EGU General Assembly 2024  

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.

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.

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.

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.

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.

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 (σ^o_hh, SE_i, K_s) and hybrid polarization parameters (σ^o_rl, CPSE_i,ρ_rh-rv, α_s) 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 20^th 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 20^th 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.

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.

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.

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.

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.

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.

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 (F_S) 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, F_S, and heat related to sea ice volume changes (Q_ice), 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.

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 95^th 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, 95^th 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 34.5^oN and 67^oN at a horizontal resolution of 1/20^o, 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 21^st 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 21^st 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 21^st 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 21^st 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (C_ant) is thereby transported to the surface, leading to a 10% higher C_ant uptake and storage. Natural CO₂ (C_nat), 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 C_nat 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 C_nat 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.

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.

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.

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.

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.

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 21^st 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Here we focus on stomach-oil deposits from Heimefrontfjella. A highly resolved radiocarbon-dated (¹⁴C-AMS) sequence spanning ~6,500 to 2,000 cal. yr BP has been investigated for organic biomarkers (fatty acids, sterols), stable isotopes of carbon and nitrogen (δ¹³C, δ¹⁵N) and inorganic composition by X-ray fluorescence (XRF).  From ~6,500 to 6,000 cal. yr BP fatty acid markers were generally high in concentration, with particularly high levels of C_14:0 mirrored by high δ¹⁵N suggesting food sources rich in Antarctic krill and periods of enhanced feeding in the open ocean. Subsequently between 6,000 and 4,500 cal. yr BP there was a marked reduction in C_14:0, C_18:0 and δ¹⁵N, although phytol concentration remained high. This trophic shift suggests a transitional Weddell Sea still rich in productivity with snow petrels feeding in both the open ocean and close to the shore on a mixture of fish, krill and squid. This is consistent with regional mid-Holocene warmth, and also suggests dynamic variable meteorological and oceanographic conditions during this period. Subsequently, between ~4,500 and 2,000 cal. yr BP organic marker concentrations were markedly lower, suggesting a relatively low productivity period, which we anticipate required more coastal feeding by snow petrels. This change is consistent with evidence from regional reconstructions suggesting movement into neoglacial conditions.

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

References: 

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

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

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

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

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

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

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

The Antarctic ice sheet is losing mass. This mass loss is primarily due to ice shelf basal melting and the subsequent acceleration of glaciers. The substantial freshwater fluxes resulting from ice shelf and iceberg melting affect the Southern Ocean and beyond. As emphasized by some studies, they slow down the decline of Antarctic sea ice and hinder mixing between surface water and Circumpolar Deep Waters, further intensifying ice shelf basal melting. In this context, most studies so far have neglected the impact of surface meltwater runoff , but recent CMIP6 projections using the SSP5-8.5 scenario challenge this view, suggesting runoff values in 2100 similar to current basal melt rates. This prompts a reassessment of surface meltwater future impact on the ocean.  We use the ocean and sea-ice model NEMO-SI3 resolving the sub-shelf cavities of Antarctica and including an interactive iceberg module. We perform thorough sensitivity experiments to disentangle the effect of changes in the atmospheric forcing, increased ice shelf basal melting, surface freshwater runoff and iceberg calving flux by 2100 in a high-end scenario. Contrary to expectations, the atmosphere alone does not substantially warm ice shelf cavities compared to present temperatures. However, the introduction of additional freshwater sources amplifies warming, leading to escalated melt rates and establishing a positive feedback. The magnitude of this effect correlates with the quantity of released freshwater, with the most substantial impact originating from ice shelf basal melting. Moreover, larger surface freshwater runoff and iceberg calving flux contribute to further cavity warming, resulting in a noteworthy 10% increase in ice shelf basal melt rates. We also describe a potential tipping point for cold ice shelves, such as Filchner-Ronne, before the year 2100.

How to cite: Kittel, C., Jourdain, N., Mathiot, P., Coulon, V., Burgard, C., Caillet, J., Maure, D., and Lambin, C.: Deciphering the impact of future individual Antarctic freshwater sources on the Southern Ocean properties and ice shelf basal melting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16331, https://doi.org/10.5194/egusphere-egu24-16331, 2024.

While the climate sensitivity and significance of subsurface fluid and greenhouse gas reservoirs have received attention in the Arctic, the presence of these features in Antarctica and their contribution to global methane and the carbon cycle remains unknown. Here, we report the discovery of extensive and emergent seafloor seeps, some initiated within the last decade, that are releasing climate-reactive fluids and gases in the coastal Ross Sea. Emission of methane in these shallow waters would expedite transfer to the atmosphere, as reported at other shallow global seep systems. While the origin, driving mechanisms, and environmental consequence of these emerging Antarctic seep systems remains unknown, we postulate that the emergent seepage results from cryospheric cap degradation, which initiates new fluid flow pathways and liberates subsurface fluids and gases. This mechanism is inherently climate sensitive with potential for positive feedback, and may be widespread around the Antarctic Continent, yet the magnitude and scale is currently undetermined. 

How to cite: Seabrook, S., Thurber, A., Ladroit, Y., Cummings, V., Tait, L., Maurice, A., and Law, C.: Cryospheric Change as a Driver of Antarctic Seep Emergence , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16511, https://doi.org/10.5194/egusphere-egu24-16511, 2024.

The Southern Ocean (SO) plays a key role in global carbon and nutrient cycles, as the SO overturning circulation feeds into both deep-water formation (lower branch) and Subantarctic intermediate and mode water formation (upper branch). While the air-sea CO₂ balance is influenced mainly by deep-water formation, global export production is more sensitive to intermediate and mode water formation, giving rise to the concept of a SO biogeochemical divide [1]. Sea ice formation, transport and melting plays a prominent role in the transformation of buoyancy for both the upper and lower branches of the overturning circulation [2]. Hence, changes in sea ice parameterisation have potential for substantially altering carbon uptake and export production in global Earth System Models (ESMs).

Global ESMs seek to simulate physical, chemical and biological processes that are relevant for the evolution of global climate, including fluxes of greenhouse gasses and aerosols between the atmosphere and ocean. The air-sea gas exchange is determined by the difference in concentration across the air-sea interface, and a gas transfer velocity that is specific for the gas in question. However, the air-sea gas exchange is inhibited by the presence of sea ice. A modified formula proposed by Steiner et al. [3], accounting for cracks and leads in the sea ice, has recently been  implemented in the Norwegian Earth System Model NorESM2 [4]. In this study we investigate how the change in this sea ice parameterisation influences the carbon uptake and export production associated with the Southern Ocean overturning circulation.

REFERENCES

[1] I. Marinov, A. Gnanadesikan, J. R. Toggweiler and J. L. Sarmiento, "The Southern Ocean biogeochemical divide", Nature, Vol. 441, 964-967, 2006. DOI: 10.1038/nature04883

[2] R. P. Abernathey, I. Cerovecki, P. R. Holland, E. Newsom, M. Mazlo and L. D. Talley, "Water-mass transformation by sea ice in the upper branches of
the Southern Ocean overturning", Nature Geoscience, Vol. 9, 596-601, 2016. DOI: 10.1038/ngeo2749

[3] N. S. Steiner, W. G. Lee and J. R. Christian, "Enhanced gas uxes in small sea ice leads and cracks: Efects on CO2 exchange and ocean acidiccation", JGR Oceans, Vol. 118(3), 1195-1205, 2013. DOI: 10.1002/jgrc.20100

[4] Ø. Seland, M. Bentsen, D. Olivié, T. Toniazzo, A. Gjermundsen, L. S. Graff, J. B. Debernard, A. K. Gupta, Y.-C. He, A. Kirkevåg, J. Schwinger, J. Tjiputra, K. S. Aas, I. Bethke, Y. Fan, J. Griesfeller, A. Grini, C. Guo, M. Ilicak, I. H. H. Karset, O. Landgren, J. Liakka, K. O. Moseid, A. Nummelin, C. Spensberger, H. Tang, Z. Zhang, C. Heinze, T. Iversen and M. Schulz, "Overview of the Norwegian Earth System Model (NorESM2) and key climate response of CMIP6 DECK, historical, and scenario simulations", Geoscientifc Model Development, Vol. 13(12), 6165-6200, 2020. DOI: 10.5194/gmd-13-6165-2020

How to cite: Torsvik, T.: Influence of changing sea ice parameterisation on Southern Ocean  carbon uptake and export production, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16957, https://doi.org/10.5194/egusphere-egu24-16957, 2024.

The West Antarctic Ice Sheet (WAIS) is rapidly losing mass due to ocean-driven melt of its ice shelves, contributing to sea level rise. This melt is associated with the intrusion of circumpolar deep water onto the continental shelf which is impacted by winds, the Amundsen undercurrent, thermodynamic processes, and buoyancy forcing. To study the sensitivity of melt to changes in these components, model configurations need to represent key processes while reducing computational cost to allow for large ensemble simulations. Regional ocean simulations have proven useful in this context, however, configurations that allow interactions between Antarctic regions would be beneficial. We will present results from present-day ocean simulations with a  ¼° circumpolar Antarctic NEMO configuration including sea ice, icebergs, and ice shelf cavities, and up-to-date forcing and bathymetry datasets. We will also discuss challenges associated with open boundary conditions and sensitivity to different forcing datasets. This configuration will provide a platform for attribution studies of ocean-driven melt of the WAIS, ocean projections, and form the starting point for coupled ocean-ice sheet simulations. 

How to cite: Rogalla, B., Naughten, K., Holland, P., Mathiot, P., Jourdain, N., and Kittel, C.: Present-day ocean simulations of the circumpolar Antarctic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17376, https://doi.org/10.5194/egusphere-egu24-17376, 2024.

Antarctic ice-shelf basal melting is a major source of uncertainty in sea level rise projections. A persistent challenge in simulating the ice-shelf-ocean interactions in z-coordinate ocean models is the introduction of artificial steps, leading to the generation of noise that impacts both melting and ocean currents. This study explores the potential of the Brinkman Volume Penalisation (BVP) method (Debreu et al. 2020, 2022) to address the recurrent issue of steps in ice-shelf-ocean models. While penalisation methods are typically applied to land topography, here, the method is generalised to ice-shelf interactions with oceans. This approach introduces porous cells that are half-ice, half-ocean, combined with a permeability parameter (friction within porous cells) to model the blocking effect of the ice draft. A unique aspect of this method is its ability to spread the penalisation region, thereby reducing model sensitivity to numerical level changes. We assess the potential benefits of the BVP approach within the idealised ice-shelf configuration ISOMIP+ as presented by Asay-Davis et al. (2016). First, a new calculation of the horizontal pressure gradient is formulated using the BVP approach, which eliminates residual biases in ocean currents down to zero machine precision. Second, the spreading of the penalised interface significantly reduces noise in the melt rates, enabling a smooth response of the ocean beneath the ice-shelf without the need for further mesh refinement. Other simulations are used to investigate the sensitivity of basal melting and freezing in the penalised configuration to changes in numerical parameters (e.g. spatial resolution). These results pave the way for a better numerical treatment of ice-shelves in earth system models.

How to cite: Nasser, A.-A., Jourdain, N. C., Mathiot, P., and Madec, G.: Benefits of the Brinkman Volume Penalisation Method for the Ice-Shelf Melt Rates Produced by Z-coordinate Ocean Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17676, https://doi.org/10.5194/egusphere-egu24-17676, 2024.

A new monthly climatology of Southern Ocean hydrography is constructed with updated observational dataset. CTD casts from World Ocean Database, Pangaea Database, CLIVAR and Carbon Hydrographic Data Office, Southern Ocean Database and Korean Polar Data Centre were assembled. All ‘Delayed Mode’ Argo floats profiles and ‘Real-time Mode’ or ‘Real-time Adjusted Mode’ over the within 2000 m isobath near the continental shelves are included. All flagged-good Seal-tag profiles are included. The interpolation scheme employs an elliptical detecting area to select profiles to be averaged into gridded product. The ellipse is designed to align with the dynamic height contour to consider the effect of large-scale circulation. The detecting radius confined by ellipse size varies with the bathymetry and facilitate to resolve local gradient in temperature and salinity field over the continental shelves. A timeseries is constructed by removing the temperature and salinity climatology from the individual profiles in Weddell Sea, and a clear subsurface warming is revealed. An entrainment of warm and saline anomalies from subsurface into the surface layer is captured around 2016 corresponding to the recent sea ice extent decline. A further regional analysis on the temperature and salinity anomaly signal will shed light on the heat delivery pathway and the cause of the subsurface heat entrainment.

How to cite: Zhou, S., Dutrieux, P., and Meijers, A.: Weddell Sea subsurface warming revealed by an updated Southern Ocean climatology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20002, https://doi.org/10.5194/egusphere-egu24-20002, 2024.

The Arctic Ocean covers only 3 % of the Earth’s surface but contributes 5 - 14 % of the global ocean carbon sink. Sparse and unevenly distributed observations of the partial pressure of CO₂ (pCO₂) hinder our understanding of the magnitude and the controlling mechanisms of this carbon sink. In order to constrain the magnitude of this flux, we adapt the Self-Organising Map – Feed-Forward neural Network (SOM-FFN) method of Landschützer et al. (2016) to interpolate existing observations and construct a monthly 1 x 1 degree pCO₂ product for the Arctic Ocean from 1991 - 2022. We first divide the Arctic Ocean (i.e., the region ≥ 55° N) into five biogeochemical provinces; four obtained from using the SOM method and a fifth for all grid cells with greater than 85 % ice cover. For each province, we then derive non-linear relationships between pCO₂ and predictor variables (i.e., biogeochemical drivers) using the FFN method. The monthly reconstructed Arctic pCO₂ product is then evaluated against existing observations of surface ocean pCO₂, chiefly from SOCATv2023 and from independent timeseries stations. Our study shows that biogeochemical properties previously selected as predictor variables at the global scale are not well suited to the Arctic Ocean. Limiting the spatial domain from which relationships are derived also improves performance, with less biased p(CO₂) values predicted when excluding the Baltic Sea. 

How to cite: Dutch, V., Bakker, D., Landschützer, P., Roobaert, A., and Kaiser, J.: Improving Estimates of Arctic Ocean CO2 Uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-152, https://doi.org/10.5194/egusphere-egu24-152, 2024.

Quantification of air-sea CO₂ exchange over coral reefs has relied primarily on measurements of the CO₂ partial pressure (pCO₂) gradient between the water overlying a reef and the lower atmosphere. A gas transfer velocity based on wind speed is then used to estimate the air-sea CO₂ mass exchange. While this approach may be suitable over the oceans or where instrumented buoys have been deployed for long-term monitoring, the method overlooks many factors that influence turbulent transport and air-sea CO₂ exchange. These include surfactants, bubble exchange, atmospheric turbulence, and wave breaking, which may be particularly important over near shore fringing coral reefs.

Using eddy covariance (EC) systems deployed at the shoreline adjacent to coral reefs and on pontoons we show through direct measurements these ecosystems may be net sinks of atmospheric CO₂. Results show sequestration of atmospheric CO₂ by healthy coral reefs and adjacent lagoons at time scales of several days to several months exceed published CO₂ sequestration rates of mature pine plantations measured by EC by an order of magnitude. These findings highlight the importance of coral reefs in carbon budgets in addition to their widely known ecosystem services and societal benefits. Conserving coral reef ecosystems and ensuring they remain healthy and resilient to the threats of climate change, pollution, overfishing, tourism, and mining should be a priority. Future research will aim to track the CO₂ influx through coral reef ecosystems.        

How to cite: McGowan, H., Lensky, N., Abair, S., and Saunders, M.: Coral Reefs: Sinks of Atmospheric CO2 ?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-232, https://doi.org/10.5194/egusphere-egu24-232, 2024.

Regional climate models (RCMs) over Europe often exhibit wet precipitation biases, primarily attributed to excess oceanic evaporation across time scales. One likely source for such wet biases are therefore imperfections in the models’ lower boundary condition (LBC) over the ocean, as realized by time-evolving sea surface temperature (SST) fields. SST data from atmospheric reanalyses (e.g., ERA5) are commonly adopted in RCMs, but ambiguity exists about the exact SST variable in these products (e.g., foundation or skin temperature) and the manner with which they represent the diurnal cycle and spatial gradients. Here we explore these questions with a ~12-km setup of ICON-CLM (Icosahedral Nonhydrostatic Model in Limited-Area Mode) over the EURO-CORDEX domain, run repeatedly for 6 years with various SST datasets. We use ERA5-based daily SST and skin temperature and hourly upper-layer SST drawn from our own global ocean simulations with FESOM2 (Finite Element Sea-Ice Ocean Model) at ~10-km node spacing in the eastern North Atlantic. Specifically, prescribing the FESOM2 SST fields in ICON-CLM both with and without spatial smoothing allows us to examine the effects of oceanic eddies and fronts on precipitation characteristics onshore. Preliminary results from 7 months of integration with ICON-CLM suggest that the choice of the SST data appreciably impacts latent heat fluxes, moisture transport onto land, and cumulative continental precipitation, generally in areas of pronounced moisture recycling. “Mind your SST” is therefore the advice we can give to ongoing dynamical downscaling efforts aimed at modeling future precipitation changes over land.

How to cite: Da Silva Lopes, F. and Schindelegger, M.: How the treatment of sea surface temperature affects the water cycle in EURO-CORDEX simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-290, https://doi.org/10.5194/egusphere-egu24-290, 2024.

 The Arctic Ocean is responsible for around 5-10% of oceanic CO₂ uptake, despite the region only accounting for approximately 4% of the world's oceans (Bates & Mathis, 2009). In this study, we investigate the exchange of CO₂ between the atmosphere and the ocean in the Arctic Ocean for the period 2000-2017. Our estimates are obtained using the GEOSChem-LETKF inverse model system (Chen et al. 2021), in combination with data from the NOAA surface CO₂ monitoring network (ObsPack, Cooperative Global Atmospheric Data Integration Project, 2018). We evaluate the impact of alternative representations of the prior flux distribution for air-sea CO₂ fluxes. These include the following datasets: Landschutzer et al. (2016), Rodenbeck et al. (2014), and Watson et al. (2020). We present estimates of the long-term trend, year-to-year fluctuations, and regional and seasonal variability in air-sea CO₂ exchange in the Arctic Ocean, with a focus on the region north of 58˚N. The sea ice extent of the regional seas of the Arctic Ocean has an influence on the magnitude and seasonality of the regional air-sea CO₂ flux. We also investigate the potential links between changes in sea-ice extent and changes in air-sea CO₂ fluxes.

How to cite: Ghosh, J., Suntharalingam, P., Chen, Z., Kaiser, J., Bakker, D., and Dutch, V.: Estimates of Arctic Ocean carbon uptake from atmospheric inverse analyses for the period 2000-2017, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-730, https://doi.org/10.5194/egusphere-egu24-730, 2024.

The global oceans are a major sink of anthropogenic carbon dioxide (CO₂), playing a critical role in mitigating climate change. The ocean CO₂ uptake estimate contains significant uncertainties due to a lack of mechanistic understanding of the role of bubbles in air-sea CO₂ exchange. Bubbles resulting from wave breaking may mediate about 40% of the global air-sea CO₂ flux.  However, bubble-mediated transfer is poorly quantified and under-represented in CO₂ flux estimates. In this study, we will present a synthesis analysis of the bubble-mediated gas transfer measurements in the last decade. We show contrasting evidence regarding the importance of bubbles in the air-sea CO₂ exchange, particularly in the comparison between laboratory and field measurements. This suggests a lack of mechanistic understanding of the air-sea gas exchange processes. Through innovative cross-linking of comprehensive field and laboratory observations using multiple techniques, we aim to make a step change in understanding the mechanisms of bubble-mediated transfer and reconcile field and laboratory measurements.  We also aim to provide novel parameterisations of gas transfer velocity with explicit representation of bubbles, thereby reducing uncertainty in air-sea CO₂ flux estimates.

How to cite: Dong, Y., Jähne, B., and Marandino, C.: Cross-linking laboratory and field measurements to quantify the role of bubbles in air-sea CO2 exchange, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-732, https://doi.org/10.5194/egusphere-egu24-732, 2024.

The sea surface microlayer (SML) is the uppermost thin oceanic surface layer in the range of 100µm with properties that are distinct from the water below. With an ocean coverage of up to 70% it is supposed to have a significant impact on air-sea gas exchange rates. In global studies, the SML is often supposed to be a missing source of trace gases, when oceanic production and the subsequent emissions alone cannot explain observed atmospheric mixing ratios. Despite the attention in the past 20 years, also in the SOLAS science plan, it remains difficult to sample volatile trace gases from the SML with existing sampling techniques. Consequently, an incomplete process understanding of trace gas cycling within the SML inhibits its effect on air-sea gas exchange.

In this study, we focus on existing and common SML sampling methods (glass plate, Garrett screen) in order to ensure that trace gas samples are comparable to other parameters sampled with the same method. A series of laboratory experiments was set up to determine a correction factor which quantifies the loss of trace gases due to the sampling method itself. Dimethyl sulfide, isoprene and carbon disulfide were sampled with a glass plate and with a Garrett screen under varying surfactant concentrations and environmental conditions (salinity, temperature). Based on physiochemical properties of the examined trace gases, we extended the correction factor to nitrous oxide and methane. Losses are high, but not as variable as expected. Around 90% are lost due to sampling with small variations between different gases. The presence of surfactants has a small effect on the losses.

The lab-based correction factors are applied to in-field SML samples from a mesocosm study in May/June 2023 conducted within the DFG research unit BASS. Those results clearly indicate that the composition of the SML highly influences the correction factor for each trace gas individually. Comparing corrected SML concentrations with underlying bulk water concentrations reveal the accumulation of specific traces gases in the SML which highly influence the magnitude of trace gas emissions to the atmosphere.

How to cite: Lange, L., Booge, D., Karnatz, J., Bange, H., and Marandino, C.: Sampling the SML for traces gases: a case study of sampling technique and resulting correction factors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1139, https://doi.org/10.5194/egusphere-egu24-1139, 2024.

Modeling the air-sea flux of CO₂ is a key factor in understanding climate change and predicting its effects. The contribution of sea spray to this flux is highly uncertain yet important for reducing error margins in global estimates. In this work, a modified CO2SYS routine is used to quantify the effect of evaporation on aqueous carbonate reactions in sea spray in order to assess this flux. Factors that affect these reactions are the increasing salinity and temperature changes of the droplet as it evaporates. The size of the droplet is also a determining factor as it affects the time aloft and thus the amount of evaporation and gas exchange that can occur. Using these factors and a number of simplifying assumptions, we model the change in DIC, TA, pCO₂ and pH in an evaporating sea spray droplet.

How to cite: Hendrickson, L., Vlahos, P., and Romero, L.: Carbonate System Changes Within an Evaporating Sea Spray Droplet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1372, https://doi.org/10.5194/egusphere-egu24-1372, 2024.

The Mediterranean basin is well recognized as one of the main climate change hotspots; besides, this region is one of the most active cyclogenetic area of the Northern Hemisphere with a large number of intense cyclones occurring every year. Intense Mediterranean cyclones are often responsible for extreme precipitation and strong wind events leading to severe socio-economic and environmental impacts especially over densely populated coastal areas. Complex feedback between the Mediterranean Sea and the atmosphere on various temporal and spatial scales plays a major role in the variability in and extremes of the regional climate system.

This study aims to investigate the impact of the ocean-atmosphere coupling on the regional climate during intense Mediterranean cyclones. To this end, two simulations are performed using the ENEA-REG regional earth system model at 12 km atmospheric horizontal resolution over the Med-CORDEX domain, both driven by ERA5 reanalysis. The first experiment uses the mesoscale WRF model with prescribed ERA5 Sea Surface Temperature (SST), while the second is coupled to the MITgcm ocean model at horizontal resolution of 1/12°. Cyclones are tracked by applying a Lagrangian algorithm to the mean sea level pressure field. The 500 most intense cyclones mainly occur in winter over the Thyrrenian, Adriatic, Ionian and Aegean Sea. They are similarly reproduced between WRFs and ERA5 in terms of seasonal and spatial distribution, due to the same large-scale atmospheric conditions. The coupled simulation is compared with the standalone WRF in terms of sub-daily fields, such as evaporation, precipitation and wind speed, during the mature stage of the cyclones. The different SST distribution between the models appears to be the main controlling factor for the differences in the atmospheric properties affecting not only the surface, but also the entire atmospheric boundary layer (ABL) and its height, due to the mixing of the turbulent processes, enhanced during intense cyclones. A statistically significant higher specific humidity and wind speed are found in the coupled model from the surface to the top of the ABL, as well as higher precipitation over sea and coastal areas. These results are consequences of higher turbulent heat and moisture fluxes in the coupled model that destabilize the ABL and provide higher moisture content available for convection.

We conclude that the use of the coupled model is crucial for a more realistic representation of the energy redistribution in both the ocean mixed layer and the ABL during intense Mediterranean cyclones. This highlights the importance of the coupled model to study the influence of climate change on intense Mediterranean cyclones and associated impacts under different future scenarios.

How to cite: Chericoni, M., Fosser, G., and Anav, A.: Impact of the ocean-atmosphere coupling on Mediterranean cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2187, https://doi.org/10.5194/egusphere-egu24-2187, 2024.

The climatological mean and trend of salinity change evidently due to the acceleration of hydrological cycle under global warming. However, no systematic research has been focused on the decadal and long-term changes of salinity and their contributions to ocean stratification during 1940-2019. In this study, the nonlinear trend of salinity is firstly exacted using the ensemble empirical mode decomposition method, and the corresponding long-term trends and their impacts on stratification in the tropical Pacific are analyzed emphatically. The results confirm that the sea surface water becomes fresher in the tropical western Pacific and southern Pacific convergence zone, while saltier in the southeastern Pacific under global warming. Moreover, the salinity changes are regional- and time-dependent, which the salinity trends in different regions of the tropical Pacific show differences at different periods responding to SST trend. As results, under the combined effects of temperature and salinity, the sea surface density reduces significantly in the tropical Pacific, with the largest reduction centered in the warm pool, while the subsurface density in the tropical western Pacific increases. These opposite changes enhance the contrast for the density between the surface and the subsurface water, leading to more stable ocean stratification. Then, the mixed layer becomes shallower near the equatorial dateline and deeper in the warm pool, mainly due to salinity variations. Salinity and temperature contribute differently to the variations of barrier layer thickness in different regions, where the changes of salinity (temperature) correspond to the thickening of barrier layer located at 160°E east (west). It is suggested that the salinity variations in the Pacific affect the ocean thermodynamic processes under global warming, which then modulate the climate variability.

How to cite: hai, Z.: Salinity Change and Its Implications for Ocean Stratification in the Tropical Pacific under Global Warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2734, https://doi.org/10.5194/egusphere-egu24-2734, 2024.

The cool skin effect, known as the temperature difference (ΔT) across the skin layer of sea surface, is of vital importance for the accurate computation of the latent heat flux (LHF). The observed features of ΔT in the South China Sea are analyzed using in situ data from a buoy platform over an approximately six-week period. Only nighttime data are used to exclude the possible warm layer effect. The positive values of ΔT falling into the range of 0 to 1 K comprise 95% of the data, and the most frequently observed values occur in the range of 0.4 to 0.6 K (38%). The cool skin model in the COARE 3.0 algorithm is then validated against those observations. The cool skin model has an efficient but insufficient ability to reduce the overestimation of the LHF. The overestimation of the LHF is reduced to 9.5% from 18.0%, leaving nearly half of the biases in the LHF unresolved. The Saunders constant (λ) in the cool skin model is markedly underestimated, leading to a much weaker prediction of ΔT. A strong linear relationship exists between the mean values of λ and the LHF with a slope of -0.9 W m^-2. With an approximately doubled λ, the biases in ΔT and in the LHF could be eliminated. Considering the possible uncertainties in sensors, the value of λ is estimated as 11.6±6.7 in the current study.

How to cite: Zhang, R.: Cool Skin Effect and its Impact on the Computation of the Latent Heat Flux in the South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3766, https://doi.org/10.5194/egusphere-egu24-3766, 2024.

In recent decades, global climate change has strongly affected the Arctic region, leading to a rapid decline in sea ice extent. This decline affects the interactions between sea ice, the atmosphere, and the ocean, driven by complex thermodynamic and dynamical processes. The Arctic mixed layer (ML), located in the upper ocean, plays a key role in regulating the interactions between the deep ocean, sea ice, and the atmosphere. This region is strongly affected by exchanges of mass (such as freshwater and saltwater fluxes) and momentum driven by various forces like ocean currents, tides, waves, and winds. Here, we study the ad-hoc vertical turbulent kinetic energy (TKE) mixing scheme within the NEMO-SI3 model. Specifically, we focus on the influence of surface and internal wave breaking in sea ice-covered regions. The critical parameters are the fraction of surface TKE that penetrates below the ML, the nature of the exponential TKE penetration decrease beneath the ML, and the damping effect on Langmuir and surface wave breaking beneath the ice cover. We aim to assess how these parameters affect the ML and various sea ice properties.

Our findings reveal significant impacts on Arctic sea ice thickness under two scenarios: when ice cover does not affect wave dynamics and when the mixing process weakens. Stronger mixing leads to a deeper ML and reduced sea ice thickness by 30 to 40 centimeters, while weaker mixing results in a shallower ML and a moderate sea ice increase of 10 to 20 centimeters. Results also show that reduced sea ice models exhibit a larger volume of freshwater content in the ocean with consistent spatial patterns. Conversely, increased sea ice simulations reveal reduced freshwater content, although clear spatial patterns are not evident. Differences in upper ocean properties, particularly in ocean stratification, highlight the significant impact of strong sea ice attenuation in the mixing parametrization. These findings underscore the substantial influence of enhanced ocean mixing on the physical properties of ocean and sea ice.

How to cite: Allende, S., Treguier, A. M., Lique, C., de Boyer Montégut, C., Massonnet, F., and Fichefet, T.: Impact of ocean vertical mixing parametrization on sea ice properties using NEMO-SI3 model in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3811, https://doi.org/10.5194/egusphere-egu24-3811, 2024.

The Southern Ocean is the dominant marine sink for anthropogenic carbon, absorbing around 40% of carbon emitted since industrialisation, but it is a remote and challenging region to measure. Sparsity of observational data is the main cause of uncertainty in air-sea carbon flux in the Southern Ocean. Year-round observations of CO₂ mixing ratios can aid understanding of air-sea flux in this critical region and provide valuable insight into how the carbon sink is changing over time as well as its seasonal and interannual variability.

This work presents ten years of high frequency in situ carbon dioxide mixing ratios measured from two coastal Antarctic research stations; Halley, operated by the British Antarctic Survey, and the German research station, Neumayer. This data set provides a rare long-term measurement of CO₂ in the Southern Ocean region, allowing annual growth rates, seasonal changes and interannual variability to be studied. The mean annual growth rate was calculated to be ~2.4 ppm year^-1 between 2013 and 2022.

The coastal location of these stations mean they are ideally placed to explore air-sea CO₂ exchange in the Southern Ocean. Both the Halley and Neumayer records show short-term fluctuations in CO₂ mixing ratios during the summer, with up to ~0.5 ppm decreases in CO₂ over the course of a day, about one fifth of the average annual growth rate. Air mass trajectory analysis carried out using Hysplit with ERA5 meteorological data, suggests that these decreases in CO₂ correspond to periods where the air sampled has spent time over the Southern Ocean, suggesting CO₂ uptake has occurred. This work explores the possible drivers for the short-term variability in CO₂ mixing ratios, focusing on the role of ocean uptake in the summer.

How to cite: Squires, F., Jones, A., Phillips, T., France, J., Wullenweber, N., and Weller, R.: Daily to decadal changes: Insights from a high resolution 10-year record of atmospheric carbon dioxide, observed from coastal Antarctica., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5785, https://doi.org/10.5194/egusphere-egu24-5785, 2024.

We analyze interannual and seasonal variability of surface density fluxes and water mass transformation rates over the global oceans for 1979-2018 using data from CFSR reanalysis and historical simulations by climate models. By analyzing density fluxes we quantify the the effect of surface heat and mass fluxes onto the formation of surface waters in the World Ocean. First, by using net fluxes from CFSR reanalysis we derive global climatology of surface density flux and further integrate it for density classes and T,S-classes, providing global and regional view of surface water mass transformation and its variability in space and in time. We precisely looked onto the role of salinity and sea ice formation in the density flux during the winter period. On average, the contribution of salinity to sea ice formation results in the differences of 9% in the density flux with the maximum effect of 12% identified in 1989. Interdecadal variability in surface transformation of the subpolar modal water and Labrador Sea waters shows opposite tendencies for the last decades. Then we analyze historical experiments from CMIP6 model ensemble and compare characteristics of surface water mass transformation with those revealed from reanalysis. We conclude that surface density fluxes and transformation rates derived from INM, MPI and MIROC are stronger compared to those diagnosed by CFSR with the largest differences identified over the Gulf Stream and the North Atlantic Current. For the same models we derive projections of surface density fluxes and surface water mass transformation for 2100 under ssp126, ssp370 and ssp585 scenarios. For all SSP scenarios, computations show a decrease in the magnitude of surface water mass transformation by the end of the century.

How to cite: Kukushkin, V., Gulev, S., and Markina, M.: Surface density fluxes and water mass transformation over global oceans from reanalysis and climate models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5985, https://doi.org/10.5194/egusphere-egu24-5985, 2024.

Carbohydrates, amino acids, and lipids are important contributors to organic carbon (OC) in the marine environment. To study their sea-to-air transfer, including their enrichment in the sea surface microlayer (SML), potential atmospheric in situ formation or degradation, and their oceanic contribution to the ambient marine aerosol particles, we provide measurements from the tropical Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO) where the above compounds were investigated in both surface seawater and in ambient submicron aerosol particles.

In bulk seawater and the SML, similar distributions among species were found for the lipids and carbohydrates with moderate SML enrichments (enrichment factor EF_SML = 1.3 ± 0.2 and 1.1 ± 0.5 respectively). In contrast, the amino acids exhibited a higher enrichment in the SML with an average EF_SML of 2.3 ± 0.4 although they are less surface-active than lipids. The same compounds studied in the seawater were found on the ambient submicron aerosol particles whereas the lipids were more pronounced enriched (EF_aer. = 1.6x10⁵) compared to the amino acids and carbohydrates (EF_aer. = 1.5x10³ and 1.3x10³ respectively), likely due to their high surface activity and/or the lipophilic character. Detailed molecular analysis of the seawater and aerosol particles revealed changes in the relative abundance of the individual organic compounds. They were most pronounced for the amino acids and are likely related to an in situ atmospheric processing by biotic and/or abiotic reactions.

On average 49% of the OC on the aerosol particles (≙ 97 ng m^-3) could be attributed to the specific components or component groups investigated in this study. The majority (43%) was composed of lipids. Amines, oxalic acid, and carbonyls, comprised an OC fraction of around 6%. Carbohydrates and amino acids made up less than 1% of the OC. This shows that carbohydrates, at least when resolved via molecular measurements of single sugars, do not comprise a very large fraction of OC on marine aerosol particles, in contrast to other studies. However, carbohydrate-like compounds are also present in the high lipid fraction (e.g., as glycolipids), but their chemical composition could not be revealed by the measurements performed here.

Since the identified compounds constituted about 50% of the OC and belong to the rather short-lived biogenic material probably originating from the surface ocean, a pronounced coupling between ocean and atmosphere was indicated for this oligotrophic region. The remaining, non-identified OC fraction might in part contain recalcitrant OC, however, this fraction does not constitute the vast majority of OC in the aerosol particles here investigated.

The study contributes to the international SOLAS program.

Ref: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates, and lipids in the tropical oligotrophic Atlantic Ocean: sea-to-air transfer and atmospheric in situ formation, Atmos. Chem. Phys., 23, 6571–6590, https://doi.org/10.5194/acp-23-6571-2023, 2023.

How to cite: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates and lipids in the tropical oligotrophic Atlantic Ocean: Sea-to-air transfer and atmospheric in situ formation , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6195, https://doi.org/10.5194/egusphere-egu24-6195, 2024.

The increasing amount of data in earth-observing systems allows us to move from considering low-order moments (means and variances) of fluctuating observations to their PDFs (Probability Density Functions). For two years of HFR (High Frequency Radar) sea surface current increments in the Gulf of Trieste (Northern Adriatic Sea) we found the analytical fat-tailed PDF form (a combination of a gaussian and a convolution of two exponentials) using superstatistics and the maximum entropy principle twice: on a short and on a longer time scale. The data observed under different wind regimes (Bora, Sirocco and low wind, from the WRF model local forecasts) follow the same analytical PDF, pointing towards a universal behaviour.

We developed an idealised deterministic-stochastic model of the wind-driven sea surface currents in the Gulf of Trieste. The deterministic model consists of a time-dependent Ekman layer system, including the tidal signal, with a quadratic drag. It describes 57% of the variability, missing the fast fluctuations. The stochastic part accounts for the fast fluctuations, reproducing the superstatistical PDFs from the observations. The model, providing a huge amount of data, allows for studying the PDF of the mechanical power-input into the ocean and the associated extreme events.

How to cite: Flora, S., Ursella, L., and Wirth, A.: Superstatistical analysis of HF Radar sea surface currents in the Gulf of Trieste, their idealized wind-driven stochastic modeling and extreme power-input events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6484, https://doi.org/10.5194/egusphere-egu24-6484, 2024.

The Kuroshio Extension (KE) bimodality has important effects on the ocean environment, ecosystem and climate. Previous studies have revealed that the Kuroshio Extension (KE) bimodality is mainly determined by the westward-propagating Rossby wave triggered by the North Pacific decadal variability such as PDO or NPGO: the positive (negative) phase of NPGO corresponds to the stable (unstable) KE state. However, the KE state and the NPGO seem to be decoupled since 2017, during which the NPGO takes a negative phase but the KE is in a stable state. This study employs the Convergent Cross Mapping (CCM) method to investigate the causality between the KE bimodality and NPGO. Simultaneously, we divide the KE region into the upstream (west of 146°E) and downstream regions. It is found that the NPGO has a significant causal impact on the downstream KE state. But the effect on the upstream KE state significantly weakens around 2017. Further analysis indicates that the upstream KE state is mainly caused by eddy activity in the Kuroshio large meander region south of Japan. In particular, the changes in the eddy activity affect the downstream advection of eddies and induce changes in the Kuroshio position over the Izu ridge, which cause different states in the KE upstream region. Therefore, we should not only consider the NPGO change, but also the eddy activity change in the Kuroshio region south of Japan when understanding and predicting the KE low-frequency variability.

How to cite: Wang, Q.: Revisiting the relationship between the North Pacific decadal variability and the Kuroshio Extension bimodality, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7259, https://doi.org/10.5194/egusphere-egu24-7259, 2024.

Precipitation alters sea surface physical and biogeochemical properties locally. However, due to its high temporal and spatial variations, it has largely been overlooked in studies assessing global ocean carbon uptake. Air-sea CO₂ flux is mainly due to the interfacial exchange of CO₂ molecules between the liquid and gaseous phases media. Rain may impact this interfacial air-sea CO₂ flux by (i) enhancing the turbulence at the air-sea interface and (ii) diluting the CO₂ concentration near the ocean surface. At the same time, rain directly injects into the ocean CO₂ absorbed by the raindrops during their fall. This latter component, known as wet deposition, contributes to the CO₂ flux into the ocean. This study provides the first comprehensive global estimate of these effects and their combined influence on the global ocean carbon uptake during the period 2008-2018. We use different representations of the ocean surface response to rain and different rain products with different rain rate distributions (ERA5 and IMERG) to quantify the uncertainty of the global impact of rain on CO₂ sink. We show that rain increases the global ocean carbon sink by +0.14 to +0.19 PgC yr^-1 over 2008-2018, representing an increase of 5 to 7% of the global carbon uptake (2.66 PgC yr^-1). Both interfacial flux and wet deposition have comparable orders of magnitude. Rain mainly increases the CO₂ sink in the tropics, where strong rain rates and weak winds induce noticeable dilution at the ocean surface, in the storm track regions, and in the Southern ocean.

How to cite: Parc, L., Bellenger, H., Bopp, L., Perrot, X., and Ho, D.: The impact of rain on the global ocean carbon uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7477, https://doi.org/10.5194/egusphere-egu24-7477, 2024.

The prototype of a short-term forecasting system of the ocean dynamics of Southern European Seas (SEAS), was developed. It is based on a regional coupled ocean-atmosphere model, with NEMO and WRF codes implemented on the same computational grid, with 1/24° resolution, which encompasses Mediterranean Sea, Marmara Sea and Black Sea. The domain extends also westward and northward in the Atlantic Ocean to downscale properly the mid-latitudes atmospheric perturbations from the parent ECMWF HRES model.

The forecasting uncertainty of the atmospheric regimes in such a complex Euro-Mediterranean region must be considered along with the uncertainties of the parametrizations of the surface processes at the ocean-atmosphere interface. Therefore, the goal of coupling oceanic and atmospheric models is to reduce these uncertainties and exploit the second type predictability to increase the forecast skills of the ocean dynamics.

The uncoupled ocean model has been validated against Sea Surface Temperature (SST) satellite observed data, and the skills compared to those of the Copernicus Mediterranean Forecasting System (MedFS hereafter) both in the short-term forecast over two seasonal periods and in the simulation of the medicane Ianos.

Various physical schemes, domain extensions, boundary, and initial conditions were initially tested using the uncoupled atmospheric model to obtain the best representation of the medicane Ianos. Furthermore, these experiments were also useful to determine the coupling strategy more appropriate to reduce the heat fluxes imbalance between the two components.

The SST differences between coupled and uncoupled experiments are determined by the heat fluxes computation in the atmospheric component rather than using the MedFS bulk formulae implemented in the ocean model. These differences are largely dependent on the surface boundary layer scheme used in WRF, therefore, several coupled experiments were conducted.

In terms of SST, the coupled model replicates the skills of the MedFS in the winter period while in the summer period the skills are worsened due to the larger heat fluxes. Numerical experiments focused on the parametrizations of the atmospheric boundary layer are still ongoing work.

The skill of the coupled model in reproducing the observed SST during the medicane Ianos is comparable with the one of the uncoupled oceanic model in the Ionian Sea. In terms of heat fluxes, the coupling changes significantly the heat budget locally in the Ionian Sea, mainly through the latent heat flux and the shortwave radiation. The coupling is not that relevant for the intensification of the cyclone, whereas it enhances the representation of its path and the time of the landfall on the Ionian Islands.

How to cite: Maicu, F., Pinardi, N., Guadi, S., Clementi, E., Trotta, F., and Coppini, G.: SEAS: a simulation system for forecasting the atmosphere-coupled ocean dynamics in the Southern EuropeAn Seas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8197, https://doi.org/10.5194/egusphere-egu24-8197, 2024.

The ocean's upper layer is inherently turbulent and constantly forced by momentum and buoyancy fluxes, and their interplay operates to mix and/or stratify the first meters of the water column. Incoming solar short-wave radiation acts to stabilize the upper layer, whereas the wind transfers momentum to the ocean and acts to vertically mix the water column. However, if the wind is not strong enough to trigger mixing, stratification in the near-surface is immediately formed in a layer of O(10) m thickness, called the diurnal warm layer (DWL). Above the bottom boundary of the DWL, shear production can be enhanced leading to large dissipation of turbulent kinetic energy (TKE) rates, i.e. high turbulence. Based on observational data from an ocean glider with a mounted microstructure package and drifters, we show the evolution of three DWLs sampled on the rim of a mesoscale eddy in the South Atlantic ocean (32^oS, 4^oE) with respect to temperature and buoyancy anomalies, potential energy and dissipation of TKE. In the near-surface, temperature and buoyancy anomalies increase with the evolution of the DWL, and the latter has the same magnitude as the time-integrated surface buoyancy flux. We also show the development of a diurnal jet with magnitude of O(10) cm/s that veers with the wind. Late in the afternoon, when the diurnal jet is fully developed, the bulk Richardson number (Ri_b) indicates that the stratified layer related to the DWL becomes marginally unstable (Ri_b ~ 0.25). During this period, the potential energy also decays, suggesting that the enhanced turbulence within the DWL acts to destroy stratification through turbulent mixing. We further assess whether a one-dimensional turbulence model is able to reproduce the observed DWL’s characteristics and the change in stability throughout the day.

How to cite: Miracca Lage, M., Ménesguen, C., Merckelbach, L., Dräger-Dietel, J., Griesel, A., and Carpenter, J.: The evolution of turbulence, stratification, and the surface jet in Diurnal Warm Layers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8431, https://doi.org/10.5194/egusphere-egu24-8431, 2024.

Eastern Boundary Upwelling Systems (EBUS) are characterised by wind-triggered upwelling of deep waters along the coast. They are hotspots of biological productivity and therefore have a high economic, ecological and social importance. Here we investigate the evolution of the two Atlantic EBUS during the historical period and in a future high-emission scenario in CMIP6 models from two modelling centres, with spatial resolutions ranging from 1° to 1/12° in the ocean. The decomposition of the upwelling systems into subregions reveals differences between the equatorward and poleward parts. Our analysis is focused on the modelled vertical transport, which is shown to be consistent with the wind-derived Ekman index. Integrating the vertical transport provides a synthetic view of the upwelling cells, their strength, depth and distance to the coast. The models show high interannual variability over the 21st century century, which explains why significant trends could only be found in few subregions of the Atlantic EBUS. The results suggest a poleward migration of upwelling systems with climate change and a change of the upwelling cells, rather than the uniform intensification which had been hypothesised by Bakun in 1990.

How to cite: Flügel, R., Herbette, S., Treguier, A. M., Waldman, R., and Roberts, M.: Spatial variation of future trends in Atlantic upwelling cells from CMIP6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10253, https://doi.org/10.5194/egusphere-egu24-10253, 2024.

In state of the art Earth System Models (ESM), the variables at the ocean-atmosphere interface (wind, air temperature, humidity, surface currents and SST) are linked to turbulent surface fluxes (momentum, sensible and latent heat) in a complex manner via bulk closures.

Understanding how turbulent fluxes interact between them and with the ocean-atmosphere interface variables is a major scientific challenge because it connects local interactions with large scale energy and water cycles.

These interactions between the different air-sea turbulent fluxes are difficult to diagnose from fully coupled ocean-atmosphere simulations due to the fact that in most modelling groups coupled and stand alone components do not necessarily use consistent forcing or representation of the air-sea fluxes. Also rigorous protocols between coupled and stand alone atmosphere and ocean simulations need to be implemented to be able to properly disentangle the role of different physical representation at the air-sea interface from global ocean-atmosphere-land adjustment feedbacks that may counteract the direct effects of air-sea fluxes modeling.

Here we use an ensemble of fully coupled and stand alone simulations using a version of the IPSL ESM [1] based on the new DYNAMICO atmospheric dynamical core [2] and the ocean engine NEMO [3]. We analyse an ensemble of experiments differing by the the bulk formulation of the air-sea turbulent fluxes (NCAR, COARE3.6, ECMWF and LMDZng). The analyses will focus on the adjustment of the system in the different cases, especially on the differences in the transport of heat and water, mixed layer depth adjustement, feedback on ocean surface properties, intertropical convergence zone (ITCZ) and mid-latitude storm tracks.


[1] Boucher O., Servonnat, J., Albright, A. L., Aumont, O., Balkanski, Y., Bastrikov, V., et al. (2020). Presentation and evaluation of the IPSL‐CM6A‐LR climate model. Journal of Advances in Modeling Earth Systems, 12, e2019MS002010. https://doi.org/10.1029/2019MS002010

[2] Dubos, T., Dubey, S., Tort, M., Mittal, R., Meurdesoif, Y., and Hourdin, F.: DYNAMICO-1.0, an icosahedral hydrostatic dynamical core designed for consistency and versatility, Geosci. Model Dev., 8, 3131–3150, https://doi.org/10.5194/gmd-8-3131-2015, 2015.
 
[3] “NEMO ocean engine”, Scientific Notes of Climate Modelling Center, 27 — ISSN 1288-1619, Institut PierreSimon Laplace (IPSL), doi:10.5281/zenodo.1464816

How to cite: Dehondt, C., Braconnot, P., Fromang, S., and Marti, O.: Feedbacks between turbulent air-sea fluxes and their role in the adjustment of the Earth Climate System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11098, https://doi.org/10.5194/egusphere-egu24-11098, 2024.

Sea spray aerosol (SSA) formed after wave breaking at the ocean surface influences our climate by scattering incoming solar radiation and acting as cloud condensation nuclei. Furthermore, they provide a microenvironment for aqueous phase chemistry, selective uptake of surfactants, and gas-to-particle partitioning of compounds by providing an acidic pH at the air-water interface (Angle et al., 2022). Despite these known effects, a crucial question remains unanswered: which volatile organic compounds (VOCs) are emitted from SSA, and how do they change over time via atmospheric aging?

To address this, we designed a novel experimental setup during the AGENA* Campaign 2022 at Graciosa Island, Portugal. For the first time, we connected a sea spray simulation chamber to a chemical ionization mass spectrometer (CIMS) to measure the freshly emitted gases from both seawater and SSA. Additionally, we aged the samples for an equivalent period of about 3-3.5 days in an oxidation flow reactor to investigate compositional changes after ageing.

Surprisingly, our findings reveal that nearly half of the mass-spectrometer signal from the fresh samples constituted fluorinated compounds, specifically short-chain perfluoroalkyl carboxylic acids - a class of perfluoroalkyl substances (PFAS). While, previous studies have shown that SSA can release and play a key role in the long-range transport of PFAS, these studies have primarily focused on particle-phase emissions (Johansson et al., 2019, Sha et al, et al., 2022). In contrast, our study provides new insights into oceanic PFAS emissions and transport to the atmosphere by examining gas-phase emissions.  

Furthermore, we observed that the gas-phase PFAS almost completely disappears after ageing. Our hypothesis is that these compounds partition into the particle phase. We plan to test this hypothesis by analyzing the particle filters collected during the campaign.

*Aerosol Growth in the Eastern North Atlantic (AGENA) https://www.arm.gov/research/campaigns/ena2022agena

Angle, K. J., Crocker, D. R., Simpson, R. M., Mayer, K. J., Garofalo, L. A., Moore, A. N., ... & Grassian, V. H. (2021). Acidity across the interface from the ocean surface to sea spray aerosol. Proceedings of the National Academy of Sciences, 118(2), e2018397118.

Johansson, J. H., Salter, M. E., Navarro, J. A., Leck, C., Nilsson, E. D., & Cousins, I. T. (2019). Global transport of perfluoroalkyl acids via sea spray aerosol. Environmental Science: Processes & Impacts, 21(4), 635-649.

Sha, B., Johansson, J. H., Tunved, P., Bohlin-Nizzetto, P., Cousins, I. T., & Salter, M. E. (2021). Sea spray aerosol (SSA) as a source of perfluoroalkyl acids (PFAAs) to the atmosphere: field evidence from long-term air monitoring. Environmental Science & Technology, 56(1), 228-238.

How to cite: Aggarwal, S., Garmash, O., Kilgour, D., Jernigan, C., Zinke, J., Gong, X., Zhou, S., Zhang, J., Wang, J., Bertram, T., Thornton, J., Salter, M., Zieger, P., and Mohr, C.: Can sea spray aerosol be a source of gas-phase perfluoroalkyl substances (PFAS)? A study in the Eastern North Atlantic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11282, https://doi.org/10.5194/egusphere-egu24-11282, 2024.

The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model has been employed to simulate the anomalous post-monsoon tropical cyclone (TC) Jawad that originated over the Bay of Bengal (BoB) in December 2021. The atmospheric initial and boundary conditions (IC and BC) have been obtained from the Global Forecasting System (GFS) Analyses and Forecasts and two contrasting ocean IC and BCs, viz., HYCOM (experiment name GFS-HYCOM) and INCOIS (experiment name GFS-INCOIS), are implemented in two separate coupled experiments to evaluate the influence of TC Jawad on the surface and sub-surface characteristics of BoB. The track of the TC, including its recurvature, was well captured by both experiments with significant accuracy. A proper contrast in temperature between the two sides of the TC track was noted in the surface and sub-surface temperatures observed by two buoys, i.e., (1) BD11 (west of the TC track) and (2) BD13 (east of the TC track), and the simulated temperatures were validated with these observations. Contrary to the usual scenario, the higher sub-surface warming on the eastern side of the TC track was captured by GFS-HYCOM, but with a significant overestimation. The lower temperature on the western side of the TC track can be attributed to the weak upwelling associated with the cyclonic circulation caused by the interaction of the TC with the southward coastal currents. An unusually higher downwelling on the eastern side of the TC track was observed in the vertical distribution of the temperature across the longitudes, which suggested the existence of a strong clockwise circulation near the location of BD13. GFS-HYCOM, which simulated a higher current magnitude in the sub-surface than GFS-INCOIS on the eastern side of the TC track, captured the circulation near BD13 more rigorously. From further analysis, it was inferred that the interaction of the cyclonic wind flow of TC Jawad (westerly) near the surface with the easterly flow caused the generation of the clockwise circulation over the ocean surface on the eastern side of the TC track, leading to intense downwelling and warming of the sub-surface temperature. This scenario was further corroborated by the simulated Ekman transport and higher convective activity in the eastern quadrants of the TC. The present study not only emphasizes the capability of the coupled ocean-atmosphere models to simulate TCs but also highlights the necessity of investigating the air-sea interaction processes and their responses to the passage of an anomalous TC like Jawad.

How to cite: Chakraborty, T., Pattnaik, S., and Joseph, S.: Modulation of surface and sub-surface circulation in the Bay of Bengal by the passage of tropical cyclone Jawad: coupled ocean-atmosphere feedback , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11375, https://doi.org/10.5194/egusphere-egu24-11375, 2024.

Estimates of global scale air-sea CO₂ fluxes have traditionally been derived from ocean biogeochemistry models and ocean surface pCO₂ data products (Friedlingstein et al. 2022). An alternative means of estimating ocean carbon uptake is provided by atmospheric inversions; these use optimization procedures and data assimilation methods to combine atmospheric CO₂ measurements with numerical transport model simulations and prior knowledge of air-sea fluxes. 

Here we use the GEOSChem-LETKF (GCLETKF) inverse system (Chen et al. 2021) in conjunction with atmospheric observations from the NOAA-GML surface CO₂ measurement network to derive grid-scale air-sea CO₂ flux estimates for the period 2000-2017. We focus, in particular, on estimates of CO₂ uptake by the polar oceans (Southern and Arctic oceans). These  regions have accounted for a significant component of global oceanic carbon uptake  in recent decades (e.g., more than 20% of global ocean uptake, in comparison to their ocean areal  extent of < 10%).

We present GCLETKF estimates of ocean CO₂ uptake at global and regional scales, and assess the robustness of our results with a suite of metrics that include model concentration bias, CO₂ flux error reduction, and comparison to independent atmospheric measurements. GCLETKF flux estimates for the 2000-2017 period indicate regional CO₂  uptake of 0.1-0.2 PgC/year for the Arctic,  and  0.45-0.55 PgC/yr for the Southern Ocean. We also provide summary estimates of the  interannual variations and  decadal-scale trends of the polar ocean carbon fluxes, and compare the GCLETKF results  to estimates derived from global ocean biogeochemistry models and surface ocean pCO₂ data products.  

How to cite: Suntharalingam, P., Chen, Z., and Ghosh, J.: Estimates of Polar Ocean CO2 Uptake from Atmospheric Inverse Analyses , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11879, https://doi.org/10.5194/egusphere-egu24-11879, 2024.

Meteorological models are important simulation tools to improve our understanding of the climate behavior on seasonal, annual, decadal and centennial scales. Their complexity has increased considerably since 1990. The output fieds of several widely available meteorological such as GFS, ECMWF, WRF, ARPEGE and MERRA are evaluated, by comparing the output fields to in situ data performed during six campaigns with the wave-following platform OCARINA (Ocean Coupled with the Atmosphere, Research on the Interface on Annex Ship) developed at MIO. Following a recent comparison for wind and SST by Benjeddou et al. (2024), emphasis will now be laid on the comparison of heat fluxes and associated bulk variables, in open sea conditions, versus close to the shore line.

How to cite: Benjeddou, S., Bourras, D., and Luneau, C.: Evaluation of several meteorological models by comparison with qualified Air-Sea observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12623, https://doi.org/10.5194/egusphere-egu24-12623, 2024.

Estimates of air-sea fluxes rely on the knowledge of the gas transfer velocity. Despite more than half a century of field measurements, starting with the GEOSECS program in the 70ies, there are still many open questions. At low wind speeds, no reliable measurements are available, because all available techniques (dual-tracer, eddy covariance and active thermography) are either not suitable for measurements under these conditions or deliver too uncertain results. At high wind speeds beyond 25 m/s, almost no measurements are available. In the intermediate wind range enough reliable data are available. But the data are partly contradictionary. The effect of the many other parameters influencing the transfer velocity besides the wind speed is still uncertain. This includes the effect of the sea state (wave age), bubbles, and surfactants.

In wind-wave tunnels, it is easy to perform systematic studies. But the conditions deviate significantly from those at the open ocean in traditional linear facilities because of the short interaction length between wind. Therefore, only young wind seas can be generated, far away from a wind sea in equilibrium with the wind (“fetch gap”).

In 2021, we started a laboratory program, funded by a Reinhart Koselleck Project of the German Science Foundation. It includes three innovative key elements, which together overcome most disadvantages of previous wind-wave tunnel experiments. Firstly, a large annular facility is used, the Heidelberg Aeolotron. Because of the infinite fetch, wind waves come into equilibrium with the wind as at the ocean. Secondly, two imaging techniques are used to measure transfer velocities locally and instantaneously. Active thermography is used to measure the heat transfer velocity across the aqueous viscous boundary layer and a novel fluorescence technique to image the concentration fields in the mass boundary layer and to estimate the gas transfer velocity. Thirdly, measurements are performed under non-stationary conditions. In this way the whole fetch range can be investigated, when the wind speed is turned on, and decaying wind seas, when the wind speed is lowered.

In this talk first results of these measurements will be shown:

At low wind speeds, a significant overshoot in the transfer velocity occurs at low-fetch wind-wave fields.

The change in the Schmidt number exponent of the transfer velocity from 2/3 to 1/2 is related to the increasing frequency of microscale wave breaking.

An insoluble monomolecular monolayer of hexadecanol has the same effect as the soluble surfactant TritonX-100 (5 ppm by volume): Wind waves are completely suppressed up to wind speeds of about 8 m/s and the spatial patterns of the concentration field in the boundary layer are the same. In contrast, lowering the surface tension to about 43 mN/m by adding 1-hexanol to the water (2.4 kg/m³) did not suppress wind waves and transfer velocities at all.

How to cite: Jähne, B.: On the Crucial Role of Wind-Wave-Tunnel Studiesto Reveal the Mechanisms of Air-Sea Gas Exchange, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13372, https://doi.org/10.5194/egusphere-egu24-13372, 2024.

The oceans are steadily warming, which affects global weather and climate and leads to an increase in extreme events such as storms, hurricanes and marine heatwaves (MHWs). Future warming scenarios predict an increase in the frequency and intensity of such events. In the Mediterranean region, both extratropical cyclones and occasional Mediterranean hurricanes (medicanes) occur, causing considerable damage to infrastructure and major socio-economic losses in coastal regions. Using ERA-5 atmospheric reanalysis data and satellite-derived sea surface temperatures (SST), this study looks at medicanes that occurred between 2011 and 2023 and examines their characteristics and impacts on the water column. The interaction with simultaneous MHWs in the Mediterranean is also investigated. A total of 15 medicanes occurred during the study period. Of these, 5 occurred in the western Mediterranean (WMed), mainly in November; 9 in the central Mediterranean and Ionian (CMed) between September and December, two of which terminated in the eastern basin; and 1 event was localised entirely in the eastern Mediterranean (EMed) in October. During the study period, 2014 recorded the highest number of medicanes with three events. One event, Ilona, occurred in January in the WMed, while the other two events, Qendresa and Xandra, occurred in November in the CMed and WMed respectively. Two events took place in both 2020 and 2021. In 2020, both Ianos in September and Elaina in December were in the CMed. In 2021, the CMed and WMed witnessed the passage of Apollo in October and Blas in November respectively. In the 15 medicanes, the mean sea level pressure (MSLP) was between 988 and 1005 hPa, while the wind speeds (Ws) were between 17 and 23 m/s. Among the events, Ilona in January 2014 had the lowest MSLP and highest Ws and the lowest associated MSLP anomaly. Of the 15 events, 11 (73%) were associated with anomalously high sea surface temperatures (SSTA) and five of these SSTAs were defined as MHW events. Moreover, the high SST anomalies were observed three or more days before the onset of these medicanes, which may have contributed to the intensification of the passing storms and amplified their impact through air-sea heat exchange. In turn, the medicanes were also observed to influence the MHWs, as the heat released from the ocean during the medicanes prevented the MHWs from deepening beyond the surface layer, demonstrating a dynamic interplay between these events. In summary, as the oceans warm, medicanes and MHWs in the Mediterranean increase, with complex interactions determining their behavior and impacts. Understanding these dynamics is crucial for predicting and mitigating the impacts of these events on marine ecosystems and coastal regions. 

How to cite: Uba, K., Hamdeno, M., Barth, A., and Alvera-Azcárate, A.: Two extremes: Investigating the impact of the co-occurrence of medicanes and marine heatwaves in the Mediterranean Sea., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13779, https://doi.org/10.5194/egusphere-egu24-13779, 2024.

Marine protected area (MPA) networks are fundamental for restoring and conserving ecosystem functions like biodiversity and general ecosystem health. Ideally, the effects of local conservation measures are not limited to one particular MPA alone but influence and connect regions beyond, or even other MPAs, through the spreading, replenishment and potential recovery of populations and communities. Connectivity defines, in a probabilistic sense, the functional linkage exchange between individual MPAs or key regions, and it depends on the features of the selected tracers (including the specific biological traits of target organisms), but it is also to a large degree determined by the hydrodynamic circulation patterns in the area. For the German Bight (south-eastern North Sea), we are focusing in particular on potential spillover from a restoration site for the European flat oyster (Ostrea edulis) through the spread of planktonic life stages. 
The circulation regimes are determined mainly by tidal and wind forcings. The prevailing wind-driven surface circulation in the area is cyclonic, influenced by frequent south-westerly to westerly winds. However, winds from other directions, for instance from the North-West, have the potential to modify and even reverse this circulation pattern. Wind intensity and directions have a clear seasonal variability, with higher magnitudes in winter and lower in summer, but also exhibit a significant interannual variability driven by the strength and location of high and low mean sea level atmospheric pressure centres. Moreover, winds from the East are relatively rare compared to the other patterns but can be extremely persistent (up to hundreds of hours) and thus affect the hydrodynamics and, hence, the connectivity between the MPAs. In this study, we catalogued the wind events according to their typical duration and magnitude using 10m eastward and northwards components retrieved from ERA5 reanalysis data and characterized them according to their seasonality and interannual variability. The results can be used to define realistic atmospheric scenarios to numerically simulate the sea dynamics in the southern North Sea and, consequently, assess the connectivity among different sites, including established MPAs. These efforts are crucial for a proper planning of conservation and restoration measures in the German Bight, which is one of the most exploited marine regions in the world. 

How to cite: Rubinetti, S., Sidorenko, V., Arnone, E., Androsov, A., Beng, K. C., Klemm, K., Sell, A. F., Akimova, A., Pineda-Metz, S. E. A., Pogoda, B., Brand, S., Wegner, M., Shama, L., Laakmann, S., Horn, S., and Wiltshire, K. H.: A catalogue of wind events for assessing the connectivity among Marine Protected Areas in the German Bight (North Sea), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15234, https://doi.org/10.5194/egusphere-egu24-15234, 2024.

Mesoscale structures are key dynamical features of the ocean. They are associated with a variety of short lived and small-scale dynamics linked to physical, biological, and chemical processes at the submesoscale, such as cascading energy, impacting ocean stratification, and guiding ocean carbon and oxygen uptake. In the high latitudes, the spatial extent of the mesoscale is only tens of kilometres, making it challenging to observe the submesoscale processes. In August-September 2022, an extensive submesoscale-resolving multiplatform experiment was conducted across an Irminger Ring in the Labrador Sea. The experiment leveraged two underwater electric gliders equipped with nitrate, microstructure shear, chlorophyll fluorescence, oxygen, and turbidity sensors, operated in concert with a variety of ship operated instruments including underway-CTD’s, a moving vessel profiler, Thermosalinograph, ADCPs and a X-band radar system. Observations were acquired both, along the peripheries and within the core of the eddy, and offered insight into submesoscale dynamics of the ring. Making use of nearly concurrent turbulence and nutrients observations, we estimated the vertical flux pattern across the eddy’s frontal and interior regions. From the recorded and expected glider vehicle motion a vertical water velocity could be inferred and compared with the nutrient flux pattern. The stability of the ring was tracked with surface drifters, for weeks after the ship and glider survey ended, and a link between the disintegration of the ring and an atmospheric event was investigated

How to cite: Dilmahamod, A. F., Karstensen, J., Horstmann, J., and Krahmann, G.: Vertical fluxes in subpolar eddies from a high-resolution, multiplatform experiment in the Labrador Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16489, https://doi.org/10.5194/egusphere-egu24-16489, 2024.

Sharp fronts with temperature differences of approximately 0.5°C across a remarkably small lateral scale of order 10 m were observed in a subtropical region with strong mesoscale and submesoscale activity in the southeast Atlantic at 34°S, 6.5°E, far away from any coastal freshwater sources. These fronts were formed at the leading edge of a buoyant gravity current of 20-40 m thickness that propagated at a speed of order 0.1 m/s relative to the colder and thus denser surrounding waters. High-resolution turbulence microstructure observations revealed strongly enhanced turbulence inside the nose of the gravity current, while turbulence in the trailing bulk region was mainly wind- and convectively-driven and showed a strong diurnal modulation. Satellite and meteorological data suggest that the gravity current was triggered by the mesoscale strain-induced sharpening and final collapse of a larger-scale front at the edge of a mesoscale eddy during a period with decaying winds. In contrast to previous studies that have identified similar buoyant gravity currents in the equatorial ocean, our data suggest that they can also form at a mid-latitude location where rotational effects are strong. This suggests that even balanced fronts can decay into gravity currents under certain conditions, indicating a potentially important pathway for mesoscale energy dissipation and mixing.

How to cite: Boskamp, G., Holtermann, P., and Umlauf, L.: Buoyant gravity currents triggered by a collapsing mid-latitude submesoscale front, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16669, https://doi.org/10.5194/egusphere-egu24-16669, 2024.

Extreme weather and climate events, such as extratropical cyclones and droughts, represent a topic of paramount importance in the Iberian Peninsula and the North Atlantic Ocean plays an important role in shaping their frequency and intensity. Sea surface temperature (SST) variations, which are important indicators of ocean variability, can result in anomalous diabatic heating or cooling of the overlying atmosphere. In this study, the contributions of different physical processes to the development of North Atlantic explosive extratropical cyclones (EC) affecting the Iberian Peninsula are investigated using the ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). Results suggest that the North Atlantic Ocean SST contributed to the formation and intensification of extratropical cyclones, and particularly to the formation and development of intense storms. Furthermore, the combined analysis of SST and net surface heat flux (QN) also shows the cooling of the ocean associated with the EC tracks caused by the heat exchanges between the ocean and the atmosphere.

Acknowledgements

This work is supported by national funds by FCT - Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020 (https://doi.org/10.54499/UIDB/04033/2020).

How to cite: Liberato, M. L. R.: North Atlantic SST variability during strong winter extratropical cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16888, https://doi.org/10.5194/egusphere-egu24-16888, 2024.

The mesoscale horizontal stirring (MHS) is closely linked to various mesoscale dynamical phenomena, encompassing not only eddies but also meanders, filaments, and fronts. A clear understanding of its seasonality holds the potential to enhance our understanding of horizontal mixing and material dispersion. Here, we analyze the seasonal variation of MHS in the North Pacific surface ocean using ocean reanalysis (GLORYS12) current velocity data with a horizontal resolution of 1/12° from 1993 to 2019. Based on the characteristic of stirring to separate adjacent fluid trajectories, MHS is quantified using the finite-size Lyapunov exponent (FSLE), one of the Lagrangian diagnostics. The FSLE in the North Pacific shows clear seasonality but the phases of its evolution differ regionally. We identify two major modes, which contribute to over 80% of the seasonality of FSLE in the North Pacific, through the application of empirical orthogonal function (EOF) analysis to the climatological monthly mean FSLE. The first mode (57%) exhibits a variation peaking in April within the Kuroshio Extension region and the Subtropical Countercurrent region, where baroclinic instability plays a significant role. The second mode (25%) peaks during the summer season over the Kuroshio area and coastal upwelling areas of western North America. It is found that the strong seasonality in the upwelling area is induced by the North Pacific High.

How to cite: Yi, G., Park, W., and Lee, J.-Y.: Seasonal Variation of Mesoscale Horizontal Stirring in the North Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18069, https://doi.org/10.5194/egusphere-egu24-18069, 2024.

Microplastics can support biomass production by acting as substrates for microbial activity. This may imply potentially relevant effects for the sea-surface microlayer, the interface mediating air-sea gas exchange and where biological organic compounds can accumulate.

We tested this hypothesis by using six large scale mesocosms to simulate a future “high plastic ocean”. During the course of a 12-days experiment, we explored microbial organic matter dynamics in the sea-surface microlayer in the presence and absence of microplastics in the underlying water. We used as a reference a known number of polystyrene beads of 30 µm diameter and compared the three treatment mesocosms to an equal number of plastic-free control mesocosms.

The presence of microplastics represented a spur for microbial activity, and in the treated mesocosms biomass production was enhanced, leading to an increased concentration of organic compounds accumulating in the sea-surface microlayer. This initial boost in biological productivity led to a ∼3 % reduction of dissolved CO₂ in the underlying water, which we could imagine potentially reversed once the degradation phase took off. Based on our results and on other recent studies, we will discuss potential interference of plastic with the composition of the sea-surface microlayer, with direct and indirect impacts on the uptake of CO₂ and the marine carbon cycle. 

How to cite: Galgani, L., Tzempelikou, E., Kalantzi, I., Tsiola, A., Tsapakis, M., Pitta, P., Esposito, C., Tsotskou, A., Magiopoulos, I., Benavides, R., Steinhoff, T., Boldrini, A., Polvani, A., and Loiselle, S. A.: Plastics Affect the Ocean's Uptake of Atmospheric CO₂ across the Marine Boundary Layer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19544, https://doi.org/10.5194/egusphere-egu24-19544, 2024.

An improved understanding of wave breaking is still a hot topic owing to its relevance in the coupling of ocean and atmosphere. Multiple communities are focusing on numerical simulations of the fully coupled two-phase flow, the validation of such models remains challenging. Herein, we demonstrate how coherent marine radars can help to shed light on how different wave and wind parameters influence the evolution of waves towards breaking in the nearshore. The interpretation of the results is undermined by SWASH simulations of shoaling waves for different wave spectra and two beaches and simulations of radvarimages of these waves. Data of three independent measurement campaigns shows that the shoaling characteristics are strongly influenced by the wave steepness, relative depth the Ursell number and the wind. The influence of individual parameters cannot be isolated, but must be understood in its entirety.

How to cite: Støle-Hentschel, S., Catalán, P., Streßer, M., Horstmann, J., and Dias, F.: On the influence of hydrodynamic and environmental conditions on wave breaking in the nearshore, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20158, https://doi.org/10.5194/egusphere-egu24-20158, 2024.

The production by biological and photochemical mechanisms of short-lived Volatile Organic Compounds (VOC) in the surface ocean is regulated by environmental parameters and nutrient abundance, and hence climate change. These gases then enter the atmosphere through the air–sea interface and contribute to photochemical pollution, affect the cloud properties, the radiative forcing and precipitation. Despite the improved understanding of the temporal and spatial distribution of marine trace gases of biogenic origin and their potential effects, further investigation is needed in different geographical regions and especially in polluted marine environments and populated coastal regions (Tinel et al., 2023). In this study we estimate the spatiotemporal distribution of seawater VOC concentrations in the climate sensitive geographical region of Eastern Mediterranean. State-of-the art empirical models linking remotely-sensed data of phytoplankton biomass (EU Copernicus Marine Environment Monitoring Service, CMEMS) and environmental parameters such as sea-surface temperature, and photosynthetically available radiation are used (Gali et al., 2018). Ocean-model data such as mixed layer depth, and euphotic zone are also exploited. The impact of the sea-to-air VOC emission fluxes on photochemistry, marine aerosols and cloud properties are assessed and quantified through advanced atmospheric simulations with the WRF-Chem atmospheric model coupled to chemistry and aerosols during typical conditions but also extreme events.

References

Gali M., Levasseur, M., Devred, E., Simo, R. and Babin, M., Sea-surface dimethylsulfide (DMS) concentration from satellite data at global and regional scales, Biogeosciences, 15, 2018, pp. 3497-3519, https://bg.copernicus.org/articles/15/3497/2018 , doi:10.5194/bg-15-3497-2018.

Tinel L., J. Abbatt, E. Saltzman, A. Engel, R. Fernandez, et al.. Impacts of ocean biogeochemistry on atmospheric chemistry. Elementa: Science of the Anthropocene, 2023, 11 (1), ff10.1525/elementa.2023.00032ff. ffhal-04221390f

How to cite: Bossioli, E., Kourakos, D., Raitsos, D. E., Kournopoulou, A., Karagiorgos, J., Methymaki, G., Portalakis, P., Karatasou, S., and Sofianos, S.: Impact of marine biogenic VOC emissions on the marine boundary layer of the Eastern Mediterranean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20277, https://doi.org/10.5194/egusphere-egu24-20277, 2024.

How to cite: Renault, L.: Submesoscale air-sea interactions: atmospheric response and impacts on the ocean dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20391, https://doi.org/10.5194/egusphere-egu24-20391, 2024.

With the aim of studying the momentum flux in wind-wave modulation situations, a 7 month field experiment was set-up in 2023 from the Belle-Ile-en-Mer island off the West coast of France. The site was selected for its exposure to dominant wind and swell, the proximity of a wave buoy, and the rapidly increasing water depth to allow a focus on deep to intermediate wave dispersion regimes. A scanning wind LiDAR [1,2] installed on the coast of the island was used to measure the vertical profile of the horizontal wind speed and direction from 1 to 3 kilometers from the coast. The configuration allowed for measurements of the wind speed and direction profiles starting at some meter above the water surface and going up to some 150m. This original approach enables to obtain quasi-instantaneous vertical planes of the wind speed as well as 30-min mean profiles simultaneously with a wave parameter.

The wide range of wind and wave combinations observed during the deployment allows statistical analysis. Significant wave heights, wave peak periods, and U10 wind speeds were observed in the range 0-6.5 m, 2-20 s, and 0.5-18 m/s, respectively. From this rich database, the near-surface momentum flux estimated by the wind profile close to the water surface appears to match well with results from COARE 3.5 algorithm. The possibility of the scanning wind LiDAR to measure mean wind profiles allows an original point of view to analyze wind-wave interactions. It was observed that for young seas, the profile can be in equilibrium, following Monin-Obukov similarity theory from close to the water surface up to some 100m. In contrast, for fast-travelling waves, significant deviations of the wind profile are observed compared to the surface fluxes. These deviations are parametrized as function of height and analyzed as function of the wave age.

[1] Paskin, L., Conan, B., Perignon, Y., & Aubrun, S. (2022). Evidence of Ocean Waves Signature in the Space–Time Turbulent Spectra of the Lower Marine Atmosphere Measured by a Scanning LiDAR. Remote Sensing, 14(13), 3007.

[2] Conan, B., & Visich, A. (2023). Measurement and analysis of high altitude wind profiles over the sea in a coastal zone using a scanning wind LiDAR–application to wind energy. Wind Energy Science Discussions, 2023, 1-23.

How to cite: Conan, B. and Bruch, W.: Analysis of wind profiles above the water surface in wind-wave interaction thanks to a scanning wind LiDAR, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21643, https://doi.org/10.5194/egusphere-egu24-21643, 2024.

Tropical Pacific quasi-decadal (TPQD) climate variability is characterized by quasi-decadal sea surface temperature variations in the central Pacific. This low-frequency climate variability is suggested to influence extreme regional weather and substantially impact global climate patterns and associated socio-economy through teleconnections. Previous studies mostly attributed the TPQD climate variability to basin-scale air-sea coupling processes. However, due to the coarse resolution of the majority of the observations and climate models, the role of sub-basin-scale processes in modulating the TPQD climate variability is still unclear. Using a long-term high-resolution global climate model, we find that energetic small-scale motions with horizontal scales from tens to hundreds of kilometers (loosely referred to as equatorial submesoscale eddies) act as an important damping effect to retard the TPQD variability. During the positive TPQD events, compound increasing precipitation and warming SST in the equatorial Pacific intensifies the upper ocean stratification and weakens the temperature fronts along the Pacific cold tongue. This suppresses the growth of submesoscale eddies as well as their associated upward vertical heat transport by inhibiting baroclinic instability and frontogenesis; Conversely, during the negative TPQD events, the opposite is true. Using a series of coupled global climate models that participated in the Coupled Model Intercomparison Project Phase 6 with different oceanic resolutions, we show that the amplitude of the TPQD variability becomes smaller as the oceanic resolution becomes finer, providing evidence for the impacts of submesoscale eddies on damping the TPQD variability. Our study suggests that explicitly simulating equatorial submesoscale eddies is necessary for gaining a more robust understanding of low-frequency tropical climate variability.

How to cite: Qu, Y., Wang, S., Jing, Z., Zhang, Y., Wang, H., and Wu, L.: Tropical Pacific Quasi-Decadal Variability Suppressed by Submesoscale Eddies , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-162, https://doi.org/10.5194/egusphere-egu24-162, 2024.

The sensitivity of the sea surface height anomaly (SSHA) forecasting on the accuracy of mesoscale eddies over the Kuroshio Extension region, which was
determined by the conditional non-linear optimal perturbation (CNOP) method using a two-layer quasigeostrophic model, is evaluated by adopting multiply realistic marine datasets through an advanced particle filter assimilation method. It is shown that, if additional observations are preferentially assimilated to the sensitive area of mesoscale eddies identified by the CNOP, where the eddies present a clear high- to low-velocity gradient along the eddy rotation, the forecasting skill of the SSHA can be more significantly improved. It is also demonstrated that the forecasts of the SSHA in the region where the large-scale mean flow possesses much stronger barotropic and/or baroclinic instability tend to exhibit stronger sensitivity to the accuracy of the initial field in the sensitive area of mesoscale eddies. Therefore, more attention should be preferentially paid to the assimilation of the additional observations of the mesoscale eddies for the SSHA forecast in the region with a strong velocity shear of ocean circulation. The present study verifies the sensitivity on mesoscale eddies of SSHA forecasts derived by the two-layer quasigeostrophic model using multiply sets of realistic oceanic data, especially including observation and reanalysis data, which further additionally demonstrates the importance of targeted observations of mesoscale eddies to the SSHA forecast in the regions of strong velocity shear of ocean circulation and provides a more credible scientific basis for the field campaign of the targeted observations for mesoscale eddies associated with the SSHA forecasting.

How to cite: Jiang, L., Duan, W., and Liu, H.: The Most Sensitive Initial Error of Sea Surface Height Anomaly Forecasts andIts Implication for Target Observations of Mesoscale Eddies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2229, https://doi.org/10.5194/egusphere-egu24-2229, 2024.

The Oxygen Minimum Zone (OMZ) off the coast of Mauritania and Senegal is characterized by a shallow and a deep oxygen minimum, each with potentially different formation mechanisms. Although the shallow OMZ has been linked to the en-route degradation of organic matter within highly productive, coastal-generated eddies, less attention has been paid to hypoxic Dissolved Oxygen (DO) concentrations observed along the coastal region, where low-oxygen eddies are formed. This study aims to clarify the spatio-temporal dynamics and underlying mechanisms that lead to the formation of the shallow OMZ along the northwestern African coast.

To achieve this, a Eulerian-Lagrangian numerical framework was employed by combining a coupled physical-biogeochemical model with a Lagrangian particle-tracking simulation. The model domain covers the entire Tropical Atlantic with an horizontal resolution of 3 km, achieving a good representation of the horizontal and vertical structure of the North Atlantic OMZ. To assess the pathways and evolution of the water masses that form the shallow OMZ, lagrangian particles were released in grid cells with DO < 40 μmol.l^-1 and traced backwards in time.

Our results reveal distinct seasonal and latitudinal variations of DO concentrations along the coast, with DO concentrations significantly decreasing in the transition from the upwelling to the relaxation season (from May to July). Associated with the transport of more oxygenated South Atlantic Central Waters (SACW), the influence of the Poleward Undercurrent (PUC) on the ventilation of the coastal region is evident, especially when the current loses intensity and becomes a surface-intensified feature in summer. When the PUC reaches its maximum intensity in autumn, its core deepens below the mixed layer and replaces the older, oxygen-poor waters with ventilated waters of southern origin.

The impact of eddies on coastal dynamics was also explored. A quasi-permanent Anticyclonic Modewater Eddy (ACME) formed during the upwelling season by the interaction of the PUC with the Cap-Vert headland is the main mechanism behind the import of offshore waters to the coastal region. Lagrangian particle trajectories suggest that this eddy prevents the direct northward transport of SACW by the PUC. Whilst some of the particles are trapped and subsequently transported offshore inside the eddy, other particles are stirred with an older, less oxygenated SACW variety in the offshore region and re-circulate to the coastal region. Similar particle re-circulation patterns are also observed further north, coinciding with cyclonic and ACME formation hotspots.

Our findings suggest that in addition to their role in the formation and advection of oxygen-depleted waters to offshore, coastal-generated eddies play a crucial role in modulating DO levels along the northwestern African coast.

How to cite: Cardoso, C., Calil, P., Caldeira, R. M. A., and Peliz, Á.: A model perspective on the drivers of the shallow oxygen minimum zone off the northwestern African coast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6196, https://doi.org/10.5194/egusphere-egu24-6196, 2024.

How to cite: Nummelin, A. and Isachsen, P. E.: Parameterizing mesoscale eddy buoyancy transport over sloping topography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6690, https://doi.org/10.5194/egusphere-egu24-6690, 2024.