2.4.1). Here, we present a new high-resolution climate and surface mass balance product by applying RACMO2.4.1 to the Antarctic and Greenland ice sheet for the historical period (starting in 1960 and 1945, respectively). In addition, RACMO output is now available for the first time on a pan-Arctic domain, starting in 1980. We assess these products by comparing model output of the surface mass balance and its components and the near-surface climate with in-situ and remote sensing observations, and study differences with the previously operational RACMO iteration, RACMO2.3p2. 

Among other changes, RACMO2.4.1 includes new and updated parameterizations related to surface and atmospheric processes. Most major updates are part of the physics package of cycle 47r1 of the Integrated Forecast System (IFS) of the European Center for Medium-Range Weather Forecasts (ECMWF), which is embedded in RACMO2.4.1. This includes updates to the cloud, radiation, convection, turbulence, aerosol and lake scheme. Other major changes are directly related to the cryosphere, such as the introduction of a new spectral albedo and radiative transfer scheme for glaciated snow, fixes to the snow drift scheme, a new multilayer snow scheme for seasonal snow and an updated ice mask. These updates lead to changes in the near-surface climate. For example, the horizontal transport of snow that is present in the atmosphere leads to a redistribution of snowfall. Furthermore, the spatial resolution for the Antarctic domain is increased to 11 km, which is also used for the pan-Arctic domain, while 5.5 km is used for Greenland. Here, we also discuss the impact that aforementioned changes have on the climate of the polar regions and the surface mass balance and its components of the ice sheets.

How to cite: van Dalum, C., van de Berg, W. J., Nagarada Gadde, S., and van den Broeke, M.: A new climate and surface mass balance product for the Antarctic and Greenland ice sheet using RACMO2.4.1, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10162, https://doi.org/10.5194/egusphere-egu24-10162, 2024.

EGU24-10256 | ECS | Orals | CR2.2

Reconstructing the Greenland ice Sheet during the last two deglaciations 

Majbritt Kristin Eckert, Mikkel Lauritzen, Nicholas Rathmann, Anne Solgaard, and Christine Hvidberg

The Parallel Ice Sheet Model (PISM) is used to build up a glacial Greenland ice sheet, simulate the evolution of the Greenland ice sheet through glacial terminations I and II and investigate the evolution during previous warmer climates, the Eemian and the Holocene thermal maximum. During the Holocene, surface elevation changes derived from ice cores suggest a large thinning in the North, suggesting that the Greenland ice sheet was connected to the North American ice sheet in Canada during the last glacial. By including Canada in the modelling domain this thinning in the early Holocene as the connecting ice bridge broke up will be investigated. 

How to cite: Eckert, M. K., Lauritzen, M., Rathmann, N., Solgaard, A., and Hvidberg, C.: Reconstructing the Greenland ice Sheet during the last two deglaciations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10256, https://doi.org/10.5194/egusphere-egu24-10256, 2024.

EGU24-12773 | ECS | Orals | CR2.2

Improved treatment of snow over ice sheets in the NASA GISS climate model: towards ice sheet–climate coupling 

Damien Ringeisen, Patrick Alexander, Lettie Roach, Ken Mankoff, and Igor Aleinov

Representing the interactions between ice sheets and climate is essential for more accurate prediction of climate change and sea level rise. Ice sheets interact with the overlying atmosphere via the accumulation of snow and its compaction into firn, then ice, as well as the melting of surface snow and ice and the creation of runoff water. Getting an adequate representation of heat transfer, compaction, and melting processes is essential for an accurate representation of snow on land ice in global climate models. We are implementing an improved snow model on top of land ice as part of an effort to couple the NASA GISS climate model with the PISM ice sheet model. The new snow model includes additional layers and processes that are not currently incorporated (e.g., liquid water retention, percolation and refreezing, and snow densification), and mass and energy transfer methods that are consistent with both static ice sheets (with implicit iceberg fluxes) and interactive ice sheets (with explicit dynamics). We are tuning the densification scheme of this snow model with temperature and density data from common FirnCover and SumUp observations at locations in the accumulation zone of Greenland, and we compare the resulting density profiles to other SumUp density profiles in Greenland and Antarctica. We will assess the impact of this new snow model in climate model simulations with a static ice sheet compared with the previous (simpler) 2-layer snow model. Finally, we aim to use the non-coupled simulations as a baseline to assess the impact of dynamic coupling with an interactive ice sheet model.

How to cite: Ringeisen, D., Alexander, P., Roach, L., Mankoff, K., and Aleinov, I.: Improved treatment of snow over ice sheets in the NASA GISS climate model: towards ice sheet–climate coupling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12773, https://doi.org/10.5194/egusphere-egu24-12773, 2024.

EGU24-13618 | ECS | Orals | CR2.2

Reconstructing the coupled Greenland Ice Sheet–climate evolution during the Last Interglacial warm period 

Matt Osman, Jessica Tierney, and Marcus Lofverstrom

During the Last Interglacial (LIG), approximately 130-118 thousand years ago (ka), the Arctic experienced relative warmth and global sea levels considerably higher than modern.  While this interval is thus considered key for understanding long-term ice–climate feedbacks under warm-state climate conditions, large uncertainties remain surrounding i. the magnitude and spatial expression of LIG global temperature change, ii. the relative contributions of the Antarctic vs. Greenlandic Ice Sheets (GrIS) to LIG sea level rise, and iii. the sensitivity of the GrIS to centennial- to millennial-scale ocean-atmospheric forcing.  Here, we present, to our knowledge, a first attempt at reconstructing the coupled GrIS–climate evolution during the LIG using an internally consistent offline “paleoclimate data assimilation” approach.  Our methodology combines a newly compiled database of nearly 400 chronologically consistent marine geochemical and ice sheet-derived climate-proxy records (spanning 250 sites globally) with recently developed, state-of-the-art transient simulations of the LIG using the coupled Community Earth System Model v2 featuring an interactive Community Ice Sheet Model v2 (CESM2-CISM2).  Our preliminary assimilations suggest LIG peak global mean surface warming of +0.1-0.5˚C (±2 range) above the pre-industrial state, arising from enhanced and widespread (>2-5˚C) high Arctic warming.  Leveraging our CESM2-coupled CISM2 results, we further identify a max GrIS contribution of 2.0 (±0.6) meters of sea level rise equivalent at around 125 ka, nearly ~two millennia after peak LIG climate forcing.  These initial results provide a new proxy-model integration framework for reconciling past GrIS contributions to global sea level rise and benchmark the potential long-term sensitivity of the GrIS to ongoing Arctic warming.

How to cite: Osman, M., Tierney, J., and Lofverstrom, M.: Reconstructing the coupled Greenland Ice Sheet–climate evolution during the Last Interglacial warm period, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13618, https://doi.org/10.5194/egusphere-egu24-13618, 2024.

Mass loss from ice sheets under the ongoing anthropogenic warming episode is a major source for sea-level rise. Due to the slow responses of ice sheets to changes in atmospheric and oceanic boundary conditions, ice sheets are projected to undergo further retreat as the climate reaches a new equilibrium, producing a long-term commitment to future sea-level rise that is fulfilled on multi-millennial scale. Future projections of ice sheets beyond 2100 have routinely employed end-of-the-century atmosphere-ocean conditions from climate model output under specified emission scenarios. This approach, however, does not account for long-term responses of the climate system to external forcings. Here we analyze the long-term atmospheric and oceanic responses to a variety of emission scenarios in several climate models and show that polar climates may see substantial changes after the atmospheric CO2 level stabilizes. With a 3-D ice sheet model, we demonstrate that the long-term climate responses are crucial for evaluating ice sheets' commitment to future sea-level rise.

How to cite: Li, D.: Effects of long-term climate responses on ice sheets' commitment to future sea-level rise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15160, https://doi.org/10.5194/egusphere-egu24-15160, 2024.

EGU24-15323 | ECS | Posters on site | CR2.2

Investigating the evolution and stability of the Greenland ice sheet using remapped surface mass balance forcing 

Charlotte Rahlves, Heiko Goelzer, and Michele Petrini

Surface mass balance (SMB) forcing for projections of the future evolution of the Greenland ice sheet with stand-alone modeling approaches is commonly produced on a fixed ice sheet geometry. As changes of ice sheet geometry become significant over longer time scales, conducting projections for the long-term evolution and stability of the Greenland ice sheet usually requires a coupled climate-ice sheet modeling setup. In this study we use an SMB remapping procedure to capture the first order feedbacks of a coupled climate-ice sheet system with a stand-alone modeling approach. Following a remapping procedure originally developed to apply SMB forcing to a range of initial ice sheet geometries (Goelzer et al., 2020), we produce SMB forcing that adapts to the changing ice sheet geometry as it evolves over time. SMB forcing from a regional climate model is translated from a function of absolute location to a function of surface elevation depending on 25 regional drainage basins, thereby reducing biases that would arise by applying the SMB derived from a fixed ice sheet geometry. We use forcing for different emission scenarios from the CMIP6 archive to compare results from the remapping approach with results from commonly used methods of parameterizing the SMB-height feedback, as well as with results from a semi-coupled climate-ice sheet simulation.

How to cite: Rahlves, C., Goelzer, H., and Petrini, M.: Investigating the evolution and stability of the Greenland ice sheet using remapped surface mass balance forcing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15323, https://doi.org/10.5194/egusphere-egu24-15323, 2024.

EGU24-15401 | Posters on site | CR2.2

Development and implementation of a refined climate index forcing for paleo ice-sheet modeling applications  

Antoine Hermant, Christian Wirths, and Johannes Sutter

The contribution of the Antarctic Ice Sheet (AIS) to sea-level rise under future scenarios remains uncertain. Simulations of the AIS covering past-climate periods provide valuable insights into its response to a range of climatological background states and transitions, as well as its past contributions to sea-level change. However, data to constrain the modelled ice-flow and the paleo-climate forcing is often lacking, leading to considerable uncertainties with respect to paleo ice sheet evolution. Here, we implement and test a framework to provide paleo-climate scenarios for continental scale ice sheet models. Our approach involves the use of an improved climate index based on ice-core records to translate paleo forcing snapshots from Earth System Models and regional models into transient paleo-climate scenarios, specifically to simulate the dynamics of the AIS throughout the last glaciation and deglaciation. Additionally, we refine paleo-accumulation scenarios by introducing a regionally-specific and temperature-dependant scaling of accumulation. Our study aims to enhance our understanding of AIS dynamics on glacial-interglacial time-scales and provide improved paleo-informed initializations for AIS projections. 

How to cite: Hermant, A., Wirths, C., and Sutter, J.: Development and implementation of a refined climate index forcing for paleo ice-sheet modeling applications , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15401, https://doi.org/10.5194/egusphere-egu24-15401, 2024.

EGU24-15987 | ECS | Posters on site | CR2.2

Assessing Antarctic Ice Sheet dynamics under temporary overshoot and long-term temperature stabilization scenarios   

Emma Spezia, Fabrice Kenneth Michel Lacroix, Vjeran Visnjevic, Christian Wirths, Antoine Hermant, Thomas Frölicher, and Johannes Sutter

Current projections of Antarctic Ice Sheet dynamics during the next centuries are subject to large uncertainties both reflecting the ice sheet model setup as well as the climate pathways taken into consideration. Assessing both we present model projections of the Antarctic Ice Sheet’s response to a range of temporary temperature overshoot and stabilization scenarios until the year 2500 accounting for various ice sheet sensitivities. We employ the ice sheet model PISM at continental scale forced by Earth system model data tailored to specific global temperature scenarios via an adaptive greenhouse gas emissions approach. These scenarios reflect both emission pathways which result in a transient temperature overshoot during the 21st and 22nd century as well as stabilization of global temperatures without overshoot. We contrast these simulations with the well- known CMIP6 scenarios to illustrate the diverse potential pathways of Antarctic Ice Sheet dynamics under uncertain future climate trajectories. 

How to cite: Spezia, E., Lacroix, F. K. M., Visnjevic, V., Wirths, C., Hermant, A., Frölicher, T., and Sutter, J.: Assessing Antarctic Ice Sheet dynamics under temporary overshoot and long-term temperature stabilization scenarios  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15987, https://doi.org/10.5194/egusphere-egu24-15987, 2024.

EGU24-16455 | ECS | Posters on site | CR2.2

Ice-dammed lake-glacier interactions: Modelling the impact on Fennoscandian Ice Sheet retreat 

Ankit Pramanik, Sarah Greenwood, Carl Carl Regnéll, and Richard Gyllencreutz

Ice-dammed lakes expedite glacier retreat, leading to the expansion of lakes and an elevated risk of Glacial Lake Outburst Floods (GLOFs), and delay the freshwater inflow to the ocean. The escalating number of ice-dammed lakes in Greenland, High Mountain Asia, and Patagonia, driven by the swift retreat of glaciers amid rapid warming, poses a significant threat of natural disasters. In the geological record, evidence indicates the rapid retreat of the Fennoscandian ice sheet, marked by the formation, expansion, and drainage of large (10s-1000s km2 surface area and up to 100s m deep) ice-dammed proglacial lakes along the entire length of the late-deglacial ice margin. The deglaciation and ice-lake interactions of the Fennoscandian Ice Sheet (FIS) provide a valuable analogue for projecting the future retreat of the Greenland ice sheet, where a manifold increase in the number and volume of ice-dammed lakes is anticipated.

Despite extensive research on marine-terminating glaciers, the dynamics of lacustrine-terminating glaciers remain poorly understood. While there are some notable differences in thermo-mechanical processes between marine and lacustrine glaciers, a significant contrast lies in the fact that the calving of lake-terminating glaciers is governed by the stress balance induced by rapidly fluctuating lake levels and thermodynamics inherent of lakes. Our study delves into accessing the impact of critical factors, such as lake size and bathymetry, on the retreat of the Fennoscandian Ice Sheet, using the Ice-sheet and Sea-level System Model (ISSM). Furthermore, we aim to evaluate the influence of calving, subaqueous melt, and rapidly fluctuating lake levels on the FIS retreat. The model's accuracy will be ensured through calibration and validation against geologically reconstructed ice sheet boundaries and lake levels.

How to cite: Pramanik, A., Greenwood, S., Carl Regnéll, C., and Gyllencreutz, R.: Ice-dammed lake-glacier interactions: Modelling the impact on Fennoscandian Ice Sheet retreat, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16455, https://doi.org/10.5194/egusphere-egu24-16455, 2024.

EGU24-16702 | ECS | Posters on site | CR2.2

Isochronally constrained ice flow evolution of Dronning Maud Land, Antarctica during the Last Glacial Period 

Vjeran Visnjevic, Julien Bodart, Antoine Hermant, Christian Wirths, Emma Spezia, and Johannes Sutter

To improve the robustness of future simulations of ice flow across the Antarctic continent as well as the projections of sea-level rise accompanying it, it is necessary to improve our understanding of the past evolution of ice dynamics. This is specially the case considering the lack of constraints on climate and basal conditions on the regional scale. To address this, we use high resolution regional ice flow modeling combined with radar obtained repositories of internal reflection horizons and ice core data, to constrain the ice flow evolution of both grounded and floating ice across the Dronning Maud Land during the Last Glacial Period. Combining the modeling results obtained using the ice sheet model PISM with radar and ice core data will enable us to improve our knowledge of conditions at the ice base, but also provide an opportunity to test and compare a range of potential climate reconstructions. The presented workflow will further be expanded to other basins in Antarctica as well as to the interglacial-glacial transition, and the results will be used to improve future simulations of ice flow across Antarctica.

How to cite: Visnjevic, V., Bodart, J., Hermant, A., Wirths, C., Spezia, E., and Sutter, J.: Isochronally constrained ice flow evolution of Dronning Maud Land, Antarctica during the Last Glacial Period, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16702, https://doi.org/10.5194/egusphere-egu24-16702, 2024.

EGU24-17391 | ECS | Orals | CR2.2

Critical thresholds of the Greenland Ice Sheet from the LGM to the future 

Lucía Gutiérrez-González, Jorge Alvarez-Solas, Marisa Montoya, Ilaria Tabone, and Alexander Robinson

In recent decades the Greenland Ice Sheet (GrIS) has undergone accelerating ice-mass loss. The GrIS is thought to be a tipping element of the Earth system due to the existence of positive feedbacks with the climate. Previous work has shown threshold behavior in the system, and its stability has been studied in a range of temperatures of the present to a global warming of +4K. However, there is still no consensus on the values of its critical thresholds for the future. Furthermore,  its stability at  lower temperatures hasn’t been studied yet. Here we use the ice-sheet model Yelmo coupled with the regional climate model REMBO and a parametrization of the ice-ocean interactions to obtain the bifurcation diagram of the GrIS from temperatures representative of the LGM (-12K) to a warmer scenario (+4K). The preindustrial simulated equilibrium volume is larger than the observations, a feature common to many other ice-sheet models. This could indicate model biases, but also that the GrIS could currently not be fully in equilibrium with the preindustrial forcing, with implications for future projections. To investigate this issue, we simulated the transient evolution of the GrIS since the LGM to the present day in the context of the bifurcation diagram, with equilibrium states acting as attractors. 

How to cite: Gutiérrez-González, L., Alvarez-Solas, J., Montoya, M., Tabone, I., and Robinson, A.: Critical thresholds of the Greenland Ice Sheet from the LGM to the future, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17391, https://doi.org/10.5194/egusphere-egu24-17391, 2024.

EGU24-18501 | Posters on site | CR2.2

Protocol for a Last Interglacial Antarctic ice-sheet model inter-comparison 

Lauren Gregoire, Maxence Menthon, Edward Gasson, and Louise Sime

During the last interglacial, geological records show evidence that the sea level peaked between 6 and 9 m above pre-industrial sea level, with a major contribution from the Antarctic ice sheet. However, ice-sheet models give a very large range of values due to a lack of understanding of the mechanisms leading to the Antarctic ice sheet retreat during the Last Interglacial

Here, we propose a protocol to apply systematically to multiple ice-sheet models to better understand the climate and ice-sheet model uncertainties as well as mechanisms leading to a smaller Antarctic ice sheet. We present the climate forcing choices and methodology, ice-sheet model requirements and the group of simulations suggested. The protocol includes transient penultimate deglaciation and last interglacial equilibrium simulations to make it accessible to all types of ice-sheet models. The protocol includes also sensitivity experiments such as hosing.

Inputs from the community are welcome to improve the protocol under development and make it relevant to all ice-sheet modelling groups interested in participating!

How to cite: Gregoire, L., Menthon, M., Gasson, E., and Sime, L.: Protocol for a Last Interglacial Antarctic ice-sheet model inter-comparison, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18501, https://doi.org/10.5194/egusphere-egu24-18501, 2024.

EGU24-19165 | Posters on site | CR2.2

Oceanic gateways in Antarctica - Impact of relative sea-level change on sub-shelf melt 

Moritz Kreuzer, Torsten Albrecht, Lena Nicola, Ronja Reese, and Ricarda Winkelmann

Relative sea level (local water depth) on the Antarctic continental shelf is changing by the complex interplay of processes associated with Glacial Isostatic Adjustment (GIA). This involves near-field visco-elastic bedrock displacement and gravitational effects in response to changes in Antarctic ice load, but also far-field interhemispheric effects on the sea-level pattern. On glacial time scales, these changes can be in the order of several hundred meters, potentially affecting the access of ocean water masses at different depths to Antarctic grounding lines and ice sheet margins. Due to strong vertical gradients in ocean temperature and salinity at the continental shelf margin, basal melt rates of ice shelves could change significantly just by variations in relative sea level alone.
Based on a coupled ice sheet – GIA model setup and the analysis of bathymetric features such as troughs and sills that regulate the access of open ocean water masses onto the continental shelf (oceanic gateways), we conduct sensitivity experiments to derive maximum estimates of Antarctic basal melt
rate changes, solely driven by relative sea-level variations.
Under Last Glacial Maximum sea-level conditions, this effect would lead to a substantial decrease of present-day sub-shelf melt rates in East Antarctica, while the strong subsidence of bedrock in West Antarctica can lead up to a doubling of basal melt rates. For a hypothetical globally ice-free sea-level
scenario, which would lead to a global mean (barystatic) sea-level rise of around +70 m, sub-shelf melt rates for a present-day ice sheet geometry can more than double in East Antarctica, but can also decrease substantially, where bedrock uplift dominates. Also for projected sea-level changes at the
year 2300 we find maximum possible changes of ±20 % in sub-shelf melt rates, as a consequence of relative sea-level changes only.

How to cite: Kreuzer, M., Albrecht, T., Nicola, L., Reese, R., and Winkelmann, R.: Oceanic gateways in Antarctica - Impact of relative sea-level change on sub-shelf melt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19165, https://doi.org/10.5194/egusphere-egu24-19165, 2024.

EGU24-20197 | ECS | Posters on site | CR2.2

Constraining projections of future freshwater fluxes from Antarctica  

Violaine Coulon, Javier Blasco, Qing Qin, Jan De Rydt, and Frank Pattyn

As global temperatures rise, Antarctica's grounded ice sheet and floating ice shelves are experiencing accelerated mass loss, releasing meltwater into the Southern Ocean. This increasing freshwater discharge poses significant implications for global climate change. Despite these consequences, interactive ice sheets and ice shelves have generally not been included in coupled climate model simulations, such as those in CMIP6. Consequently, CMIP6 projections lack a detailed representation of spatiotemporal trends in ice-sheet freshwater fluxes and their impact on the global climate system, introducing major uncertainties in future climate and sea-level projections. To address this, we provide future Antarctic freshwater forcing data and uncertainty estimates for climate models. These are derived from an ensemble of historically calibrated standalone ice sheet model projections, produced with the Kori-ULB ice flow model, under different climate scenarios up to 2300. Here, we analyse spatiotemporal trends in calving rates, ice shelf basal melt and surface mass balance for all Antarctic ice shelves. 

How to cite: Coulon, V., Blasco, J., Qin, Q., De Rydt, J., and Pattyn, F.: Constraining projections of future freshwater fluxes from Antarctica , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20197, https://doi.org/10.5194/egusphere-egu24-20197, 2024.

EGU24-20332 | ECS | Orals | CR2.2

The effect of Pacific climatology on the North American Ice Sheet at the Last Glacial Maximum 

William J. Dow, Sam Sherriff-Tadano, Lauren J. Gregoire, and Ruza Ivanovic

Surface ocean conditions and atmospheric dynamics can affect the surface mass balance (SMB) of remote ice sheets via their influence on heat and moisture transport. Here, we use the FAMOUS-ice coupled climate-ice sheet model, coupled to a slab ocean, to simulate the Last Glacial Maximum (LGM). The model was run hundreds of times to produce a large ensemble that captures a range of uncertain model inputs (parameter values). We investigate the range of simulated atmospheric circulation patterns in the 16 ‘best’ ensemble members based on constraints, such as global temperature, their relationship to sea surface conditions in the North Pacific and the interactions with the North American ice sheet. We present evidence of upper tropospheric planetary waves that facilitate communication between the tropical Pacific and extratropical Laurentide ice sheet region, yet there are clear differences in upper tropopsheric dynamics when compared to recent historical period. There is limited evidence for this tropical-extra-tropical relationship being directly responsible for regional differences in Laurentide SMB evolution.

How to cite: Dow, W. J., Sherriff-Tadano, S., Gregoire, L. J., and Ivanovic, R.: The effect of Pacific climatology on the North American Ice Sheet at the Last Glacial Maximum, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20332, https://doi.org/10.5194/egusphere-egu24-20332, 2024.

EGU24-21079 | Orals | CR2.2

Understanding conditions leading to WAIS collapse, from the Last Interglacial to the modern 

Mira Berdahl, Gunter Leguy, Eric Steig, William Lipscomb, Bette Otto-Bliesner, Nathan Urban, Ian Miller, and Harriet Morgan

It is virtually certain that the West Antarctic Ice Sheet (WAIS) collapsed during past warm periods in Earth’s history, prompting concerns about the potential recurrence under anthropogenic climate change. Despite observed ice shelf thinning in the region, the combination of climate forcing and ice sheet sensitivity driving these changes remains unclear. Here, we investigate the joint effects of climate forcing and ice sheet sensitivity to evaluate conditions leading to WAIS collapse. We run ensembles of the Community Ice Sheet Model (CISM), spun up to a pre-industrial state, and apply climate anomalies from the Last Interglacial (LIG, 129 to 116 yr ago), and the future (SSP2-4.5).  Forcing is derived from Community Earth System Model (CESM2) global simulations. We find that only modest ocean warming is required to cause significant WAIS mass loss, though such loss takes multiple centuries to millennia to manifest.

How to cite: Berdahl, M., Leguy, G., Steig, E., Lipscomb, W., Otto-Bliesner, B., Urban, N., Miller, I., and Morgan, H.: Understanding conditions leading to WAIS collapse, from the Last Interglacial to the modern, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21079, https://doi.org/10.5194/egusphere-egu24-21079, 2024.

EGU24-348 | ECS | Orals | CR3.3

Ridge-formation simulations in three dimensions using discrete element methods 

Marek Muchow and Arttu Polojärvi

Sea-ice ridges form as a part of sea-ice deformation, while the ice is moved by winds and ocean currents. While ridging is a localized process, it is assumed to limit the compressive strength of sea ice in large scale. However, formulations of large-scale ice strength, as used in Earth System Models, do not consider individual ridge formation processes in detail. Thus, it is necessary to understand the energy spend in ridge formation and various processes related to generating ice rubble and redistributing it. To investigate ridge formation in detail, we use the Aalto University in-house discrete-element-method (DEM) model. This three-dimensional DEM model features deformable, multi-fracturing, ice floes, which can fail and form ridges when coming into contact, while recording the ridging forces. With this, we discuss why three-dimensional simulations are important to investigate ridge formation process.

How to cite: Muchow, M. and Polojärvi, A.: Ridge-formation simulations in three dimensions using discrete element methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-348, https://doi.org/10.5194/egusphere-egu24-348, 2024.

EGU24-1671 | ECS | Posters on site | CR3.3

Monthly Arctic sea ice prediction based on a data-driven deep learning model  

Xiaohe Huan, Jielong Wang, and Zhongfang Liu

There is growing interest in sub-seasonal to seasonal predictions of Arctic sea ice due to its potential effects on midlatitude weather and climate extremes. Current prediction systems are largely dependent on physics-based climate models. While climate models can provide good forecasts for Arctic sea ice at different timescales, they are susceptible to initial states and high computational costs. Here we present a purely data-driven deep learning model, UNet-F/M, to predict monthly sea ice concentration (SIC) one month ahead. We train the model using monthly satellite-observed SIC for the melting and freezing seasons, respectively. Results show that UNet-F/M has a good predictive skill of Arctic SIC at monthly time scales, generally outperforming several recently proposed deep learning models, particularly for September sea-ice minimum. Our study offers a perspective on sub-seasonal prediction of future Arctic sea ice and may have implications for forecasting weather and climate in northern midlatitudes.

How to cite: Huan, X., Wang, J., and Liu, Z.: Monthly Arctic sea ice prediction based on a data-driven deep learning model , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1671, https://doi.org/10.5194/egusphere-egu24-1671, 2024.

EGU24-2377 | ECS | Posters on site | CR3.3

Multivariate state and parameter estimation using data assimilation in a Maxwell-Elasto-Brittle sea ice model 

Yumeng Chen, Polly Smith, Alberto Carrassi, Ivo Pasmans, Laurent Bertino, Marc Bocquet, Tobias Sebastian Finn, Pierre Rampal, and Véronique Dansereau

In an idealised setup, a dynamics-only sea ice model is used to investigate the fully multivariate state and parameter estimations that uses a novel Maxwell-Elasto-Brittle (MEB) sea ice rheology. In the fully multivariate state estimation, the level of damage, internal stress and cohesion are estimated along with the observed sea ice concentration, thickness and velocity. In the case of parameter estimation, we estimate the air drag coefficient and the damage parameter of the MEB model. The air drag coefficients adjust the strength of the forcing on the sea ice dynamics while the damage parameter controls the mechanical behaviour of the internal property of sea ice. We show that, with the current observation network, it is possible to improve all model state forecast and the parameter accuracy using data assimilation approaches even though problems could arise in such an idealised setup where the external forcing dominates the model forecast error growth.

How to cite: Chen, Y., Smith, P., Carrassi, A., Pasmans, I., Bertino, L., Bocquet, M., Finn, T. S., Rampal, P., and Dansereau, V.: Multivariate state and parameter estimation using data assimilation in a Maxwell-Elasto-Brittle sea ice model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2377, https://doi.org/10.5194/egusphere-egu24-2377, 2024.

EGU24-3367 | ECS | Orals | CR3.3

Perscribing Antarctic landfast sea ice in a sea ice-ocean model. 

Noé Pirlet, Thierry Fichefet, Martin Vancoppenolle, Clément Rousset, Pierre Mathiot, Alexander Fraser, Antoine Barthélemy, and Christoph Kittel

The coastal polynyas of the Southern Ocean play a crucial role in the formation of dense water and have an impact on the stability of ice shelves. Therefore, it is important to accurately simulate them in climate models. To achieve this goal, the relationship between grounded icebergs, landfast ice and polynyas appears to be central. Indeed, grounded icebergs and landfast ice are believed to be key drivers of coastal polynyas. However, ESMs do not simulate Antarctic landfast ice. Moreover, at a circumpolar scale, there are no observations of grounded icebergs available. Hence, we must seek model representations that can overcome these issues. To address these gaps, we conducted a study using an antarctic circumpolar configuration of the ocean–sea ice model NEMO4.2-SI–3 at the 1/4° resolution. We ran two simulations for the period 2001–17, with the only difference being the inclusion or exclusion of landfast ice information based on observations. All other factors, including initial conditions, resolution and atmospheric forcings, were kept the same. We then compared the results of these simulations with observations from the advanced microwave scanning radiometer to evaluate the performance of the new simulation. Our analysis allowed us to determine the extent to which prescribing the distribution of landfast ice and setting the sea ice velocity to zero on landfast ice regions influenced various aspects of the sea ice, such as polynyas, landfast ice and sea ice distribution in the model. In the future, we plan to look at the impact on the ocean and to develop a physical parameterization in order to model landfast ice and consequently polynyas on a permanent basis.

How to cite: Pirlet, N., Fichefet, T., Vancoppenolle, M., Rousset, C., Mathiot, P., Fraser, A., Barthélemy, A., and Kittel, C.: Perscribing Antarctic landfast sea ice in a sea ice-ocean model., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3367, https://doi.org/10.5194/egusphere-egu24-3367, 2024.

EGU24-4144 | ECS | Orals | CR3.3

A model for ice-mélange based on particle and continuums mechanics 

Saskia Kahl and Carolin Mehlmann

Ice mélange (a mixture of sea ice, bergy bits and icebergs) can have a strong influence on the sea-ice-ocean interaction. So far, ice mélange is not represented in climate models as numerically efficient realizations are missing. This motivates the development of an ice-mélange model based on the viscous-plastic sea-ice rheology, which is currently the most commonly used material law for sea ice in climate models. Starting from the continuum mechanical formulation, we modify the rheology so that icebergs are represented by thick, highly compact pieces of sea ice. These compact pieces of sea ice are held together by a modified tensile strength in the material law. In this framework, the ice mélange is considered as one single fluid, where the icebergs are realised by particles.
Using idealized test cases, we demonstrate that the proposed changes in the material law are crucial to represent icebergs with the viscous-plastic rheology. Similar to the viscous-plastic sea-ice model, the ice-mélange model is highly nonlinear. Solving the model at the resolution needed to represent the typical size of icebergs in ice mélange (< 300m) is therefore challenging. We show that the ice-mélange formulation can be approximated efficiently with a modified Newton's method. Overall, the simple extension of the viscous-plastic sea-ice model is a promising path towards the integration of ice mélange into climate models.

How to cite: Kahl, S. and Mehlmann, C.: A model for ice-mélange based on particle and continuums mechanics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4144, https://doi.org/10.5194/egusphere-egu24-4144, 2024.

Rapid decline of Arctic sea ice has created more open water for ocean wave development and highlighted the importance of wave-ice interactions in the Arctic. Some studies have made contributions to our understanding of the potential role of the prognostic floe size distribution (FSD) on sea ice changes. However, these efforts do not capture the full interactions between atmosphere, ocean, wave, and sea-ice. In this study, a modified joint floe size and thickness distribution (FSTD) is implemented in a newly-developed regional atmosphere-ocean-wave-sea ice coupled model and a series of pan-Arctic simulation is conducted with different physical configurations related to FSD changes, including FSD-fixed, FSD-varied, lateral melting rate, wave-fracturing formulation, and wave attenuation rate. Firstly, atmosphere-ocean-wave-sea ice coupled simulations show that the prognostic FSD leads to reduced ice area due to enhanced ice-ocean heat fluxes, but the feedbacks from the atmosphere and the ocean partially offset the reduced ice area induced by the prognostic FSD. Secondly, lateral melting rate formulations do not change the simulated FSD significantly, but they influence the flux exchanges across atmosphere, ocean, and sea-ice and thus sea ice responses. Thirdly, the changes of FSD are sensitive to the simulated wave parameters associated with different wave-fracturing formulations and wave attenuation rates, and the limited oceanic energy imposes a strong constraint on the response of sea ice to FSD changes. Finally, the results also show that wave-related physical processes can have impacts on sea ice changes with the constant FSD, indicating the indirect influences of ocean waves on sea-ice through the atmosphere and the ocean.

How to cite: Yang, C.-Y. and Liu, J.: Understanding influence of ocean waves on Arctic sea ice simulation: A modeling study with an atmosphere-ocean-wave-sea ice coupled model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4502, https://doi.org/10.5194/egusphere-egu24-4502, 2024.

EGU24-5177 | ECS | Posters on site | CR3.3

Improving the representation of snow over sea-ice in the SI3 model 

Theo Brivoal, Virginie Guemas, Clement Rousset, and Martin Vancoppenolle

Snow plays a crucial role in the formation and sustainability of sea ice. Due to its thermal properties, snow acts as an insulating layer, shielding the ice from the air above. This insulation reduces the heat transfer between the sea-ice and the atmosphere. Due to its reflective properties, the snow cover also strongly contributes to albedo over ice-covered region, which gives it a significant role in the Earth's climate system.

Current state-of-art climate models use over-simple representations of the snow cover. The snow cover is often represented with a one-layer scheme, assuming a constant density, no wet or dry metamorphism or assuming that no liquid water is stored in the snow. Here, we present the integration of a more advanced snow scheme (ISBA-ES) into the sea-ice model SI3, which serves as the sea-ice component for upcoming versions of the CNRM climate model (CNRM-CM). We compare 1D simulations over the Arctic using this new scheme with observational data and simulations utilizing the previous SI3 snow scheme. Overall, the snow simulated by the ISBA-ES scheme is realistic. We also present a sensitivity analysis of the snow and sea-ice in the SI3 model, exploring various options in the ISBA-ES scheme. Our findings reveal a strong sensitivity of both the snow and the sea-ice to the representation of liquid water in snow and the parameterization employed for calculating snowfall density.

How to cite: Brivoal, T., Guemas, V., Rousset, C., and Vancoppenolle, M.: Improving the representation of snow over sea-ice in the SI3 model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5177, https://doi.org/10.5194/egusphere-egu24-5177, 2024.

EGU24-5374 | ECS | Orals | CR3.3

Floe-scale ocean / sea ice energy transfers in the marginal ice zone 

Mukund Gupta, Andrew Thompson, and Patrice Klein

Marginal ice zones are regions where individual sea ice floes interact mechanically and thermodynamically with turbulent ocean currents at the (sub-)mesoscale. Fine scale exchanges of momentum, heat and salinity at the interface between the ocean and the sea ice floes have important effects on upper-ocean energetics, under-ice tracer mixing, and the ice-pack melt rates. The dynamics of these moving floes remain poorly constrained, notably due to the challenge of numerically resolving sub-mesoscale processes and modelling the discrete behavior of sea ice in traditional climate models. 

Here, we use oceanic Large Eddy Simulations (LES), two-way coupled to a Discrete Element Model (DEM) of disk-shaped sea ice floes, to quantify the kinetic energy transfers between ocean and sea ice during summer-like conditions, varying sea ice concentration and floe size distribution. The damping of oceanic currents by floes is found to be important for a sea ice concentration as low as 40%, when the sizes of floes are comparable to the characteristic eddy size. This damping is largely compensated by the generation of kinetic energy due to melt-induced baroclinic instability at the edge of sea ice floes, leading to a net energy sink of approximately 15%, relative to a simulation with no floes. At higher sea ice concentrations, the oceanic kinetic energy production weakens, while energy loss due to ice/ocean damping and floe-floe collisions both increase. These energy fluxes are mediated by the spatial aggregation of sea ice floes that occurs within the high-strain regions surrounding ocean mesoscale eddies. Eddy-driven aggregation can also reduce the melt rate of small floes as they become shielded from warm waters by neighboring larger floes. These results highlight the need for scale-aware, and specifically floe-scale parameterizations of sea ice and its coupling to ocean turbulence, within global climate models.

How to cite: Gupta, M., Thompson, A., and Klein, P.: Floe-scale ocean / sea ice energy transfers in the marginal ice zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5374, https://doi.org/10.5194/egusphere-egu24-5374, 2024.

EGU24-6441 | ECS | Posters on site | CR3.3

Application of mixed least-squares FEM to study sea ice dynamics 

Sonja Hellebrand, Carina Schwarz, and Jörg Schröder

The behavior of sea ice has been studied for many decades. In order to model its viscous-plastic behavior at scales spanning several thousand kilometers, different numerical models have been proposed. Based on the established approach in [1], this contribution presents a simulation model for sea ice dynamics to describe the sea ice circulation and its evolution over one seasonal cycle. In course of that, the sea ice concentration and the sea ice thickness are considered, of which the physical behavior is governed by transient advection equations. Here, the sea ice velocity serves as coupling field.

Recently developed approaches base on a finite element implementation choosing a (mixed) Galerkin variational approach, see e.g. [2] and [3]. But therein, challenges may occur regarding the stability of the numerically complex scheme, especially when dealing with the first-order advection equations. Thus, we propose the application of the mixed least-squares finite element method, which has the advantage to be also applicable to first-order systems, i.e., it provides stable and robust formulations even for non-self-adjoint operators, such as the tracer equations (for sea ice thickness and sea ice concentration).

For solving the instationary sea ice equation the presented least-squares finite element formulation takes into account the balance of momentum and a constitutive law for the viscous-plastic flow. The considered primary fields are the stresses σ, the velocity v, the concentration Aice and the thickness Hice. In relation, four residuals are defined for the derivation of a first-order least-squares formulation based on the balance of momentum, the constitutive relation for the stresses, and two tracer-equations. Different approaches can be made with respect to the approximation functions of the primary fields, i.e., choosing e.g. conforming (H(div) interpolation functions) or non-conforming (Lagrangian interpolation functions) stress approximations, while Lagrangian interpolation functions are chosen for the remaining fields. In order to compare such approaches, the box test case is utilized, cf. [3], which is well described in literature.

References:

[1] W.D. Hibler III. A dynamic thermodynamic sea ice model. Journal of Physical Oceanography, 9(4):815-846, 1979.

[2] S. Danilov, Q. Wang, R. Timmermann, M. Iakovlev, D. Sidorenko, M. Kimmritz, T. Jung. Finite-Element Sea Ice Model (FESIM), Version 2. Geoscientific Model Development, 8:1747-1761, 2015.

[3] C. Mehlmann and T. Richter. A modified global Newton solver for viscous-plastic sea ice models. Ocean Modelling, 116:96-107, 2017.

How to cite: Hellebrand, S., Schwarz, C., and Schröder, J.: Application of mixed least-squares FEM to study sea ice dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6441, https://doi.org/10.5194/egusphere-egu24-6441, 2024.

EGU24-6569 | ECS | Posters on site | CR3.3

Using discrete element methods to understand in-plane fragmentation of sea ice floes 

Adam Bateson, Daniel Feltham, David Schröder, Scott Durski, Jennifer Hutchings, Rajlaxmi Basu, and Byongjun Hwang

Sea ice floe size can impact several processes that determine the evolution of the Arctic sea ice, including lateral melt volume, momentum exchange, and rheology. Floe size distribution (FSD) models are applied within continuum sea ice models to capture the evolution of the FSD through parameterisations of the processes that modify floe size such as lateral melting and wave break-up of floes. FSD models do not yet adequately resolve in-plane fragmentation processes of floes such as the breakup of floes under wind forcing, through interactions between neighbouring floes, or through thermal weakening. It is challenging to characterise and therefore parameterise these in-plane floe breakup processes due to limited availability of in-situ observations. Discrete element models (DEMs) offer an alternative way to understand the different mechanisms of floe fragmentation. By resolving relevant properties such as shear and normal stress and sea ice strength at the sub-floe scale, it is possible to use DEMs as a virtual laboratory and directly simulate the break-up of floes into smaller fragments.

In this study, we describe how in-situ observations of sea ice can be combined with output from sea ice DEMs to develop parameterisations of in-plane breakup of floes that can then be applied in continuum models. We then discuss the necessary model developments in order to apply a sea ice DEM to floe fragmentation at smaller scales. We will also present results from a series of DEM simulations used to model the fracture of sea ice under different forcing conditions and with varying sea ice states to identify the important sea ice parameters and processes in determining the size of the floes that form from in-plane breakup events.

How to cite: Bateson, A., Feltham, D., Schröder, D., Durski, S., Hutchings, J., Basu, R., and Hwang, B.: Using discrete element methods to understand in-plane fragmentation of sea ice floes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6569, https://doi.org/10.5194/egusphere-egu24-6569, 2024.

Arctic sea ice has experienced a differential decline in speed due to the same anthropogenic greenhouse gas forcing, as evidenced by rapid decline after the end of the last century. Our convergent observations, last-millennium reanalysis, and model analyses have revealed that large tropical volcanic eruptions can lead to a decadal increase in Arctic sea ice, and the 1982 and 1991 large volcanic eruptions slowed down the decline of Arctic sea ice during the last century. The models, selected based on the observed sensitivity of Arctic sea ice to volcanic eruptions, suggest that the earliest ice-free summer year in the Arctic will be around 2040 in high-emission sceneria of SSP585. These findings emphasized the crucial need to incorporate volcanic influences when projecting future Arctic changes amid global warming.

How to cite: Wang, X.: Historical volcanic eruptions slowed down rapid decline in Arctic sea ice linked to global warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9554, https://doi.org/10.5194/egusphere-egu24-9554, 2024.

EGU24-9802 | ECS | Posters virtual | CR3.3

The sea ice component of MUSE, the unstructured-mesh global ocean model of CMCC 

Francesco Cocetta, Lorenzo Zampieri, and Doroteaciro Iovino

The rapidly evolving sea ice cover requires novel modeling approaches and mathematical techniques to accurately simulate the sea ice dynamics, thermodynamics, and its interactions with the atmosphere and ocean at varying spatiotemporal resolutions. In this context, the CMCC is developing the Multiscale Unstructured model for Simulating the Earth’s water environment (MUSE), a novel global ocean-sea ice model on unstructured meshes.

MUSE employs a finite-element numerical discretization on unstructured meshes, aiming at offering flexibility in simulating the global ocean for various applications, ranging from physical process understanding to operational sea ice predictions. The ongoing implementation of the sea ice component utilizes the traditional continuous sea ice formulation and the 2+1 split assumption, meaning that the sea ice dynamics and advection are solved for horizontal motions while the thermodynamics and radiative processes are parameterized at the subgrid scale.   

MUSE employs a modified elastic-viscous-plastic (mEVP) solver for the sea ice dynamics and a Flux Corrected Transport (FCT) advection scheme, alongside the state-of-the-art column physics package "Icepack" maintained by the CICE consortium.

Here, we describe the global implementation of the sea ice component in MUSE and its coupling with the ocean. We present the resulting representation of vertical thermodynamic processes and horizontal dynamics of sea ice.

How to cite: Cocetta, F., Zampieri, L., and Iovino, D.: The sea ice component of MUSE, the unstructured-mesh global ocean model of CMCC, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9802, https://doi.org/10.5194/egusphere-egu24-9802, 2024.

EGU24-10098 | Posters on site | CR3.3

Best of SIDFEx: Highlights and lessons learned from six years of sea-ice drift forecasting 

Simon F. Reifenberg, Valentin Ludwig, and Helge F. Goessling and the SIDFEx Team

We showcase the Sea Ice Drift Forecast Experiment (SIDFEx) database. SIDFEx is a collection of close to 225,000 lagrangian drift forecasts for the trajectories of assets (mostly buoys) on the Arctic and Antarctic sea ice, at lead times from daily to seasonal with mostly daily resolution. The forecasts are based on systems with varying degrees of complexity, ranging from free-drift forecasts to forecasts by fully coupled dynamical general circulation models. Combining several independent forecasts allows us to construct a best-guess consensus forecast, with a seamless transition from systems with lead times of up to 10 days to systems with seasonal lead times. The forecasts are generated by 13 research groups using 23 distinct forecasting systems and sent regularly to the Alfred-Wegener-Institute, where they are archived and evaluated. Many groups send forecasts operationally in near-real time.

In our presentation, we will introduce the motivation behind and setup of SIDFEx, as well as an overview on the general forecast skill. We will focus on selected highlights, comprising the operational support of research cruises, short-term predictions of sea-ice deformation and regular contributions to the Sea Ice Outlook competition.

How to cite: Reifenberg, S. F., Ludwig, V., and Goessling, H. F. and the SIDFEx Team: Best of SIDFEx: Highlights and lessons learned from six years of sea-ice drift forecasting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10098, https://doi.org/10.5194/egusphere-egu24-10098, 2024.

EGU24-11288 | Orals | CR3.3

Towards improving numerical sea ice predictions with data assimilation and machine learning 

William Gregory, Mitchell Bushuk, Yongfei Zhang, Alistair Adcroft, and Laure Zanna

In this presentation we highlight recent developments in the implementation of Machine Learning (ML) algorithms into the large-scale sea ice model, SIS2. Specifically, we show how a Convolutional Neural Network (CNN) can be used to systematically reduce global sea ice biases during a 5-year ice-ocean simulation. The CNN has been trained to learn a functional mapping from model state variables to sea ice concentration Data Assimilation (DA) increments. Therefore, during model integration, the CNN ingests information about the numerical model's atmosphere, ocean, and sea ice conditions, and predicts the appropriate correction to the sub-grid category sea ice concentration terms (without seeing any actual sea ice observations). We also show how this combined DA+ML approach leads to a natural framework for augmenting training data for neural networks; one which can lead to significant improvements in online performance, without the need for direct online learning. The bias reductions over the 5-year simulation period for this CNN correction scheme are even competitive with the bias reductions achieved from DA. These findings therefore suggest that our approach could be used to reduce systematic sea ice biases in fully coupled climate model predictions on seasonal-to-climate timescales.

How to cite: Gregory, W., Bushuk, M., Zhang, Y., Adcroft, A., and Zanna, L.: Towards improving numerical sea ice predictions with data assimilation and machine learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11288, https://doi.org/10.5194/egusphere-egu24-11288, 2024.

EGU24-11413 | Posters virtual | CR3.3

Sea ice strength in SI3 

Emma Fiedler, Ed Blockley, Clement Rousset, and Martin Vancoppenolle

The NEMO sea ice model, SI3, includes the simple formulation of Hibler (1979; H79) to parameterise the compressive strength of sea ice. This assumes that thick and compact sea ice has more strength than thin and low concentration sea ice. However, the H79 strength scheme does not consider physical assumptions around energy conservation. The strength scheme of Rothrock (1975; R75) is based on the amount of potential energy gained and frictional energy dissipated during ridging, and has been introduced to SI3. Additionally, the option for a negative exponential redistribution of ridged ice among thickness categories, to better approximate observations and improve stability compared to the existing uniform redistribution when using R75, has been included. The R75 strength formulation is stable and works well in SI3 at version 4.2 with an EVP rheology, under a Met Office forced NEMO/SI3 model configuration. Sea ice strength is generally reduced for the R75 scheme compared to H79. The most notable effect on the model output is a greater number of, and sharper, features in the resulting modelled ice field when using the R75 scheme compared to the H79 scheme, which are particularly apparent in the ice thickness field. An increase in the model effective resolution is therefore demonstrated.

How to cite: Fiedler, E., Blockley, E., Rousset, C., and Vancoppenolle, M.: Sea ice strength in SI3, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11413, https://doi.org/10.5194/egusphere-egu24-11413, 2024.

EGU24-11908 | ECS | Orals | CR3.3

A data-driven sea-ice model with generative deep learning 

Tobias Sebastian Finn, Charlotte Durand, Flavia Porro, Alban Farchi, Marc Bocquet, Yumeng Chen, and Alberto Carrassi

The current generation of sea-ice models with Brittle rheologies can represent the observed temporal and spatial scaling of the sea-ice dynamics at resolutions of around 10 km. However, running those models is expensive, which can prohibit their use in coupled Earth system models. The promising results of neural networks for the fast prediction of the sea-ice extent or sea-ice thickness offer an opportunity to remedy this shortcoming. Here, we present the development of a data-driven sea-ice model based on generative deep learning that predicts together the sea-ice velocities, concentration, thickness, and damage. Trained with more than twenty years of simulation data from neXtSIM, the model can extrapolate to previously unseen conditions, thereby exceeding the performance of baseline models.

Relying on deterministic data-driven models can lead to overly smoothed predictions, caused by a loss of small-scale information. This is why the ability to perform stochastic predictions can be instrumental to the success of data-driven sea-ice models. To generate stochastic predictions with neural networks, we employ denoising diffusion models. We show that they can predict the uncertainty that remains unexplained by deterministic models. Furthermore, diffusion models can recover the information at all scales. This resolves the issues with the smoothing effects and results in sharp predictions even for longer horizons. Therefore, we see a huge potential of generative deep learning for sea-ice modelling, which can pave the way towards the use of data-driven models within coupled Earth system models.

How to cite: Finn, T. S., Durand, C., Porro, F., Farchi, A., Bocquet, M., Chen, Y., and Carrassi, A.: A data-driven sea-ice model with generative deep learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11908, https://doi.org/10.5194/egusphere-egu24-11908, 2024.

EGU24-12451 | ECS | Posters on site | CR3.3

Development of ship navigation risk indicator in sea ice-infested water 

Xinfang Zhang

There's increasing transpolar shipping in both the Arctic and Antarctic as a result of the reduction of sea ice and the desire from social economics.  Sea ice is a hazard for shipping in ice-infested water, Ship navigability in ice-covered sea depends on sea ice concentration, ice thickness, fraction of pressure ridges, and multi-year ice as well as ice speed and compression, it also depends on the vessel ice class. IMO introduced Risk Index Outcome(RIO) to provide guidelines for safe navigation, calculation of RIO requires accurate sea ice information including sea ice concentration and thickness. We developed a method similar to RIO to calculate navigation risk indicators using forecasting models including ECMWF S2S data, Copernicus data, and DMI data. Other than conventional sea ice parameters sea ice concentration and sea ice thickness, ice salinity, and ice age are also taken into account in risk indicator calculation. We select the time March 2019 -Oct 2020 and adopt the initial condition of the model forecast for sea ice to demonstrate the capabilities of seasonal forecasting of this navigation risk indicator in different models. In future, the calculation method will be implemented within the ClimateDT environment.

How to cite: Zhang, X.: Development of ship navigation risk indicator in sea ice-infested water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12451, https://doi.org/10.5194/egusphere-egu24-12451, 2024.

EGU24-12804 | Posters on site | CR3.3

 A laboratory model of fragmentation of a 2D membrane by waves. Analogies and differences with sea ice. 

Michael Berhanu, Louis Saddier, Mathéo Aksil, Palotai Ambre, and Michel Tsamados

The marginal ice zone is the transition region between the dense floating ice pack and the open ocean. In this zone, the interaction of surface waves with sea ice is highly complex. The sea ice is broken up into fragments, the floes, which can split into smaller parts and drift under the action of waves and underwater current. Although the downscaling is challenging, laboratory model experiments can contribute to a better understanding of this process coupling fluid and solid mechanics on a large range of time and space scales. We propose to study the fragmentation of a floating membrane, made up of 10 µm graphite particles arranged in a monolayer, by gravity surface waves with a wavelength of around 15 cm [1]. For a sufficiently strong wave amplitude, the raft progressively breaks up, developing cracks and producing fragments whose sizes decrease over a time scale that is long relative to the wave period. We then study the distribution of the fragments produced during the fragmentation process. The visual appearance of the size-distributed fragments surrounded by open water bears a striking resemblance to the floes produced by the fracturing of sea ice by waves. The fragmentation concepts and morphological tools developed for sea ice floes can be applied to our macroscopic analog. Although the mechanics of the two systems differ in their physical properties and in the fracture process, our experiment provides a model laboratory system for studying the fragmentation of floating 2D materials

 

[1] Saddier, L., Palotai, A., Aksil, M., Tsamados, M., & Berhanu, M. (2023). Breaking of a floating particle raft by water waves. In arXiv preprint arXiv:2310.16188.

How to cite: Berhanu, M., Saddier, L., Aksil, M., Ambre, P., and Tsamados, M.:  A laboratory model of fragmentation of a 2D membrane by waves. Analogies and differences with sea ice., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12804, https://doi.org/10.5194/egusphere-egu24-12804, 2024.

EGU24-12912 | Orals | CR3.3

Improvements in September Arctic sea ice predictions via assimilation of summer CryoSat-2 sea ice thickness observations 

Yong-Fei Zhang, Mitch Bushuk, Michael Winton, Bill Hurlin, William Gregory, Jack Landy, and Liwei Jia

Because of a spring predictability barrier, the seasonal forecast skill of Arctic summer sea ice is limited by the availability of melt-season sea ice thickness (SIT) observations. The first year-round SIT observations, retrieved from CryoSat-2 from 2011 to 2020, are assimilated into the GFDL ocean–sea ice model. The model's SIT anomaly field is brought into significantly better agreement with the observations, particularly in the Central Arctic. Although the short observational period makes forecast assessment challenging, we find that the addition of May–August SIT assimilation improves September local sea ice concentration (SIC) and extent forecasts similarly to SIC-only assimilation. Although most regional forecasts are improved by SIT assimilation, the Chukchi Sea forecasts are degraded. This degradation is likely due to the introduction of negative correlations between September SIC and earlier SIT introduced by SIT assimilation, contrary to the increased correlations found in other regions.

How to cite: Zhang, Y.-F., Bushuk, M., Winton, M., Hurlin, B., Gregory, W., Landy, J., and Jia, L.: Improvements in September Arctic sea ice predictions via assimilation of summer CryoSat-2 sea ice thickness observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12912, https://doi.org/10.5194/egusphere-egu24-12912, 2024.

EGU24-13528 | ECS | Orals | CR3.3

A MAGICC Arctic Sea Ice Emulator 

Sian Chilcott, Malte Meinshausen, and Dirk Notz

CMIP6 models present our best understanding of the Earth system, yet they currently fail to simulate a plausible evolution of sea ice area to changes in the global-mean temperature. We aim to assess whether correcting the temperature and Arctic Amplification biases between CMIP6 models and observations can simulate a sensitivity of sea ice loss to global warming that is within the plausible range. To do this, we develop an emulator that is calibrated to physically-based CMIP6 models and then constrained to observations. Such a tool efficiently translates the global-mean temperature of a specific year into a physically-based and observationally constrained probabilistic ensemble of SIA in each month. This setup allows our emulator to capture the core physical processes of CMIP6 projections, while capturing the observed sensitivity of sea ice loss to global warming through the observational constraint of Arctic Amplification. While there are many application possibilities of our emulator, we use our model here to probabilistically diagnose the timing of an ice-free Arctic Ocean. We find that under a high (SSP5-8.5), medium (SSP2-4.5) and low (SSP1-2.6) emission scenario, an ice-free September Ocean is ‘likely’ at 1.73 of global warming above the pre-industrial level, however we note that the probability in the lower emission scenario reduces to ‘unlikely’ in the late 21st century as the global temperature partially recovers. Our projections suggest that the probability of an ice-free summer ocean rises rapidly from ‘unlikely’ at 1.5 of global warming to ‘likely’ at 2 of global warming, stressing the importance of preventing global temperatures rising above 1.5, as the probability of losing sea ice coverage in September rises sharply thereafter. For March, we also find that the observational constraints increase the probability of an ice-free ocean under SSP5-8.5, becoming ‘likely’ in early 2200, while the probability remains very low under SSP2-4.5 and SSP1-2.6 as less than 5% of models reach ice-free conditions. Our projections suggest an ice-free summer ocean could occur at 0.5 cooler levels than the CMIP6 multi-model ensemble mean implies. Likewise, our approach suggests the probability of an ice-free Arctic Ocean year-round is increased when constraining the Arctic Amplification to observations.

How to cite: Chilcott, S., Meinshausen, M., and Notz, D.: A MAGICC Arctic Sea Ice Emulator, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13528, https://doi.org/10.5194/egusphere-egu24-13528, 2024.

EGU24-15156 | Posters on site | CR3.3

Calibration of a  hybrid sea-ice model based on particle and continuums mechanics 

Carolin Mehlmann and Thomas Richter

Presently, climate models employ a continuum approach to describe sea ice. This approach assumes that statistical averages can be derived from a large number of ice floes. However, employing continuum rheological models at or below the scale of individual floes is only valid if the failure mode of a single floe aligns with that of an aggregate of floes. Initially, continuum models were designed for a grid resolution of 100 km. With recent advancements in computing power, sea-ice models are frequently operated at higher mesh resolutions, potentially leading to grid cells that no longer contains a representative sample of sea-ice floes.

We are addressing these shortcomings of current continuum sea-ice models by developing a hybrid model. The idea of the hybrid approach is to nest a particle model into a continuum sea-ice model in order to predict sea ice on fine spatial scales in a region of interest. An important component of particle models is a drag law to describe the influence of ocean and atmospheric currents on the floes. Measurements obtained onboard the Polarstern expedition PS 138 have shown that the correlation cannot be described fully locally, in regions with strongly heterogenous ice cover. Instead, larger surrounding flows have a substantial effect on the motion of small ones. Detailed numerical simulations of idealised test cases do confirm these findings.   

How to cite: Mehlmann, C. and Richter, T.: Calibration of a  hybrid sea-ice model based on particle and continuums mechanics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15156, https://doi.org/10.5194/egusphere-egu24-15156, 2024.

EGU24-15487 | Posters on site | CR3.3

Extended seasonal forecast of Antarctic Sea Ice using ANTSIC-UNet 

Ziying Yang, Jiping Liu, and Rune Grand Graversen

Antarctic sea-ice variability affects the ocean and atmosphere both locally through thermodynamic processes and beyond the Antarctic regions remotely through dynamic processes, which may all change due to global warming. In this study, we develop the ANTSIC-UNet, a deep-learning model trained on physically enriched climate variables, to predict the extended seasonal Antarctic sea ice concentration of up to 6 months in advance. We assess the predictive skill of ANTSIC-UNet as regards linear trend prediction and anomaly persistence prediction in the Pan- and regional Antarctic areas using comparative analyses with two baseline models. Our results exhibit superior performance of ANTSIC-UNet for the extended seasonal Antarctic forecast. The predictive skill of ANTSIC-UNet is notably season-dependent, showing distinct variations across regions. Optimal prediction accuracy is found in winter, while diminished skill found during the summer can be largely attributed to the ice-edge error. High predictive skills are found in the Weddell Sea throughout the year, which suggests that regional Antarctic sea-ice predictions beyond 6 months are possible. We further quantify variable importance through a post-hoc interpretation method which indicates that ANTSIC-UNet has learned the relationships between SIC and other climate variables and the method therefore provides information on the physics of the model. At short lead times, on timescales of up to two months, ANTSIC-UNet predictions exhibit heightened sensitivity to sea surface temperature, radiation conditions and vertical atmospheric circulation conditions in addition to the sea-ice itself. At longer lead times, predictions are dependent on stratospheric circulation patterns at 7-8 months lead in addition to sea-ice. Furthermore, we discuss the potential of implementing physical constraints to enhance sea-ice-edge predictability.

How to cite: Yang, Z., Liu, J., and Grand Graversen, R.: Extended seasonal forecast of Antarctic Sea Ice using ANTSIC-UNet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15487, https://doi.org/10.5194/egusphere-egu24-15487, 2024.

EGU24-18506 | ECS | Orals | CR3.3

Direct Numerical Simulation of shear turbulence interacting with a melting-freezing ice layer 

Diego Perissutti, Francesco Zonta, Alessio Roccon, Cristian Marchioli, and Alfredo Soldati

When a turbulent flow of water interacts with an ice boundary at near-freezing temperature, the fluid can undergo freezing or melting, depending on the local temperature. The turbulence structures that develop in proximity to the ice layer can affect the convective heat transport patterns, leading to the formation of complex phase-boundary morphologies. The ice layer evolves as part of the solution and modifies the near-boundary fluid structures, resulting in heat transfer perturbations. We investigate these ice-water interactions at small scales by performing Direct Numerical Simulations of an open channel flow at shear Reynolds number in the range between 10^2 and 10^3. The upper section of the channel is occupied by ice, while free shear conditions are applied at the bottom. Temperature is imposed on both walls. The ice melting/freezing is simulated using a phase field method [1] combined with a volume penalization immersed boundary method. A pseudo-spectral scheme [2] is used to solve the equations for momentum and energy transport and for phase evolution. We investigated how the behavior of the system changes with the flow conditions (i.e. Reynolds number), with a specific focus on characterizing the features of the ice morphology. In particular, we observed a remarkable influence of turbulence intensity on the ice morphology: at low shear Reynolds, the typical streamwise-oriented canyons already reported in similar studies [3] are present. However, at higher shear Reynolds, spanwise instabilities are triggered, making the final ice morphology more complex.

FIgure1: Render view from below of the open channel flow at a low Reynolds number. On the top section of the channel, the corrugated ice layer is shown. On the ice boundary, the normalized heat flux passing through it is displayed (high heat flux is shown in red, low heat flux in blue). The local temperature field is reported on the side domain boundaries. The typical streamwise-oriented canyons at the ice interface are visible and the heat flux correlates well with those patterns (the heat flux is higher inside the canyons).

[1]R. Yang et al., Morphology evolution of a melting solid layer above its melt heated from below, Journal of Fluid Mechanics, 956, A23, 2023.

[2]F Zonta et al., Nusselt number and friction factor in thermally stratified turbulent channel flow under non-Oberbeck–Boussinesq conditions, International journal of heat and fluid flow, 44:489–494, 2013.

[3]L. A. Couston et al., Topography generation by melting and freezing in a turbulent shear flow, Journal of Fluid Mechanics, 911, A44, 2021.

How to cite: Perissutti, D., Zonta, F., Roccon, A., Marchioli, C., and Soldati, A.: Direct Numerical Simulation of shear turbulence interacting with a melting-freezing ice layer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18506, https://doi.org/10.5194/egusphere-egu24-18506, 2024.

EGU24-19333 | Orals | CR3.3

Recent progress in nesting a DEM- based regional sea ice model within a continuum model 

Wenjun Lu, Andrei Tsarau, Yuan Zhang, Raed Lubbad, and Sveinung Løset

Understanding sea-ice dynamics at the floe scale is crucial to improve regional ice forecast and comprehend the polar climate systems. Continuum models are commonly used to simulate large-scale sea-ice dynamics. However, they have both theoretical and computational limitations in accurately representing sea-ice behaviour at small scales. Discrete Element Models (DEMs), on the other hand, are well-suited for modelling the behaviour of individual ice floes but face limitations due to computational constraints. To address the limitations of both approaches while combining their strengths, we explored the feasibility of nesting a DEM within a continuum model. This paper reports recent progresses in addressing two challenges associated with this method: 1) how to couple a discrete element method (DEM) – based model (a Lagrangian model explicitly tracking each element in space) into a continuum model (a Eulerian model with fixed spatial mesh transferring state variables within); 2) how to explicitly model fracture of sea ice at large scales. Based on our assessment, integrating DEM and continuum model simulations showed potential for offering accurate, high-resolution predictions of sea ice, particularly in coastal areas and near islands. Simulating fracture of sea ice still poses great computational challenges. However, we see a potential in a data-driven approach to accelerate the computational efficiency in resolving floe-scale ice fractures.  

How to cite: Lu, W., Tsarau, A., Zhang, Y., Lubbad, R., and Løset, S.: Recent progress in nesting a DEM- based regional sea ice model within a continuum model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19333, https://doi.org/10.5194/egusphere-egu24-19333, 2024.

EGU24-22361 | Posters on site | CR3.3

neXtSIM-DG – A next-generation discontinuous Galerkin sea ice model 

Einar Ólason, Timothy Spain, and Thomas Richter and the The neXtSIM team

We present neXtSIM-DG, the novel sea ice model created as part of the Scale Aware Sea Ice Project (SASIP). NeXtSIM-DG is a continuum sea ice model that combines several new model paradigms at once: besides established rheologies, we use the newly developed Brittle Bingham–Maxwell rheology. The discretization is based on higher-order continuous and discontinuous finite elements. We take advantage of the object orientation of the C++ implementation of the model to create a flexible, maintainable, and easily modifiable code base ready for adaptation and adaptation by the user. Finally, the C++ implementation uses modern data structures that allow for efficient shared-memory parallelization and are ready for GPU acceleration. These aspects reflect better the different scales of sea ice dynamics in space and time. In this poster, we review the basic modelling features and present some details of numerical realization. In particular, we study the effect of high-order discretization and the role of different rheologies. 

How to cite: Ólason, E., Spain, T., and Richter, T. and the The neXtSIM team: neXtSIM-DG – A next-generation discontinuous Galerkin sea ice model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22361, https://doi.org/10.5194/egusphere-egu24-22361, 2024.

EGU24-3232 | Posters on site | AS4.2

A climatological satellite view of marine cold air outbreaks in the northeast Atlantic 

Abhay Devasthale and Michael Tjernström

Given the high rate of sea ice loss and the Arctic amplification, the dynamical processes responsible for airmass transport into or out of the Arctic, thus affecting the seasonal melt and recovery of sea ice, need to be understood and scrutinized from different observational perspectives. In a classical, rather binary view of transport “into or out of the Arctic”, a lot of attention in the recent years has rightfully been given on understanding the role of heat and moisture transport into the Arctic in regulating the sea ice melt. However, the cold and dry Arctic airmasses with long residence times are more than occasionally transported out of the Arctic over the open ocean waters, creating one of the most spectacular air mass transformations: the marine cold air outbreaks (MCAOs). The most tangible manifestation of MCAOs are the convectively rolled, narrow cloud streets formed over open water off the edges of the Arctic sea ice in the Nordic and Barents Seas, seen vividly in visible satellite imageries. MCAOs can also locally influence the onset of sea ice melt as they often happen in spring.  

By combining nearly 20 years of remotely sensed data from the hyperspectral Atmospheric Infrared Sounder (AIRS), the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth’s Radiant Energy System (CERES) instruments onboard NASA’s Aqua satellite, this study presents a climatological view of the vertical structure of atmosphere and the cloud radiative effects during MCAOs in the northeast Atlantic.

How to cite: Devasthale, A. and Tjernström, M.: A climatological satellite view of marine cold air outbreaks in the northeast Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3232, https://doi.org/10.5194/egusphere-egu24-3232, 2024.

EGU24-3662 | ECS | Orals | AS4.2

Non-conservative nature of Boron in low salinity Arctic ice and ice melt zones 

Samantha Rush, Chang-Ho Lee, Kitack Lee, Penny Vlahos, and Lauren Barrett

The Arctic Ocean is one of the most rapidly changing environments on the planet as sea ice extent and thickness have declined extensively over the last 40 years. It is predicted that by 2050, Arctic summers will become mostly ice-free, and the Arctic Ocean will be dominated by seasonally annual, rather than multiyear, sea ice. Arctic sea ice serves as a mediator of biogeochemical processes globally, though the impacts of increased ice melt and water column freshening on Arctic biogeochemistry are uncertain. Specifically, declining sea ice raises significant concerns regarding the future carbon uptake potential of the Arctic and the buffering capacity, or alkalinity, of seawater. Boron (B) is a major element in seawater, and in the form of the borate ion, it serves as the third largest contributor to alkalinity. Boron concentrations in the open ocean are typically conservative and accounted for through relationships with other water components, such as with salinity (S) in the boron to salinity ratio (B/S). Well established B/S ratios have been defined for the open ocean; however, salinity variability can create discrepancies in the open ocean boron corrections for alkalinity. In 2021, work in the marginal ice zone of the Bering and Chukchi Seas revealed non-conservative boron behavior and significant alkalinity system inaccuracies based on the deviation in computed B/S ratios in ice cores and brine. In this study, we investigate the B/S ratio in ice melt zone waters, snow, brine, annual, and multiyear sea ice from the eastern Arctic basin. A total of 169 samples were collected during the onset on melt (May-June 2023) on the ARTofMELT expedition across a range of salinities (2 - 63). High salinity samples (S>29) included 1 lead, 7 brine, 16 under-ice, and 28 open ocean water samples. Low salinity samples (S<29) included 1 brine, 10 snow, and 106 ice core samples. Excluding snow, results indicate deviations from the accepted open ocean B/S ratio (0.1336 mg/kg). For both the entire high salinity sample set and the open ocean subset within it, the B/S average value (0.1304 ± 0.001 mg/kg) was lower. For low salinity samples, the average B/S value (0.1328 ± 0.003) was higher than the high salinity sample value but still lower than the accepted field value. The range of B/S ratios was much larger in low salinity samples (0.1260-0.1425 mg/kg) than high salinity samples (0.1275-0.1350 mg/kg); however, both ranges were significantly smaller than the 2021 B/S ratio range (0.0900-0.1850 mg/kg). The smaller deviation from the accepted B/S ratio in this study resulted in carbon system analysis inaccuracies less than 2 µmol/kg across the entire salinity range. We present the computed B/S ratios and the differences in these datasets using the δ18O isotopic ratios to understand the heterogeneity of western, annual ice in the marginal ice zone and eastern, multiyear ice in pack ice regions. The marked distinction in the datasets allows potential insight into boron concentrations and the conversion of total alkalinity to carbonate alkalinity across current and future systemic climate-change shifts in the Arctic.

How to cite: Rush, S., Lee, C.-H., Lee, K., Vlahos, P., and Barrett, L.: Non-conservative nature of Boron in low salinity Arctic ice and ice melt zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3662, https://doi.org/10.5194/egusphere-egu24-3662, 2024.

EGU24-4403 | ECS | Posters on site | AS4.2

Near-surface particle concentration profiles above the Arctic sea ice 

Theresa Mathes and Andreas Held

The Arctic region is warming rapidly, and aerosol-cloud-sea-ice interactions are considered to be one of the key features of the Arctic climate system. It is therefore crucial to identify Arctic particle sources and sinks in order to study their impact on cloud formation and properties. Scott and Levin (1972) were the first to describe open leads as potential sources of atmospheric particles and thus a local source of particle emissions in the central Arctic. Held et al. (2011) found that open leads and ice ridges in particular emit high levels of particles. Particle concentrations have also been shown to be altered by the intrusion of warm and moist air masses and can be strongly enhanced in turbulence-dominated cases (You et al., 2022). Despite significant progress in Arctic research in recent years, there is still a lack of information on near-surface particle concentrations over different surface types, especially before and during the ice-melting period.

Here, we present measurements of near-surface particle concentration profiles to help to quantify the vertical aerosol exchange between Arctic sea ice and the atmosphere. In spring 2023, during the research cruise ARTofMELT on board the icebreaker Oden, we successfully carried out vertical particle measurements. From 17 May to 9 June 2023, near-surface particle concentration profiles were measured during 16 individual measurement periods. Due to the early season, measurements could be taken both before and during the melting process.

For the profile measurements, an aersol inlet was automatically moved up and down by a 1.50 m linear actuator. A plate was attached to the lift to hold sensors for the distance, wind and temperature as well as the aerosol inlet. An  box containing the condensation particle counter (CPC 3007, TSI, St. Paul, MN, USA) was connected to the inlet. Total particle number concentrations with a lower cut-off diameter of 10 nm were then determined at six different heights from 6 cm above the surface to 1.30 m. These measurements were carried out on the ice close to an open lead or surrounded by a closed ice surface.

Figure 1 shows an example for two days of fluxes at 79.8 ° N and 1.9° W. Due to the proximity to the open lead, an emission (red) of aerosols predominates, which is partially alternated by a deposition (blue). The flow calculations are based on 26 height profiles measured on 17 May and 24 on 18 May.

We thank our colleagues from Leibniz Institute for Tropospheric Research, Stockholm University, Swedish polar research secretariat as well as all expedition participants who provided insight and expertise that greatly assisted the research.

Held, A., Brooks, I.M., Leck, C., and Tjernström, M. (2011) On the potential contribution of open lead particle emissions to the central Arctic aerosol concentration. Atmos.Chem.Phys. 11, 3093-3105.
Scott, W. D. and Z. Levin (1972) Open channels in sea ice as ion sources. Science 177, 425-426.
You, C., Tjernström, M., Devasthale, A. (2022) Warm and moist air intrusions into the winter Arctic: a Lagrangian view on the near-surface energy budgets. Atmos.Chem.Phys. 22, 8037–8057.

How to cite: Mathes, T. and Held, A.: Near-surface particle concentration profiles above the Arctic sea ice, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4403, https://doi.org/10.5194/egusphere-egu24-4403, 2024.

EGU24-5124 | ECS | Orals | AS4.2 | Highlight

Is spring melting in the Arctic detectable by under-ice radiation? 

Philipp Anhaus, Christian Katlein, Marcel Nicolaus, Noémie Planat, and Martin Schiller

A trend towards earlier sea-ice melt is detected in many ice-covered regions in the Arctic. The timing of the melt onset has a strong impact on the sea-ice energy budget. Melt onset changes the radiative properties of the ice due to increasing snow wetness and meltwater. So far, satellite passive microwave data are used to detect the melt onset. We analyzed transmitted radiation spectra as collected underneath drifting sea-ice using a remotely operated vehicle during the ARTofMELT expedition in the Fram Strait in spring 2023. We colocated those spectra with measurements of snow depth, sea ice and surface topography, chlorophyll-a concentration in the water column, and with aerial images. This combined dataset enables us to track down possible subsurface pathways and accumulation pools of meltwater. Areas of low snow load and depressed surface topography are characterized by higher transmitted radiation compared to areas with a thick snow cover. Those areas overlapped with areas that showed the first signs of surface melt. Chlorophyll-a concentrations varied only slightly in magnitude and did not match with the heterogeneous pattern of snow depth and ice topography. Here we discuss how to disentangle the influences of chlorophyll a and the subsurface meltwater on the spectral shape of transmitted radiation. We propose that upon successful disentanglement, the spectra can be used as an indicator for subsurface melting. Our study suggests that sea-ice melting starts subsurface and that measurements of transmitted solar radiation spectra could be used to identify the melt onset prior to surface melting. This can provide an interesting complementary information on melt occurrence and on the location of the water in the snowpack in addition to satellite passive microwave data.

How to cite: Anhaus, P., Katlein, C., Nicolaus, M., Planat, N., and Schiller, M.: Is spring melting in the Arctic detectable by under-ice radiation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5124, https://doi.org/10.5194/egusphere-egu24-5124, 2024.

EGU24-5372 | Orals | AS4.2

Impact of warm and moist intrusions on black carbon deposition and summer snow melt in the central Arctic 

Hélène Angot, Marion Réveillet, and Julia Schmale and the MOSAiC team

Warm and moist intrusions (WAMIs) into the central Arctic, predominantly observed in winter and early spring, are becoming more frequent, significantly affecting the region’s near-surface energy budget. This study focuses on the deposition pulses of black carbon (BC) triggered by WAMIs and their subsequent impact on snow properties and melting during the summer, using a modeling approach and comprehensive datasets from the 2019–2020 Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition. Our findings reveal that WAMIs induce episodes of intense BC wet deposition in the central Arctic shoulder season (Nov–Apr) due to transported pollution and moisture. We demonstrate that WAMIs result in exceptionally high BC deposition (> 4 orders of magnitude compared to typical winter/spring conditions) across an area of nearly 1 million km2, approximately 20% of the central Arctic Ocean. Furthermore, we establish a direct connection between these winter/spring BC deposition pulses and subsequent summer increases in absorbed solar energy (> 4 W/m2) and snowpack melt rate (+15%). Despite their sporadic occurrence (only 8% of the time), WAMIs play a significant role in the central Arctic surface energy budget through the BC snow albedo effect.

How to cite: Angot, H., Réveillet, M., and Schmale, J. and the MOSAiC team: Impact of warm and moist intrusions on black carbon deposition and summer snow melt in the central Arctic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5372, https://doi.org/10.5194/egusphere-egu24-5372, 2024.

EGU24-5901 | ECS | Posters on site | AS4.2

Aerosol-Cloud-Precipitation Interactions in the Arctic: Insights from the ARTofMELT Campaign 

Lea Haberstock, Julia Asplund, Almuth Neuberger, Luisa Ickes, Gabriel Freitas, Fredrik Mattsson, Darrel Baumgardner, Ilona Riipinen, and Paul Zieger

Aerosol-cloud interactions play a crucial role in the Arctic’s radiative budget. During the campaign ‘Atmospheric rivers and the onset of sea ice melt’ (ARTofMELT 2023) we aimed to improve our understanding of aerosol-cloud interactions by conducting in-situ measurements of microphysical and chemical properties of aerosols, cloud droplets, and precipitation in the Arctic during the onset of sea ice melt. A ground-based fog and aerosol spectrometer (GFAS) and a fog monitor (FM-120) from Droplet Measurement Technologies (DMT) were used to measure among other things droplet size, number concentration, and liquid water content. Precipitation was measured with a meteorological particle spectrometer (MPS, DMT). Throughout the campaign, we observed several fog and blowing snow events, along with occasional precipitation. These events provided an opportunity to investigate and compare the distinctive microphysical properties associated with each event. Our findings reveal significant variations in the size distribution and particle phase of blowing snow, precipitation, and fog.

How to cite: Haberstock, L., Asplund, J., Neuberger, A., Ickes, L., Freitas, G., Mattsson, F., Baumgardner, D., Riipinen, I., and Zieger, P.: Aerosol-Cloud-Precipitation Interactions in the Arctic: Insights from the ARTofMELT Campaign, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5901, https://doi.org/10.5194/egusphere-egu24-5901, 2024.

EGU24-5950 | ECS | Posters on site | AS4.2

What we can learn from aerosol size distribution measurements over the spring Arctic pack ice 

Julia Asplund, Lea Haberstock, Jessica Matthew, Fredrik Mattson, Lovisa Nilsson, Erik Swietlicki, Megan Willis, Cort Zang, and Paul Zieger

Aerosol- cloud interactions remain among the most uncertain key parameters in the fast-changing Arctic climate system, in large part due to a lack of observational data from this hardly accessible region. The spring-summer transition is a particularly under sampled time period, due to harsh ice conditions. Here, we present five weeks of aerosol size distribution measurements over the spring Arctic pack ice, including more than 30 hours of in-cloud data, obtained during the ARTofMELT 2023 expedition. A setup of three inlets, including a whole-air, an interstitial, and a counterflow virtual impactor inlet, were used to cover the full aerosol population as well as both the activated and interstitial aerosol when in cloud. We will show an overview of the collected observations and the link between the size distribution properties and parallel measured aerosol parameters such as chemical tracers, as well as an air mass source analysis. Fog events were recorded during a range of aerosol conditions, allowing us to study the activated fraction when concentrations span from under 20 particles per cc, to over 150. The dataset also features several distinct regimes where different processes such as blowing snow, new particle formation, and secondary ice production dominate or influence the aerosol population, and we will demonstrate how the regimes are characterized by the dominant mode of the size distribution.

How to cite: Asplund, J., Haberstock, L., Matthew, J., Mattson, F., Nilsson, L., Swietlicki, E., Willis, M., Zang, C., and Zieger, P.: What we can learn from aerosol size distribution measurements over the spring Arctic pack ice, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5950, https://doi.org/10.5194/egusphere-egu24-5950, 2024.

EGU24-6663 | Orals | AS4.2

Perspectives on limitations and mechanisms for atmospheric initiation of onset of the summer melt season over sea ice 

Christopher Cox, Amy Solomon, Ola Persson, Matthew Shupe, Michael Gallagher, Von Walden, Michael Town, Donald Perovich, Sarah Webster, and Jacob Anderson

Onset of surface melt over sea ice is a factor in the duration of the melt season. Onset is often triggered by advection of warm, moist air from lower latitudes. This is especially characteristic of early dates of onset, but such events have also been hypothesized to precondition the ice for an earlier onset even when they don’t act as the trigger. The importance of atmospheric advection to the melt season is well-recognized by the community. Less attention has been given to the potential limitations of these events and to what alternate mechanisms may also be important for initiation, which is the subject of this presentation. We discuss two case studies.

In the first case, atmospheric advection from the North Atlantic in late May 2020 caused onset to occur over a wide area of the sea ice north of Greenland, including the floe being measured by the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Approximately 6 weeks prior, in April, an anomalously warm advection event also impacted the MOSAiC floe and was responsible for ~40% of the total warming the ice underwent that spring. Using a diffusion model for the ice forced by surface temperatures that both include (observationally) and exclude (synthetically) the April event, we show that its influence relative to its absence was reduced by ~80% within 10 days. The result is explained by a negative feedback that suppresses conduction within the ice when warming events occur. Consequently, despite the apparent influential nature of the April event suggested by the observations, the ice temperatures would likely have been similar several weeks before onset if the April event had not occurred. This implies there are limitations to such events in preconditioning the sea ice for early onset.

Our second case examines data collected from a buoy in the Beaufort Sea during a regional onset event observed in June 2022. In this case, the air that caused melt at the buoy came from the north during a period of generally zonal flow of the polar jet (and lack of poleward moisture transport). Analysis of back trajectories indicates that the air had a residence time in the Arctic of 7-10 days prior to causing melt. The air began at mid-tropospheric levels near the pole then circulated around persistent, large-scale high pressure over the East Siberian Sea, descending along its track. Reanalysis data suggests the adiabatic contribution to the subsidence was sufficient to warm the air to the freezing point when it reached the surface, moving southward across the Beaufort Sea. This case indicates that subsidence is a mechanism internal to the Arctic that is capable of causing melt onset, though its climatological significance remains an open question.

How to cite: Cox, C., Solomon, A., Persson, O., Shupe, M., Gallagher, M., Walden, V., Town, M., Perovich, D., Webster, S., and Anderson, J.: Perspectives on limitations and mechanisms for atmospheric initiation of onset of the summer melt season over sea ice, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6663, https://doi.org/10.5194/egusphere-egu24-6663, 2024.

EGU24-11158 | ECS | Orals | AS4.2

Synoptic situation during the ARTofMELT 2023 spring expedition 

Sonja Murto and Michael Tjernström

A 6-week long expedition ARTofMELT (Atmospheric rivers and the onset of Arctic sea-ice melt) with the Swedish Icebreaker Oden took place in the Arctic Ocean during late winter and spring of 2023. The aim was to collect observations and study processes leading up to the sea-ice melt onset. One of the targets was to assess the role of atmospheric rivers (ARs), i.e., southerly warm and moist-air injections, in advancing the melt-timing. This paper presents the synoptic situation during the expedition, based on observations measured onboard Oden and reanalysis data (ERA5). Additionally, the origin and paths of airmasses reaching Oden are determined using 7-day backward trajectories computed with the Lagrangian analysis tool LAGRANTO. The meteorological conditions were quite dynamic during these 35 days, strongly influenced by several (at least 6) surface cyclones passing Oden and only two warming events accompanied by rather weak ARs were observed, the latter one leading to the melt onset at the end of the expedition.

 

Based on meteorological conditions from 6-hourly launched radiosoundings, the expedition can be divided into six periods. The first short period encompasses the first days of the expedition, when Oden was located at the marginal ice zone. The winds were variable, mainly southerly, and it was moist with slightly below-freezing temperatures. As Oden was moving northwestwards, a one-week cold (~-15 - -10) and dry period followed. This period was mainly governed by northerly winds, guided by a persistent family of surface cyclones located over the Laptev and Kara Seas. The first major storm, that coincided with an atmospheric blocking over Scandinavia, was related to a cyclone forming to the southwest of Greenland and moving northeast, bringing winds over 25 m/s as it hit Oden on 13 May.  Northerly winds followed after the stormed had passed, guided by a surface pressure dipole between a high over Greenland and a low over the Arctic Ocean.

 

The first one-week long ice camp was built at the end of the second period, extending into the third period. A low-pressure over Greenland and high-pressure and an upper-level blocking over Scandinavia resulted in a pathway for a transient warm-air mass from the south, and melting was observed for the first-time. However, this warming was only temporary, as temperatures dropped below freezing after the AR had passed. Several weaker storms governed this third milder period, ending with the second major storm associated with a cyclone on 25 May. Again, winds turned northerly after the storm passed, which made the entry to the fourth longer, colder and drier period. The second 2-week long ice camp was established at the beginning of this period and expanded over the two last periods. These captured the forecasted (6 June) and the real melt onset (10 June). A surface pressure dipole with a high over Greenland and a low over the Arctic Ocean dominated at the beginning of the fifth period, and warm but dry air aloft was observed. As the winds turned southerly, the melt-onset period was characterized as warm and moist.

How to cite: Murto, S. and Tjernström, M.: Synoptic situation during the ARTofMELT 2023 spring expedition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11158, https://doi.org/10.5194/egusphere-egu24-11158, 2024.

EGU24-11515 | ECS | Posters on site | AS4.2

Overview of SMÄLTA: Secondary Marine Aerosol precursors and Links to aerosol growth at ice-melT onset in the Arctic 

Cort Zang, Megan Willis, Julia Asplund, Fredrik Mattsson, Paul Zieger, and Michael Tjernström

The sources, composition, and reactive transformation of reactive organic carbon (ROC, non-methane organic carbon) as well as the processing, abundances, and distribution of organosulfur compounds in the Arctic marine atmosphere are unconstrained partially due to a lack of targeted measurements.  Understanding the emission, transport and processing of ROC and organosulfur compounds is important for improving our understanding of the impacts of gaseous precursors on aerosol nucleation and growth, and atmospheric oxidation capacity. There is a shift in aerosol size distribution that occurs with the Arctic spring-to-summer transition period and there are very few Arctic marine measurements of trace gases during this same period. Constraining the composition of organosulfur compounds and ROC is important for understanding the drivers in the shift of aerosol size distribution.

We present shipborne gas-phase measurements of ROC and organosulfur compounds in the Arctic marine atmosphere as part of the Atmospheric Rivers and the onseT of sea ice MELT (ARTofMELT) campaign. ARTofMELT took place from May 7th to June 15th of 2023 over pack ice and within the marginal ice zone between 78 and 81°N in the Fram Strait. We deployed a reagent ion switching chemical ionization mass spectrometer to target ROC and organosulfur compounds using H­3O+ ionization for the detection of reduced compounds and NH4+ ionization for the detection oxidized species. The measurements encompass a variety of different conditions including ozone depleted air masses (<10ppbv), cloud influenced air masses, a range of aerosol concentrations, and air masses with southern and northern airmass history with influences from biologically rich marine regions as well as transport from over pack ice. Additionally, measurements of ROC show the presence of ≥C5 organics in the environment with implications for aerosol size and growth. Here, we show an overview of our measurements and some initial observations of the ROC present during the campaign.

How to cite: Zang, C., Willis, M., Asplund, J., Mattsson, F., Zieger, P., and Tjernström, M.: Overview of SMÄLTA: Secondary Marine Aerosol precursors and Links to aerosol growth at ice-melT onset in the Arctic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11515, https://doi.org/10.5194/egusphere-egu24-11515, 2024.

EGU24-12340 | ECS | Orals | AS4.2

Sea ice drift and wave pattern analysis of the early melt onset during the ARTofMELT cruise 2023 

Thibault Desjonquères, Leif E. B. Eriksson, Malin Johansson, Denis Demchev, Truls Karlsen, Timo Vihma, and Bing Cheng

In May-June 2023 the ARTofMELT 2023 expedition took place, with the aim to capture the melt onset in the Arctic Ocean. For the sea ice dynamics part of the cruise, in-situ observations were collected to co-inside with satellite observations, enabling studies of changes in drift patterns, capture the breakup of ice floes and studies of changes in backscatter signatures in satellite images as a consequence of melt onset. 

Seven OpenMETbuoys-v2021 and three SIMBA buoys, were placed on four first-year ice floes, away from the Marginal Ice Zone (MIZ). The OpenMETbuoys, equipped with GNSS (Global Navigation Satellite Systems), gyro, and accelerometer, facilitated horizontal motion, rotation, potential deformation, and wave action analysis. SIMBA buoys, with GNSS and thermistor strings, focused on temperature effects connected to melt onset. Three OpenMETbuoys and one SIMBA buoy were deployed on two larger floes. The two remaining drifters were deployed on individual floes. Deploying multiple buoys on each floe allowed detailed examination of small-scale drift changes, convergence, divergence, rotational patterns, frequencies, and connections to satellite Synthetic Aperture Radar (SAR) images. This deployment provides insights into the remaining wave energy in the pack ice. 

Low noise Radarsat Constellation Missions (RCM) SAR images in dual polarization (HH+VV or HH+HV) were acquired to overlap with the campaign in space and time. The temperature sensors onboard the SIMBA buoys enables us to connect changes in  backscatter values in the SAR images from the winter conditions into the early melt season and help define limitations for the SAR sea ice drift retrieval algorithm. 

Initial findings from wave and GNSS data offer insights into the condition of ice floes, including dislocation, disintegration, melting, and interactions with neighboring floes. The dislocation of the floes is indicated by the physical dissociation of the buoys present on the same floe. The OpenMETbuoys' recorded wave height and wave period indicate the drifter's location: on ice, in a transition phase on a small piece of ice or floating in the water between pieces of brash ice, or in open water.

Regarding the two bigger floes, on the first one, the drifters were launched 2023-05-22. An OpenMET drifter was dislocated from the rest of the floe on the 26th of May, and was in the transition phase on the 1st of July. The two remaining drifters were separated on the 29th of May. The last OpenMET drifter reached the transition phase on the 25th of May. The drifters on the second floe were launched 2023-05-28. The first dislocation occurred on the 8th of June, the second one on the 18th of June. The two remaining OpenMET drifters on this floe reached the transition phase on the 13th of June and 15th of June. The third floe contained a SIMBA drifter launched 2023-06-06 and the fourth one an OpenMETbuoy launched 2023-05-28. The latter reached the transition phase on the 10th of June.

How to cite: Desjonquères, T., Eriksson, L. E. B., Johansson, M., Demchev, D., Karlsen, T., Vihma, T., and Cheng, B.: Sea ice drift and wave pattern analysis of the early melt onset during the ARTofMELT cruise 2023, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12340, https://doi.org/10.5194/egusphere-egu24-12340, 2024.

EGU24-15627 | Orals | AS4.2 | Highlight

Arctic spring and the onset of sea-ice melt: Early impressions from the ARTofMELT expedition 

Michael Tjernström, Paul Zieger, and Sonja Murto and the ARTofMELT Science Team

The spring season in the Arctic Ocean has gained relatively little attention with detailed observations from expeditions, due to difficulties to navigate in the ice at this time of the year. This paper reviews experiences from the ARTofMELT (Atmospheric rivers and the onset of sea-ice melt) expedition in spring of 2023.

ARTofMELT had two objectives: To study processes leading up to the onset of the sea-ice melt and to explore links to so-called atmospheric rivers (ARs). ARs are spatially and temporally distinct inflows of warm and moist air from farther south. To fulfill these goals, we instrumented the Swedish research icebreaker Oden and planned to locate her in the Atlantic sector of the Arctic Ocean north of Svalbard from early May to mid-June. Oden was equipped with advanced meteorological instrumentation including standard meteorology and 6-horly radiosoundings, radar and lidars for cloud and wind measurements, and a surface flux tower with eddy-covariance. An advanced suite of atmospheric chemistry and aerosol observations were also deployed along with water isotope measurements, and also sampled and profiled the upper ocean structure. To identify upcoming ARs, we used ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) at lead time up to seven days, to allow time to navigate the icebreaker to optimal positions and establish ice camps. While carrying out most of the observations on board, in-situ observations on the ice provide valuable details on the impact of ARs on the ice. On ice camps we therefore deployed a surface energy budget station and an ROV surveying the ice from below and also flew a tethered balloon HELIKITE system from the aft of the ship. Additionally, we also used the helicopter to deploy scientists on the ice (sampling snow, ice and water) and deploying buoys, and for flying the HELIPOD instrument package.

ARTofMELT left Svalbard on 8 May and returned on 15 June. Starting with quite cold later winter conditions there was a brief warming period around mid-May, with an AR that brought air temperatures above the melting point twice (19 and 20 May). This was interrupted by a major storm, followed by a cooler period. From the end of May the surface started to gain heat, culminating in the onset of the melt at a second AR on 10 June. Both ARs were documented from ice stations.

A major uncertainty was the navigation in the ice during late winter and this also tuned out to be the most difficult part of the deployment. The ice was thick and hard to break, the size of the largest ice floes was much larger than expected and short-term variations of the ice pressure made navigation very difficult. The maximum latitude obtained was ~80.5 °N, hence, we stayed in the Fram Strait ice pack. Also, only two brief ARs were encountered, less than expected. In spite of this we were able to gain a large amount of unique observations, both from the icebreaker when in transit and from two ice camps.

How to cite: Tjernström, M., Zieger, P., and Murto, S. and the ARTofMELT Science Team: Arctic spring and the onset of sea-ice melt: Early impressions from the ARTofMELT expedition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15627, https://doi.org/10.5194/egusphere-egu24-15627, 2024.

EGU24-17193 | ECS | Orals | AS4.2

The composition and sources of airborne bacteria and proteinaceous Ice Nucleating Particles in the High Arctic marine region during Spring 

Jennie Spicker Schmidt, Marianne Glasius, Camille Mavis, Jessie Creamean, Gabriel Freitas, Paul Zieger, Kai Finster, and Tina Šantl-Temkiv

The Arctic is a particularly vulnerable region on Earth, where climate change takes place at an intense pace. Clouds represent an essential element within the Arctic atmosphere and play a crucial role in the regional radiative balance. The physical properties of clouds are tightly interlinked with the presence of aerosols that can serve as cloud condensation nuclei (CCN) and as ice nucleating particles (INPs), which facilitate the formation of cloud droplets and ice crystals, respectively. Consequently, they affect cloud thickness, lifetime, and albedo.

More studies propose that various biological aerosols e.g., aerosolized microbial cells, proteinaceous compounds and fragments actively contribute to cloud processes serving as INPs active at high subzero temperatures (>-15°C). However, our understanding of microorganisms responsible for producing compounds serving as INPs, their source environments, and their level of activity, remains highly uncertain.

Given the profound impact of climate change in the Arctic region, understanding the role of biological INPs in the atmosphere becomes particularly critical during Arctic melt season. Here, we present an overview of bioaerosol observations and sources tracking from the recent Arctic expedition ”Atmospheric rivers and the onset of Arctic melt” (ARTofMELT 2023).

Biological INPs are thought to originate from the ocean and meltwater sources during the Arctic Spring and Summer. To assess the potential contribution of these sources to INP active aerosols, aerosols were generated from bulk seawater and sea ice melt water with a temperature-controlled sea spray simulation chamber. The presence of microorganisms in the bulk water and aerosol was quantified using flow cytometry and qPCR while the composition of the microbial communities was determined by amplicon sequencing. Additionally, fluorescent bioaerosols generated by the chamber were  analyzed using a Multiparameter Bioaerosol Spectrometer (MBS). Simultaneously, ambient air samples were analyzed for the presence of microbial cells, bioaerosols, and the composition of the collected microbial community. The ice nucleating properties of water, sea ice melt, and aerosols from the chamber and ambient aerosol were also measured to determine their relevance for Arctic cloud formation.

Preliminary results from the ambient measurements revealed low concentrations of airborne bacterial cells and highly active INPs. From the sea spray simulations, we found that ice melt, snow melt and seawater samples generated a high flux of bacterial cells which were accompanied by INPs active predominantly at low freezing temperatures (<-15°C). Therefore, it seems that the local sea spray is not a likely source of proteinaceous INPs detected in the Arctic spring atmosphere, which will be further explored through bacterial community analysis. Our results will thus provide comprehensive insights into the contribution of local and long-range transported sources of bioaerosols to the Arctic.

How to cite: Schmidt, J. S., Glasius, M., Mavis, C., Creamean, J., Freitas, G., Zieger, P., Finster, K., and Šantl-Temkiv, T.: The composition and sources of airborne bacteria and proteinaceous Ice Nucleating Particles in the High Arctic marine region during Spring, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17193, https://doi.org/10.5194/egusphere-egu24-17193, 2024.

EGU24-17589 | Posters on site | AS4.2

Intense formation of low liquid clouds over the Arctic sea-ice during May.   

Jean Lac and Hélène Chepfer

Low-liquid stratiform clouds are ubiquitous in the Arctic. Their high surface longwave warming induces change in the surface radiative budget that might have effects on the sea-ice melt especially during transitioning seasons. In particular, low liquid clouds formed in Spring may trigger early melt onset that might have an impact on the following evolution of the sea-ice during summer. 

However, relatively little is known about the existence and the drivers of such clouds in the early melt season. Here we used 13 years of space based lidar cloud profile observations with complementary data to show that the predominance of low clouds happens in May. First, we showed that the low cloud fraction reaches 75% of the Arctic Ocean in May over the sea-ice only with a low interannual variability. This cover increase in May seems to be homogeneous over the whole Arctic Ocean. Second, we investigated potential early summer drivers forming those low liquid clouds. One feature is the moisture sources that could explain the availability of such liquid droplets to form liquid clouds. While the other feature is the boundary layer structure, that might affect the stability and the ocean/atmosphere interaction over sea-ice leads.  

Overall, this study suggests a peak of Arctic low liquid clouds occurring in May that might impact the sea-ice summer melt by triggering early Spring melt. 

How to cite: Lac, J. and Chepfer, H.: Intense formation of low liquid clouds over the Arctic sea-ice during May.  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17589, https://doi.org/10.5194/egusphere-egu24-17589, 2024.

EGU24-17977 | ECS | Posters on site | AS4.2

Springtime observations of black carbon aerosols in and outside of low-level Arctic clouds 

Lovisa Nilsson, August Thomasson, Paul Zieger, Julia Asplund, Pontus Roldin, Fredrik Mattson, Erik Ahlberg, and Erik Swietlicki

Few expeditions have ventured into the Arctic to observe the processes that take place in the transition from winter to summer. Particularly, direct observations of aerosol-cloud interactions are scarce, and comprise a large source of uncertainty in radiative forcing estimations in the Arctic.

Light absorbing aerosol particles, such as black carbon (BC) from incomplete combustion, exert a positive forcing upon direct absorption of sunlight, and affect clouds by serving as cloud condensation nuclei (CCN). During the icebreaker expedition ARTofMELT in spring 2023, we measured BC with a multi-angle absorption photometer (MAAP) and a single particle soot photometer (SP2) for five weeks. The two instruments differ by principle and can be used to inform on complementary aspects of the light absorbing aerosol. For example, the MAAP provides the total mass concentrations of so-called equivalent BC (eBC), whereas the single particle instrument SP2 determines the mass of individual refractory BC (rBC) aggregates. Most of the time, the MAAP and SP2 sampled the total BC concentration on the same inlet (whole-air). However, during cloud-events, the SP2 measured downstream of a counterflow virtual impactor (CVI) inlet that samples just cloud droplets or ice crystals without the interstitial or non-activated aerosol.

Our first results indicate overall low out-of-cloud BC mass concentrations for both instruments (median and interquartile range, IQR: 4.4 (1.6-8.5) ngm-3 for the MAAP and 2.5 (1.2-4.7) ngm-3 for the SP2). The variation in mass concentration was small, although the tendency of a gradual decrease was observed towards the onset of the melt.

The SP2 instrument enables studies of the BC mass size distribution. For example, during a cloud event we observed that the geometric mean diameter (GMD, mass equivalent diameter) shifted from smaller (171 nm, whole-air inlet) to larger sizes (175-192 nm), as the SP2 switched to sampling the cloud-residual BC (CVI inlet). Further investigation is needed to examine the underlying causes for this observation (e.g. variation in airmass origin). 

The total aerosol concentration is influenced by local natural sources and production from gaseous precursors, as opposed to the BC concentration which is mainly affected by anthropogenic activities. BC source footprints from the Lagrangian dispersion model FLEXPART, indicate little influence from industrialized regions during the whole campaign. This may explain the comparably low median concentration of rBC-particles (1.1 cm-3, IQR: 0.5-2.1) to the total aerosol number concentration (in the range ~20-150 cm-3).

How to cite: Nilsson, L., Thomasson, A., Zieger, P., Asplund, J., Roldin, P., Mattson, F., Ahlberg, E., and Swietlicki, E.: Springtime observations of black carbon aerosols in and outside of low-level Arctic clouds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17977, https://doi.org/10.5194/egusphere-egu24-17977, 2024.

EGU24-19851 | ECS | Orals | AS4.2

Characteristics of natural Arctic aerosols emitted from a wide range of local sources during ARTofMELT2023 

Gabriel Freitas, Kouji Adachi, Julia Asplund, Jessie Creamean, Fredrik Mattsson, Camille Mavis, Lovisa Nilsson, Matthew Salter, Jennie Spiecker Schmidt, Tina Šantl-Temkiv, and Paul Zieger

The Arctic has been experiencing a rise in ambient temperature several times higher than the global average. This warming trend has led to a continuous decline in sea ice coverage and snowpack prevalence. Aerosol sources, such as those from the open ocean and tundra, have become more prevalent throughout the year. These sources emit primary biological aerosol particles (bioaerosols) some of which exhibit ice nucleating properties at high temperatures (>-15C). Ice nucleating particles (INPs) play a crucial role in cloud ice formation, affecting cloud physical and optical properties, as well as their lifetime. Consequently, this has a substantial impact on the Arctic climate. 

During the ARTofMELT2023 expedition (“Atmospheric Rivers and the Onset of Sea Ice Melt 2023”) conducted aboard the Swedish icebreaker Oden in the Atlantic sector of the Arctic Ocean, we assessed the relative importance of several natural bioaerosol sources, such as sea ice, snow melt (to simulate melt ponds) and bulk ocean water. This involved several sea spray simulation chamber and nebulizer experiments, referred to as “source experiments”. The aerosol particles generated in the 61 source experiments conducted were analyzed using single-particle ultraviolet fluorescence spectroscopy along with other complementary aerosol measurements. These included particle size, black carbon content, particle chemical composition, as well as the microbial community and INP concentration of emitted particles. Additionally, filter samples were obtained for transmission electron microscopy (TEM) analysis. 

Our findings indicate that sea ice and snow melt are more significant sources of bioaerosols compared to the bulk ocean water, including the sea surface microlayer, indicating the potential importance of melt ponds as a local Arctic bioaerosol source. Furthermore, we found significant differences in the chemical composition, black carbon content and size distribution of the various analyzed aerosol sources.

How to cite: Freitas, G., Adachi, K., Asplund, J., Creamean, J., Mattsson, F., Mavis, C., Nilsson, L., Salter, M., Spiecker Schmidt, J., Šantl-Temkiv, T., and Zieger, P.: Characteristics of natural Arctic aerosols emitted from a wide range of local sources during ARTofMELT2023, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19851, https://doi.org/10.5194/egusphere-egu24-19851, 2024.

EGU24-19946 | Orals | AS4.2 | Highlight

Helicopter borne measurements during melt onset in the Fram strait as part of ARTofMELT23 

Falk Pätzold, Lutz Bretschneider, Magnus Asmussen, Barbara Altstädter, Evelyn Jäkel, Hendrik Stapel, Tim Sperzel, Manfred Wendisch, Birgit Wehner, Ralf Käthner, and Astrid Lampert

In the Arctic climate system, the onset of melting is a crucial point, and timing is still difficult to predict. Therefore, the expedition ARTofMELT was dedicated to exploring atmospheric conditions and processes that are involved in triggering the onset of melting.

The helicopter borne sensor system HELIPOD was deployed in this expedition to measure the spatial variability of atmospheric dynamics, radiation, aerosols, trace gases and surface properties on a horizontal scale up to 40 km around the icebreaker ODEN. During the ARTofMELT23 expedition, the HELIPOD conducted 12 measurement flights in the FRAM strait around 80° North and the prime meridian between 9 May and 9 June 2023 with 26.5 hours in the air. The flights covered an area of about 20 NM around the location of the icebreaker ODEN and a vertical range from 50 m to 2700 m above sea level. The flight patterns were aligned parallel and perpendicular to dominating directions as the sea ice edge and the wind direction. In one case a cloud layer edge apparently structured the atmospheric situation. The flights covered pre-melt onset conditions, refreezing situations and the melt onset. Synoptic air mass changes were probed as well.    

The presentation gives an overview of the temporal changes of the ambient conditions during the research flights, and a first assessment of the flights during transient weather situations.

How to cite: Pätzold, F., Bretschneider, L., Asmussen, M., Altstädter, B., Jäkel, E., Stapel, H., Sperzel, T., Wendisch, M., Wehner, B., Käthner, R., and Lampert, A.: Helicopter borne measurements during melt onset in the Fram strait as part of ARTofMELT23, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19946, https://doi.org/10.5194/egusphere-egu24-19946, 2024.

EGU24-20594 | Posters on site | AS4.2

Sea ice, snow caps, and freshwater lenses: The hurdles local Arctic aerosols must overcome to become airborne 

Jessie Creamean and the MOSAiC and ARTofMELT field teams

Aerosol particles and clouds play a critical role in regulating radiation reaching the Arctic, which is warming faster than anywhere else globally. However, the magnitude of their effects is not adequately quantified, especially in the Arctic Ocean over sea ice. Specifically, particles generated from open leads, melt ponds, and the snow-covered sea ice surfaces remain poorly understood, yet could have significant impacts on cloud condensation nuclei (CCN) and ice nucleating particle (INP) concentrations, and thus, central Arctic cloud formation. While marine biological processes have been demonstrated to be potentially key primary aerosol sources in the Arctic summer, exact sources and emission processes of these particles remain highly uncertain. 

For this presentation, we provide an overview of aerosol observations from two recent Arctic field campaigns: the 2019–2020 Multidisciplinary drifting Observatory for Study of Arctic Climate (MOSAiC) and the 2023 Atmospheric rivers and the onset of Arctic melt (ARTofMELT) expeditions. We highlight preliminary findings focused on aerosols that have the potential to impact cloud phase and lifetime over the Arctic Ocean, specifically those from local sources in the early spring and summer melt periods. The evolution of open water within the pack ice in late spring and the Arctic melt season coincides with an increase in aerosol particle concentration, which may be attributed to biological activity within seawater and sea ice. However, the emission of aerosol particles is contingent on features like open leads and melt ponds, and whether they are covered by snow, freshwater melt layers, or ice lids. This integrative study involves the use of detailed aerosol, meteorological, oceanographic, and sea ice observations from MOSAiC and ARTofMELT. Overall, this work will enable us to assess local aerosol processes associated with cloud formation to better understand the Arctic system through a holistic approach.

How to cite: Creamean, J. and the MOSAiC and ARTofMELT field teams: Sea ice, snow caps, and freshwater lenses: The hurdles local Arctic aerosols must overcome to become airborne, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20594, https://doi.org/10.5194/egusphere-egu24-20594, 2024.

EGU24-21926 | Posters on site | AS4.2

Water Isotope measurements contribute to the understanding of atmospheric, sea ice, ocean interactions during the ArtofMelt expedition, Fram Strait, spring 2023 

Jeff Welker, Ben Kopec, Eric Klein, Julia Muchowski, Timo Vihma, Paul Zieger, Falk Paetzold, Astrid Lampert, Penny Vlahos, John Prytherch, Valtteri Hyöky, and Truls Karlsen

Transitions periods between seasons in the Arctic are phases when the atmosphere-sea ice-ocean interactions are heightened, especially during these periods of exceptional warming.  These transition periods may be accompanied by shifts in atmospheric transport patterns, the distribution of sea ice and extreme events, such as atmospheric rivers.  Atmospheric Rivers may act as accelerants of sea ice melt and its redistribution, leading to spatial complexity in ice-ocean-atmosphere exchanges of mass and energy.

As part of an interdisciplinary team aboard the I/B Oden from early May to mid-June, four main water isotope measurement packages were collected to maximize collaborations and to resolve nuisances of the Arctic System throughout the cruise track between Svalbard and NE Greenland (Figure 1).  First, in order to delineate longitudinal distribution of the warm and salty W Svalbard current compared to the cold and fresh E Greenland current, we continuously measured the near surface water δ18O, δ2H and d-excess values. Second, in order to source water vapor and moisture sources from the warm, moist, and isotopically enriched subpolar & N Atlantic, compared to cold, dry and isotopically depleted Arctic air, we also continuously measured the δ18O, δ2H and d-excess values of water vapor collected from the ship’s, bow-mounted, eddy covariance tower. Third, in order to understand the horizontal and altitudinal patterns of water vapor parcels that surround the ship; in-situ water vapor isotopes were measured during fHeliPod flight lines that extended up to 30 km N-S-E-W of the Oden and from ~ 50 m above the sea ice and open water to over 2k in altitude.  Fourth, in order to delineate the source of moisture (sea water vs. meteoric water) throughout the sea ice core profiles and the patterns and sources of moisture in the snow pack profiles; ice cores and snow pits were collected (drilled) and dug at ~10 different locations and water isotope samples were analyzed for δ18O, δ2H and d-excess values back in the laboratory.

Four major discoveries will be presented: A) mixing of the surface W Svalbard and NE Greenland current is found to be farther east than previously reported and the surface water masses may differ by up to 5 ‰ δ18O during spring; B) water vapor isotopes responded at hourly time scales as moisture sources during Atmospheric River events begin with northward fluxes of warm, moist air masses but passing cyclones deliver N-S cold-dry, isotopically depleted water vapor in extreme Arctic-sourced storm events lasting a day or more; C) Horizontal and vertical transects during Heliopod flights captured horizontal and altitudinal variation in water vapor isotopes during periods when the weather of the ship was dominated by cold-dry Arctic air, interrupted by periods when the ship was experiencing pulses of warm, moist, and high humidity conditions; D) ice cores and snow packs exhibit vertical isotopic variation indicative of different moisture sources and morphogenesis processes.

How to cite: Welker, J., Kopec, B., Klein, E., Muchowski, J., Vihma, T., Zieger, P., Paetzold, F., Lampert, A., Vlahos, P., Prytherch, J., Hyöky, V., and Karlsen, T.: Water Isotope measurements contribute to the understanding of atmospheric, sea ice, ocean interactions during the ArtofMelt expedition, Fram Strait, spring 2023, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21926, https://doi.org/10.5194/egusphere-egu24-21926, 2024.

EGU24-1026 | ECS | Orals | CL1.2.6

Input and output fluxes of surface CO2 over the Late Quaternary 

Luca Castrogiovanni, Pietro Sternai, Claudia Pasquero, and Nicola Piana Agostinetti

Ice core archives allow us to retrieve the atmospheric CO2 concentration of the past 800,000 years characterized by periodically lower and higher CO2 levels corresponding to ice ages and interglacials, respectively. However, there is no broadly accepted consensus regarding the leading drivers of such variability. To a large extent, what prevents us from identifying the mechanisms that underlie changes in atmospheric CO2 concentrations is our inability to split the overall atmospheric CO2 budget into its sources and sinks terms, thereby assessing the fluxes of carbon among different reservoirs. Here, we use a reversible-jump Markov chain Monte Carlo (rj-McMC) algorithm to invert the atmospheric CO2 concentration dataset provided by the EPICA ice core based on a general forward formulation of the geological carbon cycle. We can quantify the most likely temporal variability of atmospheric carbon fluxes in ppm/yr throughout the last 800,000 years. Results suggest that temperature changes have been driving the variations of atmospheric CO2 until the Mid-Brunhes Event (MBE), when the onset of a progressive cyclic increase of  the atmospheric carbon fluxes marks a distinct behavioral change of the climate system. We ascribe such change to mechanisms internal to the Earth system, possibly related to the deglacial triggering of global volcanism and associated feedbacks on climate or a combination of geological, biological, and physical processes. Regardless, our unprecedented quantification of past atmospheric input and output CO2 fluxes provide (1) new constraints for climate carbon cycle and paleoclimate models to assess dominant climate-driving mechanisms, and (2) the benchmark for climate models intercomparison projects and better assessing the anthropogenic perturbation to the geological carbon cycle an associated climatic effect.

 

How to cite: Castrogiovanni, L., Sternai, P., Pasquero, C., and Piana Agostinetti, N.: Input and output fluxes of surface CO2 over the Late Quaternary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1026, https://doi.org/10.5194/egusphere-egu24-1026, 2024.

One of the critical features of deglaciations is the sudden increase in atmospheric CO2 levels. Regulating the Pleistocene atmospheric CO2 variations requires the involvement of oceanic carbon storage changes. However, the mechanisms and pathways for air-sea carbon exchanges remain elusive, partly resulting from the insufficiency of marine carbonate system proxy data with a robust age control beyond Termination I.

The deglacial CO2 rise toward Marine Isotope Stage (MIS) 9e (Termination IV) started from 197.1 ppm to 300.7 ppm[1], representing the highest natural atmospheric CO2 recorded in the Antarctic ice cores over the past 800 ka[2]. Our high-resolution carbonate system records from the Iberian Margin with a robust age control suggest an expansion of southern-sourced Glacial Antarctic Bottom Water at the onset of the deglaciation, followed by a net release of CO2 from the Atlantic sector of the Southern Ocean. However, our results indicate a different ocean circulation pattern during Termination III, when atmospheric CO2 increases by 85 ppm[2]. Unlike Termination III, the north-sourced water seems to take a large proportion of the deep Atlantic Ocean during this period.

References:

[1] Nehrbass-Ahles, C. et al. (2020), Science vol. 369 1000–1005.

[2] Bereiter, B. et al. (2015), Geophys. Res. Lett. 42, 542–549.

How to cite: Ji, X. and Yu, J.: The mechanism controlling air-sea CO2 exchange under different ocean circulation conditions, a case study from Iberian Margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1157, https://doi.org/10.5194/egusphere-egu24-1157, 2024.

EGU24-3128 | Orals | CL1.2.6

Bølling-Allerød warming as a part of orbitally induced climate oscillation  

Yuchen Sun, Xu Zhang, Gregor Knorr, Martin Werner, Lev Tarasov, and Gerrit Lohmann

Deglacial abrupt warming event is a ubiquitous feature of deglaciations during the Late Pleistocene. Nevertheless, during the last deglaciation an unusually early onset of abrupt Northern Hemisphere warming event, known as Bølling/Allerød (B/A) warming, complicates our understanding of their underlying dynamics, especially due to the large uncertainty in histories of ice sheet retreat and meltwater distributions. Here applying the latest reconstruction of ice sheet and meltwater flux, we conduct a set of transient climate experiments to investigate the triggering mechanism of the B/A warming. We find that the realistic spatial distribution of meltwater flux can stimulate the warming even under a persistent meltwater background. Our sensitivity experiments further show that its occurrence is associated with an orbitally induced climate self-oscillation under the very deglacial climate background related to atmospheric CO2 level and ice sheet configurations. Furthermore, the continuous atmospheric CO2 rising and ice sheet retreating appear to mute the oscillation by freshening the North Atlantic via modulating moisture transport by the Westerly. 

How to cite: Sun, Y., Zhang, X., Knorr, G., Werner, M., Tarasov, L., and Lohmann, G.: Bølling-Allerød warming as a part of orbitally induced climate oscillation , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3128, https://doi.org/10.5194/egusphere-egu24-3128, 2024.

EGU24-4269 | ECS | Posters on site | CL1.2.6

Impact of iron fertilisation on Southern Ocean ecosystems and global carbon cycle during the last glacial cycle 

Himadri Saini, Katrin Meissner, Laurie Menviel, and Karin Kvale

Rising atmospheric CO2 concentration is a major driver of climate change. One of the several processes proposed to explain the lower atmospheric CO2 concentration during the last glacial period is an increase in aeolian iron flux into the Southern Ocean. As the Southern Ocean is a high-nutrient-low-chlorophyll region, increased iron deposition can impact Southern Ocean marine ecosystems,  increase export production, and reduce surface Dissolved Inorganic Carbon (DIC) concentration. Here, we investigate the responses of Southern Ocean marine ecosystems to changes in iron flux and their impact on ocean biogeochemistry and atmospheric CO2 during the last glacial period. We use a recently developed complex ecosystem model that includes four different classes of phytoplankton functional types and fully incorporated iron, silica and calcium carbonate cycles. We show that the changes in atmospheric CO2 are more sensitive to the solubility of iron in the ocean than the regional distribution of the iron fluxes. If surface water iron solubility is considered constant through time, we find a CO2 drawdown of ∼4 to ∼8 ppm. However, there is evidence that iron solubility was higher during glacial times. A best estimate of solubility changing from 1 % during interglacials to 3 % to 5 % under glacial conditions yields a ∼9 to 11 ppm CO2 decrease at 70 ka, while a plausible range of CO2 drawdown between 4 to 16 ppm is obtained using the wider but possible range of 1 % to 10 %. We also show that the decrease in CO2 as a function of Southern Ocean iron input follows an exponential decay relationship, which arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations.

We also investigate the role of iron flux changes on the abrupt atmospheric CO2 increase during Heinrich Stadials, which are associated with a near collapse of the Atlantic Meridional Overturning Circulation (AMOC), a sudden decrease in Greenland temperature and warming in the Southern Ocean. Previous modelling studies have investigated the role of the ocean circulation in driving changes in atmospheric CO2 concentration during these abrupt events, while the role of reduced aeolian iron input during Heinrich stadials remained poorly constrained. We show that a weakened iron fertilisation during Heinrich Stadials can lead to ~6 ppm rise in CO2 out of the total increase of 15 to 20ppm as observed. This is caused by a 5% reduction in nutrient utilisation in the Southern Ocean, leading to reduced export production and increased carbon outgassing from the Southern Ocean.

How to cite: Saini, H., Meissner, K., Menviel, L., and Kvale, K.: Impact of iron fertilisation on Southern Ocean ecosystems and global carbon cycle during the last glacial cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4269, https://doi.org/10.5194/egusphere-egu24-4269, 2024.

EGU24-4451 | Posters on site | CL1.2.6

Simulating glacial-interglacial CO2 variations: What's right with CLIMBER? 

Malte Heinemann, Victor Brovkin, Matteo Willeit, Joachim Segschneider, and Birgit Schneider

Despite intense efforts, current generation comprehensive Earth system models have, to our knowledge, not been able to simulate the full extent of the atmospheric pCO2 drawdown (as recorded in ice cores) during the Last Glacial Maximum (LGM). Yet, the intermediate complexity model CLIMBER-2 has successfully been used to simulate not only the LGM drawdown but also the transient evolution of CO2 concentrations during entire glacial–interglacial cycles. To better understand why this is the case, we compare the CLIMBER-2 results to pre-industrial and LGM simulations using two related models with increasing complexity, namely, the recently developed intermediate complexity model CLIMBER-X and the state-of-the-art comprehensive Earth system model MPI-ESM as used in the PalMod project, focusing on ocean carbon cycle changes.

How to cite: Heinemann, M., Brovkin, V., Willeit, M., Segschneider, J., and Schneider, B.: Simulating glacial-interglacial CO2 variations: What's right with CLIMBER?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4451, https://doi.org/10.5194/egusphere-egu24-4451, 2024.

EGU24-4464 | ECS | Posters on site | CL1.2.6

Polar Twins: Glacial CO2 outgassing reduced in the Southern Ocean by upwelling of well-ventilated waters from the North Pacific  

Madison Shankle, Graeme MacGilchrist, William Gray, Casimir de Lavergne, Laurie Menviel, Andrea Burke, and James Rae

The Southern Ocean is widely thought to have played a driving role in the atmospheric CO2 fluctuations of the ice ages, ventilating carbon-rich deep waters to the atmosphere during interglacial periods and limiting this CO2 leakage during glacial periods. A more efficient Southern Ocean biological pump during glacial periods is one of the leading hypotheses for how this “leak” might have been stemmed, but the exact dynamics responsible are still debated. Previous hypotheses have invoked reduced upwelling and/or enhanced stratification in reducing the carbon and nutrient supply to the glacial Southern Ocean surface, thus enhancing the net efficiency of its biological pump. Here we consider an alternative, complementary scenario in which the nutrient and carbon content of the upwelled water itself is reduced. Noting the striking similarity between proxy records from the North Pacific and Southern Ocean over the Last Glacial Cycle and given that carbon-rich waters upwelling in the Southern Ocean today are largely fed by the North Pacific, we propose that low-carbon/nutrient glacial Southern Ocean surface waters could have been sourced from a well-ventilated, low-carbon/nutrient glacial North Pacific. We then show in intermediate-complexity Earth system model simulations how a well-ventilated North Pacific can directly reduce the outgassing potential of waters upwelled in the Southern Ocean. While not precluding the possibility of changes to upwelling or mixing, our results demonstrate the ability of changes in the upwelled waters’ carbon content – outside of any changes to Southern Ocean physical dynamics (e.g., upwelling rate) – to change Southern Ocean carbon content and outgassing. This provides a novel mechanism linking Northern Hemisphere climate to Southern Ocean carbon cycling and may thus help explain the cyclic CO2 variations of the ice ages.

How to cite: Shankle, M., MacGilchrist, G., Gray, W., de Lavergne, C., Menviel, L., Burke, A., and Rae, J.: Polar Twins: Glacial CO2 outgassing reduced in the Southern Ocean by upwelling of well-ventilated waters from the North Pacific , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4464, https://doi.org/10.5194/egusphere-egu24-4464, 2024.

EGU24-5280 | ECS | Orals | CL1.2.6

Sensitivity of millennial-scale climate oscillations to boundary conditions in HadCM3 glacial simulations 

Brooke Snoll, Ruza Ivanovic, Lauren Gregoire, Yvan Rome, and Sam Sherriff-Tadano

Romé et al. (2022) present a new set of long-run Last Glacial Maximum experiments with millennial-scale climate oscillations between cold and warm modes. These oscillations are triggered by different snapshots of ice-sheet meltwater derived from the early stages of the last deglaciation. The overall characteristics of the oscillating events share similarities with δ18O records of the last glacial period. We test the robustness of these oscillations under different climate conditions, i.e., changes in atmospheric carbon dioxide concentration and orbital configuration. These experiments were run with intentions to better understand the range of conditions the oscillations can be sustained within the model and provide additional insight into the triggering mechanisms that control abrupt climate changes. The results of our sensitivity analysis show that small changes in carbon dioxide concentrations can impact the periodicity and existence of oscillations. A decrease in carbon dioxide concentration decreases periodicity, and an increase in carbon dioxide concentration increases periodicity, leading to an end of the oscillations. Our results also show that for changes in orbital configuration, an increase in Northern Hemisphere summer insolation decreases periodicity and potentially also amplitude. The results show that small changes in climate conditions can impact the shape and existence of oscillations and how this could relate to the changing periodicity and amplitude of observed Dansgaard-Oeschger events as well as transitions from glacial to interglacial states.

How to cite: Snoll, B., Ivanovic, R., Gregoire, L., Rome, Y., and Sherriff-Tadano, S.: Sensitivity of millennial-scale climate oscillations to boundary conditions in HadCM3 glacial simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5280, https://doi.org/10.5194/egusphere-egu24-5280, 2024.

Pleistocene temperatures correlate well with glacial-interglacial changes in global ice volume. While a discharge of ice-rafter debris (IRD) into the ocean directly reflects the rates of growth and decay (deglaciations) of glacial ice sheet margins at sea level, it is also the result of a rapidly changing global environment which affected both the meridional overturning in the ocean and the patterns in ocean-atmosphere circulation on a regional scale.  Circum-arctic land regions and adjacent ocean basins hold clues of varying ice sheet sizes through time. Understanding these records correctly is therefore an important asset to better appreciate Quaternary climate change also within a much broader global context. Marine sediment core data from the Nordic Seas show a stepwise trend of decreasing fluxes of IRD during major glaciations of the last 500 ka, i.e., marine isotope stages (MIS) 12, 6, and 2. Strongest IRD deposition occurred in MIS 12 (Elsterian), while it was lower in MIS 6 (Saalian) and 2 (Weichselian). These marine results of iceberg discharge rates from the western European margins, in particular, point to significant temporal changes in the ice-sheet coverage over northern Eurasia. Indeed, field data provide evidence for several major pre-Weichselian glaciations. Although their maximum limits were likely asynchronous in certain places, it seems evident that these ice sheets not only pre-date the Saalian time, they also extended much farther south (and east) than at any time later. The stepwise decreases in Eurasian ice-sheet extents during glacial maxima terminated in quite contrasting deglaciations and subsequent interglacial developments. It appears evident that such a systematic change in ice-sheet sizes were the result of specific ocean heat circulation, which influenced the pathways of atmospheric moisture transfer across northern Eurasia.

How to cite: Bauch, H.: Impact of waxing and waning of Northern Ice sheets on Pleistocene climate , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5426, https://doi.org/10.5194/egusphere-egu24-5426, 2024.

EGU24-5549 | Orals | CL1.2.6

Simulated radiocarbon cycle revisited by considering the bipolar seesaw and benthic 14C data 

Peter Köhler, Luke Skinner, and Florian Adolphi

Carbon cycle models used to interpret the IntCal20 compilation of atmospheric Δ14C have so far neglected a key aspect of the millennial-scale variability connected with the thermal bipolar seesaw: changes in the strength of the Atlantic meridional overturning circulation (AMOC) related to Dansgaard/Oeschger and Heinrich events. Here we implement such AMOC changes in the carbon cycle box model BICYCLE-SE to investigate how model performance over the last 55 kyr is affected, in particular with respect to available 14C and CO2 data. Constraints from deep ocean 14C suggest that the AMOC in the model during Heinrich stadial 1 needs to be highly reduced or even completely shutdown. Ocean circulation and sea ice coverage combined are the processes that almost completely explain modelled changes in deep ocean 14C age, and these are also responsible for a glacial drawdown of ~60 ppm of atmospheric CO2. A further CO2 drawdown of ~25 ppm is caused by the colder ocean surface at the last glacial maximum. We find that the implementation of AMOC changes in the model setup that was previously used for the calculation of the non-polar mean surface marine reservoir age, Marine20, leads to differences of less than ±100 14C years. The representation of AMOC changes therefore appears to be of minor importance for deriving mean ocean radiocarbon calibration products such as Marine20, where atmospheric carbon cycle variables are forced by reconstructions. However, simulated atmospheric CO2 exhibits minima during AMOC reductions in Heinrich stadials, in disagreement with ice core data. This mismatch supports previous suggestions that millennial-scale changes in CO2 were probably mainly driven by biological and physical processes in the Southern Ocean. By modifying the 14C production rate (Q), between one that varies so as to fit simulated atmospheric ∆14C to IntCal20 and an alternative constant Q, we can furthermore show that in our model setup the variability in deep ocean 14C age, especially during the Bølling/Allerød—Younger Dryas—Early Holocene climate transition, has its root cause in the carbon cycle, while a Q that achieves agreement with the IntCal20 atmospheric ∆14C record only enhances deep ocean age anomalies and thus optimizes agreement with the benthic 14C data.

How to cite: Köhler, P., Skinner, L., and Adolphi, F.: Simulated radiocarbon cycle revisited by considering the bipolar seesaw and benthic 14C data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5549, https://doi.org/10.5194/egusphere-egu24-5549, 2024.

EGU24-6271 | ECS | Orals | CL1.2.6

High-resolution sedimentological and palaeoceanographic investigation of the Last Glacial Termination (T1) recorded on the western margin of the Svalbard (Arctic) 

Fernando Sergio Gois Smith, Renata Giulia Lucchi, Monica Bini, Caterina Morigi, Patrizia Ferretti, Laura Bronzo, and Nessim Douss

The Last Glacial Maximum (LGM) was defined based on the low stand sea-level records from the most recent period when global ice sheets reached their maximum volume, between 26,500 and 19,000 years before present. The end of this cold period was the last glacial termination (T1), occurred between 20 and 11.7 ka BP marking the transition to the current interglacial. During T1, the sea level rise responded to a variety of processes although the melting of the world widely distributed ice sheets was initially the main contributor and responsible for abrupt relative sea level rises known as meltwater pulses (MWPs) that deeply changed the Earth’s physiography. MWPs are short-lived global acceleration in sea-level rise resulting from intense glacial melting, surge of large ice streams into oceans and intense iceberg discharge during ice sheet disintegration. Nowadays, the main concerns related to the present fast global climate change is the possibility that sudden drastic ice loss from the Greenland and/or in the West Antarctic Ice Sheet would lead to a new abrupt acceleration of the relative sea level with consequent inundation of vast coastal areas and/or to cause an abrupt slowdown of the Atlantic Meridional Overturning Circulation (i.e. Golledge et al., 2019). To better understand the dynamics and risks associated with the onset of those events, their impact on thermohaline ocean circulation and climate it is pivotal the study of the well-preserved polar marine sediment records of the events occurred during the T1. Here, we present the results of a high-resolution sedimentological, micropaleontological and geochemical investigation of 3 sediment cores collected on the western margin of Svalbard and eastern side of the Fram Strait (Artic). The sediment cores were collected between 1322 m and 1725 m of water depth, in correspondence of the southern IODP sites that will be drilled during the IODP Exp-403 (June-August 2024).

How to cite: Gois Smith, F. S., Lucchi, R. G., Bini, M., Morigi, C., Ferretti, P., Bronzo, L., and Douss, N.: High-resolution sedimentological and palaeoceanographic investigation of the Last Glacial Termination (T1) recorded on the western margin of the Svalbard (Arctic), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6271, https://doi.org/10.5194/egusphere-egu24-6271, 2024.

EGU24-6599 | ECS | Posters virtual | CL1.2.6

A two-phased Heinrich Stadial 11 as revealed by alkenone-based temperature record from the western tropical North Atlantic  

Anastasia Zhuravleva, Kirsten Fahl, and Henning A. Bauch

Paleo-data and models show that reductions in the strength of the Atlantic meridional overturning circulation (AMOC) lead to significant subsurface warming in the western tropical North Atlantic. The thermal response at the sea surface is less constrained due to the competing nature of the atmospheric and oceanic processes that produce opposite signs of temperature change. Here, we used alkenone unsaturation in sediments to reconstruct sea surface temperature (SST) evolution in the southeastern Caribbean (core MD99-2198, 1330 m water depth) during the last glacial-interglacial cycle, including Heinrich Stadial 11, which was a period of intense AMOC weakening. Our data show a 1 °C SST warming associated with the onset of Heinrich Stadial 11, and a 1 °C cooling during the late Heinrich, followed by a gradual 1 °C warming during the early last interglacial. Although stadial events are generally associated with wind-induced surface cooling in the tropical North Atlantic, the positive Caribbean SST anomaly during Heinrich Stadial 11 is consistent with previous findings. It likely originates from the upwelling of subsurface water that warmed in response to the initial AMOC weakening. Reduction in the Caribbean SST during the late Heinrich, associated with a particularly weak AMOC strength as suggested by our benthic d13C values, can indicate that the subsurface warming has diminished in the tropical North Atlantic possibly due to a general cooling in the source region (i.e., the subtropical gyre). A two-phased Heinrich is supported by the planktic foraminifera assemblage data, indicating that cooling occurred in the late Heinrich. In addition, this late phase is characterized by coarser sediments, which can be due to a strongly reduced outflow of the Orinoco and a particularly southern position of the intertropical convergence zone. For the last interglacial, our alkenone-derived SST record suggests stable conditions. However, the obtained interglacial values are characterized by very high alkenone unsaturation indexes that can incorporate large measurement and calibration errors due to the lack of Caribbean sediment traps and core-top data. These results, therefore, emphasize the need to better quantify the effectiveness of alkenones in reconstructing interglacial SST history in the Caribbean.

How to cite: Zhuravleva, A., Fahl, K., and Bauch, H. A.: A two-phased Heinrich Stadial 11 as revealed by alkenone-based temperature record from the western tropical North Atlantic , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6599, https://doi.org/10.5194/egusphere-egu24-6599, 2024.

Understanding the impact of freshwater discharge from the late Pleistocene Laurentide Ice Sheet (LIS) to the North Atlantic has been considered pivotal due to its direct regulating influence on the climate of the surrounding continents. Numerous studies using indirect paleo-proxies for iceberg discharge and fine-grained sediment supply have reconstructed the instabilities of the LIS. This study employs direct proxies for iceberg discharge and fine-grained sediment supply using the ice-rafted detritus (IRD) and X-ray fluorescence (XRF) scan combined with published X-ray diffraction (XRD) data from the same samples of the Integrated Ocean Drilling Program (IODP) Site U1313 (41°N; 32.57°W). Prominent Heinrich IRD events (H-events) of the last glacial cycle were accompanied by Ti/Ca and Fe/Ca peaks, consistent with the dolomite and calcite peaks, suggesting their Ordovician and Silurian carbonate rocks source that floor the Hudson Bay and Hudson Strait. However, despite the lack of an IRD/g peak, Ti/Ca and Fe/Ca peaks in H-event 3 suggest the arrival of fine-grained sediments in the southern edge of the IRD belt, most likely by sediment plume. In contrast to the last glacial cycle, IRD/g and Ti/Ca and Fe/Ca peaks, often assigned as Heinrich-like events, were only identified during the cold marine isotope stage (MIS) 6, 8, 10, and 12. The IRD/g, Ti/Ca, and Fe/Ca peaks, in addition to the dolomite and calcite peaks during the MIS 7, suggest a fundamentally different configuration of the LIS compared to the other warm MISs of the last 500 ka. Our data suggest that the LIS-sourced icebergs impacted the northern edge of the subtropical gyre (STG) by directly injecting meltwater and modifying the upper water masses, which most likely resulted in the southward movements of the Polar and Arctic fronts. These frontal movements were accompanied by frequent encroachment of the subpolar to transitional water masses to the STG. The polar water-dwelling planktonic foraminifera Neogloboquadrina pachyderma coupled to the IRD/g or Fe/Ca and Ti/Ca peaks support this hypothesis. The new sedimentological and micropaleontological data suggest that the instability and configuration of the LIS were not uniform during all the warm MISs of the last 500 ka.

How to cite: Rashid, H., Zeng, M., and Menke, M.: Impact of the Laurentide Ice Sheet instabilities on the mid-latitude North Atlantic and subtropical gyre during the last five glacial cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6815, https://doi.org/10.5194/egusphere-egu24-6815, 2024.

EGU24-7296 | Orals | CL1.2.6

The Southern Ocean’s role in the global carbon cycle over the last 800 kyr constrained using reconstructions of the CO2 system 

Chen Xu, Jessica Crumpton-Banks, Madison Shankle, Megan Pelly, Hana Jurikova, Jimin Yu, Bradley Opdyke, Claus-Dieter Hillenbrand, Andrea Burke, and James Rae

The critical role of the Southern Ocean in controlling the Pleistocene atmospheric CO2 oscillations is widely acknowledged. However, owing to sampling difficulties surrounding Antarctica, the underlying mechanism and associated pathway of ocean-atmosphere CO2 exchange in the Antarctic Zone (AZ) of the Southern Ocean remains mysterious. Here, we present a new planktonic δ11B record from sediment core PS1506 (68.73°S, 5.85°W) that tracks the pH and surface pCO2 of the AZ over the last 8 glacial cycles. These data are complemented by benthic B/Ca and carbonate preservation indices; due to the location of this core on the continental margin of the eastern Weddell Sea, these data allow us to track the source CO2 chemistry of the dense Antarctic waters that feed the ocean’s lower overturing cell. We find coherent CO2 change between surface and deep waters, indicating persistent formation of AABW that transfers Antarctic surface water signals to depth. Critically, we discover abrupt AZ CO2 decline during glacial onset conditions, coinciding with initial atmospheric CO2 drawdown, highlighting the AZ’s key control on glacial-interglacial CO2 change. After assessing proposed drivers, our findings implicate shifts in Southern Ocean circulation linked to changes in sea ice and/or the Southern Hemisphere westerlies in this glacial onset CO2 change, while productivity, solubility, and sea ice 'lid' effects appear insignificant or counterproductive in this region and time interval. Overall, these reconstructed CO2 system dynamics provide critical insights into Southern Ocean carbon cycling and the associated influence on the atmosphere.

How to cite: Xu, C., Crumpton-Banks, J., Shankle, M., Pelly, M., Jurikova, H., Yu, J., Opdyke, B., Hillenbrand, C.-D., Burke, A., and Rae, J.: The Southern Ocean’s role in the global carbon cycle over the last 800 kyr constrained using reconstructions of the CO2 system, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7296, https://doi.org/10.5194/egusphere-egu24-7296, 2024.

Global sea-level changes are significantly associated with variations in Northern Hemisphere ice sheets (NHISs) during the last glacial cycle. However, their responses to glacial millennial-scale climate variability (also known as Dansgaard - Oeschger (DO) cycles), especially during the Marine Isotope Stage 3 (MIS3, ~30-65ka), remains poorly studied, in addition to the contrast lines of geological evidence regarding paleo-sea level changes. Instead of applying Glacial Index Method which overlooks potential tempo-spatial heterogeneity of temperature and precipitation in the northern high latitudes, in this study, we conducted transient PISM ice sheet modeling by imposing full climate forcing derived from fully coupled climate model experiments which are characterized by spontaneous millennial variability. Our results show that control factors of ice volume changes in Laurentide and Eurasian ice sheets are different due to spatially heterogenous climate forcing. During stadial periods, North American Ice sheets is growing because of increased precipitation especially along the margins of the ice sheets, despite spatially heterogenous but trivial changes in the surface air temperature. Meanwhile, dramatic cooling on the southern regions of Eurasian Ice sheets effectively reduces ice loss and hence promote the overall ice growth. In brief, NHIS ice volume grows during stadials while declines during interstadials. We hence propose that stadial-to-interstadial duration ratio is the key to the net change in NHIS volume in a signal DO cycle, providing dynamic understanding of accelerated sea level drop during 40-30ka.

How to cite: Zhang, Y. and Zhang, X.: Millennial-scale Northern Hemisphere ice sheet growth controlled by stadial-versus-interstadial duration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7339, https://doi.org/10.5194/egusphere-egu24-7339, 2024.

EGU24-8676 | Posters on site | CL1.2.6

Abrupt climate changes triggered with GLAC-1D ice sheet, but not with ICE-6G_C, in simulations of the Last Glacial Maximum/Deglaciation 

Ruza Ivanovic, Yvan Rome, Lauren Gregoire, Brooke Snoll, Oliver Pollard, and Jacob Perez

In the course of glacial terminations, the increases in greenhouse gas concentrations, summer insolation and the ice sheet demise can trigger episodes of millennial-scale variability. Such variability was observed during the last deglaciation, between 19 ka BP (thousand years ago) and 8 ka BP, in the form of  the abrupt North Atlantic temperature shifts of the Bølling–Allerød Warming (14.5 ka BP) and Younger Dryas (12.900 ka BP).

In some climate models, abrupt climate changes are generated by modifications to the boundary conditions and freshwater discharge. Despite much study, the sensitivity of climate simulations to ice sheet geometry and meltwater is complex and not fully understood, which is a caveat when considering the impact of the rapid demise of the Northern Hemisphere ice sheets during the last deglaciation. In a new set of last glacial maximum HadCM3 simulations that can produce millennial-scale variability, we studied the influence of two ice sheet reconstructions, ICE-6G_C and GLAC-1D, and their associated deglacial meltwater history, on the simulated chain of events of the last deglaciation.

In this experiment, our simulations using the GLAC-1D ice sheet reconstruction produced abrupt climate changes. Triggered by freshwater released close to the Nordic Seas and Iceland Basin deep water formation sites, these simulations display abrupt shifts in the Atlantic Meridional Overturning Circulation (AMOC) that are decoupled from the meltwater flux. In contrast, the reconstructed ICE-6_G ice sheet modifies the North Atlantic wind patterns in the model, preventing convection in the Nordic Seas and intensifying the Iceland Basin deep water formation. As a result, no abrupt climate changes are simulated with ICE-6G_C ice sheets and the AMOC decreases almost linearly with the introduction of freshwater.

The simulations do not capture the timing of the last deglaciation chain of events, but the modelled abrupt changes replicate the main Northern Hemisphere characteristics of the Bølling Warming/Younger Dryas transitions, and are very similar to Dansgaard-Oeschger events.

How to cite: Ivanovic, R., Rome, Y., Gregoire, L., Snoll, B., Pollard, O., and Perez, J.: Abrupt climate changes triggered with GLAC-1D ice sheet, but not with ICE-6G_C, in simulations of the Last Glacial Maximum/Deglaciation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8676, https://doi.org/10.5194/egusphere-egu24-8676, 2024.

EGU24-8715 | ECS | Orals | CL1.2.6

Tracking the fate of meltwater from different Northern ice sheet sectors over Heinrich Stadial 1 

Laura Endres, Ruza Ivanovic, Yvan Romé, Julia Tindall, and Heather Stoll

The addition of meltwater from continental ice sheets to the North Atlantic is thought to have played a pivotal role in the reorganization of climate and ocean circulation over the last deglaciation as well as during Heinrich events. This is supported by recent analysis of PMIP and CMIP results, which shows that meltwater addition into the North Atlantic can largely alter global climate, and remains a key uncertainty for both reconstruction and climate projections. 

To date, most model studies of freshwater “hosing” assume a relatively uniform distribution and apply meltwater to a large portion of the North Atlantic basin. However, AMOC weakening is sensitive to the actual input position of the typically cold and non-saline meltwater perturbation, and, on a centennial-millennial timescale, the resulting temperature and salt anomaly will only partially disperse over the entire North Atlantic surface ocean. In contrast, most proxy data sensitive to meltwater record the signal at a specific location. It is unclear if spatial heterogeneity of the ocean’s distribution of the meltwater anomaly may contribute to disagreements between freshwater proxy records and model simulations with freshwater additions tuned to reproduce the record of past AMOC weakenings.

To enhance understanding of the spatial distribution of meltwater anomalies during deglaciations, we present the results of a model sensitivity study using HadCM3 and artificial dye tracers to track the fate of meltwater originating from different Northern ice sheet sectors. We consider different meltwater scenarios consistent with Heinrich Stadial 1 ice sheet reconstructions and compare the results under different AMOC states. The results confirm that, on a centennial timescale, meltwater distribution is not uniform over the North Atlantic Ocean. The emerging patterns expose that the efficiency of a meltwater injection to produce a surface ocean anomaly (in, e.g., salinity or δ18Osw) at a given proxy location differs between different ice sheet sectors by orders of magnitudes. Further, besides the direct effect of meltwater, the sensitivity of climate indicators, such as temperature, to changes in AMOC strength also shows regional discrepancies. 

How to cite: Endres, L., Ivanovic, R., Romé, Y., Tindall, J., and Stoll, H.: Tracking the fate of meltwater from different Northern ice sheet sectors over Heinrich Stadial 1, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8715, https://doi.org/10.5194/egusphere-egu24-8715, 2024.

EGU24-9143 | Orals | CL1.2.6

Bipolar control on millennial atmospheric CO2 changes over the past glacial cycle 

Jimin Yu, Robert Anderson, Zhangdong Jin, Xuan Ji, David Thornalley, Lixin Wu, Nicolas Thouveny, Yanjun Cai, Liangcheng Tan, Fei Zhang, Laurie Menviel, Jun Tian, Xin Xie, Eelco Rohling, and Jerry McManus

Ice-core measurements show diverse atmospheric CO2 variations – increasing, decreasing or remaining stable – during millennial-scale North Atlantic cold periods called stadials. The reasons for these contrasting trends remain elusive. Ventilation of carbon-rich deep oceans can profoundly affect atmospheric CO2, but its millennial-scale history is poorly constrained. In this study, I will show a high-resolution deep-water acidity record from the Iberian Margin in the North Atlantic, a unique setting that allows us to construct a robust chronology for confident comparisons between marine and ice-core records. The new data combined with ice-core CO2 records reveal multiple ocean ventilation modes involving an interplay of the two polar regions, rather than by the Southern Ocean alone. These modes governed past deep-sea carbon storage and thereby atmospheric CO2 variations on millennial timescales. Overall, our record suggests a bipolar control on millennial atmospheric CO2 changes during the past glacial cycle.

How to cite: Yu, J., Anderson, R., Jin, Z., Ji, X., Thornalley, D., Wu, L., Thouveny, N., Cai, Y., Tan, L., Zhang, F., Menviel, L., Tian, J., Xie, X., Rohling, E., and McManus, J.: Bipolar control on millennial atmospheric CO2 changes over the past glacial cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9143, https://doi.org/10.5194/egusphere-egu24-9143, 2024.

EGU24-9411 | ECS | Orals | CL1.2.6

An Early Warming Over the Southern Ocean During the Last Deglaciation 

Peisong Zheng, Matthew Osman, and Thomas Bauska

The last deglaciation, spanning approximately 23 to 6 thousand years before present (ky BP), represents the most recent period in which Earth’s climate underwent large-scale reorganizations comparable (albeit not strictly analogous) to those projected under future climate changes. However, the precise sequence of events – in particular, the timing and spatial manifestation of the initial warming – remains uncertain. Here we present a new method using Gaussian Mixture Model clustering to objectively decompose a model and proxy-based climate reconstruction (LGMR; Osman et al., 2021) into four patterns of temperature change across the last deglaciation. Broadly speaking, the patterns allow us to delineate the impact of retreating Northern Hemisphere ice sheets, the rise in greenhouse gases, and the influence of the bipolar seesaw. Crucially, our analysis reveals that the high latitudes of the Southern Hemisphere exhibited the earliest signs of warming onset around 21 kyr BP, coincident with a retreat of sea ice across the Southern Ocean. A similar pattern is observed when decomposing a solely model-based climate reconstruction (TraCE-21k; He et al., 2013).  Using a combination of both highly-simplified energy balance-type models and fully-coupled climate models forced with insolation alone, we show that the early warming and sea ice retreat was likely linked to an initial rise in high latitude summertime energy that is dominated by enhanced obliquity-driven forcing. Our findings collectively suggest that insolation dynamics in the Southern Hemisphere were a critical trigger of the Last Deglacial onset and, further, may represent one of the key prerequisites for glacial terminations during the late Pleistocene.

How to cite: Zheng, P., Osman, M., and Bauska, T.: An Early Warming Over the Southern Ocean During the Last Deglaciation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9411, https://doi.org/10.5194/egusphere-egu24-9411, 2024.

EGU24-9419 | ECS | Orals | CL1.2.6

Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling  

Hidetaka Kobayashi, Akira Oka, Takashi Obase, and Ayako Abe-Ouchi

Atmospheric carbon dioxide concentrations (pCO2) have increases by approximately  from the Last Glacial Maximum (LGM) to the late Holocene (last deglaciation). These changes in atmospheric greenhouse gases are recognized as climate system responses to gradual changes in insolation. Previous modeling studies have suggested that the deglacial increases in atmospheric pCO2 are primarily attributed to the release of CO2 from the ocean. In addition, it has been suggested that abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, there is still limited understanding in oceanic circulation changes, factors responsible for changes in chemical tracers in the ocean of the last deglaciation and its impact on atmospheric pCO2.

In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes as in reconstructions. We validate the simulated ocean carbon cycle changes and discuss possible biases and missing or underestimated processes in the model by comparing simulated carbon isotope ratios with sediment core data.

The qualitative changes in atmospheric pCO2 are consistent with ice core records: during Heinrich Stadial 1 (HS1), atmospheric  increases by . This is followed by a decrease of  during the Bølling–Allerød (BA) and an increase of  during the Younger Dryas (YD). However, the model underestimates the changes in atmospheric  during these events compared to ice core data. Radiocarbon and stable isotope signatures ( and ) indicate that the model underestimates the activated deep ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean, and active ventilation in the North Pacific Intermediate Water during HS1. The relatively small changes in simulated atmospheric  changes during HS1 may be attributed to these underestimations of ocean circulation changes. The changes in  associated with strengthening and weakening in the AMOC during the BA and YD are generally consistent with the sediment core record. On the other hand, while the data show a continuous  increase in the deep ocean throughout the YD, the model shows the opposite trend. This suggests that the model simulates excessive weakening of the AMOC during the YD, or limited representations in geochemical processes in the model including marine ecosystem responses and terrestrial carbon storage.

Decomposing the factors behind changes in ocean  reveals that changes in temperature and alkalinity have the main effects on atmospheric  changes. The compensation of the effects of temperature and alkalinity suggests the AMOC changes and associated bipolar climate changes contribute to a slight decrease or increase in atmospheric  during the BA and YD periods, respectively.

How to cite: Kobayashi, H., Oka, A., Obase, T., and Abe-Ouchi, A.: Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9419, https://doi.org/10.5194/egusphere-egu24-9419, 2024.

Be10 in ice cores provides a uniquely well resolved indication of past radionuclide production rates, with a direct bearing on past radiocarbon production.  In the absence of past carbon cycle perturbations (e.g. involving ocean-atmosphere carbon exchange), Be10-based radiocarbon production rate anomalies should correlate directly with atmospheric radiocarbon anomalies, as confirmed by models.  Over the past ~30ka, Be10-inferred radiocarbon production rates and atmospheric radiocarbon (i.e. Intcal20) both exhibit recurrent millennial anomalies, typically of ~5ka duration.  A correlation between these anomalies breaks down during the deglaciation.  This is intriguing and suggests a mix of millennial carbon cycle and radionuclide production influences. Here, global compilations of marine carbon isotope data (radiocarbon and 𝛿13C) are used to assess the potential contribution of ocean circulation and air-sea gas exchange to the apparent millennial component of variability in Intcal20, and atmospheric CO2. We find that existing marine 𝛿13C data provide strong support for a marine influence on atmospheric radiocarbon. Support from marine radiocarbon data is more complex, due to the influence of ‘attenuation biases’ (arising from radiocarbon production changes), and due to a distinct regionalism in the ocean’s impact on atmospheric radiocarbon, versus atmospheric CO2, with air-sea gas-exchange playing a significant role. Major differences in the long-term evolution of radiocarbon and 𝛿13C across the last deglaciation further point to distinct and independent controls on these isotopes systems, providing clues as to the nature and timing of different carbon cycle processes during deglaciation.

How to cite: Skinner, L.: Globally resolved marine carbon isotope data spanning the last 25ka: what do they tell us about the drivers of atmospheric radiocarbon and CO2 on millennial and deglacial timescales? , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9678, https://doi.org/10.5194/egusphere-egu24-9678, 2024.

EGU24-9708 | ECS | Orals | CL1.2.6

Glacial-interglacial variability using a low-complexity, physically based model 

Sergio Pérez-Montero, Jorge Alvarez-Solas, Marisa Montoya, and Alexander Robinson

Pleistocene glacial-interglacial variability is still under debate, as the many hypotheses proposed are subject to the models used and assumptions made. The longer time scales involved in glacial cycles make it difficult to use comprehensive climate models because of its large computational cost. In this context, conceptual models are built to mimic complex processes in a simpler and more computationally efficient way. Here we present a conceptual climate-ice sheet model that aims to represent the state-of-the-art physical processes affecting glacial-interglacial variability. Our model was constructed using linear equations that explicitly represent ice-sheet modeling approaches. Preliminary results are consistent with Late Pleistocene variability and point to the existence of nonlinearities related to both ice dynamics and ice aging that determine the timing and shape of deglaciations.

How to cite: Pérez-Montero, S., Alvarez-Solas, J., Montoya, M., and Robinson, A.: Glacial-interglacial variability using a low-complexity, physically based model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9708, https://doi.org/10.5194/egusphere-egu24-9708, 2024.

EGU24-10190 | Orals | CL1.2.6

Perspective on ice age Terminations from absolute chronologies provided by global speleothem records 

Nikita Kaushal, Heather Stoll, and Carlos Pérez-Mejías

Glacial Terminations represent the largest amplitude climate changes of the last several million years.  Over ~ 10 ky timescale, large northern hemisphere ice sheets retreat and sea level rises, and atmospheric CO2 and global temperatures make a full transition from glacial to interglacial levels.  Several possible orbital-insolation triggers have been described to initiate and sustain glacial Terminations, and feedbacks between ice sheet retreat, ocean circulation and ocean carbon storage are invoked to explain the unstoppable progression. 

Because of the availability of radiocarbon dating, the most recent termination (TI) has been extensively characterized. Yet, it is widely discussed whether this sequence of feedbacks and millennial events, and rate of warming is recurrent over previous Terminations or is unique.  Beyond the limit of radiocarbon dating, the chronologies of climate records from ice and marine cores are often developed by tuning to orbital parameters which limits their use in understanding climate dynamics, particularly the response to orbital forcing.

Speleothems provide absolute age control and high-resolution proxy measurements. This archive therefore provides unique records of climate change across Terminations, and additionally may provide the opportunity to tune ice and marine core archives.  However, speleothem climate signals are encoded in a number of proxies. Unlike proxies in other archives like ice or marine cores, the climatic interpretation of a given proxy can vary quite significantly among different regions.

In this study, we

  • synthesize the available speleothem records providing climate information for Terminations: TII, TIII, TIV and TV.
  • present the records based on the aspect of climate encoded in the available records.
  • examine the effects of different ice volume corrections on the final climate proxy record.
  • evaluate whether there are leads and lags in the manifestation of Terminations across different aspects of the climate systems and different regions.
  • we speculate on suitable tuning targets among marine and ice core proxies, and discuss what model outputs maybe most suitable for comparison.

How to cite: Kaushal, N., Stoll, H., and Pérez-Mejías, C.: Perspective on ice age Terminations from absolute chronologies provided by global speleothem records, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10190, https://doi.org/10.5194/egusphere-egu24-10190, 2024.

EGU24-10579 | Orals | CL1.2.6

A mechanism for reconciling the synchronisation of Heinrich events and Dansgaard-Oeschger cycles 

Clemens Schannwell, Uwe Mikolajewicz, Marie-Luise Kapsch, and Florian Ziemen

The evolution of the northern hemispheric climate during the last glacial period was shaped by two prominent signals of glacial climate variability known as Dansgaard-Oeschger cycles and Heinrich events. Dansgaard- Oeschger cycles are characterised by a period of rapid, decadal warming of up to 14°C in the high northern latitudes, followed by a more gradual cooling spanning several centuries. Temperature reconstructions from ice cores indicate a dominant recurrence interval of ∼1,500 years for Dansgaard-Oeschger cycles. Heinrich events are quasi-episodic iceberg discharge events from the Laurentide ice sheet into the North Atlantic. The paleo record places most Heinrich events into the cold phase of the millennial-scale Dansgaard-Oeschger cycles. However, not every Dansgaard-Oeschger cycle is accompanied by a Heinrich event, revealing a complex interplay between the two prominent modes of glacial variability that remains poorly understood to this day. Here, we present simulations with a coupled ice sheet-solid earth model to introduce a new mechanism that explains the synchronicity between Heinrich events and the cooling phase of the Dansgaard-Oeschger cycles. Unlike earlier studies, our proposed mechanism does not require a trigger mechanism during the cooling phase. Instead, the atmospheric warming signal associated with the interstadial phase of the Dansgaard-Oeschger cycle causes enhanced ice stream thickening such that a critical ice thickenss and temperature threshold is reached faster, triggering the Heinrich event during the early cooling phase of the Dansgaard-Oeschger cycle. An advantage of our mechanism in comparison to previous theories is that it is not restricted to marine-terminating ice streams, but applies equally to land-terminating ice streams that only become marine-terminating during the actual Heinrich event. Our simulations demonstrate that this mechanism is able to reproduce the Heinrich event characteristics as known from the paleo record under a wide range of forcing scenarios and provides a simple explanation for the observational evidence of synchronous Heinrich events from different ice streams within the Laurentide ice sheet.

How to cite: Schannwell, C., Mikolajewicz, U., Kapsch, M.-L., and Ziemen, F.: A mechanism for reconciling the synchronisation of Heinrich events and Dansgaard-Oeschger cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10579, https://doi.org/10.5194/egusphere-egu24-10579, 2024.

Despite decades of research, the cause of glacial-interglacial CO2 cycles is not fully understood, leaving a critical gap in our understanding of Earth’s climate system. One hypothesis is that shoaling of the boundary between Northern Source Water (NSW) and Southern Source Water (SSW) enhanced oceanic carbon sequestration during glacial intervals, resulting in lower atmospheric pCO2. To test this hypothesis, we generated vertical profiles of [CO32-], δ13C, and δ18O using a depth transect of cores from the Brazil Margin, focusing on the two major drops in atmospheric pCO2 during the last glacial cycle at ~115 ka and ~70 ka. Given that [CO32-] is inversely related to the concentration of dissolved inorganic carbon, [CO32-] should decrease if the Atlantic sequestered CO2. We observe no significant change in the [CO32-] across the first decrease in atmospheric pCO2 and no evidence for watermass boundary shoaling in the δ13C and δ18O profiles.  [CO32-] decreased only at ~3600 m, the core site most influenced by SSW.  During the second pCO2 decline at ~70 ka, [CO32-] decreased by ~30 µmol/kg below 2000 m water depth, coincident with marked shoaling in the δ13C and δ18O profiles. The lack of evidence for shoaling and deep Atlantic carbon sequestration at ~115 ka, a time of intermediate ice sheet extent and moderate global cooling, but the clear evidence for shoaling and carbon sequestration at ~70 ka, a time of near glacial maximum ice sheet extent, implies that the deep Atlantic’s capacity to store carbon depends on the Earth’s mean climate state. Our results highlight that distinct mechanisms are necessary to explain the two major drops in atmospheric pCO2 of the last glacial cycle. 

How to cite: Garity, M. and Lund, D.: Investigating oceanic carbon sequestration during glacial inception using vertical profiles of [CO32-], d13C, and d18O from the Southwest Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12112, https://doi.org/10.5194/egusphere-egu24-12112, 2024.

EGU24-12199 | ECS | Orals | CL1.2.6

Reconstructing the global mean surface temperature of the last 130 thousand years 

Jean-Philippe Baudouin, Nils Weitzel, Lukas Jonkers, Andrew M. Dolman, and Kira Rehfeld

Global mean surface temperature (GMST) is a fundamental measure of climate evolution in both past and present and a key quantity to evaluate climate simulations. However, for paleoclimate periods, its reconstruction hinges on uncertain and indirect observations which are distributed sparsely and non-uniformly in both space and time. Large datasets of homogenised proxy records help to reduce the sparsity. Then, the records can be aggregated in an algorithm retrieving the GMST signal. Here, we build on the algorithm designed in Snyder 2016, and on a recent database of ocean temperature proxy records to reconstruct the GMST evolution over the last glacial cycle (the last 130 thousand years). First, we evaluate the algorithm and quantify the sources of uncertainty. This analysis draws on pseudo-proxy experiments using a range of simulations of the last glacial cycle. We find that the over-representation of some regions (e.g. coasts, the Atlantic), to the detriment of others (e.g. the central Pacific) significantly impacts the reconstructed temperature anomaly and its variations. Additionally, millennial and shorter timescale variability cannot be reconstructed by the algorithm, due to bioturbation and age uncertainty. However, these experiments also demonstrate the ability of our algorithm to reconstruct the amplitude and timing of GMST variations occurring at orbital timescale (>10.000 years). Second, we reconstruct the GMST evolution during the last glacial cycle. We compare our result to previous studies, and discuss the improvements coming from the use of the recent proxy database. The high number of proxy records allow us to additionally investigate smaller regions (e.g. hemisphere) and overall further our understanding of the driver of orbital-scale GMST variability.

How to cite: Baudouin, J.-P., Weitzel, N., Jonkers, L., Dolman, A. M., and Rehfeld, K.: Reconstructing the global mean surface temperature of the last 130 thousand years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12199, https://doi.org/10.5194/egusphere-egu24-12199, 2024.

EGU24-13226 | Posters on site | CL1.2.6

Super-cooled glacial deep waters 

Miho Ishizu, Axel Timmermann, and Kyung-Sook Yun

Sea-ice formation in the Southern Ocean can generate supercooled waters, which can even remain below the in-situ freezing point at depths below 1,000 m. These water masses can play an important role in carbon transport to the abyssal ocean and may have therefore also played an important role in glacial-interglacial CO2 cycles.

To address this question, we examined model outputs from the transient 3 Ma simulation conducted with the CESM1.2 model (Community Earth System Model version 1.2, ~3.75 horizontal resolution. This simulation was driven by time-varying orbital forcing and estimates of atmospheric greenhouse gas concentrations and northern hemispheric ice-sheet orography and albedo. Our analysis shows the presence of large swaths of supercooled glacial deep waters mainly in the northern Pacific. This water is originally formed in the seasonal sea-ice formation regions in the subarctic North Pacific during periods of brine release and rapid mixed layer deepening. During interglacial periods, the volume of supercooled water decreases, which may hint towards a possible positive climate-carbon cycle feedback.

In climate models the freezing condition is usually only applied at the surface. Hence, they are incapable of simulating brinicles – vertical sea-ice structures that can extend from the surface to shallower depths, sometimes even reaching the ocean floor. In my presentation, I will address whether such structures may have played a more prominent role during glacial periods, and whether localized deep ocean freezing may have been a possibility.

How to cite: Ishizu, M., Timmermann, A., and Yun, K.-S.: Super-cooled glacial deep waters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13226, https://doi.org/10.5194/egusphere-egu24-13226, 2024.

EGU24-14346 | Orals | CL1.2.6

Weathering of shelf sediments exposed during a glacial period: Evidence from geochemistry and Sr-Nd isotopes 

Gyana Ranjan Tripathy, Priyasha Negi, Rakesh Kumar Rout, and Ravi Bhushan

Erosion of continental rocks controls nutrient and sediment supply, soil formation and global climate. Intensity of this land-surface process is driven by both climatic (runoff, and temperature) and non-climatic (vegetation, lithology and basin slope) factors. Additionally, climatic-driven fluctuations in sea-level may also influence the exposed land-area, which is available for weathering. The coupling between exposed shelf sediments and weathering, however, has received limited attention. In this contribution, geochemical and Sr-Nd isotopic compositions of a sediment core (VM29-17PC) from the western Bay of Bengal have been investigated to reconstruct weathering and climate interaction during last glacial-interglacial cycle. Radiocarbon dating of foraminifera samples establishes that the core preserves a continuous erosional record for last 35 kyr.  Average Sr-Nd isotopic data for the decarbonated sediments confirm dominant sediment supply from the Higher Himalaya to the core site, with sub-ordinate input from the Deccan region. Temporal changes in the isotopic data hint at a sudden increase in the Himalayan source around 15 kyr BP, which is synchronous with the Bølling-Allerød (B/A) warm phase and the strengthening of the south-west (SW) monsoon. Downcore variation of Chemical Index of Alteration (CIA) and K/Al ratios indicates intensification of chemical weathering around 25 kyr BP. This change in weathering intensity is synchronous to dropping of sea level due to onset of glaciation. This sea-level regression and sudden rise in CaCO3 concentration during this period point to weathering of additional surface exposed in the shelf regions. This enhanced weathering of the shelf sediments may have contributed to the atmospheric CO2 level during the glacial period.

How to cite: Tripathy, G. R., Negi, P., Rout, R. K., and Bhushan, R.: Weathering of shelf sediments exposed during a glacial period: Evidence from geochemistry and Sr-Nd isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14346, https://doi.org/10.5194/egusphere-egu24-14346, 2024.

EGU24-14882 | ECS | Orals | CL1.2.6

Impact of marine productivity on atmospheric pCO2 during the Last Glacial Maximum: a model-data comparison 

Pauline Depuydt, Stéphanie Duchamp-Alphonse, Nathaelle Bouttes, Chiara Guarnieri, Alice Karsenti, Ji-Woong Yang, Jean-Yves Peterschmitt, and Amaëlle Landais

Measurements of the air trapped in Antarctic ice cores reveal that atmospheric CO2 concentration (pCO2) during the Last Glacial Maximum (LGM) was about 80 ppmv lower than that recorded during the current Holocene interglacial (Bereiter et al., 2015). Studies also show a strong link between pCO2, ice volume and Antarctic temperature, suggesting pCO2 as a forcing or amplifying factor behind glacial/interglacial cycles (Petit et al., 1999; Parrenin et al., 2013). Despite such importance in the global climate changes, mechanisms behind rapid variations in pre-anthropic pCO2 remain elusive. Numerical models emphasized the crucial role of exported marine productivity Pexp, (namely, the Soft Tissue Pump) in such changes. In particular, they feature marine productivity patterns from the Southern Ocean and show that a decrease in Pexp in the Sub-Antarctic zone, linked to a reduction in iron inputs from aeolian dusts, could have increased pCO2 by 20 to 50 ppmv (Köhler and Fischer, 2006; Martínez-Garcia et al., 2009; Lambert et al., 2012). However, these studies are usually compared to proxy data from the Atlantic sector of the Subantarctic Zone i.e., an area under the direct influence of wind fields that makes it possible to test the “Fe-hypothesis” (Martin et al., 1990) but that is not necessarily representative of the entire ocean (e.g. Lambert et al., 2015). Due to a lack of recent Pexp data compilation but also of direct comparisons with model outputs integrating marine biogeochemistry­­, it remains difficult to understand the role marine biological productivity exerted on the carbon cycle and more specifically on the low pCO2 during the LGM.

The aim of this study is to explore Pexp patterns during the LGM compared to the pre-industrial Holocene and understand the mechanisms driving their global changes, in order to try and estimate the contribution of marine productivity to the pCO2 signalbased on (i) a new compilation of Pexp proxy data using the strategy previously proposed by Kohfeld et al. (2005) after Bopp et al., (2003), and (ii) a comparison of these data to outputs from the iLOVECLIM intermediate complexity.

Proxy data show that Pexp is generally higher during the LGM compared to the pre-industrial Holocene. This is particularly the case in the sub-Antarctic and sub-Arctic areas, in the equatorial Atlantic Ocean and in coastal upwelling settings i.e., regions that usually witness higher nutrient content due to revigorated ocean circulation and/or intensified surface winds. Simulations generally confirm such features except from the coastal upwelling and the Southern Ocean, due to a lack of spatial resolution and of aeolian inputs in the model, respectively. However, preliminary results from sensitivity tests show (i) net marine productivity fronts around ~40°N and 45°S due to extended sea ice cover and reduced global temperature, (ii) a decreased Pexp in the Pacific Ocean due to an overall thermohaline circulation slow down and (iii) an increase of Pexp in areas where fertilization by iron-rich dusts is expected (Lambert et al., 2021).

How to cite: Depuydt, P., Duchamp-Alphonse, S., Bouttes, N., Guarnieri, C., Karsenti, A., Yang, J.-W., Peterschmitt, J.-Y., and Landais, A.: Impact of marine productivity on atmospheric pCO2 during the Last Glacial Maximum: a model-data comparison, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14882, https://doi.org/10.5194/egusphere-egu24-14882, 2024.

EGU24-14929 | ECS | Orals | CL1.2.6

Significant change in the flow regime in the deep Southern Ocean through the MPT 

Eva M. Rückert and Norbert Frank

The deep Southern Ocean (SO) circulation is of major significance for the understanding of the ocean´s impact on Earth’s climate as uptake and release of CO2 strongly depend on the redistribution of well and poorly ventilated water masses.

Neodymium isotopes (εNd) preserved in deep sea sediment have proven useful to study the Deep Ocean Circulation and water mass provenance and are of special interest over major climate changes as the Mid Pleistocene Transition (MPT). The MPT marks the change from a 41 ka to a 100 ka glacial-interglacial cyclicity and goes along with a significant intrusion of southern sourced waters (SSW) in the deep North Atlantic.

Here, we present the first millennial resolved authigenic εNd data in the Southern Atlantic spanning across  the MPT of a deep sea sediment core positioned at the polar front. The pre-MPT εNd values of ODP 1093 show a small variability of approx. 2 ε-units around the modern AABW signature of -8. In contrast, the post-MPT εNd variability increases to 6 ε-units with glacial extremes of around -3 – εNd values that can not be found in any Atlantic sourced water mass today!

This supports not only the exsiting hypotesis of stonger SSW export to the North, but rather advocates for a radiogenic  watermass influencing the flow regime in the Atlantic south of the polar front. Increasing ice volume during post-MPT glacials has been argued to lead to a reduced AABW production. Due to continuity of flow, this opens up the possiblity of glacial intrusion through the Drake passage of a water masses likely originating in the Pacific, which would generate  the strongly radiogenic glacial εNd values. At present Pacific deep waters are enriched with respired carbon. Assuming this to hold true in the past, the intrusion of such carbon rich water masses into the deep South Atlantic could further reinforce the strong glacials and the overall global cooling trend after the MPT as suggested previously.

Hence, the SO south of the polar front played a leading role in  reinjecting respired CO2 into the deep Atlantic Ocean and the Atmosphere during climate transitions. 

How to cite: Rückert, E. M. and Frank, N.: Significant change in the flow regime in the deep Southern Ocean through the MPT, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14929, https://doi.org/10.5194/egusphere-egu24-14929, 2024.

EGU24-15214 | Orals | CL1.2.6

Exploring the differing CO2 response to Dansgaard-Oeschger and Heinrich events 

Matteo Willeit, Daniela Dalmonech, Bo Liu, Tatiana Ilyina, and Andrey Ganopolski

Dansgaard-Oeschger (DO) and Heinrich (H) events are ubiquitous features of glacial climates involving abrupt and large changes in climate over the North Atlantic region, extending also to the Southern Hemisphere through the bipolar seesaw mechanism. Ice core data also indicate that the DO and H events are accompanied by pronounced changes in atmospheric CO2 concentration, but their origin remains uncertain. Here, we use simulations with the fast Earth system model CLIMBER-X, which produces self-sustained DO events as internal variability, to explore the processes involved in the atmospheric CO2 response. While the DO events are internally generated in the model, the Heinrich events are mimicked by adding a freshwater flux of 0.05 Sv over 1000 years in the latitudinal belt between 40°N and 60°N in the North Atlantic.
The simulated Greenland temperature varies by ~7-8°C between stadials and interstadials, with only small differences between H and DO stadials, while Antarctic temperature responds substantially stronger to H than to DO events, broadly in agreement with observations. In the CLIMBER-X simulations, atmospheric CO2 varies by ~5 ppm during DO events, but by ~15 ppm during H events, comparable with ice core data. The peak in CO2 concentrations is delayed by several centuries relative to both the stadial-interstadial transition and the peak in Antarctic temperature. The CO2 rise during the H stadial is driven by ocean outgassing. In contrast, the rapid CO2 increase after the transition to the interstadial results from soil carbon release from high NH latitudes originating from substantial warming.

How to cite: Willeit, M., Dalmonech, D., Liu, B., Ilyina, T., and Ganopolski, A.: Exploring the differing CO2 response to Dansgaard-Oeschger and Heinrich events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15214, https://doi.org/10.5194/egusphere-egu24-15214, 2024.

EGU24-15672 | ECS | Orals | CL1.2.6

Carbon and nitrogen stable isotopes across the last deglaciation: perspectives from snow petrel stomach oil deposits 

Thale Damm-Johnsen, Michael J. Bentley, Darren R. Gröcke, Dominic Hodgson, and Erin L. McClymont

Evidence from both marine and ice cores strongly indicates that surface ocean processes influencing air-sea gas exchange of the Southern Ocean played a crucial role in the transition from a glacial to interglacial climate state. However, few archives have been able to reconstruct how high latitude surface ocean processes affected the biogeochemical changes occurring in nutrient utilization, primary productivity, and their effects on carbon sequestering in ecosystems. An opportunity to explore these processes is provided by accumulated snow petrel (Pagodroma nivea) stomach oil deposits, defensively regurgitated by snow petrels at their nest sites. These deposits provide a record of biogeochemical processes in the austral summer, at a high trophic level and integrated over a relatively wide area defined by snow petrel foraging range. Here, we present a joint carbon and nitrogen stable isotope record from stomach oil deposits from the Lake Untersee nunataks in Dronning Maud Land (DML) integrating data from a coastal area of 65-70°S. Our results show a 3‰ offset in δ13C and 4‰ offset in δ15N between LGM and Holocene, indicating that the coastal high latitudes underwent large changes over the deglaciation. The δ15N depletion into the Holocene shows strong similarity to changes occurring in nutrient utilization along the margin of the polar front, indicating that the Southern Ocean high latitudes were not an isolated oasis during the LGM but biogeochemically connected to the surface ocean beyond the summer sea-ice margin. In addition, the presence of stomach oil deposits indicates that open water was present in summer along the coast of DML over both the LGM and Holocene. Such highly productive, open water areas were potentially an important factor in the air-sea gas exchange contributing to the deglacial atmospheric CO2 -rise.

How to cite: Damm-Johnsen, T., Bentley, M. J., Gröcke, D. R., Hodgson, D., and McClymont, E. L.: Carbon and nitrogen stable isotopes across the last deglaciation: perspectives from snow petrel stomach oil deposits, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15672, https://doi.org/10.5194/egusphere-egu24-15672, 2024.

EGU24-16591 | Orals | CL1.2.6

Bridging Proxy Discrepancies: SST Reconstructions from the Alboran Sea During the Last Glacial Maximum and Deglaciation.  

Alvaro Fernandez, Laura Rodríguez-Sanz, Victoria Taylor, Nele Meckler, and Francisca Martínez-Ruiz

The last glacial maximum (LGM) is the most recent time period in Earth’s history with a climate that was much colder than the present. Robust temperature reconstructions from this period are needed to improve estimates of Earth's climate sensitivity and aid in future climate change projections. However, reconstructing sea surface temperatures (SSTs) during this period can be challenging due to the various limitations with the commonly used proxies. Here, we present new SST estimates from the Alboran Sea in the Western Mediterranean, an area where existing SST records for the LGM (derived from UK37, TEX86, planktic foraminiferal Mg/Ca) show large disagreements. Our new SST estimates are based on clumped isotope analyses of planktic foraminifera (G. bulloides), the same species as used for the Mg/Ca measurements in this area. Due to the insensitivity of the clumped isotope thermometer to changes in seawater chemistry, our results offer new independent constraints on the range of temperature shifts between glacial and interglacial periods in this area. Our findings are evaluated against existing SST estimates, highlighting the benefits and limitations of different proxy estimates. We find that while all proxies agree on the general millennial scale temperature trends during the period of deglaciation, they diverge in the magnitude of these temperature changes. Temperature reconstructions derived from clumped isotopes align more closely with those based on alkenone and Mg/Ca proxies than with those from TEX86, which show large differences. Our research demonstrates that clumped isotopes are a potentially effective tool to improve the accuracy of climate reconstructions from the LGM and the subsequent deglacial period.

 

 

How to cite: Fernandez, A., Rodríguez-Sanz, L., Taylor, V., Meckler, N., and Martínez-Ruiz, F.: Bridging Proxy Discrepancies: SST Reconstructions from the Alboran Sea During the Last Glacial Maximum and Deglaciation. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16591, https://doi.org/10.5194/egusphere-egu24-16591, 2024.

EGU24-17501 | ECS | Posters on site | CL1.2.6

Constraining glacial ocean carbon cycle – A multi-model study 

Bo Liu, Tatiana Ilyina, Victor Brovkin, Matteo Willeit, Ying Ye, Christoph Völker, Peter Köhler, Malte Heinemann, Takasumi Kurahashi-Nakamura, André Paul, Michael Schulz, Ute Merkel, and Fanny Lhardy

The ocean contained a larger carbon content at the Last Glacial Maximum (LGM, ~21kyr before present) compared to the late Holocene, making a considerable contribution to the deglacial atmospheric CO2 rise of about 90 ppm. Yet, there’s no consensus on the mechanisms controlling the glacial-interglacial changes in oceanic carbon storage due to uncertainties and sparseness of proxy data. Numerical simulations have been widely used to quantify the impact of key factors, such as changes in sea surface temperatures, ocean circulation and biological production, on glacial ocean carbon sequestration. However, the robustness of these findings is subject to further testing due to the differences in process representation, parameterization, model architecture, or external forcing employed by models.

Towards further constraining the LGM ocean carbon cycle, we conducted a multi-model comparison with three comprehensive Earth System Models (Alfred Wegener Institute Earth System Model, AWI-ESM; Community Earth System Model, CESM; Max Planck Institute Earth System Model, MPI-ESM) and one Earth system Model of Intermediate Complexity (CLIMBER-X). We carried out three coordinated experiments with each model: 1) PI (the pre-industrial control simulation), 2) LGM-PMIP (following PMIP4 LGM protocol) and 3) LGM-LowCO2 (as LGM-PMIP, but with boosted alkalinity inventory to lower atmospheric CO2 to about 190 ppm. All experiments were conducted with the prognostic CO2 for the carbon cycle, considering only the atmosphere and ocean reservoirs, and prescribed CO2 for radiative forcing.

All models consistently show that applying the PMIP4 LGM boundary conditions alone leads to only a 5-40 ppm decrease in atmospheric CO2. Globally, the glacial CO2 drawdown in LGM-PMIP is mainly controlled by the enhanced solubility pump. The spatial distribution of the increased glacial DIC depends on the ocean circulation state in each model. In MPI-ESM and CLIMBER-X, the shallower and weaker AMOC facilitates carbon storage in the deep Atlantic. An LGM atmospheric CO2 of 190 ppm can be achieved by boosting alkalinity by 5-8% in scenario LGM-LowCO2. In all models, boosting LGM alkalinity inventory increases DIC in the bottom water. However, comparison to proxy data reveals that the models lack respired carbon, particularly in the deep Pacific. This suggests a need to enhance the glacial biological carbon pump in the models.

How to cite: Liu, B., Ilyina, T., Brovkin, V., Willeit, M., Ye, Y., Völker, C., Köhler, P., Heinemann, M., Kurahashi-Nakamura, T., Paul, A., Schulz, M., Merkel, U., and Lhardy, F.: Constraining glacial ocean carbon cycle – A multi-model study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17501, https://doi.org/10.5194/egusphere-egu24-17501, 2024.

EGU24-17778 | ECS | Orals | CL1.2.6

Physical and biological controls on deep Pacific carbon storage over the last glacial cycle 

Megan Pelly, Madison Shankle, Molly Trudgill, Bruno Millet, Chen Xu, Gwyn Owens, Hermione Owen, Alan Foreman, Thomas Bauska, Andy Ridgwell, Elisabeth Michel, William Gray, Andrea Burke, and James Rae

The ability of the deep ocean to store and exchange large quantities of CO2 with the atmosphere on relatively short timescales means that it is thought to play a key role in dictating glacial-interglacial changes in atmospheric CO2, however records of deep ocean carbon storage and release remain sparse. The Pacific Ocean contains the largest store of carbon in the ocean-atmosphere system. As a result, changes in its circulation dynamics and biogeochemistry have the potential to significantly impact global climate. Despite this, changes in Pacific conditions and carbon storage over the last glacial cycle are poorly constrained.

Here we present new geochemical proxy records from abyssal, deep, and intermediate depths in the Southwestern Pacific to determine the changes in deep ocean carbon storage over the last glacial cycle and the mechanisms involved in driving these changes. Foraminiferal trace element and stable isotope data indicate that increased carbon storage occurred over the course of the last glaciation, promoting a drawdown in atmospheric CO2. The processes involved in driving glacial ocean carbon storage are debated, however proxy data from these sites indicate that changes in circulation dynamics promoting the isolation and expansion of deep Pacific waters was likely a key process involved. Comparison of δ13C data to box model and Earth system model output provides further insight into the physical as well as biogeochemical mechanisms involved and their relative contributions at different stages over the last glacial cycle. This includes the role of Southern Ocean sea-ice expansion, reduced ocean temperatures, and increased Southern Ocean stratification and biological productivity. We find that physical processes dominate the early in the glacial cycle, with biological processes promoting further drawdown as glacial conditions intensify. These results help to improve the understanding of deep ocean carbon cycling over the last glacial cycle and provide a new framework with which to interpret proxy δ13C data.

How to cite: Pelly, M., Shankle, M., Trudgill, M., Millet, B., Xu, C., Owens, G., Owen, H., Foreman, A., Bauska, T., Ridgwell, A., Michel, E., Gray, W., Burke, A., and Rae, J.: Physical and biological controls on deep Pacific carbon storage over the last glacial cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17778, https://doi.org/10.5194/egusphere-egu24-17778, 2024.

EGU24-17827 | ECS | Orals | CL1.2.6

A million-year reconstruction of global volcanism intensity: How does it link to glaciation? 

Jack Longman, Thomas M. Gernon, Thea K. Hincks, Sina Panitz, and Martin R. Palmer

Reduced ice volume during interglacials is hypothesized to amplify volcanic activity because ice-mass removal reduces pressure on magma chambers (Huybers & Langmuir, 2009). There is some evidence for this process occurring on regional (Maclennan et al., 2002) and perhaps semi-global scales (Kutterolf et al., 2019), but there is a lack of globally representative tephra production records. Therefore, our understanding of the global relationship between glacial-interglacial cycles and volcanism uncertain. As a result, we do not know whether deglaciation directly drives enhanced volcanism, or if the feedbacks are more complex. In this work we use a database of visible tephra layers in marine sediments, and a weighted bootstrap resampling method to develop a record of global tephra (the products of explosive volcanism) production which covers the past million years.

Our results show an intensification of global tephra production around 420 to 400 thousand years ago (ka), which coincides with Marine Isotope Stage (MIS) 11 – the warmest interglacial of the past million years. MIS11 was a period of high sea level (up to 10 m above present) and low ice cover, with Greenland likely largely ice free. We suggest the low ice levels drove enhanced volcanism, and consequently enhanced volcanic carbon dioxide degassing, which in turn drove further ice sheet ablation. This positive feedback may the explain this warmth, and in turn, the Mid-Brunhes transition, which heralded the arrival of generally warmer interglacials after 400 ka. Further, after 400 ka we begin to see cyclicity in the tephra record, mirroring eccentricity forcing seen in ice volume records. More pronounced ice-volcano feedbacks may therefore explain the stronger interglacials of the past 400,000 years.

References

Huybers, P., & Langmuir, C. (2009). Feedback between deglaciation, volcanism, and atmospheric CO2. Earth and Planetary Science Letters, 286(3–4), 479–491.

Kutterolf, S., Schindlbeck, J. C., Jegen, M., Freundt, A., & Straub, S. M. (2019). Milankovitch frequencies in tephra records at volcanic arcs: The relation of kyr-scale cyclic variations in volcanism to global climate changes. Quaternary Science Reviews, 204, 1–16.

Maclennan, J., Jull, M., McKenzie, D., Slater, L., & Grönvold, K. (2002). The link between volcanism and deglaciation in Iceland. Geochemistry, Geophysics, Geosystems, 3(11), 1–25.

 

How to cite: Longman, J., Gernon, T. M., Hincks, T. K., Panitz, S., and Palmer, M. R.: A million-year reconstruction of global volcanism intensity: How does it link to glaciation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17827, https://doi.org/10.5194/egusphere-egu24-17827, 2024.

The ocean plays an essential role in the rise of atmospheric CO2 by about 90 ppmv during the last deglaciation. The deglacial oceanic CO2 outgassing is jointly controlled by the physical, biological and geochemical processes, which affect the variations in ocean circulation, biological carbon pump and alkalinity inventory. Transient simulations of climate-carbon feedback, particularly using the comprehensive Earth System Models, are instrumental tools to quantify the contribution of different processes and their interactions. Nonetheless, knowledge gaps still exist in the deglacial variations of oceanic carbon and nutrient cycling because considerable model uncertainties arise from the choices of model processes and parameters, and the proxy data is too sparse to fully constrain the model outcome.

We conduct transient simulations for the last deglaciation with the Max Planck Institute Earth System Model (MPI-ESM) and examine the impact of different model tuning of the global ocean biogeochemistry component and a sediment module on the deglacial CO2 outgassing. The atmospheric CO2 is prognostically computed for the carbon cycle, considering only the atmosphere and ocean compartments, and it is prescribed for radiation computation. We force the model with reconstructions of atmospheric greenhouse gas concentrations, orbital parameters, ice sheet and dust deposition. In line with the physical ocean component, we account for the automatic adjustment of marine biogeochemical tracers in response to changing bathymetry and coastlines related to deglacial meltwater discharge and isostatic adjustment.

We find the deglacial CO2 outgassing is mainly driven by the sea surface warming in MPI-ESM, whereas variations in surface alkalinity and DIC have a relatively small contribution (~18%). Furthermore, the parameterisation of organic debris remineralisation considerably affects the deglacial increase in the global NPP due to different recycling rates of nutrients in the upper ocean. When a longer lifetime of dissolved organic matter is prescribed, the dissolved organic carbon pool in the glacial ocean increases, further facilitating the glacial ocean carbon sequestration. Including an interactive sediment module strongly impacts surface alkalinity due to input-sedimentation imbalance, affecting air-ocean CO2 flux. Thus, attention has to be given to tuning and adjustments regarding the input-sedimentation imbalance of alkalinity in ESMs to better represent proxy data and the deglacial oceanic CO2 outgassing.

How to cite: Liu, B. and Ilyina, T.: Quantifying the role of ocean biogeochemistry on the deglacial atmospheric CO2 rise using transient simulations with MPI-ESM, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18082, https://doi.org/10.5194/egusphere-egu24-18082, 2024.

EGU24-18786 | ECS | Posters on site | CL1.2.6

Mechanisms of atmospheric CO2 drawdown during Marine Isotope Stage 4 based on Atlantic deep-water temperature and bottom-water oxygenation reconstructions  

Svetlana Radionovskaya, Julia Gottschalk, David Thornalley, Mervyn Greaves, and Luke Skinner

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key to elucidating the ocean’s role in modulating atmospheric CO2 changes on millennial and orbital timescales. MIS 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO2 levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO2 drop of ~40 ppmv, and includes several millennial climatic events, for example Heinrich Stadial 6. Although various paleo proxy records suggest a weakened Atlantic overturning during MIS 4, and particularly HS 6, changes in AMOC strength and the geometric extent of NADW shoaling remain poorly understood. Here, we present deep-water temperature reconstructions based on infaunal benthic foraminiferal Mg/Ca ratios and bottom water oxygen concentration reconstructions using redox-sensitive foraminiferal U/Ca, from the deep North (~2.65km) and South (~3.8km) Atlantic to assess the changes in deep water hydrography and by extension circulation.

Our reconstructed deep-water temperature changes from the Iberian Margin (~2.65 km water depth) suggest a stronger influence of colder southern sourced waters during MIS 4 and particularly during HS 6; and a clear subsurface warming during MIS 5a stadials. Meanwhile, changes in deep-water temperatures in the Atlantic Sector of the Southern Ocean (SO) closely follow variations in Antarctic temperature, atmospheric CO2 and the mean ocean temperature, likely mediated by buoyancy forcing in the SO, which is in turn likely linked to sea-ice expansion at the MIS 5a/4 transition. Together with (arguably smaller) contributions from reduced air-sea gas exchange efficiency in the SO, these combined changes would have lowered atmospheric CO2through more efficient carbon sequestration in an expanded deep Atlantic reservoir during MIS 4, through their impact on the solubility- and soft tissue “pumps” (i.e. the ocean’s disequilibrium and respired carbon budgets). Indeed, bottom water oxygenation reconstructions from the South Atlantic support the conclusion that the Southern Ocean appears to have represented a significant reservoir for sequestering CO2 away from the atmosphere during MIS 4.

How to cite: Radionovskaya, S., Gottschalk, J., Thornalley, D., Greaves, M., and Skinner, L.: Mechanisms of atmospheric CO2 drawdown during Marine Isotope Stage 4 based on Atlantic deep-water temperature and bottom-water oxygenation reconstructions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18786, https://doi.org/10.5194/egusphere-egu24-18786, 2024.

EGU24-19515 | Posters on site | CL1.2.6

Sea surface temperature variations in the Eastern Equatorial Pacific (ODP Site 1240) over the last 160 kyr from three lipid paleothermometers (UK'37, TEXH86 and LDI) 

Eva Calvo, Lucía Quirós-Collazos, Marta Rodrigo, Stefan Schouten, Jaap Sinninghe-Damsté, Leopoldo Pena, Isabel Cacho, and Carles Pelejero

The Pacific Ocean equatorial upwelling region is of great interest to understand climate dynamics within the context of current global change. It plays a key role in global biogeochemical cycles, especially in the carbon cycle, as it stands for being one of the areas with largest CO2 fluxes from the ocean to the atmosphere. Moreover, tropical regions play a key role in regulating global climate, since they control the transfer of thermal energy from low to high latitudes. In this context, and with the aim of reconstructing paleoclimate conditions at glacial-interglacial time scales in this region, we analysed selected molecular biomarkers in the marine sediment core ODP 1240, at the easternmost region of the Eastern Equatorial Pacific (EEP), covering the last 160 kyr. We focused on long-chain alkenones, glycerol dialkyl glycerol tetraethers (GDGTs) and long-chain alkyl diols (LCDs). Upon quantification of these lipids, we calculated the UK'37, TEXH86 and LDI indices, and discussed their suitability as paleotemperature proxies to reconstruct sea surface conditions in the study region. We found that UK'37 and TEXH86 derived-temperatures track the warming and cooling trends typical of glacial-interglacial variations. However, while they provide similar temperatures during the last two interglacial maxima, they disagree during glacial periods, when the TEXH86-based estimations display significantly cooler temperatures. The LDI-derived record also shows similar temperatures to those from the UK'37 and TEXH86during the more recent interglacial but, for the last glacial-interglacial period, LDI-derived temperatures remain colder than those of the UK’37 and even colder than those of the TEXH86 at some periods. Multiple factors could be behind this variability and disagreement between the three paleothermometers: depth dwelling, production or exportation of the different biological producers of each lipid, seasonality, diagenetic processes and changes in biogeochemistry conditions of the studied marine region, amongst others. In this presentation, the factors that we believe are most important in the study region will be presented and discussed, to improve our understanding of the biological dynamics of the precursors of each proxy and of their reconstructed marine temperatures in the EEP.

How to cite: Calvo, E., Quirós-Collazos, L., Rodrigo, M., Schouten, S., Sinninghe-Damsté, J., Pena, L., Cacho, I., and Pelejero, C.: Sea surface temperature variations in the Eastern Equatorial Pacific (ODP Site 1240) over the last 160 kyr from three lipid paleothermometers (UK'37, TEXH86 and LDI), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19515, https://doi.org/10.5194/egusphere-egu24-19515, 2024.

EGU24-19780 | Orals | CL1.2.6

The influence of proglacial lakes on climate and surface mass balance of retreating ice sheets: A study of the Laurentide and Fennoscandian ice sheets at 13 ka BP 

Uta Krebs-Kanzow, Lianne Sijbrandij, Gregor Knorr, Lars Ackermann, Lu Niu, and Gerrit Lohmann

During the last deglaciation large proglacial lakes formed at the base of the retreating northern hemisphere ice sheets. We assess the effect of these ice-contact lakes on regional climate and on the ice sheet surface mass balance components of the adjacent  Laurentide (LIS) and Fennoscandian (FIS) ice sheets,  using an atmosphere general circulation model with a novel extension for proglacial lakes in combination with a surface mass balance scheme for ice sheets, which, for the first time, allows to estimate the effect of the cold surface of these extensive lakes on the surface mass balance of the adjacent ice sheets. In a set of simulations inspired by the  Allerød interstadial around 13000 years before present, we demonstrate that the presence of proglacial lakes significantly reduces summer air temperatures in a larger area around the proglacial lakes and leads to reduced precipitation with increased snow/rain ratio. In consequence surface ablation reduces by 39% over the FIS and 28% over the LIS while accumulation only changes slightly by 1% and -3%  respectively. About one quarter of the response in surface ablation is related to the perennially cold surface of the proglacial lakes.

How to cite: Krebs-Kanzow, U., Sijbrandij, L., Knorr, G., Ackermann, L., Niu, L., and Lohmann, G.: The influence of proglacial lakes on climate and surface mass balance of retreating ice sheets: A study of the Laurentide and Fennoscandian ice sheets at 13 ka BP, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19780, https://doi.org/10.5194/egusphere-egu24-19780, 2024.

EGU24-175 | ECS | Orals | CL2.4

Why do oceanic nonlinearities play a weak role in Extreme El Niño events? 

Fangyu Liu, Jérôme Vialard, Alexey V. Fedorov, Christian Éthé, Renaud Person, and Matthieu Lengaigne

Extreme El Niño events exhibit outsized impacts worldwide and considerably enhance the El Niño Southern Oscillation (ENSO) warm/cold phase asymmetries. While many mechanisms were proposed, no consensus has been reached and the relative role of atmospheric and oceanic processes remains to be illustrated. Here we quantitatively assess the contribution of oceanic nonlinearities through a state-of-the-art oceanic general circulation model, which realistically simulates extreme El Niño related characteristics and the oceanic nonlinear processes responsible for ENSO skewness. An effective way is developed to isolate sea surface temperature (SST) nonlinear response based on paired experiments forced with opposite wind stress anomalies. We demonstrate that the overall oceanic nonlinearities play a marginal role on extreme El Niño amplitude, which largely arises from the compensation between positive contributors from tropical instability waves (TIWs) and nonlinear dynamic heating (NDH) and negative contributors from subsurface processes and air-sea fluxes. The physical processes keep robust when using the other mixing scheme or mixed layer option for the heat budget. Our findings quantitively reveal the subtle contribution of oceanic nonlinearities, yielding strong evidence for the paramount role of atmospheric nonlinearities in shaping extreme El Niño events.

How to cite: Liu, F., Vialard, J., V. Fedorov, A., Éthé, C., Person, R., and Lengaigne, M.: Why do oceanic nonlinearities play a weak role in Extreme El Niño events?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-175, https://doi.org/10.5194/egusphere-egu24-175, 2024.

EGU24-626 | ECS | Posters on site | CL2.4

Dynamical evolution of ENSO in a warming background: A review of recent trends & future projections 

Sreevathsa Golla, Joël Hirschi, Jennifer Mecking, Adam Blaker, Stephen Kelly, and Robert Marsh

The wide-spread implications of El Niño–Southern Oscillation (ENSO) on global and regional climate necessitates a better understanding of how the underlying interannual dynamics have changed over recent years. Year-to-year changes in ENSO impact terrestrial and marine habitats, water availability, food security and social stability (Santoso et al., 2017). With abundant evidence of a warming climate, it is imperative to understand how a large-scale climatic oscillation such as ENSO is evolving and influencing changes in large-scale atmospheric circulation patterns (Alizadeh et al., 2022; Cai et al., 2021). Furthermore, quantifying the influence of the ocean on changes in this climatic pattern is an interesting and important question to answer. Evaluating the ability of models to appropriately represent the underlying physics and dynamical changes impacting the spatiotemporal extent and the intensity of ENSO is crucial to understanding ocean-climate teleconnections and changes in climatic extremes. In this study, we review and evaluate the representation of ENSO in several high-resolution CMIP6 and HighResMIP models and forced ocean-only simulations focusing on the ability of current state-of-the-art models to represent central equatorial pacific warming and cooling. This evaluation involves looking at the development and propagation of warm temperature anomalies on surface and sub-surface levels in the equatorial Pacific and understanding the differences in simulating surface heat budget and exchange with the overlying atmosphere and the deeper ocean. Surface and sub-surface (up to 200m depth) temperature anomalies in the Niño 3.4 region were calculated from modelled data and were then compared with anomalies from observational and reanalysis temperature datasets (like EN4, ORAS5). We find good agreement in the timing and vertical structure of surface/sub-surface temperature anomalies in the forced model simulations, particularly during strong ENSO years. Moreover, the genesis of sub-surface anomalies and their further propagation to the surface was well simulated in the forced simulations. The vertical coherence of temperature anomalies was relatively more pronounced in forced ocean-only simulations than in coupled high-resolution model runs. Furthermore, we comment on the shortcomings and suggest potential improvements that can be made in the models that could improve the model’s ability to capture ENSO strength and variability.

How to cite: Golla, S., Hirschi, J., Mecking, J., Blaker, A., Kelly, S., and Marsh, R.: Dynamical evolution of ENSO in a warming background: A review of recent trends & future projections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-626, https://doi.org/10.5194/egusphere-egu24-626, 2024.

EGU24-696 | ECS | Orals | CL2.4

Tropical SST Impacts on the Subtropical Atmospheric Circulation and Regional Precipitation 

Weiteng Qiu, Mat Collins, Adam Scaife, and Agus Santoso

The tropical Pacific Ocean hosts the Earth’s most prominent year-to-year climate fluctuation, the El Niño-Southern Oscillation (ENSO), which exerts strong impacts on remote regions of the globe through atmospheric teleconnection. In this study, we use reanalysis data and Coupled Model Intercomparison Project Phase 6 (CMIP6) historical simulations to investigate the relationship between tropical and subtropical atmospheric circulation, and the tropical SST patterns and regional precipitation.   

We find dynamical relationships between subtropical high intensity, the Hadley and Ferrel Circulation intensity, and the Eady Growth Rate from the reanalysis. A poleward shift of the maximum in Eady Growth Rate is associated with a strengthening of the descending branches of the Ferrel and Hadley Cells, with subtropical troposphere adiabatic warming and an increased intensity and poleward movement of the subtropical highs. Shifts in the poleward Eady Growth Rate are dominated by changes in vertical wind shear which, in turn, are in thermal wind balance with variations and trends in temperature. The mechanism for the intensification of the subtropical highs involves feedbacks from high-frequency transient eddies. Strong North Pacific and South Pacific Subtropical highs are associated with La-Niña conditions. We also show that the mechanisms for interannual variations are similar to those for trends in the highs.

We further analysed the performance of the coupled models in reproducing the trends (1979-2014) of the tropical zonal wind and regional precipitation. The CMIP6 historical simulations do not capture the intensification of trade winds within the Niño 4 region, and they also fail to reproduce the statistically significant precipitation trends over the Southern North America and the Amazon. However, a linear adjustment, based on ENSO teleconnections, can be applied to the coupled models to make the precipitation trends much closer to observations. The relationship between SST patterns and precipitation trends are confirmed by looking at atmosphere-only simulations. This study provides further evidence of the importance of reconciling observed and modelled SST patterns in the tropical Pacific.

How to cite: Qiu, W., Collins, M., Scaife, A., and Santoso, A.: Tropical SST Impacts on the Subtropical Atmospheric Circulation and Regional Precipitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-696, https://doi.org/10.5194/egusphere-egu24-696, 2024.

EGU24-1547 | ECS | Orals | CL2.4

Variable-resolution global atmospheric models are sensitive to driving SST in ENSO/IOD-Australian rainfall teleconnections 

Ying Lung Liu, Lisa Alexander, Jason Evans, and Marcus Thatcher

We have investigated the sensitivity of a global climate model to driving sea surface temperatures (SST) in simulating Australian rainfall characteristics, including the El Niño-Southern Oscillation (ENSO)- and Indian Ocean Dipole (IOD)-related rainfall variability. We employed the Conformal Cubic Atmospheric Model (CCAM), a global atmospheric model characterized by variable resolution, CCAM was forced by two SST datasets with different spatiotemporal resolutions: the 0.25° daily Optimum Interpolation Sea Surface Temperature (CCAM_OISST) version 2.1 and the 2° monthly Extended Reconstruction SSTs Version 5 (CCAM_ERSST5). A benchmarking framework was employed to appraise model performance, revealing strong agreement between the simulations and the Australian Gridded Climate Data (AGCD) in climatological rainfall spatial patterns, seasonality, and annual trends. It is noted that both simulations tend to overestimate rainfall amount, with CCAM_OISST exhibiting a larger bias.

Moreover, CCAM's performance in capturing ENSO and IOD correlations with rainfall was assessed during Austral spring (SON) using a novel hit rate metric. The findings underscore that only CCAM_OISST effectively reproduces observed SON ENSO- and IOD-rainfall correlations, achieving hit rates of 86.6% and 87.5%, respectively, in contrast to 52.7% and 41.8% for CCAM_ERSST5. Noteworthy disparities in sea surface temperatures were observed along the Australian coastline between OISST and ERSST5 (the so-called “Coastal Effect”). These disparities may be attributed to spatial interpolation errors arising from the differences in resolution between the model and driving SST. An additional experiment within CCAM, masking OISST with ERSST within a 5° proximity to the Australian continent, underscores the pronounced impact of the “Coastal Effect” on IOD-Australian rainfall simulations. Conversely, its influence on ENSO-Australian rainfall was constrained. Therefore, realistic local SSTs are important if model simulations are to reproduce realistic IOD-rainfall responses over Australia. Additionally, even though an SST product with a longer time span is preferred in simulating IOD-related variability, circumspection is warranted in the analysis of the impact of IOD on Australian rainfall when utilizing climate model output with a substantial discrepancy in spatial resolutions between the model and the driving SST. After showing CCAM’s ability to simulate ENSO- and IOD-rainfall, our future research will involve pacemaker experiments to isolate remaining climate modes and investigate their independent impact on Australian rainfall.

How to cite: Liu, Y. L., Alexander, L., Evans, J., and Thatcher, M.: Variable-resolution global atmospheric models are sensitive to driving SST in ENSO/IOD-Australian rainfall teleconnections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1547, https://doi.org/10.5194/egusphere-egu24-1547, 2024.

EGU24-1749 | ECS | Orals | CL2.4

Seasonality of Feedback Mechanisms Involved in Pacific Coastal Niño Events 

Daniel Rudloff, Sebastian Wahl, and Joke Lübbecke

The 2017 Pacific Coastal Niño Event was the strongest of its type. It caused torrential rainfall and devastating flooding in Peru and Ecuador and thus rapidly caught the attention of the scientific community. Multiple studies have been conducted focusing on the causes and consequences of this event. While the strong connection between SST anomalies and local rainfall, especially during boreal spring, is well established, the causes of the extreme warming are still a subject of discussion. In this study, we focus on the seasonality of the effectiveness of mechanisms and feedbacks involved in coastal Niño Events, utilising reanalysis products and historical model simulations from the Flexible Ocean and Climate Infrastructure (FOCI).

The 2017 event stands out due to its strength and timing as it occurred earlier in the year than most other events. We find that the atmospheric conditions during this time of year are very different due to the presence of atmospheric convection which modulates the SST-cloud feedback. Further, the event coincided with the season of strongest wind-driven upwelling. This combination enables a different forcing of a short but strong event. Additional model sensitivity experiments are performed for a better understanding of underlying mechanisms. We show how the same local wind stress forcing acts differently in different seasons, with its strongest impact during the months of strongest entrainment. Events forced by local heat fluxes and wind stress forcing only do not show any subsurface warming, which is found to be the main reason for their rapid decay. Even though the atmospheric response to a coastal warming varies seasonally, without any subsurface forcing, e.g., the events cannot be sustained through atmospheric feedbacks.

How to cite: Rudloff, D., Wahl, S., and Lübbecke, J.: Seasonality of Feedback Mechanisms Involved in Pacific Coastal Niño Events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1749, https://doi.org/10.5194/egusphere-egu24-1749, 2024.

EGU24-2133 | Orals | CL2.4

The El Niño Southern Oscillation (ENSO) recharge oscillator conceptual model : past achievements, future prospects. 

Jérôme Vialard and the CLIVAR ENSO conceptual model Working Group

The Recharge Oscillator (RO) is a simple mathematical model of the El Niño Southern Oscillation (ENSO). It is based on two ordinary differential equations that describe the evolution of eastern Pacific sea surface temperature and western Pacific oceanic heat content. These equations are based on physical principles that operate in nature: (i) the air-sea interaction loop known as the Bjerknes feedback, (ii) a delayed negative feedback arising from the slow oceanic response to near-equatorial winds, (iii) state-dependent stochastic forcing from intraseasonal wind variations known as Westerly Wind Events, and (iv) nonlinearities such as those related to deep atmospheric convection and oceanic advection. These elements can be combined in different levels of RO complexity. The RO reproduces the ENSO key properties in observations and climate models: its amplitude, dominant timescale, seasonality, warm/cold phases asymmetries, and the seasonal predictability decrease known as the “spring barrier”. We then discuss the RO in view of timely research questions. First, the RO can be extended to account for pattern ENSO diversity (with events that either peak in the central or eastern Pacific). Second, the core RO hypothesis that ENSO is governed by tropical Pacific dynamics is discussed under the perspective of research suggesting an influence from other basins. Finally, we discuss the RO relevance for studying ENSO response to climate change, and underline that accounting for diversity and better linking the RO parameters to the long term mean state are important research avenues. We end by proposing a list of ten important RO-based research problems.

How to cite: Vialard, J. and the CLIVAR ENSO conceptual model Working Group: The El Niño Southern Oscillation (ENSO) recharge oscillator conceptual model : past achievements, future prospects., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2133, https://doi.org/10.5194/egusphere-egu24-2133, 2024.

EGU24-2166 | Orals | CL2.4

Mechanisms of Tropical Pacific Decadal Variability 

Antonietta Capotondi and the CLIVAR Tropical Pacific Decadal Variability Working Group

Naturally-occurring variability in the Tropical Pacific at timescales in the 7-70 years range, defined here as Tropical Pacific Decadal Variability (TPDV), modulates ENSO characteristics and its global impacts, and is linked to the rate of change of the globally-averaged surface temperature. Thus, understanding TPDV is integral to robustly separate the forced climate response from internally-generated climate variability and thereby produce reliable projections of the tropical Pacific and global climate. Several oceanic mechanisms have been proposed to explain TPDV, including off-equatorial Rossby wave activity, propagation of spiciness anomalies from the subtropical to the tropical regions, and changes in the strength of the shallow upper-ocean overturning circulations, known as “Subtropical Cells”. However, uncertainties remain on the relative importance of these oceanic mechanisms. Another critical source of uncertainty concerns the nature and origin of the atmospheric forcing of those oceanic processes. Anomalous wind forcing could arise as a response to tropical Pacific sea surface temperature (SST) anomalies, be induced by Pacific extra-tropical influences or result from tropical basin interactions. This presentation critically reviews the nature and relative importance of the oceanic and atmospheric processes driving TPDV. Although uncertain, the tropical oceanic adjustment through Rossby wave activity is likely a dominant source of variability at decadal timescales. A deeper understanding of the origin of TPDV-related winds is a key priority for future research.

How to cite: Capotondi, A. and the CLIVAR Tropical Pacific Decadal Variability Working Group: Mechanisms of Tropical Pacific Decadal Variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2166, https://doi.org/10.5194/egusphere-egu24-2166, 2024.

EGU24-2466 | ECS | Posters on site | CL2.4

Asymmetric Influences of ENSO Phases on the Predictability of North Pacific Sea Surface Temperature 

Zhaolu Hou, Jianping Li, and Yina Diao

The North Pacific sea surface temperature (SST) exerts profound climatic influence. El Niño-Southern Oscillation (ENSO) significantly impacts North Pacific SST, yet the influence from ENSO’s distinct phases on SST predictability remains unclear. Overcoming model limitations, this study assesses SST predictability under diverse ENSO phases using reanalysis. Quantifying predictability limits (PL), results unveil asymmetry: El Niño PL at 5.5 months, La Niña at 8.4 months, and Neutral at 5.9 months. This asymmetry mirrors contemporary multimodal prediction skills. Error growth dynamics reveal La Niña's robust signal strength with slow error growth rate, contrasting El Niño's weaker signal and faster error growth. Neutral exhibits intermediate signal strength and elevated error growth. Physically, predictability signal strength aligns with SST variability, whereas error growth rate correlates with atmospheric-ocean heating anomalies. La Niña, inducing positive heating anomalies, minimizes atmospheric noise impact, resulting in lower error growth. The results are beneficial for improving North Pacific SST predictions.

How to cite: Hou, Z., Li, J., and Diao, Y.: Asymmetric Influences of ENSO Phases on the Predictability of North Pacific Sea Surface Temperature, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2466, https://doi.org/10.5194/egusphere-egu24-2466, 2024.

EGU24-2993 | Posters on site | CL2.4

El Niño Southern Oscillation and Tropical Basin Interaction in Idealized Worlds 

Dietmar Dommenget and David Hutchinson

In this study we discuss a set of fully coupled general circulation model simulations with idealised geometries of the tropical ocean basins and land with a focus on important characteristics of El Niño Southern Oscillation (ENSO) type of variability and tropical basin interaction. In a series of 15 simulations we first vary the zonal width of a single tropical ocean basin from 50o to 360o, while the rest of the tropical zone is set as land. Further we discuss different simplified configurations of two or three tropical ocean basins. The results show remarkable changes in ENSO characteristics as function of basin width and due to the interaction with other basins that challenge our current understanding of ENSO dynamics. A single basin ENSO has an optimal basin width of about 150o at which ENSO preferred period is the longest, the wind stress feedback is the strongest and variability is stronger than in all other basin widths, expect for the 350o basin. Tropical basin interactions substantially affect ENSO strength, periodicity, feedbacks, non-linearity, spatial scale and pattern. In experiments with two or three identical ocean basins we find highly synchronized ENSO modes that are identical between basins and far more energetic and oscillatory then the single basin modes. The results suggest that tropical basin interaction is an essential part of ENSO. The framework of these experiments can help to better understand the atmospheric dynamics of ENSO and should help to formulate an ENSO theory that incorporates tropical basin interactions as a core element.

How to cite: Dommenget, D. and Hutchinson, D.: El Niño Southern Oscillation and Tropical Basin Interaction in Idealized Worlds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2993, https://doi.org/10.5194/egusphere-egu24-2993, 2024.

This study investigates the delayed influence of the Indian Ocean dipole (IOD),  isolated and combined with ENSO, on the early winter North Atlantic-European (NAE) circulation.  Results reveal that a positive IOD induces a strong response in the NAE region during December, leading to a positive North Atlantic Oscillation (NAO)-like pattern. This circulation response also induces a north-south precipitation dipole and a positive temperature anomaly over Europe. The underlying physical mechanism involves a rainfall dipole response to the IOD in the Indian Ocean, persisting into early winter, which triggers a perturbation in the zonal wind within the subtropical South Asian jet (SAJET) region. This initiates a wave-train that propagates northeastward into the North Atlantic. Additionally, a positive IOD enhances transient eddy activity in the European region. Transient eddy forcing provides strong positive feedback to the NAO-like anomaly. While the ECMWF-SEAS5 seasonal hindcast system reproduces the sign of the response, its magnitude is considerably weaker. The possible reasons for this weak response are investigated. The model can reproduce the delayed rainfall dipole response to the IOD, however, the structure of the response shows some differences with the re-analysis. The zonal wind perturbation in ECMWF-SEAS5 in the SAJET region is only about half of the re-analysis magnitude. Moreover, the wave propagation into the stratosphere, as estimated by the 100h𝑃𝑎 eddy heat fluxes, plays a minor role in the re-analysis and the model.

How to cite: Kucharski, F., Raganato, A., and Abid, M. A.: The combined  impact of Indian Ocean dipole and ENSO on the North Atlantic-European circulation during early boreal winter in re-analysis and in the ECMWF-SEAS5 hindcast , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3110, https://doi.org/10.5194/egusphere-egu24-3110, 2024.

EGU24-3728 | Orals | CL2.4 | Highlight

Super El Niño: A product of three-ocean interactions  

Chunzai Wang, Jiazhen Wang, and Hanjie Fan

El Niño, the largest climate phenomenon on Earth, profoundly influences global climate, weather, ecosystems, and human societies. Super (or extreme) El Niño, in particular, has a significant impact on climate and extreme weather events, but its formation mechanism remains unknown. This presentation utilizes observations, climate model outputs, and coupled model experiments to demonstrate that interactions among the tropical Pacific, Indian, and Atlantic Oceans contribute to the development of super El Niño. The early onset of El Niño imparts sufficient strength in the summer and fall to trigger the Atlantic Niña and Indian Ocean dipole. Subsequently, the Atlantic Niña and Indian Ocean dipole alternately generate additional westerly wind anomalies over the equatorial western-central Pacific, reinforcing El Niño through the Bjerknes feedback and leading to the emergence of super El Niño. This novel mechanism is termed the Indo-Atlantic booster. The findings emphasize super El Niño as a product of three interactions, suggesting that incorporating both the Indian and Atlantic Oceans and their teleconnections with the Pacific will significantly enhance predictions of super El Niño and climate.

How to cite: Wang, C., Wang, J., and Fan, H.: Super El Niño: A product of three-ocean interactions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3728, https://doi.org/10.5194/egusphere-egu24-3728, 2024.

The El Niño-Southern Oscillation (ENSO) is one of the most significant integrated interannual oscillations with coupled atmosphere-ocean processes in the tropical Pacific. Most coupled climate models are weak in depicting ENSO asymmetry over equatorial Pacific subsurface. And it is still unclear how the stand-alone ocean model contributes to this bias. In this study, we found that most ocean models from the Ocean Model Intercomparison Project (OMIP), driven by JRA55, underestimate the asymmetry of ENSO in the equatorial western Pacific subsurface. We investigated the primary factors contributing to this bias using composite analysis and diagnostics, and found that the weaker responses in upwelling and stronger responses in downwelling to westerly and easterly wind stress anomalies in the models are mainly responsible for the bias. Furthermore, the underestimation of zonal current variability over western Pacific subsurface, influenced by the gradient of mean state of sea surface height along the equatorial Pacific, leads to an opposite relationship between asymmetry and the zonal component of nonlinear dynamic heating in the western Pacific subsurface comparing to that in the eastern Pacific subsurface. Our study emphasizes the importance of accurately modeling ocean currents to capture the characteristics of ENSO nonlinearity and highlights the significance of nonlinear dynamic responses to external forcing.

How to cite: Li, J. and Yu, Y.: Underestimated ENSO Asymmetry and Zonal Currents over the Equatorial Western Pacific in OMIP2 experiments , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4811, https://doi.org/10.5194/egusphere-egu24-4811, 2024.

EGU24-4820 | ECS | Posters on site | CL2.4

Synchronous Decadal Climate Variability in the Tropical Central Pacific and Tropical South Atlantic 

Chao Liu, Soon-Il An, Soong-Ki Kim, Malte Stuecker, Wenjun Zhang, Fei-Fei Jin, Jae-Heung Park, Leishan Jiang, Aoyun Xue, Xin Geng, Hyo-Jin Park, Young-Min Yang, and Jong-Seong Kug

The El Niño-Southern Oscillation (ENSO), the strongest interannual climate signal, has a large influence on remote sea surface temperature (SST) anomalies in all three basins. However, a missing map piece in the widespread ENSO teleconnection is the Equatorial Atlantic, where the ENSO footprint on local SST is less clear. Here, using reanalysis data and partially coupled pacemaker experiments, we show that the tropical Pacific SST anomalies, manifested as a Central Pacific (CP) ENSO-like structure, synchronize the tropical South Atlantic (40°W-10°E, 15°S-0°) SST anomalies over the last seven decades, but on a quasi-decadal (8-16 year) timescale. Such a decadal connection is most evident during the boreal spring-summer season, when the CP ENSO-like decadal SST anomalies induce a cooling of the South Atlantic SSTs through atmospheric teleconnections involving both Southern Hemisphere extratropical Rossby waves and equatorial Kelvin waves. The resulting subtropical South Atlantic low-level anticyclonic circulation and easterlies at its northern flank cause local ocean-atmosphere feedback and strengthen the Pacific-to-Atlantic teleconnections. In contrast, the concurrent tropospheric temperature teleconnection is less destructive to the above Atlantic SST response due to the weaker and more west decadal Pacific SST anomalies compared to the interannual ENSO counterpart. Pacific-driven coupled simulations reproduce key observational features fairly well, while parallel Atlantic-driven simulations show little forcing into the Pacific. Our results show that the tropical Central Pacific is an important source of decadal predictability for the tropical South Atlantic SST and the surrounding climate.

How to cite: Liu, C., An, S.-I., Kim, S.-K., Stuecker, M., Zhang, W., Jin, F.-F., Park, J.-H., Jiang, L., Xue, A., Geng, X., Park, H.-J., Yang, Y.-M., and Kug, J.-S.: Synchronous Decadal Climate Variability in the Tropical Central Pacific and Tropical South Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4820, https://doi.org/10.5194/egusphere-egu24-4820, 2024.

EGU24-5122 | Orals | CL2.4

The mechanism of multi-year La Niña events and their impact on spring precipitation over southern China 

Licheng Feng, Guangliang Li, and Ronghua Zhang

By diagnosing and analyzing the frequent occurrence of multi-year La Niña events in recent years, this study reveals the process and mechanism of the Southeast Pacific subsurface cold water triggering multi-year La Niña events. Revealing for the first time the propagation channels and physical processes of multi-year La Niña events triggered by subsurface cold water. In late spring and early summer, the anomalous eastward wind strengthens in the central equatorial Pacific, while abnormal wind stress divergence occurs in the eastern Pacific, which strengthens and spreads westward over time. The weak negative sea surface temperature anomaly in the eastern equatorial Pacific is accompanied by upwelling, providing a source of cold water for the surface. As the season progresses, the weakened equatorial undercurrent and the enhanced southern equatorial current cause cold water to spread westward and accumulate in the central Pacific, thereby extending upwards to expose the sea surface. The exposed cold water causes a cooling of the sea surface and triggers local sea atmosphere interactions, leading to abnormal development of sea atmosphere and ultimately forming a multi-year La Niña events. Composite analyses were performed in this study to reveal the differences in spring precipitation over southern China during multiyear La Niña events from 1901-2015. It was found that there is significantly below normal precipitation in the first boreal spring, but above normal in the second year. The differences in spring precipitation over southern China are correlative to the changes in anomalous atmospheric circulations over the northwest Pacific, which can in turn be attributed to different anomalous sea surface temperatures (SSTs) over the tropical Pacific. During multiyear La Niña events, anomalous SSTs were stronger in the first spring than those in the second spring. As a result, the intensity of abnormal cyclones (WNPC) in the western North Pacific Ocean (WNP) in the first year is stronger, which is more likely to reduce moisture transport, leading to prolonged precipitation deficits over southern China. In contrast, the tropical SST signal is too weak to induce appreciable changes in the WNPC and precipitation over South China in the second year. The difference in SST signals in two consecutive springs leads to different spatial patterns of precipitation in southern China by causing different WNPC.

How to cite: Feng, L., Li, G., and Zhang, R.: The mechanism of multi-year La Niña events and their impact on spring precipitation over southern China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5122, https://doi.org/10.5194/egusphere-egu24-5122, 2024.

Understanding external drivers of the El-Nino Southern Oscillation (ENSO) is essential for predicting its future evolution. Orbital precession has been identified as a driver of ENSO variability through both proxy records and climate model simulations, yet the exact mechanics remain unclear. This orbital cycle moderates the seasonal timing of insolation relative to Earth's revolution around the Sun, thereby adjusting the magnitude of the seasonal cycle experienced by each hemisphere. Here, we analyze output from a suite of simulations in NCAR CESM 2.1.1 designed to analyze ENSO under different precessional extremes that significantly modify the meridional temperature gradients and the cold tongue seasonal cycle in the Pacific ocean. Variations in orbital precession have a strong impact on the magnitude, periodicity, and spatial expression of tropical Pacific variability. We find a critical role for both the North and South Pacific Meridional Modes (NPMM and SPMM) in explaining changes in ENSO and decadal variability by propagating subtropical anomalies to the equatorial Pacific along with a shift in the meridional structure of equatorial winds. As an example, when the perihelion of orbit occurs during boreal winter creating a dampened (strengthened) seasonal cycle in the Northern (Southern) Hemisphere, the SPMM becomes significantly more active while the NPMM weakens. This precessional state experiences a shift toward amplified decadal variability and a greater prevalence of Eastern El Nino events in comparison with the other orbital configurations tested. Understanding the precessional control of tropical variability via subtropical pathways may help explain developments that have occurred in the past, as well as future changes which may be observed due to shifts in meridional temperature gradients.

How to cite: Persch, C. and Sanchez, S.: A Critical Role for Meridional Modes in Determining the Equatorial Pacific Response to Orbital Precession, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6660, https://doi.org/10.5194/egusphere-egu24-6660, 2024.

The winter sea surface temperature (SST) anomalies in the Kuroshio and adjacent regions (KAR), which greatly influence the East Asian–North Pacific–North American climate, are closely related to El Niño–Southern Oscillation (ENSO). This SST relationship between the KAR and the equatorial eastern-central Pacific is widely assumed to be symmetric between El Niño and La Niña. Compared to previous studies indicating the significant and strong KAR warming during El Niño winters, this study indicates weakly negative KAR SST anomalies in the composite analysis for all La Niña events. Positive winter KAR SST anomalies unexpectedly appear in approximately half of La Niña events, which counteract negative SST anomalies in the rest of La Niña events. Further analysis suggests that the impact of La Niña on KAR SST anomalies is modulated by the East Asian winter monsoon (EAWM) during early winter. The weaker-than-normal EAWM offsets the anomalous northeasterly winds in the KAR induced by La Niña and then reinforces the KAR warming through warm oceanic advection. As for strong EAWM, it enhances the northeasterly winds to the west of an anomalous Philippine Sea cyclone associated with La Niña, leading to KAR cooling with more latent heat flux loss from the ocean and anomalous cold oceanic advection. Additionally, when the EAWM is independent of ENSO and is associated with the western Pacific pattern, it also can exhibit a pronounced influence on the KAR SST anomalies via the major processes of surface latent flux and horizontal heat advection in the ocean, accompanied by a change in Kuroshio transport.

How to cite: Chen, S., Chen, J., Wang, X., and Xiao, Z.: Varying Relationship between La Nin a and SST Anomalies in the Kuroshio and Adjacent Regions during Boreal Winter: Role of the East Asian Winter Monsoon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7307, https://doi.org/10.5194/egusphere-egu24-7307, 2024.

EGU24-7849 | ECS | Orals | CL2.4

On the decadal changes of Atlantic-Pacific interactions and the effects of external forcing 

Soufiane Karmouche, Evgenia Galytska, Gerald A. Meehl, Jakob Runge, Katja Weigel, and Veronika Eyring

We show the results of a study investigating the predominant role of external forcing in steering Atlantic and Pacific ocean variability during the latter half of the 20th (and early 21st) century. By employing the PCMCI+ causal discovery method, we analyze reanalysis data, pacemaker simulations, and a CMIP6 pre-industrial control run. The results reveal a gradual (multi)decadal change in the interactions between major modes of Atlantic and Pacific interannual climate variability from 1950 to 2014. A sliding window analysis identifies a diminishing El Niño-Southern Oscillation (ENSO) effect on the adjacent Atlantic basin through the tropical route, coinciding with the North Atlantic trending toward and maintaining an anomalously warm state after the mid-1980s. In reanalysis, this is accompanied by the prevalence of an extra-tropical pathway connecting ENSO to the tropical Atlantic. Meanwhile, causal networks from reanalysis and pacemaker simulations indicate that increased external forcing might have contributed to strengthening ENSO’s opposite sign response to tropical Atlantic variability during the 1990s and early 21st century, where warming tropical Atlantic sea surface temperatures induced La Niña-like easterly winds in the equatorial Pacific. The analysis of the pre-industrial control run underscores that modes of natural climate variability in the Atlantic and Pacific influence each other also without anthropogenic forcing. Modulation of these interactions by the long-term states of both basins is observed. This work demonstrates the potential of causal discovery for a deeper understanding of mechanisms driving changes in regional and global climate variability.

 

Karmouche, S., Galytska, E., Meehl, G.A., Runge, J.,Weigel, K.,& Eyring,V. (2023b, in review). Changing effects of external forcing on Atlantic-Pacific interactions. EGUsphere, 2023, 1–36. https://doi.org/10.5194/egusphere-2023-1861

How to cite: Karmouche, S., Galytska, E., Meehl, G. A., Runge, J., Weigel, K., and Eyring, V.: On the decadal changes of Atlantic-Pacific interactions and the effects of external forcing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7849, https://doi.org/10.5194/egusphere-egu24-7849, 2024.

Processes leading to the onset and development of an El Niño event in the tropical Pacific remain elusive. Observed data and Ocean General Circulation Model (OGCM) simulations are used to reveal a well-defined pattern of sea surface temperature (SST) perturbations along the mean North Equatorial Countercurrent (NECC) pathways in association with the onset and evolution of some El Niño events. The OGCM-based sensitivity experiments are conducted to illustrate how a warm SST anomaly (SSTA) on the equator can result from a thermal forcing that is prescribed north of 10°N, similar to observed SST anomalies in December 1988. Within approximately one year, the imposed SST anomaly north of 10°N tends to be transported to the dateline region on the equator by the mean ocean circulation in the western Pacific (the low-latitude western boundary current (LLWBC) and the NECC). In due course, an upper-layer ocean warming is generated off the equator at 6-10°N and then on the equator, which acts to induce a westerly wind anomaly response; a simple statistical atmospheric wind stress model is then used to depict an expected westerly wind response. These resultant SST and surface wind perturbations can couple together over the western tropical Pacific, forming air-sea interactions and setting up a stage for El Niño onset. As such, this pathway mechanism can reasonably well explain the appearance of a warm SST anomaly on the equator in the dateline region and the corresponding development of westerly wind anomalies over the western Pacific in association with El Niño onset.

 

How to cite: Gao, C. and Zhang, R.: A Mechanism from Pathway Perspective for the Generation of a Warm SST Anomaly in the Western Equatorial Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8442, https://doi.org/10.5194/egusphere-egu24-8442, 2024.

EGU24-8581 | ECS | Posters on site | CL2.4 | Highlight

Increased predictability of extreme El Niño from decadal interbasin interaction 

Xuan Ma, Rizhou Liang, Xiaosong Chen, Fei Xie, Jinqing Zuo, Cheng Sun, and Ruiqiang Ding

Predicting extreme El Niño–Southern Oscillation (ENSO) events remains a formidable task. Utilizing eigen microstates (EMs) of complex systems, we elucidate the interplay of two key sea surface temperature (SST) anomaly modes, the newly identified North Atlantic–west Pacific Mode (NAPAM) and discovered Victoria Mode (VM). Our findings demonstrate that a cold NAPAM phase coupled with a positive VM phase markedly elevates the probability of extreme El Niño events; NAPAM's decadal variability serves as a key modulator of extreme El Niño events' frequency. Our empirical model, capitalizing on these modes, achieves robust forecasts with a 6–8 month lead time and boasts a 0.73 correlation with the observed ENSO index in hindcasts. Notably, the model precisely forecasts the intensity of four landmark extreme El Niño episodes: 1982/1983, 1987/1988, 1997/1998, and 2015/2016. Our findings offer promising avenues for refining ENSO predictive frameworks and deepen our understanding of the key climatic drivers.

How to cite: Ma, X., Liang, R., Chen, X., Xie, F., Zuo, J., Sun, C., and Ding, R.: Increased predictability of extreme El Niño from decadal interbasin interaction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8581, https://doi.org/10.5194/egusphere-egu24-8581, 2024.

EGU24-9096 | ECS | Orals | CL2.4 | Highlight

Effects of Niño1+2 and Niño3.4 ENSO Events over Euro-Mediterranean Climate Variability  

Ece Yavuzsoy-Keven, Yasemin Ezber, and Omer Lutfi Sen

El Niño Southern Oscillation (ENSO) is a climate phenomenon that affects the atmospheric circulation of the Northern Hemisphere and causes short-term variability in temperature and precipitation patterns. ENSO impacts over the Euro-Mediterranean (EM) region are commonly defined by using Niño3.4 and Niño3 indices. However, some recent studies indicate that the ENSO event represented by both Niño1+2 and Niño3.4 indices (shared ENSO) is more effective over EM region climate.

In this study, we examine the response of the EM climate to ENSO events detected by Niño1+2 and Niño3.4 regions. NCEP/NCAR Reanalysis surface air temperature, precipitation, 500 hPa geopotential height, 850 hPa wind, and 300 hPa zonal wind datasets and SST-based ENSO indices from ERSSTv4 were used in the analysis for boreal winters between 1950 and 2019. For composite analysis, we separated ENSO events as El Niño and La Niña according to those observed in Niño1+2, Niño3.4, and both regions. We also tried to understand if there is any relation between ENSO and teleconnection patterns such as NAO, East Atlantic (EA), Trough Displacement Index for the Mediterranean Trough (TDI_MedT), and East Atlantic/Western Russia (EAWR) by using the cross-correlation analysis. Additionally, investigate the winter (December, January, February, DJF) ENSO’s possible lagged impacts on the teleconnection patterns in the subsequent seasons, spring (March-April-May, MAM), summer (June-July-August, JJA), and autumn (September-October-November, SON).

The major finding of this study is that the shared ENSO event is more effective over the EM climate than the ENSO events detected only by Niño1+2 or Niño3.4 indices. Further, it is also important for the predictability of the EM climate. In the shared El Niño event, the Middle East and much of North Africa tend to become colder than climatology while Europe becomes warmer. The anticyclonic wind anomaly over western Europe causes drier air in southern Europe and wetter air in northern Europe. The shared El Niño event also modulates the westerly flows at the upper troposphere. The westerly flow accelerates over high latitudes while decelerates over European mid-latitudes, causing northern Europe to be wetter and the Mediterranean Basin to be drier. The cross-correlation analysis including all SST-based ENSO indices and teleconnection indices that the EA index has a significant correlation with the Niño1+2 index across all seasons.

How to cite: Yavuzsoy-Keven, E., Ezber, Y., and Sen, O. L.: Effects of Niño1+2 and Niño3.4 ENSO Events over Euro-Mediterranean Climate Variability , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9096, https://doi.org/10.5194/egusphere-egu24-9096, 2024.

EGU24-9334 | ECS | Orals | CL2.4

Characterizing Nonlinearities in ENSO Dynamics Using Hybrid Machine Learning Models 

Jakob Schlör, Jannik Thuemmel, Antonietta Capotondi, Matthew Newman, and Bedartha Goswami

Event-to-event differences of the El Niño Southern Oscillation (ENSO) result in different patterns of extreme climate conditions globally, which requires ENSO forecasts that accurately predict both the likelihood and the type of an event. One question regarding predictable ENSO dynamics is the extent to which they may be captured by multivariate linear dynamics and, relatedly, whether predictable nonlinearities must be accounted for or may be treated stochastically.

In this study, we combine Recurrent Neural Networks with the Linear Inverse Model (LIM) to assess the role of predictable nonlinearities and non-Markovianity in the evolution of tropical Pacific sea surface temperature anomalies. We observe that modeling nonlinearities significantly enhances the forecast accuracy, particularly in the western tropical Pacific within a 9 to 18-month lag time. Our results indicate that the asymmetry of warm and cold events is the main source of the nonlinearity. Moreover, we demonstrate that the predictability of the Hybrid-model can be reliably inferred from the theoretical skill of the LIM whereas a similar assessment is not possible in pure deep learning models.

How to cite: Schlör, J., Thuemmel, J., Capotondi, A., Newman, M., and Goswami, B.: Characterizing Nonlinearities in ENSO Dynamics Using Hybrid Machine Learning Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9334, https://doi.org/10.5194/egusphere-egu24-9334, 2024.

The interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anticyclones over the western tropical Pacific, and the NTA-related anticyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and although the response becomes stronger in the partially coupled experiments, obvious differences still exist between the simulations and observation. The ESTA-induced atmospheric circulation response features an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Ni ñ a–like SST anomalies in the central–eastern equatorial Pacific,the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavorable to enhancing the ESTA–Pacific teleconnection during boreal summer

How to cite: Ren, H.: The Impact of Tropical Atlantic SST Variability on the Tropical Atmosphere duringBoreal Summer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9772, https://doi.org/10.5194/egusphere-egu24-9772, 2024.

EGU24-10200 | ECS | Orals | CL2.4 | Highlight

Roles of Tropical-Pacific Interannual–Interdecadal Variability in Forming the Super Long La Niña Events 

Run Wang, Hong-Li Ren, and Minghong Liu

The super long La Niña phenomenon, which has an extremely long duration, like the recent 2020–2023 La Niña event, is less concerned than the super El Niño. In this study, we identify five super long La Niña events after 1950 and investigate roles of the 2–3-year quasi-biennial (QB) and 3–7-year low-frequency (LF) ocean–atmosphere coupled processes of El Niño–Southern Oscillation (ENSO), and the interdecadal background in forming the basin-scale prolonged negative sea surface temperature anomalies (SSTAs) during these events. We group the five events into the thermocline-driven type (the 1983–1986 and 1998–2002 events) and the wind-driven type (the 1954–1957, 1973–1976, and 2020–2023 events). The former inherited a sufficiently discharged state of equatorial upper-ocean heat content from the preceding super El Niño and dominated by the thermocline feedback, leading to a LF oceanic dynamical adjustment to support the maintenance of negative ENSO SSTAs. The latter were promoted by the relatively more important zonal advective feedback and Ekman pumping feedback and deeply affected by a strongly negative equatorial zonal wind stress background state that sourced from the strong negative phase of the Interdecadal Pacific Oscillation. Besides, the QB ENSO variability with casual contributions during these events is less important. Results show that both the LF ENSO variability and the interdecadal Pacific background could assist to the genesis of such elongated La Niñas.

How to cite: Wang, R., Ren, H.-L., and Liu, M.: Roles of Tropical-Pacific Interannual–Interdecadal Variability in Forming the Super Long La Niña Events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10200, https://doi.org/10.5194/egusphere-egu24-10200, 2024.

EGU24-11374 | ECS | Orals | CL2.4 | Highlight

The El Niño response to tropical volcanic eruptions and geoengineering  

Clarissa Kroll and Robert Jnglin Wills

Following tropical volcanic eruptions and in response to geoengineering efforts in climate models, the occurrence of El Niño is notably enhanced. However, the precise mechanisms leading to the preference of the El Niño state remain a subject of ongoing debate. In this study, we explore the El Niño response within the context of stratospheric aerosol injection experiments using the Community Earth System Model version 1, with the Whole Atmosphere Community Climate Model atmospheric component (CESM1 WACCM). Our investigation is centered around the Stratospheric Aerosol Geoengineering Large Ensemble Dataset encompassing three distinct scenarios: a simulation of the RCP8.5 scenario as baseline climate change scenario, a geoengineering scenario, in which surface temperature increases are completely compensated and a scenario focusing solely on the stratospheric heating derived from the geoengineering approach. Our analysis reveals that the El Niño response is primarily linked to the heating in the tropical tropopause layer and lower stratosphere, and notably, it occurs independently of tropospheric cooling effects. We explain the increased occurrence of El Niño after volcanic eruptions and simulated geoengineering interventions by a slow down of the tropical atmospheric circulation, which is caused by increases in gross moist stability due to aerosol heating in tropical tropopause layer.

How to cite: Kroll, C. and Jnglin Wills, R.: The El Niño response to tropical volcanic eruptions and geoengineering , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11374, https://doi.org/10.5194/egusphere-egu24-11374, 2024.

EGU24-11643 | ECS | Orals | CL2.4

Dynamical systems analysis of the "El Niño Southern Oscillation" phenomenon  

Julia Mindlin, Gabriel B Mindlin, and Pedro di Nezio

Since the 1980s, when the World Meteorological Organization launched the TOGA (Tropical Ocean-Global Atmosphere Program) program, great advances have been made in understanding ENSO by studying a hierarchy of models (Dijkstra, 2005). At the most complex end of this hierarchy are the Global Climate Models (GCMs), with which simulations of the entire climate system are performed, while at the most elementary end are the simple dynamical models that involve the minimum number of modes necessary to generate the phenomenon and therefore represent the dominant physical processes. Conceptually, two different ways of understanding the irregular oscillations of ENSO are still valid: it could be either a self-sustained oscillator of a chaotic nature or a stable mode excited by atmospheric noise. 

In this work, we use methods from complex systems to revisit the ideas regarding two plausible dynamics of ENSO. We ask if the dynamics can be better represented as a self-sustained oscillator of a chaotic nature or a stable mode excited by noise. For this, we analyzed the sea surface temperatures (SSTs), one of the output variables of the simulations generated with GCMs, the most complex simulations available from the extended system. This temperature field averaged in a particular region of the eastern equatorial Pacific (Niño 3.4) gives rise to a temporal signal widely used for ENSO monitoring and as a proxy for the study of the oscillation. In order to analyze the dynamics of the system, we reconstruct the phase space from an embedding of the temporal signal. We find that three modes are enough to recover the ENSO dynamics of the extended system, in principle of infinite dimension. Our conceptual model is based on the existence of a self-sustaining oscillation with a critical slowing down in phase space; that is, the system traverses a region of phase oscillation with a critical slowing down in phase space; that is, the system traverses a region of phase space more slowly, and includes a periodic forcing that gives rise to chaotic behavior for certain values of the parameters. We validate the model with a topological and statistical analysis of the periodic orbits in the system and, in addition, we show that the complexity of the signal is better represented as a self-sustained oscillator of a chaotic nature than as a stable mode excited by noise (Wang, 2018).

Dijkstra, HA, Nonlinear Physical Oceanography, volume 28. Springer, 2nd revised edition, 2005.

Wang C., A review of ENSO theories, National Science Review, Volume 5, Issue 6, November 2018, Pages 813–825

How to cite: Mindlin, J., Mindlin, G. B., and di Nezio, P.: Dynamical systems analysis of the "El Niño Southern Oscillation" phenomenon , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11643, https://doi.org/10.5194/egusphere-egu24-11643, 2024.

EGU24-12873 | ECS | Orals | CL2.4

The Dynamics and Propagation of Westerly Wind Bursts 

Inko Bovenzi, Minmin Fu, and Eli Tziperman

Westerly wind bursts (WWBs), a westerly anomaly in equatorial winds in the Pacific, occur before every major El Niño event, yet major aspects of their mechanism are still not fully understood. Proposed mechanisms include cyclones approaching the equator, eastern-propagating convective heating, and wind-induced surface heat exchange, which amplifies WWBs near their peaks (Fu and Tziperman, 2019). To better understand WWB dynamics, we study their composite momentum budget using reanalysis and examine the role of convective heating and other factors. We find that many WWBs are not directly explained by nearby tropical cyclones or convective precipitation. We study their momentum budget before, during, and after the peak of the event, finding different balances at each stage. A comparison of the deduced balance to that in atmospheric general circulation climate models should add confidence in their ability to simulate this important factor in El Niño's development.

How to cite: Bovenzi, I., Fu, M., and Tziperman, E.: The Dynamics and Propagation of Westerly Wind Bursts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12873, https://doi.org/10.5194/egusphere-egu24-12873, 2024.

EGU24-12936 | Orals | CL2.4

A Regime View of ENSO Flavors Through Clustering in CMIP6 Models 

Pradeebane Vaittinada Ayar, David Battisti, Camille Li, Martin King, Mathieu Vrac, and Jerry Tjiputra

El Niño-Southern Oscillation (ENSO) flavors in the tropical Pacific are studied from a regime perspective. Five recurring spatial patterns or regimes characterizing the diversity of ENSO are established using a clustering approach applied to the HadISST sea surface temperature (SST) anomalies. Compared to previous studies, our approach gives a monthly characterization of the diversity of the warm and cold phases of ENSO established from observations but commonly applied to models and observations. Two warm (eastern and central El Niño), two cold (basin wide and central La Niña) and a neutral reference regimes are found. Simulated SST anomalies by the models from the latest Coupled Model Intercomparison Project Phase 6 are then matched to these reference regimes. This allows for a consistent assessment of the skill of the models in reproducing the reference regimes over the historical period and the change in these regimes under the high-warming Shared Socio-economic Pathway (SSP5.8.5) scenario. Results over the historical period show that models simulate well the reference regimes with some discrepancies. Models simulate more intense and spatially extended ENSO patterns and have issues in capturing the correct regime seasonality, persistence, and transition between regimes. Some models also have difficulty simulating the frequency of regimes, the eastern El Niño regime in particular. In the future, both El Niño and central La Niña regimes are expected to be more frequent accompanied with a less frequent neutral regime. The central Pacific El Niño and La Niña regimes are projected to increase in amplitude and variability. 
Reference:
Vaittinada Ayar, P.Battisti, D. S.Li, C.King, M.Vrac, M., & Tjiputra, J. (2023). A regime view of ENSO flavors through clustering in CMIP6 modelsEarth's Future11, e2022EF003460. https://doi.org/10.1029/2022EF003460

How to cite: Vaittinada Ayar, P., Battisti, D., Li, C., King, M., Vrac, M., and Tjiputra, J.: A Regime View of ENSO Flavors Through Clustering in CMIP6 Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12936, https://doi.org/10.5194/egusphere-egu24-12936, 2024.

In recent decades, a growing body of research has highlighted the intricate interplay between the El Niño-Southern Oscillation (ENSO) and various climatic patterns across multiple ocean basins. Several studies have highlighted the significance of the South Atlantic Subtropical Dipole (SASD) and its association with ENSO.

This investigation examines the interaction between SASD and ENSO, focusing on the critical role of the South Pacific High in these dynamics. Our study proposes that the onset of the South American Monsoon (SAM) plays a crucial role in this connection, challenging the traditional perception of land's passive role in tropical interbasin interactions.

We identified two eastern Pacific and two central Pacific ENSO precursors from SAM onset period using ERA5 reanalysis data along with 1200-year CESM2 PI run. Applying partial linear regressions revealed the following patterns: initially, warm Southwestern Tropical Atlantic (SWTA) and basin-wide low pressure in the equatorial and subequatorial Atlantic, evolving into cold Southeastern Tropical Pacific (boreal spring); then, negative South Pacific Oscillation (SPO) during the following boreal summer, culminating in La Niña conditions between 12 and 15 months later (SON and DJF of the following year).

We hypothesize that anomalous upper-level divergent monsoonal circulation acts as a bridge connecting the two ocean basins. Ekman dynamics arguably transfers and amplify atmospheric signals from the SAM and SPO to the equatorial Pacific Ocean.

Random Forest and Support Vector Machines for regression analysis yielded results consistent with those from the linear model; superior skill was noted in La Niña prediction compared to under-predicted El Niño events.

Moving forward, we intend to construct causal networks to disentangle the complex interplays described herein while ensuring independence from other known teleconnections; alternatively, we plan to design appropriate numerical experiments using coupled GCMs.

This study's preliminary results present exciting opportunities to enhance early ENSO prediction by considering the impact of the South American Monsoon on aligning the variability of the tropical South Atlantic and South Pacific oceans.

How to cite: Bellacanzone, F. and Bordoni, S.: Enhancing early ENSO prediction: how the South American Monsoon onset connects the South Atlantic Subtropical Dipole and the South Pacific Oscillation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13140, https://doi.org/10.5194/egusphere-egu24-13140, 2024.

EGU24-13513 | ECS | Posters on site | CL2.4 | Highlight

Impact of summer-persistent ENSO events on the global climate and the occurrence of extreme weather events 

Anna Schultze, Zhengyao Lu, Qiong Zhang, Minjie Zheng, and Thomas Pugh

El Niño Southern Oscillation (ENSO), the most prominent climate variability in the tropical Pacific Ocean, significantly influences global climate and weather patterns, impacting ecosystems and societies worldwide. Our study focuses on the underexplored aspect of summer-persistent ENSO events, their global climatic impacts, and their role in triggering extreme weather occurrence.

ENSO events follow a distinct cycle, with El Niños more tightly bound to this cycle, while some La Niñas tend to fall below the ENSO threshold during the summer and then re-intensify in the following winter, resulting in multi-year La Niña events. However, there have been cases of slower ENSO decay, where sea surface temperature anomalies (SSTA) exceeding the ENSO threshold values into the northern-hemisphere summer, have been observed. The 2018/2019 El Niño, persisting until July, is a recent example, linked to significant events like the severe Australian bushfires in 2020 and the longest heatwave in history in the North Pacific in 2019. The El Niño was followed by a triple-dip La Niña, linked to extreme weather events in Africa, Australia and the United States. This highlights the importance of understanding the summer-persistent ENSO events.

Our study is structured based on three aims: identifying past summer-persistent ENSO events, assessing their impacts on global temperature and precipitation patterns, and examining their linkage to extreme weather events. Utilizing the Oceanic Niño Index calculated from the extended reconstructed sea surface temperature (ERSSTv5), we categorised ENSO events into conventional, summer-persistent, and multi-year summer-persistent types. The latter two were defined by events in which the Oceanic Niño Index exceeded the ENSO threshold until June for one or two consecutives summer seasons, respectively. We identified 12 summer-persistent ENSO events since 1940, separated into four summer-persistent El Niños, five summer-persistent La Niñas, and three multi-year summer-persistent La Niñas. Analyzing ERA5 reanalysis composites of 2-m temperature and precipitation, we compared the climatic impacts of these ENSO variants across winter and summer. This study advances our understanding of the climatic consequences of summer-persistent ENSO events, providing insights crucial for developing mitigation strategies for their impacts on global climate and extreme weather occurrences.

How to cite: Schultze, A., Lu, Z., Zhang, Q., Zheng, M., and Pugh, T.: Impact of summer-persistent ENSO events on the global climate and the occurrence of extreme weather events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13513, https://doi.org/10.5194/egusphere-egu24-13513, 2024.

The El Niño Southern Oscillation (ENSO) dominates tropical climate variability. While it is defined by alterations in sea surface temperatures in the eastern and central tropical Pacific, ENSO influences temperature and precipitation patterns across the globe through a network of atmospheric and oceanic teleconnections. Whether ENSO is controlled or responds to external climate factors has long remained elusive, in large part due to the lack of paleoclimate evidence of tropical variability during different climate states. Here we utilize the geochemical signatures of planktic foraminifera to reconstruct eastern and central tropical variability during the last glacial maximum (LGM), some 20-25,000 years ago. Climate conditions during the LGM were very different, featuring atmospheric CO2 concentrations, global temperatures, and sea level all substantially lower than today. However, precessional forcing, thought to be a potential control on ENSO expression, was similar to modern orbital configuration. Our reconstruction spans the central and eastern tropical Pacific during this key time frame and assesses how the patterns of variability - or ENSO ‘flavors’ - may have changed. We compare our spatial reconstructions of variability to changes in the equatorial Pacific thermocline and test hypotheses of thermocline control of ENSO. We explore the evolution of the eastern and central Pacific thermocline, and how their relationship may be an additional factor in influencing ENSO expression. Our results provide key insights into the evolution and history of tropical variability under differing background climate states, providing context for modern ENSO behavior and prediction.

How to cite: Rustic, G., Rosenheim, E., Slotter, J., and Hill, K.: Reconstructing Tropical Pacific Variability During the Last Glacial Maximum Using Individual Foraminifera: An Investigation of ENSO Flavors , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13790, https://doi.org/10.5194/egusphere-egu24-13790, 2024.

EGU24-13992 | ECS | Orals | CL2.4

Oceans outside the tropical Pacific influence ENSO when ENSO predictability is poor 

Jemma Jeffree, Nicola Maher, Dillon Amaya, and Dietmar Dommenget

Various studies demonstrate that the El Niño Southern Oscillation is influenced by each of the Atlantic Ocean, Indian Ocean, extra-tropical Pacific Ocean and Southern Ocean. However, there is no cohesive picture of the relative importance of different ocean basins. Furthermore, even when considering only one basin, there is disagreement over the strength of it's influence on ENSO. Differences between previous studies likely arise from differences in their design. Untangling interbasin influences is non-trivial, due to  the need to distinguish between correlation and causation. Investigating these interbasin interactions is additionally complicated by model bias, and computational expense limiting the breadth of model studies.

We investigate the interbasin influences on ENSO from a new angle. We use analogue forecasting instead of initialised ensemble forecasting: we select analogues similar to some target state from a long model run (e.g. pre-industrial control or single model initial-condition large ensemble), rather than initialising from that target state. The analogue forecasts, made by following the selected analogues through time in the model run, have been previously evaluated to show similar skill to an initialised forecast. These forecasts are much faster than traditional initialised forecasts, allowing us to explore multiple models, lead times and initialisation months. We explore whether these analogue forecasts are improved by considering information from regions outside the tropical Pacific, and then infer how these regions contribute to ENSO evolution.

When ENSO forecasts are skilful, before the Spring Predictability Barrier, outside influences have little impact on ENSO forecast skill. When ENSO forecasts cross the Spring Predictability Barrier and are poor, then considering information from outside the Tropical Pacific Ocean improves forecasts. We conclude that when ENSO is in a growth phase it dominates the climate system, but in a decay phase ENSO is influenced by regions outside the tropical Pacific. This behaviour is consistent across at least two global coupled climate models, despite large variability in the way these models represent ENSO's seasonal evolution. We intend to expand this investigation to more models, and to compare the impacts of verifying forecasts against observational or model data.

How to cite: Jeffree, J., Maher, N., Amaya, D., and Dommenget, D.: Oceans outside the tropical Pacific influence ENSO when ENSO predictability is poor, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13992, https://doi.org/10.5194/egusphere-egu24-13992, 2024.

EGU24-15294 | ECS | Orals | CL2.4

Towards a better understanding of ENSO diversity: a paleoclimate perspective 

Isma Abdelkader Di Carlo, Pascale Braconnot, Matthieu Carré, Mary Elliot, and Olivier Marti

El Niño-Southern Oscillation (ENSO) events are hard to put in one category because they differ in intensity, spatial pattern, and temporal evolution. Studies have characterized events into two main categories: central Pacific (CP) and eastern Pacific (EP) events. The indicators used to compute EP and CP events are varied, from area-averaged regions to Empirical Orthogonal Function (EOF) analysis. In the recent climatic period, they all show similar results. However, future projections show differing results when using two different methods of computing EP and CP events. Since the observational period is too short, we use paleoclimate reconstructions, which provide unique and quantitative measures of past climate changes over long time scales. We will first synthesize previous studies and discuss how they have used paleoclimate modeling and/or data to provide clues into how ENSO diversity may have been shaped in past climates. Our results indicate that many apparent inconsistencies in future projection studies are due to misleading use of ENSO diversity indicators and that investigating ENSO diversity with a climate change perspective requires assessing both changes in the climate mean state (annual mean and seasonality) and changes in variability. 

How to cite: Abdelkader Di Carlo, I., Braconnot, P., Carré, M., Elliot, M., and Marti, O.: Towards a better understanding of ENSO diversity: a paleoclimate perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15294, https://doi.org/10.5194/egusphere-egu24-15294, 2024.

EGU24-17071 | ECS | Posters on site | CL2.4

Present and future of Extreme El Niño teleconnections over North America in CMIP6 models 

Margot Beniche, Jérôme Vialard, and Matthieu Lengaigne

Previous studies did suggest a diversity of the ENSO teleconnection pattern, with an eastward shifted pattern for El Niño relative to La Niña or for “eastern Pacific” (EP) relative to “central Pacific” (CP) El Niño events. Recently, Beniche et al. (in revision) demonstrated that extreme El Niño events (i.e. the strongest EP events, such as those in 1982/83, 1997/98, and 2015/16) were the only events leading to a clear eastward shift of the winter ENSO teleconnection pattern over North America. This specific teleconnection is also associated with reproducible wet (warm) anomalies over the western USA coast (northern USA and Canada). They did however demonstrate it based on the limited observational dataset, and a single AMIP CNRM-CM6.1 ensemble.

The current study aims at evaluating the robustness of these results using the broader AMIP6 and CMIP6 datasets. The specificity of the Extreme El Niño North American winter teleconnection pattern, and its inter-event and inter-member reproducibility, are robust across 23 historical AMIP ensembles (1979-2014). These events are associated with 73% chances of warm conditions over the Northern USA and Canada and 68% chances of wet conditions over the Western US coast across the AMIP ensemble. The stronger reproducibility of the extreme El Niño teleconnections can be explained by a more favourable Signal to Noise (SNR) ratio (mainly due to stronger signal).

We further evaluate the realism of these teleconnections patterns in presence of the systematic biases that are present in CMIP6. We only select CMIP6 models that reproduce Extreme El Niño events based on the precipitation-index of Cai et al. (2014). In agreement with previous studies using CMIP5 (e.g. Bayr et al., 2019), we find that models with stronger cold climatological SST bias are unable to simulate extreme Niño3 rainfall anomaly events. CMIP6 models that reproduce extreme El Niño tropical rainfall reasonably also reproduce the specific extreme El Niño 500 hPa geopotential height and surface temperature winter teleconnection pattern over North America. They however do not reproduce well the specific wet anomalies over the west American coast associated with those events, casting doubt on the CMIP6 ability to project precipitation changes over this region. We end by discussing the relevance of these results for understanding projected changes in ENSO teleconnections over North America in the context of different Shared Socioeconomic Pathways (SSPs) scenarii.

How to cite: Beniche, M., Vialard, J., and Lengaigne, M.: Present and future of Extreme El Niño teleconnections over North America in CMIP6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17071, https://doi.org/10.5194/egusphere-egu24-17071, 2024.

EGU24-17210 | ECS | Posters on site | CL2.4 | Highlight

Crying wolf with the 2023 El Niño: a predicted event that failed to materialize? 

Sandro Carniel, Gian Luca Eusebi Borzelli, Aniello Russo, and Cosimo Enrico Carniel

The El Niño–Southern Oscillation (ENSO) is a phenomenon that involves the redistribution of heat in the tropical Pacific Ocean, resulting in irregular oscillations in the sea surface temperature (SST) between warm (El Niño) and cold (La Niña) phases, and impacting the global planetary climate. In July 2023 the World Meteorological Organization, formally responsible to declare the onset of El Niño, officially announced its onset to the media, urging governments to prepare for potential high impacts on health, ecosystems and economies. However, the analysis of long-term meteorological and oceanographic data updated to the end of 2023 shows that while the eastern Pacific was warmer than normal in the second half of the year, the overall configuration of the tropical Pacific climate system did not indicate a strong El Niño event. Our findings show that the 2023-24 El Niño event, initially predicted to be at least moderate and possibly strong, turned out to be weak and, de facto, the year closed confirming it as a weaker than expected event. Based on historical records, we hypothesize that the state of the Pacific climate system at the end of 2023, following the unusual 2023-24 El Niño, may lead to the development of a strong or very strong El Niño by mid-2024.

How to cite: Carniel, S., Eusebi Borzelli, G. L., Russo, A., and Carniel, C. E.: Crying wolf with the 2023 El Niño: a predicted event that failed to materialize?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17210, https://doi.org/10.5194/egusphere-egu24-17210, 2024.

EGU24-20761 | ECS | Orals | CL2.4

Visualizing the transition from LaNiño to ElNiño from NASA's model outputs 

Atousa Saberi and Gregory Shirah

The ENSO affects global weather. We used NASA GEOS Subseasonal to Seasonal (S2S) Coupled ocean-atmosphere model, NASA MERRA‐2 reanalysis, along with NOAA Niño3.4 SST anomaly index time series to visualize the transition from  LaNiño 2021 to ElNiño 2023. The visualization is a comprehensive model explainer showing changes in the top 300 meters of the Pacifc Ocean (such as thermocline flattening, movements of the temperature anomalies) coupled with the Walker Circulation and the continous coupled interaction between the ocean and the atmosphere. It's the first effort in visualizing the Walker Circulation and the moving convective branch across the Pacific without schematic plots but rather with climate model outputs.  We will also cover the effect of the two phases of ENSO on the global weather pattern. This visualization will be narrated and released to the public in the future.

How to cite: Saberi, A. and Shirah, G.: Visualizing the transition from LaNiño to ElNiño from NASA's model outputs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20761, https://doi.org/10.5194/egusphere-egu24-20761, 2024.

EGU24-21415 | Posters on site | CL2.4

How closely related are the Interdecadal Pacific Oscillation and El Niño-Southern Oscillation? 

Tim Cowan, Hanna Heidemann, Scott B. Power, and Benjamin J. Henley

Sea surface temperature (SST) patterns in the Pacific Ocean cause climate variability in many parts of the world. This is due to the El Niño-Southern Oscillation (ENSO) on interannual timescales and the Interdecadal Pacific Oscillation (IPO) acting on decadal to interdecadal timescales, modifying ENSO teleconnections. However, how both ENSO, ENSO diversity and the IPO interact with each other still requires further clarification. In this study, we use observations of Pacific Ocean SSTs from 1920 to 2022 to explore the statistical relationships between decadal ENSO variability and the IPO. More specifically, we show how ENSO event characteristics of both central and eastern Pacific El Niño, as well as all La Niña events varies between their occurrence in warm (positive), compared to cool (negative) phases of the IPO. We further show that up to 60% of the variability in the IPO Tripole Index can be reconstructed by using simple ENSO metrics such as the relative frequency of El Niño and La Niña events. While statistically a clear relationship between ENSO and the IPO exists, some of the IPO’s key features, especially North Pacific SSTs, cannot be explained by decadal ENSO variability.  

How to cite: Cowan, T., Heidemann, H., Power, S. B., and Henley, B. J.: How closely related are the Interdecadal Pacific Oscillation and El Niño-Southern Oscillation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21415, https://doi.org/10.5194/egusphere-egu24-21415, 2024.

Monsoon rainfall and year-to-year variability play an important role in Africa’s energy, agriculture, and other societal sectors. Within the African continent, east African countries are affected much by higher degrees of variability in seasonal monsoon precipitation. Two large-scale climate drivers, the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) are studied in this regard. A strong connection starting from a season ahead is identified for early austral summer (Oct-Nov-Dec, OND) monsoonal rain in eastern Africa.  This has been examined using various data sources, detrending data beforehand, analysing either recent or earlier time periods - covering two decades each, and using the analyses of regression. Results of compositing also suggested a strong significant anomaly in OND rain covering that region of east Africa (named here as region A:18˚S-12˚N, 25˚E-52˚E).  When IOD and ENSO are both negative in July-August-September(JAS) there is a significant deficit in OND rainfall, while an excess rain when both are positive. The Walker circulation plays a key role via altering descending and ascending branches in two circumstances. Based on this analysis, it is possible to deliver an estimation of cumulative rain in terms of median value, range and distribution, one season in advance, at a point location or average over a region. Results are further verified for recent two years of 2022 and 2023, where drivers were of same sign, either both negative (2022) or positive (2023). Classifications based on two drivers, starting from JAS, are not only modulating cumulative rain but also influencing onset dates; excess (deficit) rain and early (late) onset are associated with positive (negative) phases of both drivers. Interestingly, regions of east Africa, south of that box region show a complete reverse pattern in OND and that pattern continues till Dec-Jan-Feb. In terms of mechanisms, apart from Walker circulation, ocean also plays a key part.      

            Some results of compositing are confirmed for longer records (1940-2021) too and further classification of drivers, based on a threshold value (+0.4) is tested. In the recent year 2023, as both drivers were strongly positive in JAS, more analyses in such cases are presented.  We note, if either of the drivers is weak positive and lies in the range of 0 to +.04, the signal in region A weakens substantially on the eastern side of the box. The strongest weakening happens when both the drivers are of low magnitude in JAS (i.e.,  between 0 to +0.4). Rainfall (OND) variability of region A, at intra-decadal, decadal and multi-decadal scales are studied by applying the method of centered moving averages of 5-year, 11-year and 21-year respectively. A decreasing trend is noted in all situations and major peak/trough years are identified. For multi-decadal analyses, a shift at around 1958 is identified when the trend of OND rain is reversed and switched from increasing to decreasing. Our results have implications for future planning in optimizing energy and agricultural outputs and the livelihood of millions of east Africans will be impacted.   

How to cite: Roy, I. and Troccoli, A.: Important drivers of October to December rainfall season in eastern Africa and relevant mechanisms, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21764, https://doi.org/10.5194/egusphere-egu24-21764, 2024.

The latest assessment report (AR6) of the Intergovernmental Panel on Climate Change includes a new element to climate research, i.e. the Interactive Atlas (IA), which is very useful for users from different sectors. As the new CMIP6 global climate model simulations use the brand-new SSP-scenarios paired with the RCP-scenarios, the latest climate change projections should be evaluated in order to update the regional and national adaptation strategies. Keeping this in mind we focused on Europe, with a special emphasis on Hungary in our study.

Our aim was to analyse the potential future changes of different temperature indices for Europe, in order to recognize spatial patterns and trends that may shape our climate in the second half of the 21st century. For this purpose, multi-model mean simulation data provided by the IPCC AR6 WG1 IA were downloaded on a monthly base. We chose two climate indices beside the mean temperature values, which represent temperature extremes, namely, the number of days with maximum temperature above 35 °C and the number of frost days (i.e. when daily minimum temperature is below 0 °C). We focused on the end of the 21st century (2081–2100) with also briefly considering the medium-term changes of the 2041–2060 period (both compared to the last two decades of the historical simulation period, i.e. 1995–2014 as the reference period). For both future periods we used all scenarios provided in the IA, namely, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5.

Several zonal and meridional segments over the continent were defined, where we analysed the projected changes of the indices. The zonal segments provide an insight on two different effects that may induce spatial differences between future regional changes. (i) Continentality can be recognized as an increasing effect from the western parts of the segment towards the east. (ii) Topography also appears as the influence of mountains, plains, and basins emerge. The meridional segments provide information about the north-to-south differences as well, as the effects of sea cover. The changes in the indices are plotted on diagrams representing the different months, where the differences in the scenarios are also shown. These diagrams are compared to their respective landscape profiles, furthermore, statistical parameters were calculated. In addition, a monotony index was defined as the cumulative direction of differences between the neighbouring grid cells and analysed within the study.

Our results show that in the changes of mean temperature, both the zonal location and sea cover will play a key role in forming spatial differences within Europe. However, for the extreme temperature indices, topography and continentality are likely to become more dominant than sea cover, while the zonal location remains an important factor. 

Acknowledgements: This work was supported by the Hungarian National Research, Development and Innovation Fund [grant numbers PD138023, K-129162], and the National Multidisciplinary Laboratory for Climate Change [grant number RRF-2.3.1-21-2022-00014]. 

How to cite: Divinszki, F., Kis, A., and Pongrácz, R.: Analysing the projected monthly changes of temperature-related climate indices over Europe using zonal and meridional segments based on CMIP6 data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-389, https://doi.org/10.5194/egusphere-egu24-389, 2024.

EGU24-868 | ECS | Posters on site | CL4.3

Relationship of the predictability of North Pacific Mode and ENSO with predictability of PDO 

Jivesh Dixit and Krishna M. AchutaRao

PDO and ENSO are most prominent variability modes in the Pacific Ocean at decadal and interannual timescales respectively. Mutual independence between ENSO and PDO is questionable (Chen & Wallace, 2016). Linear combination of the first two orthogonal modes of SST variability in our Study Region (SR; 70oN - 20oS, 110oE - 90oW) i.e. mode 1 (interannual mode, we call it, IAM; ENSO like variability) and mode 2 (North Pacific Mode (NPM; Deser & Blackmon (1995)); a decadal mode) produces a PDO like variability (Chen & Wallace, 2016). It suggests that PDO is not independently hosted in the Pacific Ocean and can be represented by two linearly independent variability modes.

To produce credible and skillful climate information at multi-year to decadal timescales, Decadal Climate Prediction Project (DCPP), led by the Working Group on Subseasonal to Interdecadal Prediction (WGSIP), focuses on both the scientific and practical elements of forecasting climate by employing predictability research and retrospective analyses within the Coupled Model Intercomparison Project Phase 6 (CMIP6). Component A under DCPP experiments concentrates on hindcast experiments to examine the prediction skill of participating models with respect to actual observations.

As linear combination of  IAM and NPM in SR produces PDO pattern and timescales efficiently, we compared the  ability of DCPP-A hindcasts to predict  IAM, NPM, and  PDO. In this analysis we use output from 9 models (a total of 128 ensemble members), initialised every year from 1960 to 2010. To produce the prediction skill estimates.

At lead year 1 from initialisation, the prediction of NPM,  IAM and PDO is quite skillful as the models are initialised with observations. In subsequent years, skill of either IAM or NPM or both drop significantly and that leads to drop in skill of predicted PDO index. Both the deterministic estimates and probabilistic estimates of prediction skill for DCPP hindcast experiments suggest that the ability of hindcast experiments to predict NPM governs the prediction skill to predict PDO index.

Keywords: PDO, ENSO, NPM, CMIP6, DCPP, hindcast

References

Chen, X., & Wallace, J. M. (2016). Orthogonal PDO and ENSO indices. Journal of Climate, 29(10), 3883–3892. https://doi.org/10.1175/jcli-d-15-0684.1

Deser, C., & Blackmon, M. L. (1995). On the Relationship between Tropical and North Pacific Sea Surface Temperature Variations. Journal of Climate, 8(6), 1677–1680. https://doi.org/10.1175/1520-0442(1995)008<1677:OTRBTA>2.0.CO;2

How to cite: Dixit, J. and AchutaRao, K. M.: Relationship of the predictability of North Pacific Mode and ENSO with predictability of PDO, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-868, https://doi.org/10.5194/egusphere-egu24-868, 2024.

EGU24-1757 | Posters on site | CL4.3

Is the NAO signal-to-noise paradox exacerbated by severe winter windstorms? 

Lisa Degenhardt, Gregor C. Leckebusch, Adam A. Scaife, Doug Smith, and Steve Hardiman

The signal-to-noise paradox is known to be a limitation in multiple seasonal and decadal forecast models where the model ensemble mean predicts observations better than individual ensemble members. This ‘paradox’ occurs for different parameters, like the NAO, temperature, wind speed or storm counts in multiple seasonal and decadal forecasts. However, investigations have not yet found the origin of the paradox. First hypotheses are that weak ocean – atmosphere coupling or a misrepresentation of eddy feedback in these models is responsible.

Our previous study found a stronger signal-to-noise error in windstorm frequency than for the NAO despite highly significant forecast skill. In combination with the underestimation of eddy feedback in multiple models, this led to the question: Might the signal-to-noise paradox over the North-Atlantic be driven by severe winter windstorms?

To assess this hypothesis, the signal-to-noise paradox is investigated in multiple seasonal forecast suites from the UK Met Office, ECMWF, DWD and CMCC. The NAO is used to investigate the changes in the paradox depending on the storminess of the season. The results show a significant increase of the NAO-signal-to-noise error in stormy seasons in GloSea5. Other individual models like the seasonal model of the DWD or CMCC do not show such a strong difference. A multi-model approach, on the other hand, shows the same tendency as GloSea5. Nevertheless, these model differences mean that more hindcasts are needed to conclusively demonstrate that the signal-to-noise error arises from Atlantic windstorms.

How to cite: Degenhardt, L., Leckebusch, G. C., Scaife, A. A., Smith, D., and Hardiman, S.: Is the NAO signal-to-noise paradox exacerbated by severe winter windstorms?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1757, https://doi.org/10.5194/egusphere-egu24-1757, 2024.

EGU24-1940 | ECS | Orals | CL4.3

Study of the Decadal Predictability of Mediterranean Sea Surface Temperature Based on Observations 

Xiaoqin Yan, Youmin Tang, and Dejian Yang

Sea surface temperature (SST) changes in the Mediterranean Sea have profound impacts on both the Mediterranean regions and remote areas. Previous studies show that the Mediterranean SST has significant decadal variability that is comparable with the Atlantic multidecadal variability (AMV). However, few studies have discussed the characteristics and sources of the decadal predictability of Mediterranean SST based on observations. Here for the first time we use observational datasets to reveal that the decadal predictability of Mediterranean SST is contributed by both external forcings and internal variability for both annual and seasonal means, except that the decadal predictability of the winter mean SST in the eastern Mediterranean is mostly contributed by only internal variability. Besides, the persistence of the Mediterranean SST is quite significant even in contrast with that in the subpolar North Atlantic, which is widely regarded to have the most predictable surface temperature on the decadal time scale. After the impacts of external forcings are removed, the average prediction time of internally generated Mediterranean SST variations is more than 10 years and closely associated with the multidecadal variability of the Mediterranean SST that is closely related to the accumulated North Atlantic Oscillation forcing.

How to cite: Yan, X., Tang, Y., and Yang, D.: Study of the Decadal Predictability of Mediterranean Sea Surface Temperature Based on Observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1940, https://doi.org/10.5194/egusphere-egu24-1940, 2024.

EGU24-3190 | ECS | Orals | CL4.3

Seasonal forecasting of the European North-West shelf seas: limits of winter and summer sea surface temperature predictability 

Jamie Atkins, Jonathan Tinker, Jennifer Graham, Adam Scaife, and Paul Halloran

The European North-West shelf seas (NWS) support economic interests and provide environmental services to several adjacent populous countries. Skilful seasonal forecasts of the NWS would be useful to support decision making. Here, we quantify the skill of an operational large-ensemble ocean-atmosphere coupled dynamical forecasting system (GloSea), as well as a benchmark persistence forecasting system, for predictions of NWS sea surface temperature (SST) at 2-4 months lead time in winter and summer. We also identify sources of- and limits to NWS SST predictability with a view to what additional skill may be available in the future. We find that GloSea NWS SST skill is generally high in winter and low in summer. Persistence of anomalies in the initial conditions contributes substantially to predictability. GloSea outperforms simple persistence forecasts, by adding atmospheric variability information, but only to a modest extent. Where persistence is low – for example in seasonally stratified regions – both GloSea and persistence forecasts show lower skill. GloSea skill can be degradeded by model deficiencies in the relatively coarse global ocean component, which lacks a tidal regime and likely fails to properly fine-scale NWS physics. However, using “near perfect atmosphere” tests, we show potential for improving predictability of currently low performing regions if atmospheric circulation forecasts can be improved, underlining the importance of development of atmosphere-ocean coupled models for NWS seasonal forecasting applications.

How to cite: Atkins, J., Tinker, J., Graham, J., Scaife, A., and Halloran, P.: Seasonal forecasting of the European North-West shelf seas: limits of winter and summer sea surface temperature predictability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3190, https://doi.org/10.5194/egusphere-egu24-3190, 2024.

EGU24-4538 | ECS | Orals | CL4.3

Statistical downscaling of extremes in seasonal predictions - a case study on spring frosts for the viticultural sector 

Sebastiano Roncoroni, Panos Athanasiadis, and Silvio Gualdi

Spring frost events occurring after budburst of grapevines can damage new shoots, disrupt plant growth and cause large economic losses to the viticultural sector. Frost protection practices encompass a variety of vineyard management actions across timescales, from seasonal to decadal and beyond. The cost-effectiveness of such measures depends on the availability of accurate predictions of the relevant climate hazards at the appropriate timescales.

In this work, we present a statistical downscaling method which predicts variations in the frequency of occurrence of spring frost events in the important winemaking region of Catalunya at the seasonal timescale. The downscaling method exploits the seasonal predictability associated with the predictable components of the atmospheric variability over the Euro-Atlantic region, and produces local predictions of frost occurrence at a spatial scale relevant to vineyard management.

The downscaling method is designed to address the specific needs highlighted by a representative stakeholder in the local viticultural sector, and is expected to deliver an actionable prototype climate service. The statistical procedure is developed in perfect prognosis mode: the method is trained with large-scale reanalysis data against a high-resolution gridded observational reference, and validated against multi-model seasonal hindcast predictions.

Our work spotlights the potential benefits of transferring climate predictability across spatial scales for the design and provision of usable climate information, particularly regarding extremes.

How to cite: Roncoroni, S., Athanasiadis, P., and Gualdi, S.: Statistical downscaling of extremes in seasonal predictions - a case study on spring frosts for the viticultural sector, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4538, https://doi.org/10.5194/egusphere-egu24-4538, 2024.

EGU24-4873 | ECS | Orals | CL4.3

Why does the Signal-to-Noise Paradox Exist in Seasonal Climate Predictability? 

Yashas Shivamurthy, Subodh Kumar Saha, Samir Pokhrel, Mahen Konwar, and Hemant Kumar Chaudhari

Skillful prediction of seasonal monsoons has been a challenging problem since the 1800s. However, significant progress has been made in Indian summer monsoon rainfall prediction in recent times, with skill scores reaching 0.6 and beyond, surpassing the estimated predictability limits. This phenomenon leads to what is known as the “Signal-to-noise Paradox.” To investigate this paradox, we utilized 52 ensemble member hindcast runs spanning 30 years.

Through the application of ANOVA and Mutual Information methods, we estimate the predictability limit globally. Notably, for the boreal summer rainfall season, the Indian subcontinent exhibited the paradox, among several other regions, while the Equatorial Pacific region, despite demonstrating high prediction skill, does not have the Signal-to-Noise paradox. We employed a novel approach to understand how sub-seasonal variability and their projection in association with predictors are linked to the paradoxical behavior of seasonal prediction skill.

We propose a new method to estimate predictability limits that is free from paradoxical phenomena and shows much higher seasonal predictability. This novel method provides valuable insights into the complex dynamics of monsoon prediction, thereby creating opportunities for expanded research and potential improvements in seasonal forecasting skill in the coming years.

How to cite: Shivamurthy, Y., Saha, S. K., Pokhrel, S., Konwar, M., and Chaudhari, H. K.: Why does the Signal-to-Noise Paradox Exist in Seasonal Climate Predictability?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4873, https://doi.org/10.5194/egusphere-egu24-4873, 2024.

EGU24-7134 | ECS | Orals | CL4.3

Towards the Predictability of Compound Dry and Hot Extremes through Complexity Science 

Ankit Agarwal and Ravikumar Guntu

Compound Dry and Hot Extremes (CDHE) have an adverse impact on socioeconomic factors during the Indian summer monsoon, and a future exacerbation is anticipated. The occurrence of CDHE is influenced by teleconnections, which play a crucial role in determining its likelihood on a seasonal scale. Despite the importance, there is a lack of studies unravelling the teleconnections of CDHE in India. Previous investigations specifically focused on teleconnections between precipitation, temperature, and climate indices. Hence, there is a need to unravel the teleconnections of CDHE. This study presents a framework combining event coincidence analysis (ECA) with complexity science. ECA evaluates the synchronization between CDHE and climate indices. Subsequently, complexity science is utilized to construct a driver-CDHE network to identify the critical drivers of CDHE. A logistic regression model is employed to evaluate the proposed drivers' effectiveness. The occurrence of CDHE exhibits distinct patterns from July to September when considering intra-seasonal variability. Our findings contribute to the identification of drivers associated with CDHE. The primary driver for Eastern, Western India and Central India is the indices in the Pacific Ocean and Atlantic Ocean, respectively, followed by the indices in the Indian Ocean. These identified drivers outperform the traditional Niño 3.4-based predictions. Overall, our results demonstrate the effectiveness of integrating ECA and complexity science to enhance the prediction of CDHE occurrences.

How to cite: Agarwal, A. and Guntu, R.: Towards the Predictability of Compound Dry and Hot Extremes through Complexity Science, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7134, https://doi.org/10.5194/egusphere-egu24-7134, 2024.

EGU24-8028 | ECS | Orals | CL4.3

Constraining near to mid-term climate projections by combining observations with decadal predictions 

Rémy Bonnet, Julien Boé, and Emilia Sanchez

The implementation of adaptation policies requires seamless and relevant information on the evolution of the climate over the next decades. Decadal climate predictions are subject to drift because of intrinsic model errors and their skill may be limited after a few years or even months depending on the region. Non-initialized ensembles of climate projections have large uncertainties over the next decades, encompassing the full range of uncertainty attributed to internal climate variability. Providing the best climate information over the next decades is therefore challenging. Recent studies have started to address this challenge by constraining uninitialized projections of sea surface temperature using decadal predictions or using a storyline approach to constrain uninitialized projections of the Atlantic Meridional Overturning Circulation using observations. Here, using a hierarchical clustering method, we select a sub-ensemble of non-initialized climate simulations based on their similarity to observations. Then, we try to further refine this sub-ensemble of trajectories by selecting a subset based on its consistency with decadal predictions. This study presents a comparison of these different methods for constraining surface temperatures in the North-Atlantic / Europe region over the next decades, focusing on CMIP6 non-initialized simulations.

How to cite: Bonnet, R., Boé, J., and Sanchez, E.: Constraining near to mid-term climate projections by combining observations with decadal predictions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8028, https://doi.org/10.5194/egusphere-egu24-8028, 2024.

EGU24-9049 | Posters on site | CL4.3

Constraining internal variability in CMIP6 simulations to provide skillful near-term climate predictions 

Rashed Mahmood, Markus G. Donat, Pablo Ortega, and Francisco Doblas-Reyes

Adaptation to climate change requires accurate and reliable climate information on decadal and multi-decadal timescales. Such near-term climate information is obtained from future projection simulations, which are strongly affected by uncertainties related to, among other things, internal climate variability. Here we present an approach to constrain variability in future projection simulations of the coupled model intercomparison project phase 6 (CMIP6). The constraining approach involves phasing in the simulated with the observed climate state by evaluating the area-weighted spatial pattern correlations of sea surface temperature (SST) anomalies in individual members and observations. The constrained ensemble, based on the top ranked members in terms of pattern correlations with observed SST anomalies, shows significant added value over the unconstrained ensemble in predicting surface temperature 10 and also 20 years  after the synchronization with observations, thus extending the forecast range of the standard initialised predictions. We also find that while the prediction skill of the constrained ensemble for the first ten years is similar to the initialized decadal predictions, the added value against the unconstrained ensemble extends over more regions than the decadal predictions. In addition, the constraining approach can also be used to attribute predictability of regional and global climate variations to regional SST variability.

How to cite: Mahmood, R., G. Donat, M., Ortega, P., and Doblas-Reyes, F.: Constraining internal variability in CMIP6 simulations to provide skillful near-term climate predictions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9049, https://doi.org/10.5194/egusphere-egu24-9049, 2024.

There is an ongoing discussion about the contributions from forced and natural sources to the Atlantic Multi-decadal Variability (AMV).  As the AMV influences the general climate in large regions, this question has important consequences for climate predictions on decadal timescales and for a robust estimation of the influence of climate forcings.

Here, we investigate the Atlantic Multi-decadal Variability (AMV) in observations and in a large CMIP6 historical climate model ensemble. We compare three different definitions of the AMV aimed at extracting the variability intrinsic to the Atlantic region. These definitions are based on removing from the Atlantic temperature the non-linear trend, the part congruent to the global average, or the part congruent to the multi-model ensemble mean of the global average. The considered AMV definitions agree on the well-known low-frequency oscillatory variability in observations, but show larger differences for the models. In general, large differences between ensemble members are found.

We estimate the forced response in the AMV as the mean of the large multi-model ensemble.  The forced response resembles the observed low-frequency oscillatory variability for the detrended AMV definition, but this definition is also the most inefficient in removing the forced global mean signal. The forced response is very weak for the other definitions and only few of their individual ensemble members show oscillatory variability and, if they do, not with the observed phase.

The observed spatial temperature pattern related to the AMV is well captured for all three AMV definitions, but with some differences in the spatial extent. The observed instantaneous connection between NAO and AMV is well represented in the models for all AMV definitions. Only non-significant evidence of NAO leading the AMV on decadal timescales is found.

How to cite: Christiansen, B., Yang, S., and Drews, A.: The Atlantic Multi-decadal Variability in observations and in a large historical multi-model ensemble: Forced and internal variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9100, https://doi.org/10.5194/egusphere-egu24-9100, 2024.

EGU24-9274 | ECS | Orals | CL4.3 | Highlight

The Role of the North Atlantic for Heat Wave Characteristics in Europe 

Sabine Bischof, Robin Pilch Kedzierski, Martje Hänsch, Sebastian Wahl, and Katja Matthes

The recent severe European summer heat waves of 2015 and 2018 co-occurred with cold subpolar North Atlantic (NA) sea surface temperatures (SSTs). However, a significant connection between this oceanic state and European heat waves was not yet established.

We investigate the effect of cold subpolar NA SSTs on European summer heat waves using two 100-year long AMIP-like model experiments: one that employs the observed global 2018 SST pattern as a boundary forcing and a counter experiment for which we removed the negative NA SST anomaly from the 2018 SST field, while preserving daily and small-scale SST variabilities. Comparing these experiments, we find that cold subpolar NA SSTs significantly increase heat wave duration and magnitude downstream over the European continent. Surface temperature and circulation anomalies are connected by the upper-tropospheric summer wave pattern of meridional winds over the North Atlantic European sector, which is enhanced with cold NA SSTs. Our results highlight the relevance of the subpolar NA region for European summer conditions, a region that is marked by large biases in current coupled climate model simulations.

How to cite: Bischof, S., Pilch Kedzierski, R., Hänsch, M., Wahl, S., and Matthes, K.: The Role of the North Atlantic for Heat Wave Characteristics in Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9274, https://doi.org/10.5194/egusphere-egu24-9274, 2024.

EGU24-9690 | ECS | Orals | CL4.3

Hybrid statistical-dynamical seasonal prediction of summer extreme temperatures over Europe 

Luca Famooss Paolini, Paolo Ruggieri, Salvatore Pascale, Erika Brattich, and Silvana Di Sabatino

Several studies show that the occurrence of summer extreme temperatures over Europe is increased since the middle of the twentieth century and is expected to further increase in the future due to global warming (Seneviratne et al., 2021). Thus, predicting heat extremes several months ahead is crucial given their impacts on socio-economic and environmental systems.

In this context, state-of-the-art dynamical seasonal prediction systems (SPSs) show low skills in predicting European heat extremes on seasonal timescale, especially in central and northern Europe (Prodhomme et al., 2022). However, recent studies have shown that our skills in predicting extratropical climate can be largely improved by subsampling the dynamical SPS ensemble with statistical post-processing techniques (Dobrynin et al., 2022).

This study assesses if the seasonal prediction skill of summer extreme temperatures in Europe in the state-of-the-art dynamical SPSs can be improved through subsampling. Specifically, we use a multi-model ensemble (MME) of SPSs contributing to the Copernicus Climate Change Service (C3S), analysing di hindcast period 1993—2016. The MME is subsampled by retaining a subset of members that predict the phase of the North Atlantic Oscillation (NAO) and the Eastern Atlantic (EA), typically linked to summer extreme temperatures in Europe. The subsampling relies on spring predictors of the weather regimes and thus allows us to retain only those ensemble members with a reasonable representation of summer heat extreme teleconnections.

Results show that by retaining only those ensemble members that accurately represent the NAO phase, it not only enhances the seasonal prediction skills for the summer European climate but also leads to improved predictions of summer extreme temperatures, especially in central and northern Europe. Differently, selecting only those ensemble members that accurately represent the EA phase does not improve either the predictions of summer European climate or the predictions of summer extreme temperatures. This can be explained by the fact that the C3S SPSs exhibits deficiencies in accurately representing the summer low-frequency atmospheric variability.

Bibliography

Dobrynin, M., and Coauthors, 2018: Improved Teleconnection-Based Dynamical Seasonal Predictions of Boreal Winter. Geophysical Research Letters, 45 (8), 3605—3614, https://doi.org/10.1002/2018GL07720

Prodhomme, C., S. Materia, C. Ardilouze, R. H. White, L. Batté, V. Guemas, G. Fragkoulidis, and J. Garcìa-Serrano, 2022: Seasonal prediction of European summer heatwaves. Climate Dynamics, 58 (7), 2149—2166, https://doi.org/10.1007/s00382-021-05828-3

Seneviratne, S., and Coauthors, 2021: Weather and Climate Extreme Events in a Changing Climate, chap. 11, 1513—1766. Cambridge University Press, https://doi.org/10.1017/9781009157896.013

How to cite: Famooss Paolini, L., Ruggieri, P., Pascale, S., Brattich, E., and Di Sabatino, S.: Hybrid statistical-dynamical seasonal prediction of summer extreme temperatures over Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9690, https://doi.org/10.5194/egusphere-egu24-9690, 2024.

EGU24-9905 | ECS | Orals | CL4.3

Optimization-based driver detection and prediction of seasonal heat extremes 

Ronan McAdam, César Peláez Rodríguez, Felicitas Hansen, Jorge Pérez Aracil, Antonello Squintu, Leone Cavicchia, Eduardo Zorita, Sancho Saldez-Sanz, and Enrico Scoccimarro

As a consequence of limited reliability of dynamical forecast systems, particularly over Europe, efforts in recent years have turned to exploiting the power of Machine Learning methods to extract information on drivers of extreme temperature from observations and reanalysis. Meanwhile, the diverse impacts of extreme heat have driven development of new indicators which take into account nightime temperatures and humidity. In the H2020 CLimate INTelligence (CLINT) project, a feature selection framework is being developed to find the combination of drivers which provides optimal seasonal forecast skill of European summer heatwave indicators. Here, we present the methodology, its application to a range of heatwave indicators and forecast skill compared to existing dynamical systems. First, a range of (reduced-dimensionality) drivers are defined, including k-means clusters of variables known to impact European summer (e.g. precipitation, sea ice content), and more complex indices like the NAO and weather regimes. Then, these drivers are used to train machine learning based prediction models, of varying complexity, to predict seasonal indicators of heatwave occurrence and intensity. A crucial and novel step in our framework is the use of the Coral Reef Optimisation algorithm, used to select the variables and their corresponding lag times and time periods which provide optimal forecast skill. To maximise training data, both ERA5 reanalysis and a 2000-year paleo-simulation are used; the representation of heatwaves and atmospheric conditions are validated with respect to ERA5. We present comparisons of forecast skill to the dynamical Copernicus Climate Change Service seasonal forecasts systems. The differences in timing, predictability and drivers of daytime and nighttime heatwaves across Europe are highlighted. Lastly, we discuss how the framework can easily be adapted to other extremes and timescales.



How to cite: McAdam, R., Peláez Rodríguez, C., Hansen, F., Pérez Aracil, J., Squintu, A., Cavicchia, L., Zorita, E., Saldez-Sanz, S., and Scoccimarro, E.: Optimization-based driver detection and prediction of seasonal heat extremes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9905, https://doi.org/10.5194/egusphere-egu24-9905, 2024.

EGU24-10539 | ECS | Orals | CL4.3

Exploring multiyear-to-decadal North Atlantic sea level predictability using machine learning and analog methods 

Qinxue Gu, Liwei Jia, Liping Zhang, Thomas Delworth, Xiaosong Yang, Fanrong Zeng, and Shouwei Li

Long-term sea level rise and multiyear-to-decadal sea level variations pose substantial risks of flooding and erosion in coastal communities. The North Atlantic Ocean and the U.S. East Coast are hotspots for sea level changes under current and future climates. Here, we employ a machine learning technique, a self-organizing map (SOM)-based framework, to systematically characterize the North Atlantic sea level variability, assess sea level predictability, and generate sea level predictions on multiyear-to-decadal timescales. Specifically, we classify 5000-year North Atlantic sea level anomalies from the Seamless System for Prediction and EArth System Research (SPEAR) model control simulations into generalized patterns using SOM. Preferred transitions among these patterns are further identified, revealing long-term predictability on multiyear-to-decadal timescales related to shifts in Atlantic meridional overturning circulation (AMOC) phases. By combining the SOM framework with “analog” techniques based on the simulations and observational/reanalysis data, we demonstrate prediction skill of large-scale sea level patterns comparable to that from initialized hindcasts. Moreover, additional source of short-term predictability is identified after the exclusion of low-frequency AMOC signals, which arises from the wind-driven North Atlantic tripole mode triggered by the North Atlantic Oscillation. This study highlights the potential of machine learning methods to assess sources of predictability and to enable efficient, long-term climate prediction.

How to cite: Gu, Q., Jia, L., Zhang, L., Delworth, T., Yang, X., Zeng, F., and Li, S.: Exploring multiyear-to-decadal North Atlantic sea level predictability using machine learning and analog methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10539, https://doi.org/10.5194/egusphere-egu24-10539, 2024.

The inter-annual to multi-decadal variability of recurrent, synoptic-scale atmospheric circulation patterns in the Northern Hemisphere extratropics, as represented by the Jenkinson-Collison classification scheme, is explored in reanalysis data spanning the entire 20th century, and in global climate model (GCM) data from the historical, AMIP and DCPP experiments conducted within the framework of CMIP6. The aim of these efforts is to assess the effect of coupled vs. uncoupled and initialised vs. non-initialized GCM simulations in reproducing the observed low-frequency variability of the aforementioned circulation patterns.

Results reveal that the observed annual counts of typical recurrent weather patterns, such as cyclonic or anticyclonic conditions and also situations of pronounced advection, exhibit significant oscillations on multiple time-scales ranging between several years and several decades. The period of these oscillations, however, is subject to large regional variations. This is in line with earlier studies suggesting that the extratropical atmospheric circulation’s low frequency variability is essentially unforced, except in the Pacific-North American sector where the forced variability is enhanced due to ENSO teleconnections. Neither the periods obtained from historical nor those obtained from AMIP experiments align with observations. Likewise, not even the periods obtained from different runs of the same GCM and experiment correspond to each other. Thus, in an non-initialized model setup, ocean-atmosphere coupling or the lack thereof essentially leads to the same results. Whether initialization and/or augmenting the ensemble size can improve these findings, will also be discussed.

Acknowledgement: This work is part of project Impetus4Change, which has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No 101081555.

How to cite: Brands, S., Cimadevilla, E., and Fernández, J.: Low-frequency variability of synoptic-scale atmospheric circulation patterns in the Northern Hemisphere extratropics and associated hindcast skill of decadal forecasting systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10551, https://doi.org/10.5194/egusphere-egu24-10551, 2024.

EGU24-10574 | Orals | CL4.3 | Highlight

Will 2024 be the first year above 1.5 C? 

Nick Dunstone, Doug Smith, Adam Scaife, Leon Hermanson, Andrew Colman, and Chris Folland

Global mean surface temperature is the key metric by which our warming climate is monitored and for which international climate policy is set. At the end of each year the Met Office makes a global mean temperature forecast for the coming year. Following on from the new record 2023, we predict a high probability of another record year in 2024 and a 35% chance of exceeding 1.5 C above pre-industrial. Whilst a one-year temporary exceedance of 1.5 C would not constitute a breech of the Paris Agreement target, our forecast highlights how close we are now to breeching this target. We show that our 2024 forecast can be largely explained by the combination of the continuing warming trend of +0.2 C/decade and the lagged warming affect of a strong tropical Pacific El Nino event. We further highlight 2023 was significantly warmer than forecast and that much of this warming signal came from the southern hemisphere and requires further understanding.

How to cite: Dunstone, N., Smith, D., Scaife, A., Hermanson, L., Colman, A., and Folland, C.: Will 2024 be the first year above 1.5 C?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10574, https://doi.org/10.5194/egusphere-egu24-10574, 2024.

EGU24-11485 | ECS | Orals | CL4.3

Summer drought predictability in the Mediterranean region in seasonal forecasts 

Giada Cerato, Katinka Bellomo, and Jost von Hardenberg

The Mediterranean region has been identified as an important climate change hotspot, over the 21st century both air temperature and its extremes are projected to rise at a rate surpassing that of the global average and a significant decrease of average summer precipitation is projected, particularly for the western Mediterranean. On average, Mediterranean droughts have become more frequent and intense in recent years and are expected to become more widespread in many regions. These prolonged dry spells pose a substantial threat to agriculture and impact several socio-economic sectors. In this context, long-range weather forecasting has emerged as a promising tool for seasonal drought risk assessment. However, the interpretation of the forecasting products is not always straightforward due to their inherent probabilistic nature. Therefore, a rigorous evaluation process is needed to determine the extent to which these forecasts provide a fruitful advantage over much simpler forecasting systems, such as those based on climatology. 

In this study, we use the latest version of ECMWF’s seasonal prediction system (SEAS5) to understand its skill in predicting summer droughts. The Standardized Precipitation Evapotranspiration Index (SPEI) aggregated over different lead times is employed to mark below-normal dryness conditions in August. We use a comprehensive set of evaluation metrics to gain insight into the accuracy, systematic biases, association, discrimination and sharpness of the forecast system. Our findings reveal that up to 3 months lead time, seasonal forecasts show stronger association and discrimination skills than the climatological forecast, especially in the Southern Mediterranean, although the prediction quality in terms of accuracy and sharpness is limited. On the other hand, extending the forecast range up to 6 months lead time dramatically reduces its predictability skill, with the system mostly underperforming elementary climatological predictions. 

This approach is then extended to examine the full ensemble of seasonal forecasting systems provided by the Copernicus Climate Change Service (C3S) to test their skill in predicting droughts. Our findings can help an informed use of seasonal forecasts of droughts and the development of related climate services.

How to cite: Cerato, G., Bellomo, K., and von Hardenberg, J.: Summer drought predictability in the Mediterranean region in seasonal forecasts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11485, https://doi.org/10.5194/egusphere-egu24-11485, 2024.

EGU24-11930 | ECS | Posters on site | CL4.3

A global empirical system for probabilistic seasonal climate prediction based on generative AI and CMIP6 models  

Lluís Palma, Alejandro Peraza, Amanda Duarte, David Civantos, Stefano Materia, Arijit Nandi, Jesús Peña-Izquierdo, Mihnea Tufis, Gonzalo Vilella, Laia Romero, Albert Soret, and Markus Donat

Reliable probabilistic information at the seasonal time scale is essential across various societal sectors, such as agriculture, energy, or water management. Current applications of seasonal predictions rely on General Circulation Models (GCMs) that represent dynamical processes in the atmosphere, land surface, and ocean while capturing their linear and nonlinear interactions. However, GCMs come with an inherent high computational cost. In an operational setup, they are typically run once a month and at a lower temporal and spatial resolution than the ones needed for regional applications. Moreover, GCMs suffer from significant drifts and biases and can miss relevant teleconnections, resulting in low skill for particular regions or seasons. 

In this context, the use of generative AI methods that can model complex nonlinear relationships can be a viable alternative for producing probabilistic predictions with low computational demand. Such models have already demonstrated their effectiveness in different domains, i.e. computer vision, natural language processing, and weather prediction. However, although requiring less computational power, these techniques still rely on big datasets in order to be efficiently trained. Under this scenario, and with sufficiently high-quality global observational datasets spanning at most 70 years, the research trend has evolved into training these models using climate model output. 

In this work, we build upon the work presented by Pan et al., 2022, which introduced a conditional Variational Autoencoder (cVAE) to predict global temperature and precipitation fields for the October to March season starting from July initial conditions. We adopt several pre-processing changes to account for different biases and trends across the CMIP6 models. Additionally, we explore different architecture modifications to improve the model's performance and stability. We study the benefits of our model in predicting three-month anomalies on top of the climate change trend. Finally, we compare our results with a state-of-the-art GCM (SEAS5) and a simple empirical system based on the linear regression of classical seasonal indices based on Eden et al., 2015.

 

Pan, Baoxiang, Gemma J. Anderson, André Goncalves, Donald D. Lucas, Céline J.W. Bonfils, and Jiwoo Lee. 'Improving Seasonal Forecast Using Probabilistic Deep Learning'. Journal of Advances in Modeling Earth Systems 14, no. 3 (1 March 2022). https://doi.org/10.1029/2021MS002766.


Eden, J. M., G. J. van Oldenborgh, E. Hawkins, and E. B. Suckling. 'A Global Empirical System for Probabilistic Seasonal Climate Prediction'. Geoscientific Model Development 8, no. 12 (11 December 2015): 3947–73. https://doi.org/10.5194/gmd-8-3947-2015.

How to cite: Palma, L., Peraza, A., Duarte, A., Civantos, D., Materia, S., Nandi, A., Peña-Izquierdo, J., Tufis, M., Vilella, G., Romero, L., Soret, A., and Donat, M.: A global empirical system for probabilistic seasonal climate prediction based on generative AI and CMIP6 models , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11930, https://doi.org/10.5194/egusphere-egu24-11930, 2024.

EGU24-12969 | ECS | Orals | CL4.3

How unusual is the recent decade-long pause in Arctic summer sea ice retreat? 

Patricia DeRepentigny, François Massonnet, Roberto Bilbao, and Stefano Materia

The Earth has warmed significantly over the past 40 years, and the fastest rate of warming has occurred in and around the Arctic. The warming of northern high latitudes at a rate of almost four times the global average (Rantanen et al., 2022), known as Arctic amplification, is associated with sea ice loss, glacier retreat, permafrost degradation, and expansion of the melting season. Since the mid-2000s, summer sea ice has exhibited a rapid decline, reaching record minima in September sea ice area in 2007 and 2012. However, after the early 2010s, the downward trend of minimum sea ice area appears to decelerate (Swart et al., 2015; Baxter et al., 2019). This apparent slowdown and the preceding acceleration in the rate of sea ice loss are puzzling in light of the steadily increasing rate of greenhouse gas emissions of about 4.5 ppm yr−1 over the past decade (Friedlingstein et al., 2023) that provides a constant climate forcing. Recent studies suggest that low-frequency internal climate variability may have been as important as anthropogenic influences on observed Arctic sea ice decline over the past four decades (Dörr et al., 2023; Karami et al., 2023). Here, we investigate how unusual this decade-long pause in Arctic summer sea ice decline is within the context of internal climate variability. To do so, we first assess how rare this is deceleration of Arctic sea ice loss is by comparing it to trends in CMIP6 historical simulations. We also use simulations from the Decadal Climate Prediction Project (DCPP) contribution to CMIP6 to determine if initializing decadal prediction systems from estimates of the observed climate state substantially improves their performance in predicting the slowdown in Arctic sea ice loss over the past decade. As the DCPP does not specify the data or the methods to be used to initialize forecasts or how to generate ensembles of initial conditions, we also assess how different formulations affect the skill of the forecasts by analyzing differences between models. This work provides an opportunity to attribute this pause in Arctic sea ice retreat to interannual internal variability or radiative external forcings, something that observation analysis alone cannot achieve.

How to cite: DeRepentigny, P., Massonnet, F., Bilbao, R., and Materia, S.: How unusual is the recent decade-long pause in Arctic summer sea ice retreat?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12969, https://doi.org/10.5194/egusphere-egu24-12969, 2024.

EGU24-14341 | Posters on site | CL4.3

Compound Heat and Dry Events Influenced by the Pacific–Japan Pattern over Taiwan in Summer 

Szu-Ying Lin, Wan-Ling Tseng, Yi-Chi Wang, and MinHui Lo

Compound dry and hot events, characterized by elevated temperatures and reduced precipitation, pose interconnected challenges to human social economics, necessitating comprehensive strategies for mitigation and adaptation. This study focuses on the Pacific-Japan (PJ) pattern, a significant climate variability influencing summer climates in East Asia. While previous research has explored its impact on Japan and Korea, our investigation delves into its effects on Taiwan, a mountainous subtropical island with a population of approximately 24 million. Utilizing long-term temperature and rainfall data, along with reanalysis dynamic downscaling datasets, we examine the interannual impacts of the PJ pattern on summer temperature and compound heat and dry events. Our findings reveal a significant temperature increase during the positive phase of the PJ pattern, characterized by anticyclonic anomalous circulation over Taiwan. Additionally, both the Standardized Precipitation Index and soil water exhibit a decline during this phase, reflecting meteorological and hydrological drought conditions. A robust negative correlation (-0.7) between drought indices and temperature emphasizes the compound effect of heat and dry events during the PJ positive phase. This study enhances the understanding of the PJ pattern as a climate driver, describing its role in hot and dry summers over Taiwan. The insights gained, when integrated into seasonal prediction and early warning systems, can aid vulnerable sectors in preparing for potential heat and dry stress hazards.

How to cite: Lin, S.-Y., Tseng, W.-L., Wang, Y.-C., and Lo, M.: Compound Heat and Dry Events Influenced by the Pacific–Japan Pattern over Taiwan in Summer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14341, https://doi.org/10.5194/egusphere-egu24-14341, 2024.

EGU24-14379 | Posters on site | CL4.3

Linkage between Temperature and Heatwaves in Summer Taiwan to the Pacific Meridional Mode 

Chieh-Ting Tsai, Wan-Ling Tseng, and Yi-Chi Wang

Over the past century, Taiwan has gradually recognized the hazards posed by extreme heat events (EHT), prompting the development of mid-term adaptation strategies to address challenges in the coming decades. However, our understanding of decadal-scale temperature variations remains insufficient, requiring further research into influencing factors. Our study reveals the crucial role of the Pacific Meridional Mode (PMM) in modulating decadal-scale variations in summer temperatures in Taiwan. During the positive phase of PMM, warm sea surface temperature anomalies trigger an eastward-moving wave train extending into East Asia. This leads to the development of high-pressure circulations near Southeast Asia and Taiwan, enhancing the temperature increase. This mechanism has been reproduced in experiments using the Taiwan Earth System Model. Moreover, our study utilizes the calendar day 90th percentile of maximum temperature (CTX) as the threshold for extreme high-temperature events (EHT), while also employing the heatwaves magnitude scale (HWMS) as the criterion for defining heatwaves. During the positive phase of PMM, the frequency and duration of EHT increase, with variations observed across different regions. The overall intensity of heatwave events also strengthens, primarily due to extended durations. Notably, in a single city, this results in exposure of up to 800,000 person-days to EHT, presenting a tenfold increase compared to the annual effect observed in the long-term warming trend. These findings on the decadal-scale relationship between summer temperatures in Taiwan and PMM contribute to a deeper understanding of EHT and heatwaves events impacts, providing more nuanced insights for future regional strategies in mitigating heatwave disasters.

How to cite: Tsai, C.-T., Tseng, W.-L., and Wang, Y.-C.: Linkage between Temperature and Heatwaves in Summer Taiwan to the Pacific Meridional Mode, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14379, https://doi.org/10.5194/egusphere-egu24-14379, 2024.

EGU24-14688 | ECS | Orals | CL4.3

Exploring ML-based decadal predictions of the German Bight storm surge climate 

Daniel Krieger, Sebastian Brune, Johanna Baehr, and Ralf Weisse

Storm surges and elevated water levels regularly challenge coastal protection and inland water management along the low-lying coastline of the German Bight. Skillful seasonal-to-decadal (S2D) predictions of the local storm surge climate would be beneficial to stakeholders and decision makers in the region. While storm activity has recently been shown to be skillfully predictable on a decadal timescale with a global earth system model, surge modelling usually requires very fine spatial and temporal resolutions that are not yet present in current earth system models. We therefore propose an alternative approach to generating S2D predictions of the storm surge climate by training a neural network on observed water levels and large-scale atmospheric patterns, and apply the neural network to the available model output of a S2D prediction system. We show that the neural-network-based translation from large-scale atmospheric fields to local water levels at the coast works sufficiently well, and that several windows of predictability for the German Bight surge climate emerge on the S2D scale.

How to cite: Krieger, D., Brune, S., Baehr, J., and Weisse, R.: Exploring ML-based decadal predictions of the German Bight storm surge climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14688, https://doi.org/10.5194/egusphere-egu24-14688, 2024.

Atlantic meridional overturning circulation (AMOC) is one of the mechanisms for climate predictability and one of the properties that decadal climate predictions are attempting to predict. The starting point for AMOC decadal predictions is sensitive to the underlying data assimilation and/or initialization procedure. This means that different choices during the data assimilation procedure (e.g., assimilation method, assimilation window, data sources, resolution, nudging terms and strength, full field vs anomaly initialization/assimilation, etc) can result in a different mean and even variability of reconstructed ocean circulation. How coherent the AMOC initial states should be among the CMIP-like decadal prediction experiments? How good in general should the initial AMOC be for decadal predictions? And do initialization issues of the ocean circulation influence the prediction skill of other variables that are of interest for application studies? These are the questions that we were attempting to address in our study, where we analyzed twelve decadal prediction systems from the World Meteorological Organization Lead Centre for Annual-to-Decadal Climate Prediction project. We identify that the AMOC initialization influences the quality of predictions of the subpolar gyre (SPG). When predictions show a large initial error in their AMOC, they usually have low skill for predicting the internal variability of the SPG five years after the initialization.

How to cite: Polkova, I. and the Co-Authors: Initialization shock in the ocean circulation reduces skill in decadal predictions of the North Atlantic subpolar gyre, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15358, https://doi.org/10.5194/egusphere-egu24-15358, 2024.

EGU24-15476 | Posters on site | CL4.3

Statistics of sudden stratospheric warmings using a large model ensemble 

Sarah Ineson, Nick Dunstone, Adam Scaife, Martin Andrews, Julia Lockwood, and Bo Pang

Using a large ensemble of initialised retrospective forecasts (hindcasts) from a seasonal prediction system, we explore various statistics relating to sudden stratospheric warmings (SSWs). Observations show that SSWs occur at a similar frequency during both El Niño and La Niña northern hemisphere winters. This is contrary to expectation, as the stronger stratospheric polar vortex associated with La Niña years might be expected to result in fewer of these extreme breakdowns. We show that this similar frequency may have occurred by chance due to the limited sample of years in the observational record. We also show that in these hindcasts, winters with two SSWs, a rare event in the observational record, on average have an increased surface impact. Multiple SSW events occur at a lower rate than expected if events were independent but somewhat surprisingly, our analysis also indicates a risk, albeit small, of winters with three or more SSWs, as yet an unseen event.

How to cite: Ineson, S., Dunstone, N., Scaife, A., Andrews, M., Lockwood, J., and Pang, B.: Statistics of sudden stratospheric warmings using a large model ensemble, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15476, https://doi.org/10.5194/egusphere-egu24-15476, 2024.

EGU24-15709 | ECS | Orals | CL4.3

Predicting Atlantic and Benguela Niño events with deep learning  

Marie-Lou Bachelery, Julien Brajard, Massimiliano Patacchiola, and Noel Keenlyside

Extreme Atlantic and Benguela Niño events continue to significantly impact the tropical Atlantic region, with far-reaching consequences for African climate and ecosystems. Despite attempts to forecast these events using traditional seasonal forecasting systems, success remains low, reinforcing the growing idea that these events are unpredictable. To overcome the limitations of dynamical prediction systems, we introduce a deep learning-based statistical prediction model for Atlantic and Benguela Niño events. Our convolutional neural network (CNN) model, trained on 90 years of reanalysis data incorporating surface and 100m-averaged temperature variables, demonstrates the capability to forecast the Atlantic and Benguela Niño indices with lead times of up to 3-4 months. Notably, the CNN model excels in forecasting peak-season events with remarkable accuracy extending up to 5 months ahead. Gradient sensitivity analysis reveals the ability of the CNN model to exploit known physical precursors, particularly the connection to equatorial dynamics and the South Atlantic Anticyclone, for accurate predictions of Benguela Niño events. This study challenges the perception of the Tropical Atlantic as inherently unpredictable, underscoring the potential of deep learning to enhance our understanding and forecasting of critical climate events. 

How to cite: Bachelery, M.-L., Brajard, J., Patacchiola, M., and Keenlyside, N.: Predicting Atlantic and Benguela Niño events with deep learning , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15709, https://doi.org/10.5194/egusphere-egu24-15709, 2024.

EGU24-15974 | ECS | Posters virtual | CL4.3

Recalibrating DWD’s operational climate predictions: towards a user-oriented seamless climate service 

Alexander Pasternack, Birgit Mannig, Andreas Paxian, Amelie Hoff, Klaus Pankatz, Philip Lorenz, and Barbara Früh

The German Meteorological Service's (Deutscher Wetterdienst DWD) climate predictions website  (www.dwd.de/climatepredictions) offers a centralized platform for accessing post-processed climate predictions, including subseasonal forecasts from ECMWF's IFS and seasonal and decadal predictions from the German climate prediction system. The website design was developed in collaboration with various sectors to ensure uniformity across all time frames, and users can view maps, tables, and time series of ensemble mean and probabilistic predictions in combination with their skill. The available data covers weekly, 3-month, 1-year, and 5-year temperature means, precipitation sums and soil moisture for the world, Europe, Germany, and particular German regions. To achieve high spatial resolution, the DWD used the statistical downscaling method EPISODES. Moreover, within the BMBF project KIMoDIs (AI-based monitoring, data management and information system for coupled forecasting and early warning of low groundwater levels and salinisation) the DWD provides climate prediction data of further hydrological variables (e.g. relative humidity) with corresponding prediction skill on a regional scale.

However, all predictions on these time scales can suffer from inherent systematic errors, which can impact their usefulness. To address these issues, the recalibration method DeFoReSt was applied to decadal predictions, using a combination of 3rd order polynomials in lead and start time, along with a boosting model selection approach. This approach addresses lead-time dependent systematic errors, such as drift, as well as inaccuracies in representing long-term changes and variability.

This study highlights the improved accuracy of the recalibration approach on decadal predictions due to an increased polynomial order compared to the original approach, and its different impact on global and regional scales. It also explores the feasibility of transferring this approach to predictions with shorter time horizons of the provided variables.

How to cite: Pasternack, A., Mannig, B., Paxian, A., Hoff, A., Pankatz, K., Lorenz, P., and Früh, B.: Recalibrating DWD’s operational climate predictions: towards a user-oriented seamless climate service, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15974, https://doi.org/10.5194/egusphere-egu24-15974, 2024.

EGU24-16366 | ECS | Orals | CL4.3

Decadal predictions outperform projections in forecasting winter precipitation over the Mediterranean region 

Dario Nicolì, Silvio Gualdi, and Panos Athanasiadis

The Mediterranean region is highly sensitive to climate change, having experienced an intense warming and drying trend in recent decades, primarily due to the increased concentrations of anthropogenic greenhouse gases. In the context of decision-making processes, there is a growing interest in understanding the near-term climate evolution of this region.

In this study, we explore the climatic fluctuations of the Mediterranean region in the near-term range (up to 10 years ahead) using two different products: projections and decadal predictions. The former are century-scale climate change simulations initialized from arbitrary model states to which were applied anthropogenic and natural forcings. A major limitation of climate projections is their limited information regarding the current state of the Earth’s climate system. Decadal climate predictions, obtained by constraining the initial conditions of an ensemble of model simulations through a best estimate of the observed climate state, provide a better understanding of the next-decade climate and thus represent an invaluable tool in assisting climate adaptation.

Using retrospective forecasts from eight decadal prediction systems contributing to the CMIP6 Decadal Climate Prediction Project (CMIP6 DCPP) and the corresponding ensemble of non-initialized projections, we compare the capabilities of the state-of-the-art climate models in predicting future climate changes of the Mediterranean region for some key quantities so as to assess the added value of initialization. 

Beyond the contribution of external forcings, the role of internal variability is also investigated since part of the detected predictability arises from internal climate variability patterns affecting the Mediterranean. The observed North Atlantic Oscillation, the dominant climate variability pattern in the Euro-Atlantic domain, as well as its  impact on wintertime precipitation over Europe are well reproduced by decadal predictions, especially over the Mediterranean, outperforming projections. We also apply a sub-sampling method to enhance the respective signal-to-noise ratio and consequently improve precipitation skill over the Mediterranean.

How to cite: Nicolì, D., Gualdi, S., and Athanasiadis, P.: Decadal predictions outperform projections in forecasting winter precipitation over the Mediterranean region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16366, https://doi.org/10.5194/egusphere-egu24-16366, 2024.

EGU24-16985 | Posters on site | CL4.3

Investigating signals in summer seasonal forecasts over the North Atlantic/European region 

Julia Lockwood, Nick Dunstone, Kristina Fröhlich, Ramón Fuentes Franco, Anna Maidens, Adam Scaife, Doug Smith, and Hazel Thornton

The current generation of seasonal forecast models struggle to skilfully predict dynamical circulation over the North Atlantic and European region in boreal summer.  Using two different state-of-the-art seasonal prediction systems, we show that tropical rainfall anomalies drive a circulation signal in the North Atlantic/Europe via the propagation of Rossby waves.  The wave, however, is shifted eastwards compared to observations, so the signal does not contribute positively to model skill.  Reasons for the eastward shift of the Rossby wave are investigated, as well as other drivers of the signal in this region.  Despite the errors in the waves, the fact that seasonal forecast models do predict dynamical signals over the North Atlantic/Europe signifies seasonal predictability over this region beyond the climate change trend, and understaning the cause of the errors could lead to skilful predictions.

How to cite: Lockwood, J., Dunstone, N., Fröhlich, K., Fuentes Franco, R., Maidens, A., Scaife, A., Smith, D., and Thornton, H.: Investigating signals in summer seasonal forecasts over the North Atlantic/European region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16985, https://doi.org/10.5194/egusphere-egu24-16985, 2024.

EGU24-17418 | Posters on site | CL4.3

Strengthening seasonal forecasting in the Middle East & North Africa (MENA) through the WISER Programme. 

Stefan Lines, Nicholas Savage, Rebecca Parfitt, Andrew Colman, Alex Chamberlain-Clay, Luke Norris, Heidi Howard, and Helen Ticehurst

In this presentation, we introduce the WISER MENA projects SeaFOAM (Seasonal Forecasting Across MENA) and SeaSCAPE (Seasonal Co-Production and Application in MENA). These projects explore both the improvement to the regional-level seasonal forecast in the MENA region, as well as how to tailor the information in ways useful to a range of climate information stakeholders. SeaFOAM works alongside Maroc Meteo, Morocco's National Meteorological and Hydrological Service (NMHS) and the Long Range Forecasting node of the Northern Africa WMO Regional Climate Centre (RCC), to develop a framework for objective seasonal forecasting. This approach will blend techniques such as bias correction via local linear regression and canonical correlation analysis (CCA), with skill-assessed sub-selected models, to improve forecasting accuracy. Multiple drivers of rainfall variability, including the North Atlantic Oscillation (NAO) and Mediterranean Oscillation (MO), are investigated for their calibration potential. SeaSCAPE works with the WMO and various partners across MENA to understand the use of seasonal information in multiple sectors, exploring existing gaps and needs. Through stakeholder engagement workshops, training and bespoke support for the Arab Climate Outlook Forum (ArabCOF), SeaSCAPE operates collaboratively to tailor regional and national-level climate information to improve accessibility and usability of climate information on seasonal timescales.

How to cite: Lines, S., Savage, N., Parfitt, R., Colman, A., Chamberlain-Clay, A., Norris, L., Howard, H., and Ticehurst, H.: Strengthening seasonal forecasting in the Middle East & North Africa (MENA) through the WISER Programme., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17418, https://doi.org/10.5194/egusphere-egu24-17418, 2024.

EGU24-17585 | Orals | CL4.3

Skill of wind resource forecasts on the decadal time scale 

Kai Lochbihler, Ana Lopez, and Gil Lizcano

Accurate forecasts of the natural resources of renewable energy production have become not only a valuable but a crucial tool for managing the associated risks of specific events, such as wind droughts. Wind energy, alongside with solar power, now provide a substantial part to the renewable energy share of the global energy production and growth in this sector will most likely further increase. The naturally given fluctuations of wind resources, however, pose a challenge for maintaining a stable energy supply, which, at the end of the chain, can have an impact on the energy market prices.
Operational short-term forecasting products for the wind energy sector (multiple days) are already commonly available and seasonal to sub seasonal forecasting solutions (multiple months) can provide valuable skill and are gaining in popularity. On the other side of the spectrum, typically on a time scale of multiple decades, we find risk assessment based on climate change projections. In between the long and short term time scales, however, there is a gap that still needs to be filled to achieve seamless prediction of risks that are relevant for the energy sector: decadal predictions.

Here, we present the results of an evaluation study of a multi-model decadal prediction ensemble (DCPP) for a selection of wind development regions in Europe. The evaluation is based on multiple decades long hindcasts and carried out with a focus on the skill of predicting specific event types of wind resource availability in a probabilistic context, alongside with basic deterministic skill measures. We further investigate specific event constellations and their large-scale drivers that, in combination, can provide windows of opportunity with enhanced predictive skill. We conclude with a discussion on how this hybrid approach can be used to potentially increase not only forecast skill but also the trust of the end user.

How to cite: Lochbihler, K., Lopez, A., and Lizcano, G.: Skill of wind resource forecasts on the decadal time scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17585, https://doi.org/10.5194/egusphere-egu24-17585, 2024.

EGU24-19229 | ECS | Orals | CL4.3

Comparing the seasonal predictability of Tropical Pacific variability in EC-Earth3 at two different horizontal resolutions 

Aude Carreric, Pablo Ortega, Vladimir Lapin, and Francisco Doblas-Reyes

Seasonal prediction is a field of research attracting growing interest beyond the scientific community due to its strong potential to guide decision-making in many sectors (e.g. agriculture and food security, health, energy production, water management, disaster risk reduction) in the face of the pressing dangers of climate change.

Among the various techniques being considered to improve the predictive skill of seasonal prediction systems, increasing the horizontal resolution of GCMs is a promising avenue. There are several indications that higher resolution versions of the current generation of climate models might improve key air-sea teleconnections, decreasing common biases of global models and improving the skill to predict certain regions at seasonal scales, e.g. in tropical sea surface temperature.

In this study, we analyze the differences in the predictive skill of two different seasonal prediction systems, based on the same climate model EC-Earth3 and initialized in the same way but using two different horizontal resolutions. The standard (SR) and high resolution (HR) configurations are based on an atmospheric component, IFS, of ~100 km and ~40 km of resolution respectively and on an ocean component, NEMO3.6, of ~100 km and ~25 km respectively. We focus in particular on the Tropical Pacific region where statistically significant improvements are found in HR with respect to SR for predicting ENSO and its associated climate teleconnections. We explore some processes that can explain these differences, such as the simulation of the tropical ocean mean state and atmospheric teleconnections between the Atlantic and Pacific tropical oceans. 

A weaker mean-state bias in the HR configuration, with less westward extension of ENSO-related SST anomalies, leads to better skill in ENSO regions, which can also be linked to better localization of the atmospheric teleconnection with the equatorial Atlantic Ocean. It remains to be assessed if similar improvements are consistently identified for HR versions in other forecast systems, which would prompt their routine use in seasonal climate prediction.

How to cite: Carreric, A., Ortega, P., Lapin, V., and Doblas-Reyes, F.: Comparing the seasonal predictability of Tropical Pacific variability in EC-Earth3 at two different horizontal resolutions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19229, https://doi.org/10.5194/egusphere-egu24-19229, 2024.

EGU24-19251 | Orals | CL4.3 | Highlight

The opportunities and challenges of near-term climate prediction 

Hazel Thornton

Accurate forecasts of the climate of the coming season and years are highly desired by many sectors of society. The skill of near-term climate prediction in winter in the North Atlantic and European region has improved over the last decade associated with larger ensembles, improving models and boosting of the prediction signal using intelligent post processing. International collaboration has improved the availability of forecasts and promoted the uptake of forecasts by different sectors. However, significant challenges remain, including summer prediction, understanding the risk of extremes within a season, multi-seasonal extremes and how best to post process the forecasts to aid decision making. This talk will summarise recent near-term climate prediction research activities at the UK Met Office and will detail our experience of providing such forecasts to the energy and water sectors.  

How to cite: Thornton, H.: The opportunities and challenges of near-term climate prediction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19251, https://doi.org/10.5194/egusphere-egu24-19251, 2024.

This study focuses on applying machine learning techniques to bias-correct the seasonal temperature forecasts provided by the Copernicus Climate Change Service (C3S) models. Specifically, we employ bias correction on forecasts from five major models: UK Meteorological Office (UKMO), Euro-Mediterranean Center on Climate Change (CMCC), Deutscher Wetterdienst (DWD), Environment and Climate Change Canada (ECCC), and Meteo-France. Our primary objective is to assess the performance of our bias correction model in comparison to the original forecast datasets. We utilise temperature-based indices recommended by the Expert Team on Climate Change Detection and Indices (ETCCDI) to evaluate the effectiveness of the bias-corrected seasonal forecasts. These indices served as valuable metrics to gauge the predictive capability of the models, especially in forecasting natural cascading hazards such as wildfires, droughts, and floods. The study involved an in-depth analysis of the bias-corrected forecasts, and the derived indices were crucial in understanding the models' ability to predict temperature-related extreme events. The results of this research contribute valuable information for decision-making and planning across various sectors, including disaster risk management and environmental protection. Through a comprehensive evaluation of machine learning-based bias correction techniques, we enhance the accuracy and applicability of seasonal temperature forecasts, thereby improving preparedness and resilience to climate-related challenges. 

How to cite: Mbuvha, R. and Nikraftar, Z.: Machine Learning Approaches to Improve Accuracy in Extreme Seasonal Temperature Forecasts: A Multi-Model Assessment , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19297, https://doi.org/10.5194/egusphere-egu24-19297, 2024.

EGU24-19359 | ECS | Posters on site | CL4.3

Seasonal forecast of the late boreal winter temperature based on solar forcing and QBO 

Mikhail Vokhmianin, Antti Salminen, Kalevi Mursula, and Timo Asikainen

The ground temperature variability in the Northern Hemisphere winter is greatly influenced by the state of the polar vortex. When the vortex collapses during sudden stratospheric warmings (SSWs), rapid changes in stratospheric circulations propagate downward to the troposphere in the subsequent weeks. The ground effect following SSWs is typically manifested as the negative phase of the North Atlantic Oscillation. Our findings reveal a higher frequency of cold temperature anomalies in the Northern part of Eurasia during winters with SSWs, and conversely, warm anomalies in winters with a strong and stable vortex. This behavior is particularly evident when temperature anomalies are categorized into three equal subgroups, or terciles. Recently, we developed a statistical model that successfully predicts SSW occurrences with an 86% accuracy rate. The model utilizes the stratospheric Quasi-Biennial Oscillation (QBO) phase and two parameters associated with solar activity: the geomagnetic aa-index as a proxy for energetic particle precipitations and solar irradiance. In this study, we explore the model's potential to provide a seasonal forecast for ground temperatures. We assess the probabilities of regional temperature anomalies falling into the lowest or highest terciles based on the predicted weak or strong vortex state. Additionally, we demonstrate that the QBO phase further enhances the forecast quality. As the model provides SSW predictions as early as preceding August, our results carry significant societal relevance as well, e.g., for the energy sector, which is highly dependent on prevailing weather conditions.

How to cite: Vokhmianin, M., Salminen, A., Mursula, K., and Asikainen, T.: Seasonal forecast of the late boreal winter temperature based on solar forcing and QBO, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19359, https://doi.org/10.5194/egusphere-egu24-19359, 2024.

It is expected that sea level rise and resulting coastal flooding will cost us over 1 trillion dollars annually by 2050. Therefore, understanding and monitoring coastal sea level rise is vital. Tide Gauges are in-situ instruments that have been providing sea level measurements since the 1800s, but they are sparse, and data availability is limited. Therefore, ocean altimetry has been the preferred observational tool for monitoring global sea levels.  

Satellite altimetry has been providing extensive and continuous global sea level data for more than three decades now. However, extracting reliable data close to the coast has been problematic due to signal contamination from land or calm water and lack of accurate geophysical corrections. Recently dedicated coastal altimetry products were proposed to provide better coastal sea level change product.   

In this study, we compare coastal altimetry products XTRACK-1Hz, XTRACK/ALES-20Hz in observing Sea Level Anomalies (SLA) with Tide Gauges (TG) along the global coastline from 2002-2019. 458 stations were selected for the study after applying several selection criteria that address data gaps, data availability from TG, altimetry, and correction products. The SLA signals from TG were decomposed into non-linear trend, seasonal, and residual components using Seasonal-Trend Decomposition using Loess (STL) method. The correlation coefficient, Root Mean Square Error (RMSE), and Index of Agreement (IOA) were computed for interannual and residual signals from TG and coastal altimetry products. Linear sea level trends at each station were also estimated from altimetry and TG observations after correcting for GPS-derived vertical land motion (VLM). 

When using altimetry for sea level signals near the coast, it is important to select point observations carefully instead of using a search radius that may take points from adjacent regions that could behave differently due to different coastal ocean processes. We developed a dynamically varying search radius for each TG, a function of the coastal shelf width near that station, to collate satellite observations as a representative of coastal sea level change. All the altimetry observations that fall within the search radius and are less than 25 km along the coast are used for comparison. In several cases, due to the sharp changes in the coastal morphology, the sea level signals seen by the adjacent TG stations are quite different, and thus, the reliability of altimetry suffered. 

With our analysis approach, we found good agreement between all altimetry products (XTRACK, XTRACK/ALES), and TG at residual and non-linear trend scales. A few stations near the fault lines and other tectonic regions disagree with altimetry trend estimates due to strong VLM signals that are not completely resolved by the VLM product used for correction. Around 70% of stations had a good agreement (r > 0.7) with trend and 55% with residual components. High-resolution (20Hz) XTRACK/ALES provided more observations near the coast. Nevertheless, both XTRACK/ALES-20Hz and XTRACK-1Hz performed well. This novel approach to select representative observation points from altimetry for a coastal zone will provide improved coastal sea level products from satellites, which can be considered at par with TG observations. 

How to cite: Sukumaran, V. and Vishwakarma, B. D.: Comparing altimetry-derived coastal sea level anomaly with tide gauge observations along the global coastline by accounting for shelf-width , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1007, https://doi.org/10.5194/egusphere-egu24-1007, 2024.

Worldwide geological markers of former sea-level (SL), such as wave-cut benches (raised, drowned), reveal a ~3-metre(m) SL rise, loosely carbon-dated post-50AD/pre-600. This "Rottnest Transgression" is the youngest of several m-scale rises, interspersed with m-scale falls, on Fairbridge's (1961) global-compilation Holocene-interglacial SL curve.

Copious British archaeological evidence (email me for sources), far better-dated (pottery-sherds/dendrochronology/Roman coins), confirms the Rottnest ("Romano-British marine transgression" of Godwin 1955), verifies its amplitude (~3m), and shows it spanned only ~70 years(y), ~430-500AD (early Dark Ages; Romans abandoned Britain 410AD). (An equally fast global SL rise, ~3m in decades, is proven by last-interglacial reef-facies relationships in tectonically stable Yucatan.) The Rottnest explains 5th-Century(C) mass-migration, underway by 450AD (dendro/artefacts/skeletal-DNA), of Saxon- and Angle civilians to SE Britain ('pre-subjugated' by rebellious Saxon mercenaries by 441AD), their North-Sea-coastal-plain homelands intolerably 'squeezed' between west-advancing Huns and rapid eastward shore-retreat. Among other British evidence: (1) Pevensey sea-fort (Roman-built ~290AD) straddles a promontory pointing NE into Pevensey Levels (reclaimed former tidal-flat embayment, beside English Channel). Indicating that high-spring-tide-level (HSTL) rose >2m in the 5thC, a defensive-ditch fronting the fort's SW gate contains "tidal" mud, dated early-5thC (sherds), whose top is ~1m higher than the NW-wall foundation and <0.5m higher than the SE foundation. This explains wall-collapse in both sectors (outward-toppled slabs visible on GoogleEarth), undermined by waves/currents, no later than mid-or-late 5thC (age of Early-Saxon-style sherd in sediment draping excavated wall-stump). Subsequent HSTL fall enabled William the Conqueror's 1066AD disembarkation at Pevensey fort; (2) excavated remnant stumps of Londinium's Thames-estuary-side city-wall (~270-300AD), up to 2.5m tall, show their entire outer face eroded (wall thinned ~50%), implying HSTL rose 3+m post-construction. Confirming this rise and its likely 5thC timing, across the Thames (Southwark) a peat layer containing 4thC sherds is capped by 2.8m of barren "river clay", reaching 3.2m higher than Londinium's lowest-known Thames-side wall-foundation. Proving HSTL soon fell 2+m, 1km upstream, in Lundenwic (Saxon port founded late-5thC), a building-floor dated ~700-750AD (sherds) is 1.6m lower than Londinium's highest-known wall erosion, and 1.5m below the top of the river-clay.

Such a large/fast global SL rise implies a peri-Antarctic 'MICI' ice-cliff-collapse event (Greenland lacks requisite >1km-deep grounding-line). Regarding causation, the Rottnest rise began (~430AD) only ~25y after the ~405AD warmest Arctic temperature-spike of the period 1-2000AD. This spike followed ~100y after the Sun's 310AD strongest magnetic-grand-maximum (MGM) peak of the interval 1-1885AD. The ~100y lag is attributable to ocean-thermal-inertia. The additional ~25y lag in SL response (Rottnest start) may reflect AMOC 'conveyor-belt' oceanic-circulation, specifically the time needed for ocean-surface-water, 'overwarmed' by the MGM (Svensmark effect, reduced cloudiness), to down-well in the north-Atlantic (Arctic fringe), then travel south, then up-well and encircle Antarctica, unleashing ice-collapse. The resulting iceberg-armada would cool the ocean, hence the atmosphere, causing increased global snowfall (ice build-up), intrinsically lowering SL.

Due to anthropogenic warming, the Arctic's average-surface-air-temperature exceeds, since 2005, the 405AD peak. This portends another rapid, metre-scale SL rise, beginning ~2030 (25y lag, above). Before 2100 the time-lagged effect of the Sun's even-stronger 1991 MGM peak will exacerbate warming.

How to cite: Higgs, R.: British archaeology verifies 5th-Century rapid multi-metre sea-level rise and portends another before 2100, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1322, https://doi.org/10.5194/egusphere-egu24-1322, 2024.

Sea level in the Southeast Asia (SEA) seas is driven by various phenomena at global,  regional and local scales. The latest tide gauge and satellite data revealed its most recent spatial and temporal patterns. The trend of global sea level rise in Singapore region is hindered by dominant variability of El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Indian Ocean Dipole (IOD), as well as associated modulation of Asian Monsoon. It was confirmed that positive sea-level anomalies in the southern and western areas of Southeast Asia seas were significantly high (~10 cm) during the northeast monsoon, especially in the Gulf of Thailand (~25 cm). The sea level trends for these regions were basically reversed during the southwest monsoon but with a smaller magnitude of negative sea-level anomalies. The regional sea-level trend in the Sunda Shelf differed from region to region, with the rates varied greatly from 1.4 to more than 4.8 mm/year. Interestingly, the rates on the east-western side of the region were roughly 3.0-4.5 mm/year, which were higher than the ones at other regions, being 2.5-3.5 mm/year. The presentation discuss the causes and consequences of sea level rise and variability in SEA and Singapore region in particular.

This Research is supported by Singapore’s National Research Foundation and National Environment Agency under the National Sea Level Programme Funding Initiative (Award No. USS-IF-2020-4).

How to cite: Tkalich, P. and Luu, Q.-H.: Recent Sea Level Rise and Variability in Singapore Region Derived from Tide Gauges and Satellite Altimetry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2288, https://doi.org/10.5194/egusphere-egu24-2288, 2024.

Dynamical downscaling employing high-resolution ocean models is widely considered as an efficient approach for modelling of regional ocean dynamics and sea-level changes using output of original coarse-resolution global general circulation models (GCMs). In this study, the historical dynamic variability and trends of sea level in the South China Sea (SCS) and the Southeast Asia Seas (SEAS) are investigated using the high-resolution regional ocean model (NEMO). Two hindcast ocean modelling experiments are conducted for the period 1960-2014. One is driven by global reanalysis data (ERA5 and ORAS5) forcings at the lateral and surface boundaries. The other is driven by global modelling oceanic data (EC-EARTH3) at the lateral boundary and by WRF-based downscaled atmospheric fields from the same parent model (EC-Earth3) at the surface boundary. Using the hindcast model runs, variability and trends of low-frequency sea-levels, as well as the driving mechanisms and the related processes are discussed, and the model performance and biases are analysed.

This Research is supported by Singapore’s National Research Foundation and National Environment Agency under the National Sea Level Programme Funding Initiative (Award No. USS-IF-2020-4).

How to cite: Ma, P., Gangadharan, N., and Tkalich, P.: Modelling of Low Frequency Sea Level Variability Over the Maritime Continent: Historical Dynamic Variability and Changes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2405, https://doi.org/10.5194/egusphere-egu24-2405, 2024.

EGU24-3440 | ECS | Orals | CL4.9

Quantifying the impact of near-surface winds on the occurrence of extreme sea level rises along the Swedish Baltic coastline: A statistical analysis 

Lorenzo Minola, Alice Re, Shalenys Bedoya-Valestt, Corrado Motta, Cesar Azorin-Molina, Alessandro Pezzoli, and Deliang Chen

Sea level rises pose significant risks to densely populated coastal regions, threatening human lives and vital infrastructures. Coastal societies, economies, and properties face acute vulnerability from saltwater intrusion, coastal erosion, and flooding resulting from extreme sea level variations. These occurrences are a confluence of factors, including local sea level rises, tidal changes, storm surges, waves, and shifts in coastal morphology.

In the Baltic Sea basin, where tides and North Atlantic storm surges are mitigated by the Danish Straits due to its semi-enclosed nature, coastal extreme sea levels are primarily driven by storm surges propelled by atmospheric pressure and surface winds from extratropical cyclones. Consequently, the surge in extreme sea levels here is predominantly wind-induced, regulated by meteorological processes.

This research focuses on the meteorological conditions, specifically wind patterns, that contribute to sudden sea level rises along the Swedish Baltic coastline. By integrating observations and model data like the ERA5 reanalysis, the study correlates the rapid increase in relative sea levels across 14 tide-gauge stations with wind and wave data. The aim is to exclusively utilize meteorological information for identifying extreme sea level occurrences, thereby enhancing the prediction of such events through weather forecasting.

How to cite: Minola, L., Re, A., Bedoya-Valestt, S., Motta, C., Azorin-Molina, C., Pezzoli, A., and Chen, D.: Quantifying the impact of near-surface winds on the occurrence of extreme sea level rises along the Swedish Baltic coastline: A statistical analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3440, https://doi.org/10.5194/egusphere-egu24-3440, 2024.

EGU24-3445 | ECS | Posters on site | CL4.9

Are multi-decadal sea-level oscillations augmenting rates of mean sea level? 

Erin Robson, Luke Jackson, and Sophie Williams

There is evidence to show sea-level change is accelerating, with a departure from Holocene rates in the late-19th century, to more than a doubling of the rate of global mean sea-level change over the past 25-years. Although the effect of anthropogenic forcing on sea level is certain, the influence of natural internal variability on augmenting rates of change remains an important area of research. This is especially significant at ocean-climate response timescales (>30-years). Using tide-gauge data, we apply empirical mode decomposition (EMD) to separate both the global and regional sea-level records into a series of intrinsic mode functions (IMFs) that are quasi-periodic in character. From them, we identify the dominant modes of variability that are common to each ocean basin, and compare these to recognised modes of climate variability to determine the causal factors of sea-level oscillations. We also conduct a sensitivity analysis with sub-sampled tide-gauge data to test the feasibility of this approach with high-resolution proxy-based sea-level reconstructions.

How to cite: Robson, E., Jackson, L., and Williams, S.: Are multi-decadal sea-level oscillations augmenting rates of mean sea level?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3445, https://doi.org/10.5194/egusphere-egu24-3445, 2024.

EGU24-3503 | Orals | CL4.9 | Highlight

Improving, evaluating and sharing projections of global mean sea level rise 

Tamsin Edwards, Fiona Turner, and Victor Malagon Santos and the EU PROTECT project
Projections of the ice sheet and glacier contributions to sea level rise to 2100 in the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report were made by representing physical models with statistical "emulators" or machine learning techniqes (Edwards et al., 2021). This allowed estimation of the impacts of several kinds of model uncertainty on sea level projections: multiple models for the ice sheets and glaciers, multiple settings determining ice sheet sensitivity to climate change, and multiple estimates of global warming, as well as uncertainty from the emulators themselves.
 
However, there were some limitations, including: predicting each year of the century independently (i.e. not providing smooth timeseries or the possibility to assess rates of change), beginning physical model simulations in 2015 (not allowing evaluation with observations), and exploring a small number of possible model settings. Projections beyond 2100 also had to be estimated for the IPCC with other methods. These limitations presented difficulties for users.
 
We improve on these projections here in their usefulness and robustness for coastal impacts communities and decision-makers. Usefulness: by predicting ice sheet and glacier changes to 2300, not 2100; providing smooth timeseries; and incorporating the emulators into the community FACTS sea level calculation framework (Kopp et al., 2023) for use by others. Robustness: by systematically exploring many more model settings than before (including, for the first time, those for glacier models), and beginning in the past to allow calibration of the projections with observations. The result is more meaningful trajectories of sea level contribution from each land ice source, in which we have greater confidence. We combine these in FACTS with estimates of thermal expansion and land water changes to show new projections of global mean sea level rise. This work was carried out by the EU Horizon 2020 project PROTECT.
 
References:
 
Edwards et al. (2021) Projected land ice contributions to twenty-first-century sea level rise, Nature, 593, 74–82.
 
Kopp et al. (2023) The Framework for Assessing Changes To Sea-level (FACTS) v1.0: a platform for characterizing parametric and structural uncertainty in future global, relative, and extreme sea-level change, Geosci. Model Dev., 16, 7461–7489. 
 

How to cite: Edwards, T., Turner, F., and Malagon Santos, V. and the EU PROTECT project: Improving, evaluating and sharing projections of global mean sea level rise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3503, https://doi.org/10.5194/egusphere-egu24-3503, 2024.

EGU24-4335 | ECS | Orals | CL4.9

Uncertainties in the projection of dynamic sea level in CMIP6 and FGOALS-g3 large ensemble 

Chenyang Jin, Hailong Liu, Pengfei Lin, and Yiwen Li

Decision-makers need reliable projections of future sea level change for risk assessment. Untangling the sources of uncertainty in sea level projections will help narrow the projection uncertainty. Here, we separate and quantify the contributions of internal variability, intermodel uncertainty, and scenario uncertainty to the ensemble spread of dynamic sea level (DSL) at both the basin and regional scales using Coupled Model Intercomparison Project Phase 6 (CMIP6) and FGOALS-g3 large ensemble (LEN) data. For basin-mean DSL projections, intermodel uncertainty is the dominant contributor (>55%) in the near- (2021-2040), mid- (2041-2060), and long-term (2081-2100) relative to the climatology of 1995-2014.  Internal variability is of secondary importance in the near- and mid-term until scenario uncertainty exceeds it in all basins except the Indian Ocean in the long-term. For regional-scale DSL projections, internal variability is the dominant contributor (60~100%) in the Pacific Ocean, Indian Ocean and western boundary of the Atlantic Ocean, while intermodel uncertainty is more important in other regions in the near-term. The contribution of internal variability (intermodel uncertainty) decreases (increases) in most regions from mid-term to long-term. Scenario uncertainty becomes important after emerging in the Southern, Pacific, and Atlantic oceans. The signal-to-noise (S/N) ratio maps for regional DSL projections show that anthropogenic DSL signals can only emerge from a few regions. Assuming that model differences are eliminated, the perfect CMIP6 ensemble can capture more anthropogenic regional DSL signals in advance. These findings will help establish future constraints on DSL projections and further improve the next generation of climate models.

How to cite: Jin, C., Liu, H., Lin, P., and Li, Y.: Uncertainties in the projection of dynamic sea level in CMIP6 and FGOALS-g3 large ensemble, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4335, https://doi.org/10.5194/egusphere-egu24-4335, 2024.

EGU24-5010 | ECS | Orals | CL4.9

Mid Holocene relative sea-level changes from coral microatolls of Pulau Biola, Singapore  

Jennifer Quye-Sawyer, Jing Ying Yeo, Wan Lin Neo, Zihan Aw, Lin Thu Aung, Nurul Syafiqah Tan, Junki Komori, Ke Lin, Xianfeng Wang, and Aron J. Meltzner

Coral microatolls are precise proxies of relative sea-level (RSL) change in low-latitude coastal regions. These coral colonies live in the intertidal zone where partial mortality due to low-water events produces a characteristic planform ring structure. Since ring elevations reflect changes in local RSL during a coral’s lifetime, we can use the surface profiles of microatolls to quantify short-term (decadal) rates of RSL change. Therefore, Holocene fossil microatolls can produce sea-level index points (SLIPs) with relatively high spatial and temporal resolution. In this study, we present preliminary sea-level reconstructions from Pulau Biola (Violin Island), Singapore, based upon several Porites sp. and Diploastrea heliopora fossil microatolls. We calculated the difference in elevation between the fossils and local living microatolls of the same genus to quantify the magnitude of past water level. We also combined U-Th ages, structure-from-motion photogrammetry and LiDAR 3D models, and survey data to generate a RSL history spanning more than two centuries in the mid Holocene. The highest-elevation fossil microatolls at Pulau Biola are consistent with an overall rise in sea level, from 0.2 to 0.7 m above present, between 7.7 and 7.4 kyr BP. In addition, decimetre-scale sea-level fluctuations during this period are inferred from decreasing and increasing ring elevations within corals. These fluctuations indicate a more complex sea-level history than resolved by other proxies or glacial isostatic adjustment models, and ongoing work aims to reconcile conflicting Holocene sea-level models and datasets in the Singapore region.

How to cite: Quye-Sawyer, J., Yeo, J. Y., Neo, W. L., Aw, Z., Aung, L. T., Tan, N. S., Komori, J., Lin, K., Wang, X., and Meltzner, A. J.: Mid Holocene relative sea-level changes from coral microatolls of Pulau Biola, Singapore , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5010, https://doi.org/10.5194/egusphere-egu24-5010, 2024.

Southern Hemisphere observational records of sea-level change are rare prior to ~1950, making it difficult to close historical regional sea-level budgets and quantify individual contributions to sea level (e.g. barystatic, thermosteric). Recent work generated four microfossil-based high-resolution sea-level reconstructions from southeastern Australia, all of which indicated rapid regional rates of 20th century sea-level rise compared to the global average. However, unlike analogous work in the North Atlantic (for which there is a high-density network of high-resolution reconstructions), there remains a paucity of proxy data from the Southern Hemisphere, hindering a probabilistic estimate of regional drivers of relative sea level using a spatio-temporal model.

We generate two new reconstructions using salt-marsh foraminifera from King Island, Tasmania, and Venus Bay, Victoria, to add to a growing database of Common Era sea-level reconstructions from southeastern Australia (and indeed wider Australasia). Fossil foraminifera indicate a rising palaeomarsh over the last ~150 years of ~0.15-0.25 m (average); this is interpreted as relative sea-level rise consistent with patterns observed in recent reconstructions. A chronology for the core is developed using both 14C and lead isotopes. Ongoing efforts to supplement the regional database will allow us to determine local and regional drivers of relative sea-level change in the region.

How to cite: Williams, S., Carvalho, R., Perry, P., Reef, R., and Sefton, J.: Drivers of Common Era sea-level change in southeastern Australia: extending tide-gauge records and developing a network of high-resolution reconstructions for regional analyses., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5258, https://doi.org/10.5194/egusphere-egu24-5258, 2024.

EGU24-6564 | Orals | CL4.9

Emulating Thermosteric Sea-Level Rise Using a Three-Layer Energy Balance Model 

Víctor Malagón-Santos, Aimée Slangen, Tim Hermans, Tamsin Edwards, and Fiona Turner

Although the mass loss of land ice is projected to be the largest contribution to sea-level rise in the coming centuries, thermal expansion will also be an important contributor and its accurate projection is primordial to understanding sea-level change over (multi-)centennial timescales. Earth System Models (ESMs) are the main tools for projecting thermosteric sea-level rise. ESMs, however, are computationally demanding and therefore long, multi-centennial simulations are challenging. In this study, we use a physical-based emulator that simplifies the climate system by using three vertically stacked layers, allowing us to mimic the energy balance response of ESMs to reproduce their thermal expansion simulations. We use this emulator to extrapolate simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) from 2100 to 2300 and validate our method with CMIP6 runs that are available over that time scale. Overall, the three-layer emulator outperforms its two-layer predecessor in simulating thermal expansion up to 2300, providing a reduction of up to 78% in cumulative error for the projection period covering 2100 to 2300. Finally, we demonstrate how using temperature output from the three-layer model can help us capture non-linearities in dynamic sea-level change better than its two-layer counterpart. The latter is a first step towards building more reliable emulation approaches for oceanic processes affecting regional sea-level change.

How to cite: Malagón-Santos, V., Slangen, A., Hermans, T., Edwards, T., and Turner, F.: Emulating Thermosteric Sea-Level Rise Using a Three-Layer Energy Balance Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6564, https://doi.org/10.5194/egusphere-egu24-6564, 2024.

EGU24-6921 | Posters on site | CL4.9

Modelling dependence between the ice-sheet components of sea-level rise 

Benjamin S. Grandey, Justin Dauwels, Svetlana Jevrejeva, Antony J. Payne, Zhi Yang Koh, Benjamin P. Horton, and Lock Yue Chew

Sea-level projections are sensitive to statistical dependence between the East Antarctic, West Antarctic, and Greenland ice-sheet components.  The dependence is produced by climate uncertainty and ice-sheet process uncertainty.  To investigate this dependence, we model the dependence using copulas.  We use a vine copula to couple the ice-sheet components of projected sea level in 2100 under the SSP5-8.5 scenario.  Assumptions about rank correlation and copula family influence both the centre and the tails of the total ice-sheet contribution.  For example, rank correlation can influence the 95th percentile by approximately 50%.  We explore three alternative approaches for specifying the dependence: shared dependence on global-mean surface temperature, dependence derived from ice-sheet model ensembles, and dependence derived from expert judgement.  Shared dependence on global-mean surface temperature produces little dependence between the ice-sheet components.  In contrast, ice-sheet model ensembles suggest that the dependence between the East and West Antarctic ice-sheet components may be strong, amplifying the uncertainty in future sea-level rise.

How to cite: Grandey, B. S., Dauwels, J., Jevrejeva, S., Payne, A. J., Koh, Z. Y., Horton, B. P., and Chew, L. Y.: Modelling dependence between the ice-sheet components of sea-level rise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6921, https://doi.org/10.5194/egusphere-egu24-6921, 2024.

EGU24-7891 | Orals | CL4.9

Revisiting the relation between ocean heat storage and thermal expansion from a water mass perspective 

Robin Waldman, Benoît Meyssignac, Sébastien Fourest, Robin Guillaume-Castel, Karina von Schuckmann, and Jean-Baptiste Sallée

The excess anthropogenic ocean heat is causing thermal expansion, which has driven approximately 40% of the industrial-era global mean sea level rise. This relation between ocean heat uptake H and thermosteric sea level rise hθ is mediated by the so-called expansion efficiency of heat (EEH=hθ/H, in m/YJ) which characterises the expansion of a water-mass under a unit increase of its enthalpy. The EEH of a water-mass depends on its temperature, salinity and pressure. At global scale the EEH has been characterized in both historical observations and climate simulations, but the the role of regional EEH and of individual water-mass layers in the formation of this global expansion efficiency remains undocumented. Here we propose a new approach where the EEH is decomposed in temperature coordinate into a temperature plus a pressure contribution to seawater thermal expansion. We show that the temperature contribution largely dominates the global signal. We also show that the global EEH can be interpreted as a weighted global average thermal expansion coefficient.

We make use of the global EEH decomposition in temperature coordinate to estimate the contribution of individual water-mass layers to global thermal expansion in both historical reference observational datasets and Climate Model Intercomparison Project (CMIP5-6) historical and scenario simulations. Results show a contrasting picture of water mass contributions to global thermal expansion and sea level rise. Whereas ocean warming is distributed between mode, intermediate and deep waters, a disproportionate share of global ocean expansion occurs within tropical waters and subtropical mode waters. Regionally, tropical Pacific waters and subtropical north Atlantic mode waters appear as key contributors to global thermal expansion. These results show that the regional distribution of ocean heat uptake is a key driver of thermal expansion and sea level rise not only at regional scale but also at global scale. We also show that projections of future sea level rise at global scale critically depend on the ability of climate models to simulate both the regional water mass properties and their heat uptake.

How to cite: Waldman, R., Meyssignac, B., Fourest, S., Guillaume-Castel, R., von Schuckmann, K., and Sallée, J.-B.: Revisiting the relation between ocean heat storage and thermal expansion from a water mass perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7891, https://doi.org/10.5194/egusphere-egu24-7891, 2024.

EGU24-8462 | ECS | Posters on site | CL4.9

Eddy variability contribution to decadal regional sea level trends 

Benoit Laurent, William Llovel, Anne-Marie Treguier, and Antoine Hochet

Sea level rise is one of the most direct consequences of the actual global warming. Over the 20th century, global mean sea level rises at a rate of 1.5-2 mm. yr-1. Since the beginning of the 1990s, satellite altimetry measure the changes of sea level with a near global coverage (from 66oS to 75oN). The use of satellite altimetry has, for the first time, highlighted large regional variability in sea level trends that significantly differ from the global mean estimate. If global ocean warming and land ice melting (mountain glaciers and ice sheets from Greenland and Antarctica) are the main processes explaining the observed global mean sea level rise, at regional scales, other processes are involved, such as changes in salinity or temperature associated with ocean circulation or air-sea fluxes at the ocean surface.

 

Sea level projections used in IPCC reports are based primarily on coarse-resolution coupled climate models. Current projections are based on climate models in which ocean-eddy variabilities are parameterized and results deviate from observations especially in the Southern Ocean. Mesoscale processes transport heat/freshwater over very large distances in the ocean (both horizontally and vertically). They also regulate energy, moisture and carbon exchanges between the oceans and the atmosphere via coupling. Understanding these processes and how they might change in the future is critical for portraying robust regional sea level change.

 

Recently, new generations of climate models have been integrated at spatial resolutions of ¼° and 1/12°, which is sufficient to partially resolve the mesoscale eddy variability. These higher resolutions enable the study of the impact of mesoscale eddies on regional sea level changes and how these processes may change in the future.

 

In this work, we will take advantage of a suite of climate model simulations based on HadGEM3-GC3.1 at different spatial resolution (1°, ¼° and 1/12°) to assess the contribution of eddy-variability on regional sea level trends. We first present the ability of such climate models to reproduce regional sea level trends observed by satellite altimetry over decadal to multi-decadal time periods. Second, temperature and salt budget will be presented to quantify the contribution of eddy variability on these regional sea level trends.

How to cite: Laurent, B., Llovel, W., Treguier, A.-M., and Hochet, A.: Eddy variability contribution to decadal regional sea level trends, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8462, https://doi.org/10.5194/egusphere-egu24-8462, 2024.

EGU24-9008 | Posters on site | CL4.9

Sea-level projections in recent IPCC reports: how we got here, where we are and where we’re going  

Aimée Slangen, Matthew Palmer, Carolina Camargo, John Church, Tamsin Edwards, Tim Hermans, Helene Hewitt, Gregory Garner, Jonathan Gregory, Robert Kopp, Victor Malagon Santos, and Roderik van de Wal

Sea-level science has seen many recent developments in observations and modelling of the different contributions and the total mean sea-level change. Here, we focus on sea-level projections in the recent IPCC reports, and discuss (1) the evolution in IPCC projections (“how we got here”), (2) how the projections compare to observations (“where we are”) and (3) the outlook for further improving projections (“where we’re going”). We start by discussing how the model projections of 21st century sea-level change have changed from the IPCC AR5 report (2013) to SROCC (2019) and AR6 (2021), highlighting similarities and differences in the methodologies and comparing the global mean and regional projections. This shows that there is good agreement in the median values, but also highlights some differences. In addition, we discuss how the different reports included high-end projections. We then show how the AR5 projections (from 2007 onwards) compare against the observations, and find that they are highly consistent with each other. Finally, we discuss how to further improve sea-level projections in future studies.

How to cite: Slangen, A., Palmer, M., Camargo, C., Church, J., Edwards, T., Hermans, T., Hewitt, H., Garner, G., Gregory, J., Kopp, R., Malagon Santos, V., and van de Wal, R.: Sea-level projections in recent IPCC reports: how we got here, where we are and where we’re going , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9008, https://doi.org/10.5194/egusphere-egu24-9008, 2024.

EGU24-9762 | ECS | Orals | CL4.9

The interannual and decadal sea level variabilities over the Indo-Pacific Oceans in the Reanalysis and CMIP6 Historical Simulations and Projections 

Ponni Maya, José Antonio Álvarez Antolínez, Deepa Js, and Chellappan Gnanaseelan

In climatological research, understanding past and accurately simulating future sea level variability is paramount due to the considerable risk that sea level changes pose to low-lying regions, coupled with their significant influence on the occurrence and severity of extreme meteorological events  . This research insights are vital in evaluating the potential impact on renewable energy sources, particularly offshore wind, wave, and tidal energy, where changes in sea level can significantly alter the efficiency and viability of these energy converters. This study comprehensively analyses sea level variability on interannual and decadal scales in the Indo-Pacific region, integrating data from the Ocean Reanalysis System 5 (ORAS5), CMIP6 historical simulations spanning from 1850-2014, and future projections under the CMIP6 future intermediate emission scenario (rcp245/ssp245) for the period 2015 to 2100. Our investigation spans key areas such as the Northwest Central Pacific Ocean (NWCPO), the Eastern Equatorial Pacific Ocean (EEPO), and the Thermocline Ridge of the Indian Ocean (TRIO), among others.
We report findings on interannual and decadal Sea Level Anomaly (SLA) variability, especially highlighting the TRIO region and various Pacific Ocean zones such as the SWPO, NWCPO, EEPO, and NWNPO. Our study identifies a substantial increase in interannual variability in the NWNPO. We also observe consistent sea-level variability patterns across these regions, extending into future projections under moderate emission scenarios.
We find that the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole, and the Pacific Decadal Oscillation are key drivers of these variabilities. Our study reveals a strong connection between sea levels in the Equatorial Pacific and the Niño 3.4 index, suggesting its potential as a sea level-based indicator for El Niño and La Niña events.
Our research highlights the critical role of atmospheric forcing in driving sea level variability. We link high sea-level variability regions to significant wind stress curl anomalies, with distinct differences between hemispheres. We explore the mechanics of equatorial variability, emphasizing the role of equatorial Kelvin waves and local and remote Rossby waves in different oceanic regions.
Our study concludes that most CMIP6 models, despite large model uncertainty, predict an increase in sea level variability for the upcoming century, particularly in the Pacific Ocean, emphasizing the need for heightened attention to this dynamic region in the context of global climate change .

How to cite: Maya, P., Álvarez Antolínez, J. A., Js, D., and Gnanaseelan, C.: The interannual and decadal sea level variabilities over the Indo-Pacific Oceans in the Reanalysis and CMIP6 Historical Simulations and Projections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9762, https://doi.org/10.5194/egusphere-egu24-9762, 2024.

EGU24-9936 | ECS | Posters on site | CL4.9

Linking the Permanent Service for Mean Sea Level’s (PSMSL) global mean sea level dataset to the ellipsoid  

Chanmi Kim, Andrew Matthews, and Elizabeth Bradshaw

The Permanent Service for Mean Sea Level (PSMSL) is the internationally recognised global sea level data bank for long term sea level change information from tide gauges, responsible for the collection, publication, analysis and interpretation of sea level data. The PSMSL was founded 90 years ago, and today operates from the Liverpool site of the UK’s National Oceanography Centre. 

The PSMSL’s main product, a dataset of monthly and annual means from over 2000 locations worldwide aggregated from over 200 suppliers, is a cornerstone in our understanding of changes in sea level over the two centuries. For our highest quality Revised Local Reference (RLR) dataset, we ensure the data can all be referred to a fixed point on land, ensuring a consistent vertical reference frame is used throughout the record. Also, we provide GNSS solutions near the guage to estimate the ellipsoidal height and rate of movement of the site in our website.

Here we introduce the PSMSL mean sea level dataset, and explain how we present these ellipsoidal ties on our website. We also discuss ongoing efforts to improve the breadth of metadata we supply, and attempts to ensure they meet FAIR data practices (Findable, Accessible, Interoperable and Reusable). 

 

How to cite: Kim, C., Matthews, A., and Bradshaw, E.: Linking the Permanent Service for Mean Sea Level’s (PSMSL) global mean sea level dataset to the ellipsoid , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9936, https://doi.org/10.5194/egusphere-egu24-9936, 2024.

EGU24-10137 | ECS | Posters on site | CL4.9

Reconstruction of the global ocean heat content and thermosteric sea-level rise with an improved configuration of the ISAS interpolation tool 

Rémy Asselot, Nicolas Kolodziejczyk, William Llovel, Kevin Balem, and Annaïg Prigent-Mazella

Anthropogenic greenhouse gas emissions have caused an imbalance in the energy content of the Earth's system, warming the atmosphere, the land surface, the cryosphere and the ocean. On a global scale, over the last five decades, the ocean has stored more than 90% of the heat excess associated with the Earth energy imbalance. This absorption of heat by the ocean leads to an increased Oceanic Heat Content (OHC). As the OHC rises, the global mean sea-level increases due to thermal expansion, a mechanism known as the global mean thermosteric sea-level (TSL) rise. In order to monitor accurately the global OHC and global mean TSL, one of the main sources of data is in situ Temperature and Salinity profiles. These profiles need to be interpolated on a regular grid to prevent any bias due to regional over or under-sampling. However, to date, OHC and TSL estimates and their associated uncertainties are sensitive to the parameterization and a priori assumption of the interpolation tools. To address this issue in a controlled framework, we run sensitive experiments where we adjust the configuration of the In Situ Analysis System (ISAS) interpolation tool. To do so, we extracted “synthetic profiles” of Temperature and Salinity from NEMO simulations, integrated over the 1980-2020 period.  We interpolated these profiles with ISAS and compared them with the original model outputs. This comparison allows us to improve the parameterization and a priori assumption of ISAS in order to, ultimately, provide a better understanding of the sensitivity of the global and regional OHC and TSL estimates. Here we present the first results of this work.

How to cite: Asselot, R., Kolodziejczyk, N., Llovel, W., Balem, K., and Prigent-Mazella, A.: Reconstruction of the global ocean heat content and thermosteric sea-level rise with an improved configuration of the ISAS interpolation tool, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10137, https://doi.org/10.5194/egusphere-egu24-10137, 2024.

EGU24-10576 | ECS | Orals | CL4.9 | Highlight

Extreme sea-level projections along European coasts for climate adaptation services  

Maialen Irazoqui Apecechea, Angelique Melet, Guillaume Reffray, and Goneri Le Cozannet

Sea-level rise is one of the most hazardous climate-change impacts and is projected to trigger dramatic increases of coastal flooding frequency in Europe in the current century and beyond.  As such, adaptation-related effective decision making relies on the availability of authoritative and locally relevant information on future coastal sea-levels and their extremes, which include uncertainty quantification. However, current available sea-level projections are typically limited by either too low spatial resolution and therefore missing physical processes relevant at the coast, they account for only part of the sea-level signal (e.g. storm surges), and/or are typically limited to the downscaling of a single atmospheric model and therefore offer no quantification of the potentially significant inter-model uncertainty.   

In response to this knowledge gap, we present a novel extreme sea-level (ESL) projection dataset which focuses on the North-east Atlantic region. The dataset consists of a CMIP6-forced multi-model ensemble of downscaled projections until the end of the century, generated with a regional 3-dimensional ocean model at ~7km resolution. As such, the model captures not only storm-surge and tide induced ESLs, typically captured in barotropic 2-dimensional models, but also accounts for the contribution of circulation and density-driven modulations to extremes. Therefore, the ensemble dataset offers an excellent opportunity to explore ESL drivers at different spatio-temporal scales, their projected future changes, and associated uncertainties.

This dataset will help to advance scientific knowledge on climate-change induced coastal flood risk changes, but also to increase confidence in quantitative assessments of impacts of sea-level rise through its contribution to the Coastal Climate Core Service (CoCliCo), a decision-oriented platform which will inform users on present-day and future coastal risks, and which is currently under development as part of a European Union’s Horizon 2020 project.

The CoCliCo project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101003598

How to cite: Irazoqui Apecechea, M., Melet, A., Reffray, G., and Le Cozannet, G.: Extreme sea-level projections along European coasts for climate adaptation services , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10576, https://doi.org/10.5194/egusphere-egu24-10576, 2024.

EGU24-10716 | Posters on site | CL4.9 | Highlight

Exploring multi-century sea level rise commitments from 21st century cumulative emissions to inform minimum coastal adaptation needs 

Alexander Nauels, Zebedee Nicholls, Uta Klönne, Tim Hermans, Matthias Mengel, Christopher J. Smith, and Matthew D. Palmer

It is crucial to explore multi-century sea level responses under different emissions scenarios despite underlying physical uncertainties that rapidly increase over time, because resulting coastal risks fully manifest only on these longer timescales. Here, we use a set of climate and sea level emulators to investigate sea level rise commitments out to 2300 for cumulative emission levels at the start of every remaining 21st century decade under the five illustrative SSP-RCP scenarios. Our results indicate that emissions until 2030 “lock in” around 1.0 m (66% model range: 0.8 to 1.3 m) of global mean sea level rise in 2300 relative to 1995-2014. Under an intermediate emissions scenario roughly in line with current climate policies (SSP2-4.5), median 2300 global mean sea level commitments for cumulative emissions in 2050 (1.2 m) and 2100 (1.7 m) would be around 0.1 m and 0.6 m higher than under a very low emissions scenario (SSP1-1.9). Global results are also downscaled to selected regional sites and highlight that particularly vulnerable regions like low-lying Pacific Islands will experience higher local committed sea level rise than the global average. By attributing projected sea level rise commitments in 2300 to different cumulative emission levels in the 21st century, the study aims to more clearly link mitigation efforts in the near term to longer term coastal risk and to inform minimum adaptation requirements under different climate futures.

How to cite: Nauels, A., Nicholls, Z., Klönne, U., Hermans, T., Mengel, M., Smith, C. J., and Palmer, M. D.: Exploring multi-century sea level rise commitments from 21st century cumulative emissions to inform minimum coastal adaptation needs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10716, https://doi.org/10.5194/egusphere-egu24-10716, 2024.

EGU24-11869 | Orals | CL4.9

Relative sea level rise trends and projections up to 2150 along the Italian coasts: implications for coastal flooding 

Marco Anzidei, Antonio Vecchio, Tommaso Alberti, Enrico Serpelloni, and Anita Grezio

We focus on the current and future sea level (SL) trends along the Italian coasts which are affected by spatially variable rates of Vertical Land Movements (VLM) in response to tectonics and anthropic activities. Since VLM play a crucial role in local sea level rise along the coasts, they need to be estimated and incorporated in the analysis for more affordable sea level rise projections.

To estimate the current VLM rates we used geodetic data from about 27 years of continuous GNSS observations at a set of stations belonging to Euro-Mediterranean networks located within 5 km from the coast. Revised SL projections up to the year 2150 are provided at a set of points on a geographical grid and at the location of some tide gauges belonging to the PSMSL network, by including the estimated VLM in the SL projections released by the IPCC in the AR6 Report. Our results show that the current IPCC projections are not representative of the expected future sea levels since they do not properly consider the effects of tectonics and other local factors. Here we show that revised multi-temporal sea level projections at 2030-2050-2100 and 2150 show significant differences with respect to those of the IPCC for different Shared Socio-economic Pathways and global warming levels. Finally, our results indicate that about 1600 km of length and 10.000 km2 of the considered Italian coasts are yet exposed to flooding risk, with enhanced impacts on the environment, human activities and coastal infrastructures, in particular in 39 coastal plains. With the above scenarios, and especially in case of eventual instabilities of the Greenland and west Antarctica ice sheets, the effects of extreme meteorological events and tsunamis, will soon amplified along the Italian coasts, with serious concerns for main and small islands. Therefore, actions are needed to support vulnerable populations to adapt to the expected relative sea level rise by the end of this century and beyond.

How to cite: Anzidei, M., Vecchio, A., Alberti, T., Serpelloni, E., and Grezio, A.: Relative sea level rise trends and projections up to 2150 along the Italian coasts: implications for coastal flooding, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11869, https://doi.org/10.5194/egusphere-egu24-11869, 2024.

EGU24-13354 | ECS | Posters on site | CL4.9

GravIS Portal: User-friendly Ocean Bottom Pressure data from GRACE and GRACE-FO 

Christoph Dahle, Eva Boergens, Henryk Dobslaw, Ingo Sasgen, Thorben Döhne, Sven Reißland, and Frank Flechtner

The German Research Centre for Geosciences (GFZ), together with the Alfred-Wegener-Institute (AWI) and the Technische Universität Dresden, maintains the ‘Gravity Information Service’ portal (GravIS, gravis.gfz-potsdam.de). GravIS facilitates the dissemination of user-friendly data of mass variations in the Earth system, based on observations of the US-German satellite missions GRACE (Gravity Recovery and Climate Experiment, 2002-2017) and its successor GRACE-FO (GRACE Follow-On, since 2018).

The portal provides ocean bottom pressure (OBP) data on a global 1° grid. Two versions of the product are provided, based on spherical harmonic coefficients taken from either the most recent GRACE/GRACE-FO release from GFZ or from the International Combination Service for Time-variable Gravity Fields (COST-G). Corrections applied to the data include the insertion of estimates of the geocentre motion, replacement of the C20 and C30 coefficients, corrections of the co- and postseismic deformations after the three megathrust earthquakes (Sumatra-Andaman 2004, Chile 2010, Japan-Tohoku 2011), and the correction for glacial isostatic adjustment with the ICE-6G model.

The data product consists of barystatic sea-level pressures calculated from the gravity data using the sea-level equation. Residual ocean circulation is provided as well. Besides the gridded products, regional average time series are also available for predefined ocean regions.

In addition to the OBP data, GravIS provides terrestrial water storage (TWS) variations over the continents and ice mass variations over Greenland and Antarctica. These data sets are also provided either as grids or regional averages.

The data sets of all Earth system domains can be interactively displayed within the portal and are freely available for download. This contribution aims to show the features of the GravIS portal and its potential benefit to sea-level and ocean science applications.

How to cite: Dahle, C., Boergens, E., Dobslaw, H., Sasgen, I., Döhne, T., Reißland, S., and Flechtner, F.: GravIS Portal: User-friendly Ocean Bottom Pressure data from GRACE and GRACE-FO, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13354, https://doi.org/10.5194/egusphere-egu24-13354, 2024.

EGU24-14249 | ECS | Posters on site | CL4.9

Mid-Holocene relative sea-level reconstruction from digital surface models of coral microatolls at Pulau Semakau, southwestern Singapore  

Lin Thu Aung, Nural Syafiqah Tan, Jennifer Susan Quye-Sawyer, Fangyi Tab, Junki Komori, Zihan Aw, Jing Ying Yeo, Wan Lin Neo, Maya Baltz, and Aron Maltzner

Coral microatolls are coral colonies that grow with distinct morphologies consisting of living polyps on their outer perimeters and dead upper surfaces with concentric rings in planform. Their upward growth is limited by the lowest tides, allowing them to be used as precise indicators of relative sea-level (RSL) change. Therefore, detailed morphological investigation of fossil microatolls provides an important proxy for the reconstruction of past RSL. We present a preliminary RSL reconstruction from Pulau Semakau (Semakau Island), southwestern Singapore, based on digital surface models (DSMs) of fossil corals captured by portable iPhone LiDAR integrated with field survey data and radiocarbon analysis. Pulau Semakau is the largest field site in Singapore, with an intertidal flat extending more than 2 km long by 0.4 km wide, on which we observed 79 living and 65 fossil microatolls containing well preserved, concentric rings. In this study, we reconstruct mid-Holocene RSL using seven fossil, Diploastrea heliopora microatolls, relative to living counterparts on the island. DSMs indicate that three of these fossil corals are lower in elevation at the center with higher outer rings, indicating gradual RSL rise between ~7700 and 7500 cal yr BP. Conversely, three fossil corals are observed to decrease in elevation from the innermost to outermost rings, indicative of RSL fall between ~7350 and 7200 cal yr BP. These observations are consistent with but more well constrained than the existing sea-level curve of Singapore based on sea-level index points (SLIPs) and limiting dates from intertidal mangrove and shallow marine sediments. RSL records between ~7500 and 7350 cal yr BP are largely uncertain due to erosion of a fossil coral, and this remains as future work. The initial results reflect mid-Holocene RSL fluctuations at Pulau Semakau, from ~7700 to 7200 cal yr BP.

How to cite: Aung, L. T., Tan, N. S., Quye-Sawyer, J. S., Tab, F., Komori, J., Aw, Z., Yeo, J. Y., Neo, W. L., Baltz, M., and Maltzner, A.: Mid-Holocene relative sea-level reconstruction from digital surface models of coral microatolls at Pulau Semakau, southwestern Singapore , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14249, https://doi.org/10.5194/egusphere-egu24-14249, 2024.

EGU24-14926 | ECS | Posters on site | CL4.9

Sea level projections for the German Coast 

Corinna Jensen, Frank Janssen, Jens Möller, and Tim Kruschke

Sea level rise is a certain consequence and one of the most important threats associated with climate change. It increases the risk of flooding of low-lying land at the German Coast.

In cooperation of the “Network of Experts” of the German Federal Ministry for Digital and Transport and the DAS core service “climate and water”, we aim to provide high-quality projections of relative sea level change for the German coastal areas, both in terms of spatial data as well as time series for specific stations. Most of the drivers for sea level change must be considered on a continental or global scale. The main exception for this in northern Europe is land uplift as its impacts are regional and dependent on glacial isostatic adjustment as well as local processes. We therefore created a new set of sea level projections for the North Sea and Baltic Region. This dataset is based on the IPCC 6th Assessment Report (AR6) projections of absolute sea level change, which we combine with a new and high-resolution land elevation model over Fennoscandia (instead of the coarse land elevation model for this region used in the IPCC AR6). The data will eventually be published via the “DAS core service”.

 

How to cite: Jensen, C., Janssen, F., Möller, J., and Kruschke, T.: Sea level projections for the German Coast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14926, https://doi.org/10.5194/egusphere-egu24-14926, 2024.

EGU24-15877 | ECS | Posters on site | CL4.9 | Highlight

Regional variations in relative sea level changes influenced by non-linear vertical land motion  

Julius Oelsmann, Marta Marcos, Marcello Passaro, Laura Sanchez, Denise Dettmering, Sönke Dangendorf, and Florian Seitz

Vertical land movements can cause regional relative sea level changes to differ substantially from climate-driven absolute sea level changes. While absolute sea level has been accurately monitored by satellite altimetry since 1992, there are limited observations of vertical land motion. Vertical land motion is generally modeled as a linear process, despite some evidence of non-linear motion associated with tectonic activity, changes in surface loading, or groundwater extraction. As a result, the temporal evolution of vertical land motion, and its contribution to projected sea level rise and its uncertainty, remains unresolved. Here, we present a probabilistic vertical land motion reconstruction from 1995-2020 and determine the impact of regional scale and non-linear vertical land motion on relative sea level projections up to 2150. We show that regional variations in projected coastal sea level changes are equally influenced by vertical land motion and climate-driven processes, with vertical land motion causing relative sea level changes of up to 50 cm by 2150. Accounting for non-linear vertical land motion increases the uncertainty in projections by up to 1 m on a regional scale. Our results highlight the uncertainty in future coastal impacts and demonstrate the importance of including non-linear vertical land motions in sea level change projections. In addition to its application to regional sea level projections, the vertical land motion estimate is an important source of information for various sea level studies focusing on the analysis of tide gauge or satellite altimetry observations in coastal areas.

How to cite: Oelsmann, J., Marcos, M., Passaro, M., Sanchez, L., Dettmering, D., Dangendorf, S., and Seitz, F.: Regional variations in relative sea level changes influenced by non-linear vertical land motion , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15877, https://doi.org/10.5194/egusphere-egu24-15877, 2024.

EGU24-15948 | ECS | Posters on site | CL4.9

Drivers of Late Holocene relative sea-level change in the Sunda Shelf: new insights from coral microatolls in Singapore 

Fangyi Tan, Benjamin Horton, Ke Lin, Tanghua Li, Maeve Upton, Yucheng Lin, Jennifer Walker, Trina Ng, Jennifer Quye-Sawyer, Joanne TY Lim, Shi Jun Wee, Nurul Syafiqah Tan, and Aron Meltzner

Existing Late Holocene relative sea-level (RSL) records from the Sunda Shelf suffer from spatial and temporal discontinuities and/or a lack of precision, hindering an understanding of the drivers of RSL change. Here, we present the first RSL record from fossil coral microatolls in Singapore, which has high vertical (<± 0.20 m, 2𝜎) and temporal (<± 26 yrs, 95% highest density region) precision.

We applied a novel approach to produce sea-level index points and infer sea-level tendencies by combining (1) the use of photogrammetry with traditional levelling techniques; (2) 230Th dating; and (3) surface morphologies of the fossil coral microatolls. The fossil corals reveal a gradual, 0.31 ± 0.18 m (2𝜎) fall in RSL between 2.8 kyrs BP and 0.6 kyrs BP, with rates averaging 0.15 ± 0.10 mm/yr (2𝜎). Our coral record lies within uncertainty of some of the published RSL records from the region but disagrees with others, suggesting that local to regional processes may be driving spatial variability in RSL in the region. Misfits of the data with glacial isostatic adjustment (GIA) models may be explained by the influence of non-GIA processes, such as vertical land motion, and/or the need to fine-tune GIA model parameters. Work is ongoing to decompose the drivers of relative sea-level change within the region.

How to cite: Tan, F., Horton, B., Lin, K., Li, T., Upton, M., Lin, Y., Walker, J., Ng, T., Quye-Sawyer, J., Lim, J. T., Wee, S. J., Tan, N. S., and Meltzner, A.: Drivers of Late Holocene relative sea-level change in the Sunda Shelf: new insights from coral microatolls in Singapore, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15948, https://doi.org/10.5194/egusphere-egu24-15948, 2024.

EGU24-16146 | Posters on site | CL4.9

Spatially variable sea level response to erosion and deposition in Aotearoa New Zealand 

Gregory Ruetenik, John D. Jansen, and Ken L. Ferrier

Surface processes alter sea level by warping Earth’s surface and modifying the gravitational field. Recent studies show that paleo-sea level indicators are depressed by sedimentation near major depocenters, such as the Mississippi and Indus deltas, and raised by the erosion of rock in rapidly eroding coastal regions such as Taiwan. The South Island of Aotearoa New Zealand poses an interesting combination of these endmembers because the Southern Alps are eroding rapidly on the west coast, while high sediment loads are deposited along the eastern margin. Here, we use a global, gravitationally self-consistent sea-level model to demonstrate that sediment redistribution on the South Island drastically alters interpretations of sea level change since the Last Interglacial (Marine Isotope Stage 5e) by as much as +100 m on the west coast and –30 m on the east coast. The influence of sediment redistribution on sea level is highly sensitive to geodynamic properties such as effective elastic thickness, which we reconcile using the abundance of paleo-shoreline markers available.

How to cite: Ruetenik, G., Jansen, J. D., and Ferrier, K. L.: Spatially variable sea level response to erosion and deposition in Aotearoa New Zealand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16146, https://doi.org/10.5194/egusphere-egu24-16146, 2024.

EGU24-16387 | Posters on site | CL4.9

Sea-level scenarios for coastal adaptation: the example of France 

Rémi Thiéblemont and Gonéri Le Cozannet

Climate change scenarios are a typical request of adaptation practioners. Within its third national adaptation plan, France is developing a consistent set of climate scenarios based on global warming levels. The scenario currently under consideration would lead to a global mean temperature increase of 3°C with respect to the preindustrial period, which is consistent with the current climate policies to 2100. Later on, these scenarios would be integrated in the regulation, for example in order to update risk assessment guidance.

Here, we present how sea-level rise scenarios aligned with this global warming level were produced. We selected emulated simulations for each component of future sea-level rise consistently, including ocean and ice components, following a method similar to that of the 6th assessment report of the IPCC, yet with specific attention to the consistency of uncertainty treatment before and after 2100. This responds to the needs to consider impacts of sea-level rise over hundred years, that is, to 2125 within coastal risk prevention plans. Furthermore, we added simulations considering a potential collapse of ice-sheets at 3°C of global warming levels in 2100. We consider only vertical land motions related to the Glacial Isostatic Adjustment as new observations from the Copernicus Land Motion service are now available for local stakeholder’s use.

The results show that the 87th percentile of projections is close to 80cm in 2100 with respect to 1995-2014 for the majority of mainland and overseas French regions, whether ice-sheets collapse is considered or not. Conversely, median values display differences of about 10cm depending whether ice sheet collapse is hypothesized or not. In the context of the development of these new scenarios, simplicity was considered a key criterion of success to ensure that all users - and not only those with high climate literacy - can effectively use scenarios. Hence, we propose to use one single scenario corresponding to the 87th percentile of the projections. This corresponds to a cautious approach consistent with the risk prevention policy in France. This does not preclude advanced users considering additional scenarios such as low-likelihood/high-impact scenarios voluntarily.

This work was performed within a project supported by the ministry in charge of Environment. We thank the steering and scientific committees of this project for useful comments and inputs.

How to cite: Thiéblemont, R. and Le Cozannet, G.: Sea-level scenarios for coastal adaptation: the example of France, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16387, https://doi.org/10.5194/egusphere-egu24-16387, 2024.

EGU24-16743 | ECS | Posters on site | CL4.9

Inferring climatic sea-level variations from microatolls in tectonically active regions 

Sophie Debaecker, Mikhail Karpytchev, Mélanie Becker, Nathalie Feuillet, and Kenji Satake

Coral microatolls are often used to reconstruct the relative sea-level (RSL) along tropical coastlines. They grow at a constant rate, developing each year a growth band that can be observed in their internal stratigraphy. As their development is limited by the water height, they record annual variations of the relative sea-level once they have reached the sea surface. These changes are related to both climate and tectonic, and several criteria are used to decipher both signals. For example, it is commonly accepted that a local signal would rather correspond to a tectonic event, and inversely. However, majority of the criteria such as regrowth of the coral, amplitude of the RSL anomaly or matches with seismic or climatic events catalogs are mainly qualitative and most of the time, incomplete. In our study, we seek to develop a mathematically sound method to separate the climatic signal recorded by a series of microatolls. We focused on the region of the Ryukyus islands in south-west Japan, where the Philippine sea plate plunges under the Eurasia plate. In this area, up to 15 modern and living corals have been collected previously; and their RSL records extend from 1762 to 2018. They extend over 900 km along the subduction zone. Despite the seismic activity of the area, it is possible to infer that any signal common to all microatolls can be considered as climatic. We used a statistical method over the corals dataset to extract a common-mode RSL signal over the island arc. We found a long term sea-level rise for the last 200 years. We further analyze shorter time trends and annual anomalies, and compare our results from the RSL records that include years where only minimum RSL was recorded. Additionally, to refine our method we aim to compare sea level changes recorded by tide gauge in the Ryukyus with the estimates from inferred from the coral microatolls from seismically stable regions in the Pacific Ocean.

How to cite: Debaecker, S., Karpytchev, M., Becker, M., Feuillet, N., and Satake, K.: Inferring climatic sea-level variations from microatolls in tectonically active regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16743, https://doi.org/10.5194/egusphere-egu24-16743, 2024.

EGU24-17955 | ECS | Posters on site | CL4.9

Sea-level storylines to inform coastal adaptation planning and decision-making for the UK, South Africa and Southeast Asia 

Jennifer Weeks, Matthew D. Palmer, Benjamin P. Horton, Trina Ng, Susan M. Parnell, and Antony Payne

Implementing responses to sea-level rise requires accessible, credible and relevant sea-level information to facilitate effective use by practitioners and decision-makers. However, recent consultations have highlighted the need to better translate sea-level information to meet the physical and cultural diversity of decision-making and planning across the world. This includes communicating sea-level rise across a range of timescales, providing information tailored to different risk tolerances and better linking sea-level rise to impacts analysis to provide useful and usable metrics (e.g., Weeks et al., 2023, Environ. Res. Commun.). 


The presence of ambiguity in sea-level projections means there are limitations in the use of probabilistic approaches in coastal planning and decision-making (Kopp et al., 2023, Nature Climate Change). Storylines (physically consistent and plausible pathways of future climate events) are increasingly being used as a distillation tool presented alongside probabilistic sea level projections, for example to address the challenge of “deep uncertainty” associated with the future response of the ice sheets. Here, we focus on the regionalisation of sea-level projections into a set of discrete, actionable future pathways, to meet the needs of coastal adaptation planners and decision-makers. Building on the work of Palmer et al., (2020) (Earth’s Future), we generate a set of sea-level storylines for coastal city locations in the UK, South Africa and Southeast Asia, constrained by different emissions scenarios and high-end sea-level rise estimates. Locations are chosen based on their population density and geographical spread, whilst the regions allow consideration of the different risk profiles and contexts for decision-making. This work explores a range of decision-making contexts and how the storyline framework can be tailored to different user needs. 

How to cite: Weeks, J., Palmer, M. D., Horton, B. P., Ng, T., Parnell, S. M., and Payne, A.: Sea-level storylines to inform coastal adaptation planning and decision-making for the UK, South Africa and Southeast Asia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17955, https://doi.org/10.5194/egusphere-egu24-17955, 2024.

EGU24-18361 | Orals | CL4.9

New sea level scenarios for the Netherlands 

Sybren S. Drijfhout, Dewi Le Bars, and Iris Keizer

We present the framework used to develop a new set of sea level scenarios for the Dutch coast published by KNMI in October 2023, to help the Netherlands adapt to sea level rise. Based on interactions with stakeholders, the development of the scenarios focused on two main areas: the connection between observations and projections and the development of low-likelihood high-impact scenarios up to 2300. We developed a local sea level budget for the period 1993-2021 to better understand past observations and to constrain the scenarios. In particular, the contribution of Ocean Dynamic Sea Level was important in the benchmark period 1993-2021, and observational evidence was used to select CMIP6 models that were close to the observations. For the low-likelihood high-impact scenarios three lines of evidence were used: structured expert judgement, a numerical model including Marine Ice Cliff Instability, and a physical evidence discussion. We also discuss some practical applications of these scenarios.

How to cite: Drijfhout, S. S., Le Bars, D., and Keizer, I.: New sea level scenarios for the Netherlands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18361, https://doi.org/10.5194/egusphere-egu24-18361, 2024.

EGU24-18660 | ECS | Orals | CL4.9

Understanding the Regional Disparity of the Sea Level Rise during Altimetry Era 

Rong Deng and Wenjie Dong

The application of satellite altimetry allows us to acquire global sea level height data with higher spatial and temporal resolution, enabling a systematic understanding of spatial differences in sea level rise. In our study, we reconstructed the barystatic sea level and steric sea level change during the altimetry era (1993-2022). This involved utilizing mass change data and ocean heat content data from various sources. Notably, we incorporated the latest observation and model-simulation data, ensuring coverage of the entire altimetry era compared with previous reconstructions. Based on altimetry-observed relative sea level change, the global sea level rise rate is 3.38 [3.09 3.68] mm/yr, the global barystatic and steric sea level change is 1.80 [1.45 2.15] mm/my, and 1.02 [0.67 1.37] mm/yr, respectively. Subsequently, we further analyzed the regional characteristics of these sea level rises.

Over the past three decades, sea levels have exhibited a faster rate of increase in the western basins, as well as in the equatorial and mid-latitude region, surpassing the global average. Conversely, sea level rise at higher latitudes has been relatively slower than the global average. In the mid-low latitude regions, the higher rate of sea level rise is primarily dominated by the expansion of ocean water due to its heating. In high-latitude regions, the lower sea level rise rate is primarily attributed to the far-field effects of the melting of land ice. The distribution of halosteric sea level changes is nearly uniform across latitudes. However, in the western Atlantic, a significant counteracting effect against the rise in thermosteric sea level is observed. This is linked to the weakening Atlantic Meridional Overturning Circulation (AMOC).

Furthermore, we selected 8 regions, North Pacific (NP), South China Sea (SCS), Western Tropical Pacific (WTP), Bay of Bengal (BOB), Tropical Indian Ocean (TIO), Southwest Pacific (SWP), Gulf of Mexico (GOM), and North Atlantic (NA), with sea level rise rates faster than the global average. We analyzed the contributions of different components to the sea level rise in these areas. These regions are all adjacent to land or have a significant number of islands, the faster sea level rise poses a greater threat to the corresponding coastal areas. The contributions of barystatic and steric sea level components are approximately equal in most of these regions. However, in SCS and GOM, the contributions of the barystatic component exceed 60%. The halosteric sea level has a significant negative contribution to the sea level rise in the GOM and NA. The Antarctic Ice Sheet and Greenland Ice Sheet melting contribute to sea level rise in these regions by less than 15%, and more than 15%, respectively. The highest contribution of glacier melting is in the SCS, approximately 23%. Compared to the melting of land ice, changes in land water contribute limitedly to sea level rise in these regions. The contribution is less than 10%, except for in NA.

How to cite: Deng, R. and Dong, W.: Understanding the Regional Disparity of the Sea Level Rise during Altimetry Era, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18660, https://doi.org/10.5194/egusphere-egu24-18660, 2024.

EGU24-18693 | ECS | Posters on site | CL4.9

Loss of safe land on atolls highlights need for immediate emissions reductions to support coastal adaptation 

Tessa Möller, Rosanne Martyr-Koller, Scott Kulp, Tabea Lissner, Benjamin H Strauss, Zebedee Nicholls, and Alexander Nauels

The impacts of climate change and sea level rise are posing substantial threats to the long-term habitability of low-lying atolls. As of today, the sparse data coverage of these islands limits the ability to assess and respond to climate change related risks.

Advances in coastal digital elevation models provide data for very remote coastal regions with low vertical bias. Here, we combine the Intergovernmental Panel on Climate Change regional sea level rise projections under its illustrative emissions scenarios, with the coastal digital elevation model CoastalDEM and COAST-RP, a dataset of storm tide return periods to assess the exposure to rising sea levels and coastal flooding of 166 atolls. Our results show that in 2050 and under a very low emissions scenario (SSP1-1.9), atoll area exposure to SLR and coastal flooding will amount to 35% [34-36%] and that only 64% of atoll area can still be considered safe. By the end of century and under the same scenario, only 61% can be considered safe. Under an intermediate emissions scenario (SSP2-4.5), a scenario roughly capturing projected warming under current policies and actions, the share of safe land further reduces to 58% by 2100. By 2150, only 58% or 51% of the land can still be considered safe under the very low and intermediate emissions scenario respectively. Our results show that the habitability of atolls is already threatened in the near future, but that near-term mitigation can limit the pace at which atolls are flooded in particular beyond 2100. Our results imply that in addition to immediate and rapid emission reductions in line with the Paris Agreement, remaining adaptation options must be enabled and implemented today to reduce the future exposure of atolls.

How to cite: Möller, T., Martyr-Koller, R., Kulp, S., Lissner, T., Strauss, B. H., Nicholls, Z., and Nauels, A.: Loss of safe land on atolls highlights need for immediate emissions reductions to support coastal adaptation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18693, https://doi.org/10.5194/egusphere-egu24-18693, 2024.

Future sea-level rise on shallow continental shelves differs in one important aspect from open ocean sea-level rise: the local steric effect, that is the change in the water column height due to changes in sea water density, plays a minor role compared to the much deeper open ocean. Instead, the bulk of oceanic sea-level rise on continental shelves arises from an increase in ocean water mass that is being imported from the open ocean – the so-called shelf mass loading (SML). This redistribution is mainly driven by thermal expansion of water masses below shelf depth and magnifies as the subsurface ocean layers continue to warm.

 

Few studies have tried to detect SML as the signal is only expected to become dominant over decadal to multidecadal periods given the large natural variability in shallow regions.

Here, we combine hydrographic data from a section crossing the Norwegian shelf, with observations of total sea-level change from altimetry and estimates of mass changes from GRACE gravity missions to estimate the strength of SML over the past decades. We compare the residual of total sea level (from altimetry) and steric height (from hydrography) with GRACE estimates from three different solutions. Over the common period (2002 -2020), both estimates show a consistently higher trend over the shallow shelf area compared to the deep ocean. We estimate the shelf mass contribution in the order of 0.5 – 1.0 mm/yr, depending on the GRACE solution selected.

How to cite: Richter, K., Mangini, F., Bonaduce, A., and Raj, R.: Estimating the long-term sea-level contribution from shelf mass loading on the Norwegian shelf using hydrographic in-situ data, satellite altimetry and GRACE, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19016, https://doi.org/10.5194/egusphere-egu24-19016, 2024.

EGU24-19452 | Posters on site | CL4.9

The barystatic contribution to multi-decadal sea-level change in the 19th century. 

Luke Jackson, Sophie Williams, Fiona Hibbert, Sönke Dangendorf, Ed Garrett, Andrew Sole, and Roland Gehrels

Understanding long-term trends in mass loss is vital for assessing the (in)stability of ice sheets and glaciers and their subsequent contribution to global mean sea level. Observational estimates of mass loss from the Greenland and Antarctic Ice Sheets are scarce before the satellite era (i.e., 1990s), and from glaciers before the 1950s. A variety of modelling techniques (process-driven and statistical) have been employed to synthesise and extend observational estimates, so that much of the 20th century sea-level budget is closed within uncertainty. Despite this work, uncertainty remains, particularly for contributions prior to ~1940 and the 19th century. 

Sea-level fingerprinting exploits the fact that the geometry of land-based water masses (i.e., ice sheets, glaciers, hydrological storage) and any changes (via loss or gain) will generate a unique gravitational equipotential surface (fingerprint). We apply this technique in a Monte-Carlo-based linear inversion model to isolate the globally averaged barystatic contribution from Greenland, Antarctica and glaciers over pentadal periods since 1813. We use a selection of long-duration tide gauges and high-resolution proxy-based sea-level reconstructions, with model-based glacio-isostatic adjustment (GIA), stero-dynamic, and terrestrial water storage corrections. 

Our initial findings confirm the validity of the approach when comparing barystatic contributions to observed estimates for the last 50 years. Whilst uncertainty is significant for the 19th century, the barystatic contribution deviates from zero in different pentads. We also conduct a sensitivity analysis to evaluate the idealised locations/corrections required to enhance confidence in the inversion procedure.

How to cite: Jackson, L., Williams, S., Hibbert, F., Dangendorf, S., Garrett, E., Sole, A., and Gehrels, R.: The barystatic contribution to multi-decadal sea-level change in the 19th century., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19452, https://doi.org/10.5194/egusphere-egu24-19452, 2024.

EGU24-19505 | Orals | CL4.9

Progress in the Global Sea Level Fingerprints since the 20th century 

Yuxin Liu, Shanshan Deng, Wenxi Zhang, and Ange Hu

Ocean mass change is the primary driver of sea level rise. Understanding the mechanisms of mass sea level change can help coastal areas scientifically respond to climate change. Under combined the self-attraction and loading effect and the Earth's rotational feedback, land-source freshwater input leads to global spatiotemporal heterogeneity of mass sea level, known as Sea Level Fingerprints. In this study, Sea Level Fingerprints were simulated under three different scenarios, covering periods from January 1901 to July 2019, January 1981 to June 2020, and July 1979 to June 2020. These scenarios encompassed: (1) consideration of climate variability alone; (2) consideration of both climate variability and actual glacial mass balance; and (3) alignment with recent climate change trends. The study aimed to analyze the contribution of Sea Level Fingerprints to satellite-derived mass sea level across these three scenarios. Results showed that in all three scenarios, the significant seasonal amplitude regions include the South China Sea and the Bay of Bengal, with peak values ranging from 42.60 to 45.20 mm. Changes in mass sea level are primarily caused by climate variability. Sea Level Fingerprints, which considered only precipitation and temperature as key indicators of climate variability, best reproduced the variation signal of the GRACE-derived data and the Altimetry-derived mass component. The spatial similarity coefficient derived between their global change range distributions were 0.67 and 0.87, respectively. Sea Level Fingerprints, which additionally considered glacial mass balance, provided a more accurate depiction of the spatial distribution and long-term trend of mass sea level derived from Altimetry satellites and Argo systems. This was demonstrated by the similarity between the sea-level fingerprints and altimetry-derived mass components across global long-term trend distribution patterns, with a spatial similarity coefficient of 0.75. The main contributing regions to these patterns include the Greenland Ice Sheet, Alaska, the Southern Andes, the Caucasus, the Middle East, and West Antarctica.

How to cite: Liu, Y., Deng, S., Zhang, W., and Hu, A.: Progress in the Global Sea Level Fingerprints since the 20th century, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19505, https://doi.org/10.5194/egusphere-egu24-19505, 2024.

EGU24-20096 | ECS | Orals | CL4.9

Attributing low-frequency variations in ocean water mass redistribution during 2002-2020 

Shanshan Deng, Yuxin Liu, Wenxi Zhang, and Ange Hu

Studying how ocean water mass is redistributed can help with a better understanding of the regional sea level change. This study investigates the roles of the different physical processes involved in low-frequency ocean water mass, including the sea level fingerprint and the dynamic ocean mass change, from regional to global scales over the period 2004-2021. Global water mass redistribution data from the GRACE and GRACE-FO satellites were used, as well as surface wind and sea surface temperature data from the ERA5 reanalysis. The sea-level equation is used to simulate the sea level fingerprint, and the maximum covariance analysis is used to extract possible signals of the wind-forcing and temperature-gradient-forcing ocean mass redistribution. The results show that the low-frequency ocean water mass is dominated by the long-term trend and the decadal-like fluctuation. Sea level fingerprint significantly contributes to the open ocean. Compared with temperature gradients, wind forcing plays a more important role in dynamic ocean mass redistribution. The wind-forcing dynamic processes significantly drive the anomalies near the North Indian Ocean, North Atlantic Ocean, South Pacific Ocean, and some marginal seas. After removing the sea level fingerprint and ocean dynamics, some non-negligible noise, located in seismic zones, was also found, suggesting the misestimation of seafloor deformation resulting from earthquakes in the GRACE/GRACE-FO data processing. These findings may improve our understanding of the long-term anomalies in regional and global sea levels.

How to cite: Deng, S., Liu, Y., Zhang, W., and Hu, A.: Attributing low-frequency variations in ocean water mass redistribution during 2002-2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20096, https://doi.org/10.5194/egusphere-egu24-20096, 2024.

EGU24-21090 | ECS | Orals | CL4.9

Fast recovery of North Atlantic sea level in response to atmospheric CO2 removal 

Sunhee Wang, Yechul Shin, Ji-Hoon Oh, and Jong-Seong Kug

Human-induced increases in atmospheric carbon dioxide (CO2) cause global warming, which leads global mean sea level rise. Previous research has shown that even with the reduction or removal of atmospheric CO2, the global mean sea level will not return to its initial level. However, the regional effects of reducing or removing atmospheric CO2 on sea level change have not been extensively studied. In this study, we analyzed global and regional sea level changes over a 560-year period, including 140 years of a linear increase in atmospheric CO2 of 1% per year, followed by 140 years of a linear decrease, and finally 280 years of maintenance at pre-industrial CO2 levels. Our analysis showed that the sea level in the North Atlantic region increased rapidly relative to the global mean, and then recovered rapidly. We attribute these variations to fluctuations in the Atlantic Meridional Overturning Circulation (AMOC). As the AMOC weakened, heat and salt were trapped in the lower latitudes of the North Atlantic region, resulting in a slower transfer of these elements to higher latitudes. As the AMOC recovered and overshoot, the accumulated heat and salt were rapidly transferred to higher latitudes, resulting in changes in sea level. Our results suggest that the North Atlantic region is more sensitive to changes in atmospheric CO2 compared to the global mean. The North Atlantic region has a high population density and is expected to suffer significant damage as a result of sea level change. Therefore, continuous research on sea level change in this region is needed, and our study could help improve the ability to predict future sea level change in this area.

How to cite: Wang, S., Shin, Y., Oh, J.-H., and Kug, J.-S.: Fast recovery of North Atlantic sea level in response to atmospheric CO2 removal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21090, https://doi.org/10.5194/egusphere-egu24-21090, 2024.

EGU24-21107 | ECS | Orals | CL4.9

The defining roles of sterodynamic sea level in future climate projections 

Jan-Erik Tesdal, John Krasting, Robert Kopp, Praveen Kumar, Stephen Griffies, and William Sweet

Our ability to characterize and quantify the complex uncertainties surrounding future sea-level changes is crucial for coastal risk assessments and adaptation strategies. This study focuses on the role of steric and dynamic changes (i.e., sterodynamics) in sea level projections, particularly regarding their contribution to the uncertainty of global and regional sea level changes in relation to other components such as ice sheet dynamics. A probabilistic framework is used to estimate probability distributions of sea-level change for each component. Through variance decomposition, the total uncertainty in sea-level change is dissected into its constituent sources. Subsequently, the relative contribution of sterodynamics uncertainty is quantified across various regions, time frames, emission scenarios, and projection methodologies utilized to estimate future sea-level distributions. The contribution of sterodynamics to overall uncertainty reduces over time as the contribution from ice sheets becomes more pronounced. The spatiotemporal pattern of sterodynamic significance is not strongly dependent on future greenhouse gas emissions, yet its overall role is highly dependent on the representation (e.g., emulation) of ice sheets. When high-end, low-probability estimates of future Antarctic ice sheet contributions are excluded, sterodynamics remain a dominant source of regional sea-level uncertainty at the end of this century, particularly along the US East Coast and European coast. These regions are also identified as hotspots for future sea-level rise, indicating that sterodynamic processes will play a significant role in assessing coastal vulnerabilities there. This study suggests that ocean model development can most effectively reduce the overall uncertainty in future sea-level projections by focusing on these areas.

How to cite: Tesdal, J.-E., Krasting, J., Kopp, R., Kumar, P., Griffies, S., and Sweet, W.: The defining roles of sterodynamic sea level in future climate projections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21107, https://doi.org/10.5194/egusphere-egu24-21107, 2024.

EGU24-995 | ECS | Posters on site | CL4.10 | Highlight

Assessing the predictability of Euro-Mediterranean droughts through seasonal forecasts 

Thomas Dal Monte, Andrea Alessandri, Annalisa Cherchi, and Marco Gaetani

Droughts are characterized by prolonged and severe deficits in precipitation that can extend in time, over a season, a year or more. They are confined to specific climatic zones but can manifest in both high and low rainfall regions. Contributing factors include temperatures, strong winds, low relative humidity, and the characteristics of rainfall. Drought events are characterized through indices that can be categorized based on the specific impacts they are associated with, such as meteorological, agricultural, or hydrological effects. Using such indices for drought characterization serves multiple purposes, including detection, assessment, and representation of drought conditions within a particular region. Seasonal precipitatio is essential for social and economic development and activities, hence. Reliable seasonal forecasts, especially regarding extreme precipitation events, become crucial for sectors like agriculture and insurance. Europe, and in particular the Mediterranean region, is expected to be considerably affected under climate change. The northern regions are anticipated to exhibit higher variability, increasing the risk of floods, while the southern areas may face decreased rainfall, prolonged dry spells, and intensified evaporation, potentially leading to more frequent drought occurrences.

This research aims to evaluate the prediction skill for extreme drought events at the seasonal time-scale using the SPI and SPEI indices over the EURO-Mediterranean area. The use of SPEI also takes into account the effect of temperature on the water balance, given by the calculation of potential evapotranspiration within it, which can be crucial in a context of global warming. We consider the seasonal forecasts provided by the Copernicus multi-system and we use the Brier Skill Score metric for the assessment of the performance. The objective is to understand potential predictability factors of these indices within the study area. The results show a positive performance for most of the areas examined, between 60 and 80 percent of the entire area for both indices. This led us to investigate possible optimization strategies to increase the skill in the area.

Using the multi-model approach we optimize the prediction skill obtaining considerable performance in forecasting drought conditions. Different multi-model strategies are compared, including the selection or aggregation of available forecasts to achieve the best overall performance in the area. We show that multi-model optimization can indeed provide valuable probabilistic predictions of seasonal drought events in many areas of the Euro-Mediterranean that could be useful for the decision-making process of the affected end users.

How to cite: Dal Monte, T., Alessandri, A., Cherchi, A., and Gaetani, M.: Assessing the predictability of Euro-Mediterranean droughts through seasonal forecasts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-995, https://doi.org/10.5194/egusphere-egu24-995, 2024.

EGU24-1120 | ECS | Orals | CL4.10

Effects of the realistic vegetation cover representation on the large-scale circulation and predictions at decadal time scale. 

Emanuele Di Carlo, Andrea Alessandri, Fransje van Oorschot, Annalisa Cherchi, Susanna Corti, Giampaolo Balsamo, Souhail Boussetta, and Timothy Stockdale

Vegetation is a highly dynamic component of the Earth System. Vegetation plays a significant role in influencing the general circulation of the atmosphere through various processes. It controls land surface roughness, albedo, evapotranspiration and sensible heat exchanges among other effects. Understanding the interactions between vegetation and the atmosphere is crucial for predicting climate and weather patterns. This study explores how better representation of vegetation dynamics affects climate predictions at decadal timescale and how surface characteristics linked to vegetation affect the general circulation at local, regional and global scales. We used the latest satellite datasets of vegetation characteristics and developed a new and improved parameterization for effective vegetation cover. We implemented the new parameterization in the land surface scheme Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land (HTESSEL), which is embedded in the EC-Earth model. 

The enhancement of the model's vegetation variability significantly improves the prediction skill of the model for several parameters, encompassing both surface and upper-level elements such as 2-metre temperature, zonal wind at 850 hPa and mean sea level pressure. The improvement is particularly evident over Euro-Asian Boreal forests. In particular, a large-scale effect on circulation emerges from the region with the most 2-metre temperature improvement, over Eastern Europe. 

The incorporation of an effective vegetation cover also introduces heightened realism in surface roughness and albedo variability. This, in turn, leads to a more accurate representation of the land-atmosphere interactions. The regression analysis of surface roughness and albedo with 2-metre temperature, mean sea level pressure and wind (both at surface and 850 hPa) reveals a robust relationship across the entire northern hemisphere. This relation between the surface and the atmosphere is notably absent in the standard configuration model, where the vegetation is prescribed by a dynamical vegetation module.

These findings underscore the substantial impact of vegetation cover on the general circulation, particularly in the northern hemisphere, and emphasise its crucial role in improving prediction skills. Furthermore, they highlight the challenges faced by modern earth system models in accurately representing several processes connecting the land surface and the atmosphere.

How to cite: Di Carlo, E., Alessandri, A., van Oorschot, F., Cherchi, A., Corti, S., Balsamo, G., Boussetta, S., and Stockdale, T.: Effects of the realistic vegetation cover representation on the large-scale circulation and predictions at decadal time scale., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1120, https://doi.org/10.5194/egusphere-egu24-1120, 2024.

EGU24-1407 | ECS | Posters on site | CL4.10 | Highlight

Time Lag and Cumulative Effects of Extreme Climate on Coastal Vegetation in China 

Dong Tong and Dahai Liu

Rapid global changes are altering regional hydrothermal conditions, especially in ecologically vulnerable regions such as coastal areas of China. The response of vegetation growth to extreme climates and the time lag-accumulation relationship still require further exploration. We characterize the vegetation growth status by solar-induced chlorophyll fluorescence (SIF), analyzed the vegetation dynamic in coastal areas of China from 2000 to 2019, also explored the spatiotemporal pattern of vegetation, and assessed the response of vegetation to extreme climate in term of time lag-accumulation by combines gradual analysis and abrupt analysis. The results showed that (1) Coastal areas of China were sensitive to global climate change, with extreme high temperatures and extreme precipitation increasing from 2000 to 2019, and the warming in high latitudes was greater than in low latitudes, while the increase in precipitation was concentrated in the southern regions, which are already water-rich. (2) The vegetation in coastal areas of China improved significantly, with gradual analysis showed that the vegetation improvement area accounts for 94.12% of the study area, and the abrupt analysis showed that the majority (69.78%) of the vegetation change types were "monotonic increase", with 11.77% showing "increase with negative break" and 9.48% "increases to decreases." (3) Significant lag-accumulation relationships were observed between vegetation and extreme climate in coastal areas of China, and the time-accumulation effects was stronger than time-lag effects. The accumulation time of extreme temperatures was typically less than one month, and the accumulation time of extreme precipitation was 2-3 months. These findings contribute to filling gaps in understanding the time lag-accumulation effects of extreme climates on vegetation in sensitive coastal regions. It provides a foundational basis for predicting the growth trend of coastal vegetation, environmental changes and ecosystem evolution, which is essential for a comprehensive assessment of coastal ecological security.

How to cite: Tong, D. and Liu, D.: Time Lag and Cumulative Effects of Extreme Climate on Coastal Vegetation in China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1407, https://doi.org/10.5194/egusphere-egu24-1407, 2024.

EGU24-3134 | Orals | CL4.10

Decadal predictability of seasonal temperature distriubutions 

André Düsterhus and Sebastian Brune

Climate predictions focus regularly on the predictability of single values, like means or extremes. While these information offer important insight into the quality of a prediction system, some stakeholders might be interested in the predictability of the full underlying distribution. These allow beside evaluating the amplitude of an extreme also to estimate their frequency. Especially on decadal time scales, where we verify multiple lead years at a time, the prediction quality of full distributions may offer in some applications important additional value.

In this study we investigate the predictability of the seasonal daily 2m-temperature on time scales of up to ten lead years within the MPI-ESM decadal prediction system. We compare yearly initialised hindcast simulations from 1960 onwards against estimates for climatology and uninitialised historical simulations. To verify the predictions we demonstrate a novel approach based on the non-parametric comparison of distributions with the integrated quadratic distance (IQD).

We show that the initialised prediction system has advantages in particular in the North Atlantic area and allow so to make reliable predictions for the whole temperature distribution for two to ten years ahead. It also demonstrates that the capability of initialised climate predictions to predict the temperature distribution depends on the season. Finally, we will also discuss potential opportunities and pitfalls of such approaches.

How to cite: Düsterhus, A. and Brune, S.: Decadal predictability of seasonal temperature distriubutions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3134, https://doi.org/10.5194/egusphere-egu24-3134, 2024.

EGU24-3274 | ECS | Orals | CL4.10

 A Multi-year Climate Prediction System Based on CESM2 

Yong-Yub Kim, June-Yi Lee, Axel Timmermann, Yoshimitsu Chikamoto, Sun-Seon Lee, Eun Young Kwon, Wonsun Park, Nahid A. Hasan, Ingo Bethke, Filippa Fransner, Alexia Karwat, and Abhinav R.Subrahmanian

Here we present a new seasonal-to-multiyear earth system prediction system which is based on the Community Earth System Model version 2 (CESM2) in 1° horizontal resolution. A 20- member ensemble of temperature and salinity anomaly assimilation runs serves as the initial condition for 5-year forecasts. Initialized on January 1st of every year, the CESM2 predictions exhibit only weak climate drift and coupling shocks, allowing us to identify sources of multiyear predictability. To differentiate the effects of external forcing and natural climate variability on longer-term predictability, we analyze anomalies calculated relative to the 50-member ensemble mean of the CESM2 large ensemble. In this presentation we will quantify the extent to which marine biogeochemical variables are constrained by physical conditions. This analysis provides crucial insights into error growth of phytoplankton and the resulting limitations for multiyear predictability.

How to cite: Kim, Y.-Y., Lee, J.-Y., Timmermann, A., Chikamoto, Y., Lee, S.-S., Kwon, E. Y., Park, W., A. Hasan, N., Bethke, I., Fransner, F., Karwat, A., and R.Subrahmanian, A.:  A Multi-year Climate Prediction System Based on CESM2, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3274, https://doi.org/10.5194/egusphere-egu24-3274, 2024.

EGU24-4083 | Posters virtual | CL4.10 | Highlight

Enhancing Subseasonal Climate Predictions through Dynamical Downscaling: A Case Study in the Southern Plains of the United States 

Yoshimitsu Chikamoto, Hsin-I Chang, Simon Wang, Christopher Castro, Matthew LaPlante, Bayu Risanto, Xingying Huang, and Patrick Bunn

Predicting extreme precipitation events at subseasonal timescales is a critical challenge in Earth system science. This study advances climate predictability by employing dynamical downscaling, specifically focusing on convection-permitting modeling in the Southern Plains of the United States. Two contrasting extreme precipitation periods in Texas, the extremely dry May of 2011 and the abnormally wet May of 2015, were selected for analysis. To enhance subseasonal climate forecasting, we integrated the Weather Research and Forecasting (WRF) model with the decadal climate prediction system based on the Community Earth System Model (CESM). Evaluating the impact of dynamical downscaling on the prediction of extreme precipitation events, our study demonstrates how high-resolution downscaling enhances model skill in capturing these events. The findings hold the potential to significantly contribute to improving climate predictions and assessing regional climate-related risks, aligning with the session's goals.

How to cite: Chikamoto, Y., Chang, H.-I., Wang, S., Castro, C., LaPlante, M., Risanto, B., Huang, X., and Bunn, P.: Enhancing Subseasonal Climate Predictions through Dynamical Downscaling: A Case Study in the Southern Plains of the United States, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4083, https://doi.org/10.5194/egusphere-egu24-4083, 2024.

Accurate seasonal streamflow forecasts (SSF) are crucial for disaster prevention, water management, agriculture, and hydropower generation. A global approach becomes imperative in regions lacking forecast systems. The Météo-France seasonal prediction system (MF System 8 - SYS8), contributing to Copernicus Climate Change Services (C3S), employs a fully coupled Atmosphere-Ocean General Circulation Model (AOGCM) with an advanced river routing component (CTRIP) interacting with the ISBA land-surface scheme. This study evaluates the skill of the SYS8 global SSF through hindcast river discharges. This work is part of the European project CERISE, which aims to enhance the C3S seasonal forecast portfolio by improving land initialisation methodologies.

SYS8 derives land initial conditions from a historical initialisation run where land (such as soil moisture and river discharges) is weakly constrained, contrasting with the atmosphere and ocean counterparts, which are nudged to the ERA5 and GLORYS re-analysis. This study improves the initialisation run by relaxing soil moisture to fields reconstructed from an offline land simulation.  Daily streamflow ensemble hindcasts of 25 members are generated in a  0.5° grid, with a lead time of up to 4 months initialised on the first day of May and August between 1993-2017. May and August initialisations allow forecasting of summer (JJA) and fall (SON) seasons. Actual forecast skill is assessed against streamflow observations in 1608 monitored basins worldwide (with areas > 3000 km2) using deterministic and probabilistic metrics. The classical Ensemble Streamflow Prediction approach (ESP) serves as a benchmark to evaluate the control SYS8 SSF skill and the additional skill of soil moisture nudging.

Globally, hindcast skill improves with enhanced land-surface initial conditions, especially during summer. Lower latitudes (<50°N) exhibit increased skill, while higher and cooler latitudes may lead to overestimated streamflow magnitude and oscillation amplitude due to soil moisture constraints. Local skill degradation will be discussed. Still, positive results support ongoing efforts to enhance land initialisation through a global land data assimilation system.

How to cite: Narváez, G. and Ardilouze, C.: Global Streamflow Seasonal Forecasts: Impact of soil moisture initialization in a novel two-way AOGCM-River Routing coupling approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5484, https://doi.org/10.5194/egusphere-egu24-5484, 2024.

EGU24-6494 | Posters virtual | CL4.10 | Highlight

Seasonal predictions of summer humid heat extremes in the southeastern United States driven by sea surface temperatures 

Liwei Jia, Thomas Delworth, and Xiaosong Yang

Humid heat extreme (HHE) is a type of compound extreme weather event that poses severe risks to human health. Skillful forecasts of humid heat extremes months in advance are essential for developing strategies to help communities build more resilience to the risks associated with extreme events. This study demonstrates that the frequency of summertime HHE in the southeastern United States (SEUS) can be skillfully predicted 0-1 months in advance in the SPEAR (Seamless system for Prediction and EArth system Research) seasonal forecast system. The sea surface temperature (SST) at the tropical North Atlantic (TNA) basin is found as the primary driver of the prediction skill. The responses of large-scale atmospheric circulation and winds to anomalous warm SSTs in TNA favor the heat and moisture flux transported from the gulf of Mexico to the SEUS. This research demonstrates the role of slowly-varying sea surface conditions in modifying large-scale environments that contribute to the predictions of HHE in SEUS. The results are potentially applicable for developing early warning systems of HHE. 

How to cite: Jia, L., Delworth, T., and Yang, X.: Seasonal predictions of summer humid heat extremes in the southeastern United States driven by sea surface temperatures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6494, https://doi.org/10.5194/egusphere-egu24-6494, 2024.

“Synergistic Observing Network for Ocean Prediction (SynObs)” is a project of the United Nations Decade of Ocean Science for Sustainable Development. SynObs aims to find the way to extract maximum benefits from the combination among various ocean observation platforms, including satellite and in situ observations. A major ongoing effort led by SynObs is the international multi-system OSEs/OSSEs. In this activity, various operational centers and research institutes participating will conduct Observing System Experiments (OSEs) and Observing System Simulation Experiments (OSSEs) using a variety of ocean or coupled ocean-atmosphere prediction systems with the common setting to evaluate ocean observation impacts which are robust for most ocean prediction systems. More than 10 ocean prediction systems with various model resolutions and diverse data assimilation methods are used in this activity, and impacts of various observation data, including satellite sea surface temperature and height, Argo floats, and tropical mooring buoys, will be evaluated.

The activity is divided into two parts. The first part is the ocean prediction OSEs. In this part, we run several ocean reanalysis runs assimilating different observation datasets at least for 2020 (preferably extended to 2022), and conduct 10-day ocean predictions from the reanalysis fields of every 5 days. Three-dimensional oceanic temperature, salinity, and velocity fields with the 1/10-degree resolution, and several two-dimensional diagnostics with the 1/4-degree resolution will be analyzed. The second part is the subseasonal-to-seasonal (S2S) OSEs. Here, we run several ocean reanalysis runs for 2003-2022, and conduct 1-month (4-month) coupled predictions from the reanalysis fields of every month (twice a year). We will evaluate the impacts of ocean observation data on the long-term reanalysis and S2S predictions using the coupled prediction systems. We also plan to conduct OSSEs using multiple ocean prediction systems in order to assess newly emerging or future observing systems, such as SWOT, ocean gliders, etc. 

We are currently conducting the S2S OSEs using a Japanese operational global ocean data assimilation and coupled prediction system for S2S forecasts. We are now conducting OSEs assimilating no in situ observations and withholding temperature and salinity profiles observed by Argo floats. In the presentation, we will introduce the results and the perspective of the collaborative activities.

How to cite: Fujii, Y., Ishikawa, I., and Hirahara, S.: Early results of OSEs conducted for the SynObs international multi-system OSE effort using an Japanese operational system, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6970, https://doi.org/10.5194/egusphere-egu24-6970, 2024.

EGU24-7918 | ECS | Orals | CL4.10

Generation of sea ice initial conditions for the next Météo-France seasonal forecasting system 

Fousiya Thottuvilampil Shahulhameed, Jonathan Beuvier, and Damien Specq

Research and development activities around the current Météo-France operational seasonal forecasting system (System 8) are underway to upgrade it to the next version (System 9), along with efforts to improve the initialization of its components. Among these components, sea ice is particularly challenging to initialize. At present, a coupled-nudged initialisation strategy, based on a high-resolution configuration of the CNRM-CM6 climate model, is employed to initialise the System 8, except for the sea-ice. In order to get initial states of sea ice that are consistent with the forecasting model, our procedure consists in making a preliminary continuous run where the ocean and sea ice models are integrated in stand-alone mode, with forcing at the surface from an atmosphere reanalysis.

However, in the current operational System 8 – based on the NEMO 3.6 ocean model and the GELATO sea ice model – the initial states of sea ice generated with this procedure are not fully realistic. Results show that the sea ice thickness over the Arctic region in the System 8 initial states is underestimated compared to the reference data. Numerous sensitivity experiments were carried out with the current NEMOv3.6-GELATO system, leading to some minor improvements. Thus, an upgraded version of the ocean model (NEMO version 4.2) coupled to a new sea-ice component (SI3) has been tested (in stand-alone mode, not coupled to the atmosphere) to see if the use of more recent versions of ocean and sea-ice models leads to some improvements in the Arctic sea ice representation. The results are encouraging as the representation of sea ice variables in the Arctic is improved compared to the old version.

This incites our team to foresee that System 9 will indeed incorporate the NEMO4.2 and SI3 models, and that the same initialization procedure as before (using these new models) will provide sea-ice initial states closer to those observed.

 

 

How to cite: Thottuvilampil Shahulhameed, F., Beuvier, J., and Specq, D.: Generation of sea ice initial conditions for the next Météo-France seasonal forecasting system, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7918, https://doi.org/10.5194/egusphere-egu24-7918, 2024.

EGU24-11927 | Posters virtual | CL4.10 | Highlight

Seasonal prediction of solar energy resources in the United States 

Xiaosong Yang, Thomas Delworth, Liwei Jia, Nathaniel Johnson, Feiyu Lu, and Colleen McHugh

Solar energy plays a crucial role in the transition towards a sustainable and resilient energy future. One challenge that remains is the considerable year-to-year variation in solar energy resources. As a result, precise seasonal solar energy predictions become pivotal for effective energy system planning and operation.  This study employs GFDL’s GFDL’s Seamless System for Prediction and Earth System (SPEAR) to evaluate seasonal solar irradiance prediction across the United States.  Notably, SPEAR demonstrates high skill in predicting solar irradiance particularly in the western United States. Furthermore, we conduct an advanced predictability analysis to pinpoint the underlying physical drivers contributing to this skillful solar energy prediction.  The outcomes of this research offer substantial potential benefits to stakeholders within the energy sector by providing predictable information regarding year-to-year fluctuations in solar energy resources.

How to cite: Yang, X., Delworth, T., Jia, L., Johnson, N., Lu, F., and McHugh, C.: Seasonal prediction of solar energy resources in the United States, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11927, https://doi.org/10.5194/egusphere-egu24-11927, 2024.

EGU24-11948 | Posters on site | CL4.10

What is the Target for Multi-Model and Perturbed-Physics Ensembles? 

David Stainforth

Much effort goes into studying the causes of systematic errors in Earth System Models (ESMs). Reducing them is often seen as a high priority. Indeed, the development of Digital Twin approaches in climate research is founded on the idea that a sufficiently good model would be able to provide reliable and robust, conditional predictions of climate change (predictions conditioned on scenarios of future greenhouse gas emissions). Here, “reliable” encapsulates the idea that the predictions are suitable for use by society in anticipating and planning for future climate change, and “robust” encapsulates the idea that they are unlikely to change as the models are improved and developed.

Such an approach, however, begs the question, when is a model sufficiently realistic to be able to provide reliable, detailed predictions? A physical processes view of current ESMs suggests that they are not close to this level of realism while a nonlinear dynamical systems perspective raises questions over whether it will ever be possible to achieve such reliability for the types of regionally-specific, extrapolatory, climate change predictions that we may think society seeks.

Given this context, multi-model and perturbed-physics ensembles are often seen as a means to quantify uncertainty in conditional, climate change predictions (commonly referred to as “projections” in the scientific community). In the IPCC atlas (https://interactive-atlas.ipcc.ch/) the most easily accessible output is the multi-model median with the 10th, 25th, 75th and 90th percentiles of the multi-model distribution also prominent. This presentation in terms of probabilities implies that the probabilities themselves have meaning to the users of the data - most users are likely to take them as probabilities of different outcomes in reality. Unfortunately multi-model ensembles cannot be interpreted that way because we have no metric for the shape of model space nor any idea of how to explore it, so the ensemble members cannot be taken as independent samples of possible models. Perturbed-parameter ensembles work in a more defined space of possible model-versions but the shape of that space is also undefined and as a result the ensemble-based probabilities are again arbitrary.

When seeking the best possible information for society, multi-model and perturbed physics ensembles would benefit from targeting diversity: the greatest possible range of responses given a particular model structure. Model emulators could be used to systematise this process. Such an approach would provide more reliable information. It changes the question, however, from “when is a model sufficiently realistic” to “how unrealistic does a model have to be to be uninformative about extrapolatory future climatic behaviour?”

In this presentation I will discuss and elaborate on these issues.

 

References:

Stainforth, D., “What we do with what we’ve got”, Chapter 21 in “Predicting Our Climate Future: What we know, what we don’t know and what we can’t know”, Oxford University Press, 2023.

Stainforth, D.A. et al., Confidence, uncertainty and decision-support relevance in climate predictions, Phil.Trans.Roy.Soc., 2007.

Stainforth, D.A. et al., Issues in the interpretation of climate model ensembles to inform decisions, Phil.Trans.Roy.Soc., 2007.

How to cite: Stainforth, D.: What is the Target for Multi-Model and Perturbed-Physics Ensembles?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11948, https://doi.org/10.5194/egusphere-egu24-11948, 2024.

EGU24-12988 | ECS | Posters on site | CL4.10

A CNN-based Downscaling Method of C3S Seasonal Forecast: Temperature and Precipitation 

Qing Lin, Yanet Díaz Esteban, Fatemeh Heidari, Edgar Fabián Espitia Sarmiento, and Elena Xoplaki

Copernicus Climate Change Service provides seasonal forecasts for meteorological outlooks several months in advance and can provide indications of future climate risks on a global scale. Using downscaling techniques, global variables can be transferred to the high-resolution regional scale, allowing the information to be elaborated for extreme events detection and further implementing and coupling with hydrological models for regional hazard prediction, thus serving agriculture and energy, improving planning for tourism and other sectors.

In this study, we applied a new CNN-based architecture for temperature and precipitation downscaling. Both variables are downscaled from 1 degree to 1 arcminute to fulfill the requirements as an input to the hydrological models. The architecture implements an auto-encoder/decoder structure to extract the data relations. The system is trained with seasonal forecast inputs and observation data to establish the relation between both scales. The model is then evaluated with the validation period from the observation data to achieve the best performance, changing network structures and tuning different network hyper-parameters. The results show a good fit for the observation data on the monthly scale, providing enough details in the downscaling product. Finally, the best-performing networks for downscaling temperature and precipitation are selected and could be extended for further utilization.

How to cite: Lin, Q., Díaz Esteban, Y., Heidari, F., Espitia Sarmiento, E. F., and Xoplaki, E.: A CNN-based Downscaling Method of C3S Seasonal Forecast: Temperature and Precipitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12988, https://doi.org/10.5194/egusphere-egu24-12988, 2024.

EGU24-13811 | ECS | Posters on site | CL4.10

Estimating Seasonal to Multi-year Predictability of Statistics of Climate Extremes using the CESM2-based Climate Prediction System 

Alexia Karwat, June-Yi Lee, Christian Franzke, and Yong-Yub Kim

Climate extremes, such as heat waves, heavy precipitation, intense storms, droughts, and wildfires, have become more frequent and severe in recent years as a consequence of human-induced climate change. Estimating the predictability and improving prediction of the frequency, duration, and intensity of these extremes on seasonal to multi-year timescales are crucial for proactive planning and adaptation. However, climate prediction at regional scales remains challenging due to the complexity of the climate system and limitations in model accuracy. Here we use a large ensemble of simulations, assimilations, and reforecasts using Community Earth System Model version 2 (CESM2) to assess the predictability of statistics of climate extremes with lead times of up to 5 years. We show that the frequency and duration of heat waves during local summer in specific regions are predictable up to several months to years. Sources of long-term predictability include not only external forcings but also modes of climate variability across time scales such as El Niño and Southern Oscillation, Pacific Decadal Variability, and Atlantic Multidecadal Variability. This study implies opportunities to deepen our scientific understanding of sources for long-term prediction of statistics of climate extremes and the potential for the associated disaster management.

How to cite: Karwat, A., Lee, J.-Y., Franzke, C., and Kim, Y.-Y.: Estimating Seasonal to Multi-year Predictability of Statistics of Climate Extremes using the CESM2-based Climate Prediction System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13811, https://doi.org/10.5194/egusphere-egu24-13811, 2024.

EGU24-15488 | ECS | Orals | CL4.10

Phytoplankton predictability in the Tropical Atlantic - triggered by nutrient pulses from the South 

Filippa Fransner, Marie-Lou Bachèlery, Shunya Koseki, David Rivas, Noel Keenlyside, Nicolas Barrier, Matthieu Lengaigne, and Olivier Maury

The variability and predictability of the Tropical Atlantic primary productivity remains little explored on interannual-to-decadal time scales. Here, we  present the results of two studies, in which find a decadal scale variability in phytoplankton abundance that can be predicted three years ahead. The predictions are made with NorCPM, which is a fully coupled climate prediction model with ocean biogeochemistry that assimilates temperature and salinity to reconstruct past variability. From these reconstructions, predictions are initialized that are run freely ten years ahead. We find that the predictability is a result of nutrient pulses that are advected with the southern branch of the South Equatorial Current from the most southern part of the Atlantic, and that then get caught in the Equatorial undercurrent before they reach the surface in the Tropical Atlantic Ocean. A more detailed analysis is being done in order to pinpoint the underlying mechanisms in a forced ocean model, where we find a link to the Pan-Atlantic decadal oscillation.

How to cite: Fransner, F., Bachèlery, M.-L., Koseki, S., Rivas, D., Keenlyside, N., Barrier, N., Lengaigne, M., and Maury, O.: Phytoplankton predictability in the Tropical Atlantic - triggered by nutrient pulses from the South, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15488, https://doi.org/10.5194/egusphere-egu24-15488, 2024.

EGU24-15829 | Posters on site | CL4.10

The role of realistic vegetation variability in climate predictability and prediction 

Andrea Alessandri, Emanuele Di Carlo, Franco Catalano, Bart van den Hurk, Magdalena Alonso Balmaseda, Gianpaolo Balsamo, Souhail Boussetta, and Tim Stockdale

Vegetation is a relevant and highly dynamic component of the Earth system and its variability – at seasonal, interannual, decadal and longer timescales – modulates the coupling with the atmosphere by affecting surface variables such as roughness, albedo and evapotranspiration. In this study, we investigate the effects of improved representation of vegetation dynamics on climate predictability and prediction at the seasonal timescale. To this aim, the observational constraints from the latest generation satellite dataset of vegetation Leaf Area Index (LAI) have been integrated in the modeling, including a parameterization of the effective vegetation cover as a function of LAI. The improved vegetation representation is implemented in HTESSEL, which is the land surface model included in the seasonal forecasting (ECMWF SEAS5) systems used in this work.

Our results show that the realistic representation of vegetation variability has significant effects on both potential predictability and actual prediction skill at the seasonal time scale. It is shown a significant improvement of the skill in predicting boreal winter (December-January-February; DJF) 2m Temperature (T2M) at 1-month lead time especially over Euro-Asian boreal forests; the improvement is at least in part due to the more realistic representation of the interannual albedo variability that is related to the changes in vegetation shading over snow. Remarkably, from the region with the most considerable T2M improvement originates a large-scale ameliorating effect on circulation encompassing Northern Hemisphere middle-to-high latitudes from Siberia to the North Atlantic. The results indicate that the coupling with the improved vegetation might operate by amplifying locally the signal originating from the North Atlantic sector, therefore improving both potential predictability and actual skill over the region. Concurrently, the improved predictability and skill over the Euro-Asian forests appears to feedback to the large-scale circulation enhancing the representation of the circulation pattern and associated interannual anomalies.

How to cite: Alessandri, A., Di Carlo, E., Catalano, F., van den Hurk, B., Balmaseda, M. A., Balsamo, G., Boussetta, S., and Stockdale, T.: The role of realistic vegetation variability in climate predictability and prediction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15829, https://doi.org/10.5194/egusphere-egu24-15829, 2024.

EGU24-16402 | Orals | CL4.10

On the stationarity of the global spatial dependency of heat risk on drought. 

Matteo Zampieri, Karumuri Ashok, Andrea Toreti, Davide Bavera, and Ibrahim Hoteit

Compound climate anomalies pose escalating risks in the context of climate change, with anomalous heat and drought presenting significant stressors to both ecosystems and society. The simultaneous occurrence of these events can be influenced by land surface processes such as the soil moisture – air temperature coupling. However, the long-term variability of this coupling remains unexplored. Here, using a combination of observations and multi-model ensemble forecasts dating back to the 1980s, we examine the global land exposure to higher than normal probabilities of concurrent hot temperature anomalies and drought on a monthly scale. Our findings confirm that drought substantially shapes the spatial distribution of heat-related risks on a global scale, offering a crucial predictive factor for these combined events. Traditionally, defining heat anomalies for non-adaptive systems involves fixed reference temperature thresholds. Using this method, our analysis reveals that the portion of global land experiencing drought-conditioned hot temperature anomalies has tripled in less than three decades. Surprisingly, the global level of spatial coupling appears to be declining. However, this outcome heavily depends on the specific definition of heat risk employed. By employing a time-dependent temperature threshold that considers changes in the climate's mean state due to both global warming and natural variability, a different picture emerges. Using the latter method, the level of spatial coupling demonstrates persistence and stability. Importantly, this method is better suited to assessing risks for adaptive systems and is more consistent with our current understanding of the underlying processes. Our study strongly advocates for tailoring hazard definitions to the specific processes and systems under investigation. Additionally, it underscores the pivotal role of operational sub-seasonal and seasonal forecasts in early warning systems, crucial for societal adaptation in the face of global warming.

How to cite: Zampieri, M., Ashok, K., Toreti, A., Bavera, D., and Hoteit, I.: On the stationarity of the global spatial dependency of heat risk on drought., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16402, https://doi.org/10.5194/egusphere-egu24-16402, 2024.

EGU24-16456 | Orals | CL4.10

Advancements and Challenges in Assessing and Predicting the Global Carbon Cycle Variations Using Earth System Models 

Hongmei Li, Tatiana Ilyina, István Dunkl, Aaron Spring, Sebastian Brune, Wolfgang A. Müller, Raffaele Bernardello, Laurent Bopp, Pierre Friedlingstein, William J. Merryfield, Juliette Mignot, Michael O'Sullivan, Reinel Sospedra-Alfonso, Etienne Tourigny, and Michio Watanabe

The imperative to comprehend and forecast global carbon cycle variations in response to climate variability and change over recent decades and in the near future underscores its critical role in informing the global stocktaking process. Our study investigates CO2 fluxes and atmospheric CO2 growth through ensemble decadal prediction simulations using Earth System Models (ESMs) driven by CO2 emissions with an interactive carbon cycle. These prediction systems provide valuable insights into the global carbon cycle and, therefore, the variations in atmospheric CO2. Assimilative ESMs with interactive carbon cycles effectively reconstruct and predict atmospheric CO2 and carbon sink evolution. The emission-driven prediction systems maintain comparable skills to conventional concentration-driven methods, predicting 2-year accuracy for air-land CO2 fluxes and atmospheric CO2 growth, with air-sea CO2 fluxes exhibiting higher skill for up to 5 years. Our multi-model predictions for the next year, along with assimilation reconstructions, for the first time contribute to the Global Carbon Budget 2023 assessment. We plan regular updates and the involvement of more ESMs in future assessments. Ongoing efforts include implementing seasonal-scale predictions for skill improvement. Furthermore, we assess uncertainty contributions to CO2 flux and growth predictions, revealing the comparable impacts of internal climate variability and diverse model responses, particularly at a lead time of 1-2 years. Notably, the effect of CO2 emission forcing rivals internal variability at a 1-year lead time. Large uncertainties in CO2 responses to initial states of ENSO are observed, stemming from both model responses and internal variability. The challenge lies in addressing the scarcity and uncertainty of data for initialization and obtaining precise external forcings to enhance the reliability of predictions. The further advancements involve not only addressing comprehensive bias correction but also implementing statistical methods to enhance dynamical predictions.

How to cite: Li, H., Ilyina, T., Dunkl, I., Spring, A., Brune, S., Müller, W. A., Bernardello, R., Bopp, L., Friedlingstein, P., Merryfield, W. J., Mignot, J., O'Sullivan, M., Sospedra-Alfonso, R., Tourigny, E., and Watanabe, M.: Advancements and Challenges in Assessing and Predicting the Global Carbon Cycle Variations Using Earth System Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16456, https://doi.org/10.5194/egusphere-egu24-16456, 2024.

EGU24-16842 | Posters on site | CL4.10 | Highlight

Exploring Sources of Multi-year Predictability of Terrestrial Ecosystem 

June-Yi Lee, Yong-Yub Kim, and Jeongeun Yun

The demand for decision-relevant and evidence-based near-term climate information is increasing. This includes understanding and explaining the variability and changes in ecosystems to support disaster management and adaptation choices. As climate prediction from seasonal to decadal (S2D) expands to encompass Earth system dimensions, including terrestrial and marine ecosystems, it is crucial to deepen our scientific understanding of the long-term predictability sources for ecosystem variability and change. Here we explore to what extent terrestrial ecosystem variables are driven by large-scale - potentially predictable -climate modes of variability and external forcings or whether regional random environmental factors are dominant. To address these issues, we utilize a multi-year prediction system based on Community Earth System Model version 2 (CESM2).  The system consists of 50-member uninitialized historical simulations, 20-member ocean assimilations, and 20-member hindcast initiated from every January 1st integrating for 5 years from 1961 to 2021. The key variables assessed are surface temperature, precipitation, soil moisture, wildfire occurrence, and Gross Primary Productivity. Our results suggest that land surface processes and ecosystem variables over many parts of the globe can be potentially predictable 1 to 3 years ahead originating from anthropogenic forced signals and modes of climate variability, particularly El Nino and Southern Oscillation and Atlantic Multi-decadal variability. These global modes of climate variability shift regional temperature and precipitation patterns, leading to changes in soil moisture, wildfire occurrence, and terrestrial productivity.  

How to cite: Lee, J.-Y., Kim, Y.-Y., and Yun, J.: Exploring Sources of Multi-year Predictability of Terrestrial Ecosystem, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16842, https://doi.org/10.5194/egusphere-egu24-16842, 2024.

EGU24-18766 | Orals | CL4.10

Deciphering Prediction Windows of Opportunity: A Cross Time-Scale Causality Framework   

Stefano Materia, Constantin Ardilouze, and Ángel G. Muñoz

While subseasonal forecasts often exhibit limited skill across mid-latitudes, occasional improvements are observed in specific locations during certain periods, known as "windows of opportunity." Understanding the causal factors behind these windows is complex due to the diverse and interdependent nature of predictors, their spatial and temporal variability, and the challenges in establishing causality relationships. 

Traditional lagged-correlations methods provide only a partial view, lacking insights into causality. Based on previous work on the role of land surface processes, multi-model subseasonal model skill assessment and the use of causality metrics in predictions across timescales (e.g. Ardilouze et al., 2020, 2021; Materia et al 2020, 2022; Muñoz et al., 2023), here we propose an approach based on the Liang-Kleeman information flow, allowing the assessment of statistically significant causal links across various lead times.

Applied to reforecast and reanalysis data, our framework success