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.