Orals: Mon, 28 Apr | Room L3
The recent decline in Antarctic sea-ice, notably the extreme low winter cover in 2023 and 2024 is a major source of concern. Some progress has been made towards determining the drivers of ice loss but uncertainty remains regarding its impacts, particularly for ocean-atmosphere interaction. Resolution of this uncertainty is important as ice decline can significantly modify surface heat loss, and thus the ocean and atmosphere. We show that the substantial failure of ice regrowth in winter 2023 provided a major new source of turbulent ocean heat loss to the atmosphere. Ice concentration in the Weddell, Bellingshausen and Ross Seas is reduced by up to 80% and is accompanied by an unprecedented doubling of mid-winter ocean heat loss. Furthermore, peak heat loss shifts from late April to mid-June with weaker than normal heat loss in austral autumn. The strengthening of winter surface heat loss is accompanied by changes on both sides of the ocean-atmosphere interface. These include a rise in frequency of atmospheric storms and greater surface heat loss driven dense water formation. The findings reveal that the record low winter 2023 Antarctic sea-ice cover substantially modified Southern Ocean-atmosphere interaction and motivate in-depth analysis of the wider climate system impacts. The subsequent evolution of low ice conditions together with their ocean-atmosphere impacts through to 2025 will also be considered.
How to cite: Josey, S., Meijers, A., Blaker, A., Grist, J., Mecking, J., and Ayres, H.: Record Low Winter 2023 Antarctic Sea-Ice Increased Ocean Heat Loss, Dense Water Formation and Storms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9075, https://doi.org/10.5194/egusphere-egu25-9075, 2025.
The sea ice extent (SIE) in the Southern Ocean experienced a drop in the late 1970s, though less pronounced compared to the one post-2016. Though several studies explain the drop since 2016, the 1970s decline is critical in understanding the long-term variability of SIE. To investigate the underlying mechanisms for this decline, we conducted wind perturbation experiments using the general circulation model EC-Earth3. We perturb the model by adding wind stress anomalies derived from ERA5 to the model winds. Using this technique, we simulate the evolution of SIE over the period 1958-2023. Our analyses show that the perturbation induces a drop in the 1970s despite the control (no-perturbation) run showing no such trend. This strongly indicates the important role of winds in driving the drop, though ocean processes and other feedback mechanisms may also contribute to the decline. The SIE shows spatial heterogeneity in the variations driven by the wind. Presently, we are conducting multiple ensemble simulations to evaluate the influence of initial conditions on the results.
How to cite: Francis, F., Goosse, H., and Barriat, P.-Y.: Wind perturbation experiments to simulate the 1970s drop in the sea ice extent in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17472, https://doi.org/10.5194/egusphere-egu25-17472, 2025.
While the Southern Ocean is freshening, the sources of this freshening and their variability remain uncertain. Freshwater enters the ocean as Meteoric Water (MW; precipitation, river runoff, glacial discharge) and Sea Ice Meltwater (SIM). These inputs can be quantified using seawater salinity and stable oxygen isotopes in seawater, δ18O; however it involves the challenging task of determining the isotopic signature of MW (δ18OMW). Here, we apply Self-Organising Map (SOM), a machine learning technique, to water mass properties to estimate the global distribution of the isotopic signature of MW (δ18OMW) by characterizing distinct salinity-δ18O relationships from two comprehensive datasets. The inferred δ18OMW is then used in a 3-endmember mixing model to estimate MW and SIM contributions to global ocean freshwater content. Our results show the large scale distribution of MW and SIM, as well as giving insights into their role in mass transformation and interannual variability. We highlight the MW content in Ice Shelf Water and Antarctic Bottom Water linked to glacial melt, which is concurrent with brine content derived from sea ice formation. Our results also show that AABW has freshened since the 1990’s due to a reduction of sea ice formation (less brine production) rather than an increase in glacial melt, and suggest the emergence of anthropogenic forced signals in seawater δ18O.
How to cite: Davila, X., L. McDonagh, E., Jebri, F., Gebbie, G., and P. Meredith, M.: Freshwater Sources and their Variability through Salinity-δ18O Relationships: A Machine Learning Solution to a Water Mass Problem, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4254, https://doi.org/10.5194/egusphere-egu25-4254, 2025.
Pycnocline stratification is increasing across multiple ocean basins due to a warming surface ocean and changes in wind forcing. Pycnocline stratification plays a leading order role in tracer transport, shaping capacity for heat and carbon uptake, making it a key parameter of interest on timescales ranging from paleoclimate to plankton blooms. Part of the challenge in assessing the role of pycnocline stratification in global models is the two-way connection between physical processes at the mesoscale and submesoscale and stratification, with important implications for the resulting tracer transport. Using idealized runs of MITgcm, we find that the strength of pycnocline stratification influences the formation and evolution of submesoscale structure. When a constant isopycnal slope is initialized, tracers get efficiently transferred across the base of the mixed layer and get trapped in anticyclonic submesoscale vortices below the mixed layer. This leads to tracer concentrations below the mixed layer and fluxes through it to be stronger under decreased stratification conditions. In contrast, when the frontal lateral buoyancy gradient is held fixed while stratification changes, the vertical flux of tracers and the concentrations at depth stay constant across all examined stratification conditions. Understanding the relationship between pycnocline stratification and fine-scale physical motions is necessary to diagnose and predict trends in carbon uptake and storage, particularly in the Southern Ocean.
How to cite: Dove, L., Freilich, M., Siegelman, L., Fox-Kemper, B., and Hall, P.: Frontal Subduction in an Increasingly Stratified Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-213, https://doi.org/10.5194/egusphere-egu25-213, 2025.
The Southern Ocean is critical in regulating the global climate by playing a key role in absorbing, transporting, and storing atmospheric carbon dioxide and heat. This is largely due to the region's unique geography, which connects multiple ocean basins and forms a complex network of global ocean circulation. This network spans a wide range of scales, from planetary motions to small-scale processes. Major large-scale features within its circulation network include the Antarctic Circumpolar Current, Slope Current, Subpolar Gyres, and Bottom Water formation, layered over finer processes like convection and turbulence. Conventional research into the Southern Ocean relies on Global and Regional Ocean and Climate Models which can incorporate realistic forcing like wind, topography and bathymetry. While these models are effective for large-scale motions, they struggle to resolve smaller-scale dynamics, leaving an incomplete picture of the region’s physical processes. To address this, Direct Numerical Simulations (DNS) are used to solve the fundamental equations of fluid dynamics within a small, idealized domain resembling the Antarctic region. This domain is driven solely by surface density variations and planetary rotation. By leveraging dynamic similarity, the small-scale results are scaled to represent the full ocean. This approach successfully captures all scales of motion and reveals the emergence of all major oceanographic features. Remarkably, the simulations show that convection alone can drive a cascade of interconnected physical processes, recreating the Southern Ocean's complex circulation without additional complexities like wind or bathymetry.
How to cite: Chidhambaranathan, B., Gayen, B., and Vreugdenhil, C.: Exploring Southern Ocean’s hidden drivers with direct numerical simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16813, https://doi.org/10.5194/egusphere-egu25-16813, 2025.
Dense water formation on the Antarctic continental shelf drives the abyssal overturning circulation, being the main process by which Antarctic Bottom Waters form. Despite its importance, most ocean models cannot simulate dense water formation at the Antarctic coast and flow down the continental slope (i.e., overflow) due to the fine resolution required by these processes. While many studies have looked at the impact of horizontal and vertical resolution in the deep ocean on the overflows, no studies have investigated whether surface vertical resolution impacts dense water formation. In this work, we varied the surface ocean cell of two dense water-forming models from 1m to 5m thickness as a simple vertical resolution sensitivity test. We used the ACCESS-OM2 and the Pan-Antarctic ocean and sea ice models, each employing a different boundary layer parameterization. Thickening the surface cell to 5m in ACCESS-OM2 decreased the dense water formation at the Antarctic continental shelf by 45% (1.5 Sv) and ceased its overflow through the continental slope after 10 years of simulation. In the Pan-Antarctic, thickening the surface cell reduced the Antarctic dense water formation by 34% (1.5 Sv) and its overflow by 67% (2.5 Sv) after 10 simulation years. The dense water formation reduction in 5m experiments is explained by a southward shift in the surface Ekman transports, bringing light offshore waters to the coast and prohibiting dense water formation at the Antarctic continental shelf. This response is independent of the boundary layer scheme employed.
How to cite: Aguiar, W., K. Morrison, A., McC. Hogg, A., Huneke, W., Hutchinson, D., Spence, P., Colombo, P., and D Stewart, K.: Sensitivity of Antarctic dense water formation to surface vertical resolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3356, https://doi.org/10.5194/egusphere-egu25-3356, 2025.
Recent observations reveal that Antarctic Bottom Water is thinning, warming, and spreading northward more slowly into the abyssal ocean. The causes and consequences of these changes remain uncertain due to limited observations. Using historical data (1985-2024) and model projections (2041-2050), we assess abyssal ocean changes in the past, present, and future. Between 1985-2024, isopycnals below 3000~m descended at -149±5 m decade-1, and are replaced by warmer water, causing warming of 0.03±0.02 °C decade-1. Freshening of -0.004±0.003 g kg-1 decade-1 occurred due to meltwater-driven changes in continental shelf waters forming bottom water. Projections suggest bottom water thinning will continue, doubling the abyssal ocean’s contribution to Southern Ocean sea level rise by 2050. However, freshening has slowed or reversed, indicating a salinity turning point. This transition occurs as shelf waters become too fresh and light to reach the abyssal ocean, with important implications for overturning circulation, nutrient cycling, and regional sea level rise.
How to cite: Gunn, K., England, M., and Rintoul, S.: Persistent Abyssal Warming but Emerging Salinity Shifts in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1745, https://doi.org/10.5194/egusphere-egu25-1745, 2025.
Under future climate forcing, in both strong emission (SSP585) and strong mitigation (SSP126) scenarios, the deep overturning circulation of the Southern Ocean collapses by 2050 in almost all CMIP6 simulations. The SSP scenario applied ultimately has a relatively small impact on this shutdown, which appears to have commenced in the last few decades of the historical scenario runs – the present day. The ensemble mean drops to 50% (10±4 Sv) strength, from a pre Industrial strength of 20±9 Sv, with individual models decreasing by over 65%. This ‘collapse’ occurs over 30-50 years, with most of the temporal variation explained by internal variability within models.
Associated with the change in overturning strength is a reduction in Antarctic Bottom Water (AABW) volume south of 30S by >2x1016 m3, driven by a reduction in formation rates around Antarctica. Here surface warming reduces formation by approximately 4 Sv, whilst freshwater effects (due to sea ice reduction for example) are relatively weak. Walin analysis shows that subsequent entrainment of Circumpolar Deep Water (CDW) by convecting AABW also significantly decreases, further reducing AABW volume and export.
The reduction in AABW formation results in an expansion of CDW volumes. CDW upwelling reduced due to the lower cell shutdown is largely compensated by a wind driven increase in upper cell overturning of approximately 25%. The increased upper cell overturning enhances SAMW export by up to 5 Sv, with a significant boost to net heat export due to both enhanced volumes and temperatures of this water mass.
We explore the wider impact of these Southern Ocean shifts in overturning on the net warming and carbon storage of the global ocean, and the wider global climate. We contextualize climate model representations of bottom water processes against observations and other model studies to suggest that a deep overturning tipping point may have already been reached.
How to cite: Meijers, A. and Rosser, J.: Collapse of deep overturning under future climate forcing and impacts on ocean heat and carbon uptake in CMIP6 models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9932, https://doi.org/10.5194/egusphere-egu25-9932, 2025.
The interaction between ocean eddies and the large-scale ocean circulation results in a pronounced multidecadal variability in the Southern Ocean, with a dominant periodicity of 40-50 years, referred to as the Southern Ocean Mode (SOM) (Le Bars et al., GRL, 2016). The SOM plays a critical role in modifying the ocean heat content, thereby influencing both sea-ice extent and basal melt around Antarctica. Additionally, this multidecadal variability propagates northward into the Atlantic Ocean, modulating the Atlantic Meridional Overturning Circulation (AMOC) strength. The AMOC is one of the most prominent climate tipping elements on Earth and can potentially collapse as a consequence of surface freshwater input in the North Atlantic. Here, we investigate the impacts of an AMOC collapse on multidecadal variability in the Southern Ocean using the results of the first modeled AMOC collapse in a high-resolution and strongly eddying (0.1° horizontal resolution) ocean-only model, the Parallel Ocean Program (POP). Our findings indicate that the magnitude of the SOM reduces significantly following an AMOC collapse. An analysis of the SOM variability before and after an AMOC collapse allows us to study the role of background stratification, baroclinic instability, and convection in shaping the SOM.
How to cite: Smolders, E., van Westen, R., and Dijkstra, H.: The Impacts of an AMOC Collapse on Southern Ocean Multidecadal Variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12345, https://doi.org/10.5194/egusphere-egu25-12345, 2025.
The Southern Ocean is a critical player in regulating Earth’s carbon cycle and climate, yet its role under future climate change remains uncertain. Studying the Southern Ocean under past climate states can help address this knowledge gap. For example, changes in the Southern Ocean, through modulating deep ocean carbon content, are widely thought to have played a driving role in the atmospheric carbon dioxide (CO2) fluctuations of Earth’s past ice ages. Here we present three novel findings that advance our understanding of processes linking Southern Ocean changes to glacial CO2 drawdown. First, a new proxy record reveals a tight coupling between atmospheric CO2 levels and deep Southern Ocean carbon storage over the Last Glacial Cycle, providing clear evidence of the systematic transfer of carbon into the deep ocean during glaciation and its release during deglaciation. These results are used to quantify the deep Indo-Pacific’s remineralized carbon content at the Last Glacial Maximum and are found to explain a significant proportion of observed glacial CO2 drawdown. Second, we demonstrate how improved ventilation of North Pacific mid-depths (evidenced by glacial proxy data) directly impacts Southern Ocean biogeochemistry by reducing the carbon and nutrient load of waters upwelling in the Southern Ocean. This process enhances biological pump efficiency, curtailing Southern Ocean CO2 outgassing and highlighting a critical interhemispheric connection in glacial nutrient cycling. Finally, idealized numerical modelling experiments demonstrate cooling of the high northern latitudes associated with a large Northern Hemisphere ice sheet can, in isolation of any other forcing, remotely induce glacial-like changes in Southern Ocean sea ice, circulation, and biogeochemistry that work to enhance ocean carbon content. These changes include expanded southern sea ice and cooler, saltier, better-stratified bottom water, which together increase oceanic carbon storage via solubility- and disequilibrium-driven effects. Collectively, these findings underscore the Southern Ocean’s central role in mediating interhemispheric and glacial climate feedbacks, offering new insights into the processes that drove the Earth’s into low-CO2 glacial periods.
How to cite: Shankle, M., MacGilchrist, G., Rae, J., and Burke, A.: Southern Ocean mechanisms of glacial CO2 drawdown and their links to global climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-427, https://doi.org/10.5194/egusphere-egu25-427, 2025.
The ongoing increase of global mean temperature, caused by anthropogenic CO2 emissions, will most likely lead to enhanced melting and calving of Antarctic ice shelves in the coming decades. As a consequence, the freshwater input into the Southern Ocean is expected to increase as well. The resulting change in ocean salinity could have significant consequences for ocean circulation, water column stratification, and water mass formation in the Southern Ocean, which are all expected to affect the capacity of the surface ocean to remove CO2 from the atmosphere, and the sequestration of carbon in the deep ocean. However, the magnitude and spatio-temporal patterns of these changes and their links to freshwater forcing are not yet well understood. To reduce these uncertainties, increase our understanding, and better quantify the feedbacks on the climate system, the international SOFIA initiative (Swart et al., 2023) defines freshwater input protocols for consistent use in various Earth System Models. Here we study the impact of additional freshwater around Antarctica on circulation and carbon fluxes in a steady preindustrial climate state using four Earth System Models. Most of the models show a decrease in the uptake of CO2 by the surface of the Southern Ocean, caused by a strengthened outgassing of natural CO2 between 50°S and 60°S. The stronger outgassing can be attributed to an increase in sub-surface dissolved inorganic carbon concentration south of the Antarctic Circumpolar Current that is associated with a redistribution of water masses in the Southern Ocean. Furthermore the reduction of the production and downward flow of Antarctic Bottom Water is leading to a decrease of its volume, and the expansion of carbon-rich Circumpolar Deep Water, which increases the carbon content at depth and thus weakens the overall CO2 uptake. However, the models disagree in terms of the intensity of the weakened Southern Ocean CO2 uptake. This difference seems to be mainly linked to the model resolution and the representation of the ocean mean state, e.g. the strength of the stratification, which is a determining factor for the redistribution of the additional freshwater to depth. To pursue this work, experiments with additional freshwater forcing in various climate states are conducted to analyse the ocean carbon cycle’s response and quantify potential climate feedbacks.
How to cite: Jouet, O., Hauck, J., Danek, C., Haumann, A., Hattermann, T., Muilwijk, M., Pauling, A. G., Swart, N. C., and Völker, C.: Impact of additional freshwater around Antarctica on the Southern Ocean carbon cycle : an inter-model comparison, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2563, https://doi.org/10.5194/egusphere-egu25-2563, 2025.
The global ocean has taken up an estimated 89% of excess heat and 26% of annual CO2 emissions resulting from anthropogenic activity in recent decades. Despite the crucial role of the ocean in the climate system, there remains significant uncertainty around ocean heat and carbon uptake. Processes of ocean carbon and heat uptake and redistribution are among the leading processes limiting scientists’ ability to predict the rate and magnitude of global climate change. Furthermore, despite being responsible for as much as 40–50% of global annual oceanic CO2 uptake, the Southern Ocean remains the most controversial ocean basin, with large differences in both the magnitude and variability of CO2 fluxes across various estimation methods. The disagreement regarding the magnitude and variability of Southern Ocean CO2 fluxes largely arises due to the sparsity and uneven distribution of in-situ observations of carbon in the ocean, a problem which also constrains estimates of ocean heat uptake. However, machine learning has been extensively demonstrated in recent years to be an effective tool for overcoming such data limitations through gap-filling methods.
We leverage existing expertise in machine learning methods from generating the SOM-FFN and MOBO-DIC ocean carbon data products and apply this expertise to the development of a novel machine learning generated ocean heat uptake data product for the Southern Ocean. We integrate new Earth Observation data from Copernicus satellites with in-situ observation data from the Southern Ocean as input for our new data products. Here, we present a beta-version of our new machine-learning based products of carbon and heat uptake in the Southern Ocean and a first analysis of the variability and trends of the carbon and heat uptake of these data products. This work presents our first steps towards data products which will allow us to identify transport pathways of carbon and heat within the ocean interior. The novel machine learning-based products will also be used to support the advancement of Earth System Models and climate change predictions.
How to cite: Burt, D. and Landschützer, P.: Filling the gaps in Southern Ocean carbon and heat: Machine learning-based products from sparse observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18124, https://doi.org/10.5194/egusphere-egu25-18124, 2025.
The physical and biogeochemical processes governing the air-sea CO2 flux in the Southern Ocean are still widely debated. This presentation focus on an anomalously large sink of CO2 observed north of the Weddell Sea in Summer 2022. The processes behind this anomalous situation are analyzed based on the combination of in situ observations, various satellite parameters (altimetric currents, Chl-a, ice concentration and sea surface salinity) and ocean model reanalysis.
The “Southern Ocean Carbon and Heat Impact on Climate” cruise in Summer 2022 aimed at studying physical and biogeochemical processes in the Weddell Sea and in its vicinity. A “CARbon Interface OCean Atmosphere” (CARIOCA) drifting buoy was deployed in January 2022 in the subpolar Southern Ocean, providing hourly surface ocean observations of fCO2 (fugacity of CO2), dissolved oxygen, salinity, temperature and chlorophyll-a fluorescence for 17 months. An underwater glider was piloted with the buoy for the first 6 weeks of the deployment to provide vertical ocean profiles of hydrography and biogeochemistry. These datasets reveal an anomalously strong ocean carbon sink for over 2 months occuring in the region of Bouvet Island and associated with large plumes of chlorophyll-a (Chl-a). Based on Lagrangian backward trajectories reconstructed using various surface currents fields, we identified that the water mass reaching the Bouvet Island region originated from the south-west, from the vicinity of sea ice edge in Spring 2021. We suggest that a strong phytoplankton bloom developed there in November 2021 favoured by early sea ice melt in 2021 in the Weddell Sea. These waters, depleted in carbon, then travelled to the position of the CARIOCA buoy. The very low values of ocean fCO2, measured by the buoy (down to 310 μatm), are consistent with net community production previously observed during blooms occurring near the sea ice edge, partly compensated by air-sea CO2 flux along the water mass trajectory. Early sea ice retreat might therefore have caused a large CO2 sink farther north than usual in Summer 2022, in the Atlantic sector of the subpolar Southern Ocean. Such events might become more frequent in the future as a result of climate change.
How to cite: Boutin, J., Naëck, K., Swart, S., Du Plessis, M., Merlivat, L., Beaumont, L., Lourenço, A., D'Ovidio, F., Rousselet, L., Ward, B., and Sallée, J.-B.: Anomalous Summertime CO2 sink in the subpolar SouthernOcean promoted by early 2021 sea ice retreat, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2577, https://doi.org/10.5194/egusphere-egu25-2577, 2025.
The Southern Ocean, a region highly vulnerable to climate change, plays a critical role in regulating global nutrient cycles and atmospheric CO₂ via the biological carbon pump. Diatoms, a group of photosynthetically active plankton with dense opal skeletons, are central to this process, as their exoskeletons are thought to enhance the transfer of particulate organic carbon to depth, making them main vectors of carbon storage. However, conflicting observations obscure the mechanistic link between diatoms, opal, and particulate organic carbon fluxes, especially in the twilight zone where the greatest flux losses occur.
Here we present direct springtime flux measurements from different sectors of the subpolar Southern Ocean, demonstrating that across large areas of the subpolar twilight zone, carbon is efficiently transferred to depth: however, not by diatoms. Instead, opal is retained near the surface ocean, indicating that processes such as diatom buoyancy regulation and grazer repackaging can negate the ballast effects of diatoms’ skeletons. Using image data, we further reveal species-specific differences in diatom flux dynamics, highlighting the complexity of their role in the carbon cycle.
Our findings challenge the assumption that diatom-rich surface waters are necessarily associated with effective carbon export and transport in the Southern Ocean. They suggest that shifts in phytoplankton community composition driven by climate change may have a smaller impact on biological carbon storage than current models predict.
How to cite: Giering, S. L. C., Williams, J. R., Baker, C., Pabortsava, K., Blackbird, S., Martin, A., Poulton, A., Briggs, N., Carvalho, F., Le Moigne, F., Villa-Alfageme, M., Henson, S., Espinola, B., Iversen, M., Liu, Z., Moore, M., Passow, U., Romanelli, E., Thevar, T., and Sanders, R. and the The following authors are missing: The Diatom Disconnect: Retention near surface waters limits carbon transfer to the deep ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19855, https://doi.org/10.5194/egusphere-egu25-19855, 2025.
Assessing and understanding the factors that control the biological carbon pump (BCP), i.e. the transfer of organic carbon biologically fixed by primary production (PP) from the euphotic zone to the deep ocean, remains a major challenge in marine biogeochemistry. Among these factors, the intensity of PP and the structure of phytoplankton community play key roles in the biogeochemical fluxes of the BCP and depend on the physico-chemical conditions of the ocean. Although the BCP has received significant attention in the last decades, the magnitude of this process remains poorly quantified, notably for under-sampled areas such as the Indian sector of the Southern Ocean (ISSO). The latter hosts contrasting biogeochemical provinces, from low productive systems with High Nutrient Low Chlorophyll areas, to high productive regimes in the vicinity of the Subantarctic Islands as a consequence of natural iron fertilization.
This study aims to assess the links between the PP and phytoplankton community structure in the ISSO. We present results from the SOCARB (South Indian Ocean CARBon fluxes from the surface to the mesopelagic twilight zone) cruise conducted during the late austral summer of 2023. This includes: (i) PP from 13C method and on-deck incubations, (ii) relative phytoplankton chemotaxonomic groups from pigments data and total chlorophyll a (TChla) and (iii) phytoplankton size classes abundances from on board cytometry flow analyses. PP and pigments were size-fractioned (< 3 µm; 3-20 µm; > 20 µm) following the three considered phytoplankton classes (pico-; nano-; microphytoplankton) to quantify their impact on organic carbon production and to address the size structure of the phytoplankton community.
At the Antarctic zone (AZ) and the Polar Frontal zone (PFZ), integrated TChla – over the 0.01 % euphotic layer depth – was structured by TChlaNANO (46 ± 12 %) and TChlaMICRO (40 ± 14 %) and featured a community dominated by diatoms and haptophytes (68 ± 8 %). The Subantarctic zone (SAZ) differs from the rest of the Southern Ocean (i.e. south of the subtropical front) with a distinct community and a TChla structured in pico- (42 %) and nano- (36 %). In the South Indian Ocean, the Subtropical zone (STZ) exhibited a TChla structured by TChlaPICO (43 ± 8 %) and TChlaNANO (39 ± 9 %) with a diversified community. From linear correlations and relative contribution of phytoplankton groups to TChla, we show that PP in the AZ and PFZ is conditioned by diatoms and haptophytes algal biomass in both nano- and micro- size classes. In the STZ, PP is mainly conditioned by the algal biomass of cyanobacteria in the pico- and by haptophytes, chlorophytes and dinoflagellates in the nano- size classes. Our results also underline the intra-zonal variability of PP and TChla through bottom-up processes, such as cyclonic eddy in the STZ or water mass intrusion in the PFZ. This study paves the way for a better comprehension of phytoplankton productivity and community size structure, which could contribute to a more detailed knowledge on their role in the BCP.
How to cite: Deteix, V., Ridame, C., Thyssen, M., Dimier, C., Lo Monaco, C., Metzl, N., Tribollet, A., and Planchon, F.: New insights into the primary production and the structure of the phytoplankton community in the Southern Indian Ocean from the subtropical to the Antarctic zones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8253, https://doi.org/10.5194/egusphere-egu25-8253, 2025.
The Southern Ocean plays a major role in the global carbon cycle, with air-sea carbon dioxide (CO2) exchanges influenced by phytoplankton phenology. However, during summer, global ocean biogeochemistry models struggle to capture the interplay between biological and physical processes and their combined effects on air-sea CO2 fluxes. Improved constraints on phytoplankton seasonal dynamics and the mechanistic drivers of their spatial and temporal variability are crucial for refining these models. In this study, we used a 24-year (1998–2022) database of weekly vertical profiles of chlorophyll-a and particulate organic carbon concentrations. This observation-based 4D dataset was generated by matching up and merging satellite and hydrological data using a machine learning methodology trained on BioGeoChemical-Argo observations. A Functional Principal Component Analysis, coupled with a Gaussian Mixture Model, was applied to this dataset, enabling a revised regionalisation of the Southern Ocean based on the 3D seasonal variability of phytoplankton biomass and particulate organic carbon concentrations. Distinct latitudinal differences mostly align with physical oceanic fronts, while zonal differences emerge within the Antarctic Circumpolar Current region. These spatial patterns reflect regional disparities in key phytoplankton growth drivers, including light and nutrient availability, vertical mixing, and stratification. We further explore the interannual variability of this bioregionalisation, shedding light on the environmental drivers of phytoplankton seasonal dynamics and their potential biogeochemical impacts in the context of a changing climate.
How to cite: Mayot, N., Sauzède, R., Izard, L., Nerini, D., and Uitz, J.: Advancing understanding of Southern Ocean phytoplankton phenology with 4D data product, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-339, https://doi.org/10.5194/egusphere-egu25-339, 2025.
There is general agreement that iron is the main nutrient limiting primary productivity in the Southern Ocean. In contrast to the average low biological productivity elsewhere in the Southern Ocean, the Kerguelen region is home to massive blooms that extend hundreds of kilometers offshore, and serve as a backbone to rich ecosystems. The blooms have been shown to be sustained by continental iron inputs, in particular by the resuspension of iron-enriched sediments over the plateau, transported eastward by the Antarctic Circumpolar Current. However, iron inputs from glacial erosion and ice melt may be another iron source. In particular, two of the outlet glaciers of Kerguelen's Cook Ice Cap transport iron-enriched lithogenic material downstream to the coastal waters of the Golfe des Baleiniers. Whether the circulation is able to connect the glacier outlets to the pelagic area, and how much of the pelagic bloom can be influenced by glaciogenic iron, are two open questions that we address here. Using in situ and satellite data, we show the persistence, on an interannual basis, of a chlorophyll-enriched plume connected to the Golfe des Baleiniers and driven by a horizontal advection of iron. Using a Lagrangian methodology, we reconstruct the horizontal advection of iron and show that glaciogenic iron supply influences up to a third of the spatial extent of the open ocean bloom onset. We find that the new high resolution SWOT observations allow a significant reduction in altimetry biases attributable compared to previous products, allowing a better representation of fine scale biogeochemical structures. Our results are particularly relevant in the context of the negative mass balance of ice caps and glacial retreat evidenced both on Kerguelen and other Southern Ocean islands in the context of climate change.
How to cite: Nalivaev, A., D'Ovidio, F., Bopp, L., Berta, M., Azarian, C., Rousselet, L., and Blain, S.: Lagrangian reconstruction of a glaciogenic iron delivery to the Kerguelen blooms, Southern Ocean: comparison of SWOT-merged products with conventional altimetry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8611, https://doi.org/10.5194/egusphere-egu25-8611, 2025.
The Southern Ocean plays a crucial role in absorbing carbon dioxide from the atmosphere, accounting for 20% of the ocean's carbon sink despite covering only 10% of the global ocean area. Antarctic Krill (Euphausia superba) are central to this process, as their faeces help remove carbon from the upper ocean by sinking to deeper layers. However, microplastics are increasingly polluting the Southern Ocean, and have been found in zooplankton, especially krill. These buoyant microplastics may slow the sinking of krill faeces, potentially reducing the amount of carbon that is trapped in the deep ocean. Whether, and to what extent, microplastics impact faeces sinking is still an open question. To address this gap, we developed a theoretical model to study how microplastics affect the density and fragmentation of krill FP which in turn will impact their vertical sinking to the oceanic depths. Our findings suggest that in environmentally relevant concentrations, microplastics could slow down the sinking of these pellets. Larger microplastics have the most impact, causing greater fragmentation of the faeces as they settle in the water column. While the buoyancy effect of microplastics is currently marginal due to the density change, under a business-as-usual scenario. Our results highlight that future increases in microplastics will likely have a significant negative impact on the ability of krill to promote the storage of carbon in the deep ocean.
How to cite: Wu, N., Hunter, A., and Manno, C.: Impact of Microplastics on Antarctic Krill Faeces Carbon Sequestration in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8079, https://doi.org/10.5194/egusphere-egu25-8079, 2025.
Posters on site: Tue, 29 Apr, 08:30–10:15 | Hall X5
Wintering periods are vital for the survival and reproductive success of Antarctic animals, yet their winter behaviors remain poorly understood. This study investigates the winter movements and foraging behavior of Weddell seals (Leptonychotes weddellii), an indicator species for the Commission for the Conservation of Antarctic Marine Living Resources, at Terra Nova Bay in the Ross Sea. Using CTD-Satellite Relay Data Loggers, which record head movement and oceanographic data (temperature and salinity), we tracked 48 individuals for three consecutive years (2021–2023) over the migratory period. Of these, 23 seals migrated an average of 339 km (up to 911 km) northeast or southeast between March and early April, while 25 seals remained within 200 km from the summering site restricted at near Terra Nova Bay. Migratory seals exhibited higher prey capture attempts (4.88 ± 1.23 attempts per hour) compared to resident seals (3.86 ± 0.96 attempts per hour), suggesting that long-distance travel provides foraging benefits despite associated energetic costs and risks. Regions frequented by migrants, particularly near continental shelf edges, exhibited warmer water (-1.44 ± 0.37°C) intake at 200–350 m depth, indicative of nutrient-rich conditions. These findings reveal divergent wintering strategies in Weddell seals, highlighting a trade-off between migratory risks and feeding advantages. Long-term integrated monitoring of seal behavior and environmental changes is essential to advancing our understanding of their ecological adaptations and the Antarctic marine ecosystem.
How to cite: Lee, W. Y., Park, J., Park, M., Kim, Y., Chun, U., Chung, H., Choi, H. A., Yoon, S.-T., Na, J. S., Yoon, S., and Lee, W. S.: Trade-offs Between Migration and Foraging Success: Winter Behavior of Weddell Seals in the Ross Sea, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19210, https://doi.org/10.5194/egusphere-egu25-19210, 2025.
Monitoring changes in the marine environment is important for both oceanography and ecology study as it helps us understand the process by which oceanic conditions influence the entire ecosystem. In Antarctica, marine mammals encounter substantial changes due to seasonal variation of water mass composition and the complicated submarine topography. However, unbroken observations on ocean conditions are highly restricted to the summer season only when research vessels are available. In this study, we explored how seasonal variations in oceanic conditions affect the foraging behaviors of Weddell seals (Leptonychotes weddellii) in the Ross Sea, Antarctica using miniaturized CTD tags. Over the course of three consecutive years, from 2021 to 2023, 64 adult individuals were instrumented to collect data on water temperature and salinity as well as head acceleration. From the head movement, we found that seals foraged more often in modified shelf water and ice shelf water compared to Antarctic surface water. Additionally, as the lower boundary of Antarctic surface water descends from March to July, the seal dived to greater depths. Additionally, CTD profiles from CTD tags were classified using machine learning methods. Temperature and salinity from each profile were linearly interpolated across depths from 1 to 600 meters. Principal Component Analysis was then applied to extract three principal components for each profile. These components were subsequently used as input for a Gaussian Mixture Model, which classified the profiles into four distinct clusters. Each cluster had distinct temperature and salinity profiles and showed spatial and temporal separation (Warm surface Cluster: Distributed near Terra Nova Bay, predominant in summer (February); Intermediate surface Cluster: Distributed near Terra Nova Bay, predominant in fall (March); Cold Cluster: Distributed near Terra Nova Bay, predominant in winter (May, June, and July); Warm subsurface Cluster: Distributed near the shelf break, predominant in April and May). Prey capture attempts of dives were highest in Warm surface Cluster (3.16) and lowest in Intermediate surface Cluster (2.91). This study reveals the spatial and temporal shifts of foraging behavior with the surrounding oceanographic conditions, and it further emphasize that these oceanic factors should be considered for estimating their foraging activities.
How to cite: Chung, H., Park, J., Park, M., Kim, Y., Chun, U., Yun, S., Lee, W. S., Choi, H. A., Yoon, S.-T., Na, J. S., and Lee, W. Y.: Oceanographic and behavioral monitoring inferred from seal CTD tagging in the Ross Sea, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15611, https://doi.org/10.5194/egusphere-egu25-15611, 2025.
The Ross Sea is a critical region for understanding phytoplankton dynamics and their role in polar marine ecosystems in the west Antarctica, particularly in the context of rapid environmental change. Investigating the spatial variability of phytoplankton biomass and community structure is essential for assessing ecosystem productivity and biogeochemical cycling in this vulnerable area. From January to February 2023, a field survey of the Ross Sea was conducted onboard the icebreaker research vessel Araon, utilizing high-resolution, continuous observations of surface seawater with the Algae Online Analyzer and Imaging FlowCytoBot. These observations provided precise spatial distribution data on phytoplankton biomass and species abundance. The results revealed intricate spatial variability in phytoplankton community structure and biomass across the eastern and western Ross Sea, spanning coastal to offshore gradients, and in the dynamic waters near polynyas and ice shelves. The phytoplankton community was predominantly composed of diatoms, especially Fragilariopsis spp., and/or Phaeocystis spp., with their dominance varying across spatial gradients. The observed patterns suggest that multiple interacting environmental factors, including sea ice concentrations, water masses, and ocean currents, influence on phytoplankton distribution in the region. These findings highlight the critical role of physical and biological interactions in phytoplankton distributions and offer valuable insights into their potential responses to environmental changes in the Ross Sea ecosystem.
How to cite: Lee, Y., Jung, J., Park, J., and Moon, J. K.: Spatial Variability of Phytoplankton Communities in the Ross Sea: Insights from High-Resolution Observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1764, https://doi.org/10.5194/egusphere-egu25-1764, 2025.
The Amundsen Sea, located in West Antarctica, is undergoing rapid melting due to the intrusion of Circumpolar Deep Water. This intrusion results in ice sheet thinning and basal melting, which can have cascading effects on the biogeochemical cycle of dissolved organic matter (DOM) by introducing iron from sea ice and glaciers and by affecting ocean circulation. Therefore, it is crucial to understand the dynamics of the DOM in this region. Our study focused on assessing the optical properties of DOM in the oceanic areas adjacent to the West Getz Ice Shelf (WGIS) and the Dotson Ice Shelf (DIS). Notably, the WGIS regions exhibited relatively high dissolved organic carbon (DOC) and chromophoric DOM (CDOM) absorption coefficient at 350 nm (a350). Molecular weight indices, including spectral slope coefficient (S275-295) and specific UV absorbance at 254 nm (SUVA254), suggested that high molecular weight DOM with a substantial aromatic component predominated in the WGIS regions. Conversely, the DIS regions showed low CDOM values, low SUVA254 values, and elevated S275-295 values, indicative of low molecular weight CDOM with lower aromaticity. Furthermore, we observed significant negative correlations between the biomass of Phaeocystis antarctica (P. antarctica) and phosphate (PO4) in the WGIS regions. However, no such relationship was found in the DIS region. These findings imply that the high concentration and molecular weight of a350 in the WGIS regions, spanning from the surface layer to deeper depths, are predominantly driven by autochthonous sources, notably the colony-forming blooms of P. antarctica. The results of this study highlight the crucial role of bloom conditions in shaping both the quantity and quality of DOM in the Amundsen Sea.
How to cite: Jung, J., Son, J., Lee, Y., Kim, T.-W., Park, J., and Jeon, M. H.: Distinct optical properties of dissolved organic matter near Getz and Dotson ice shelves in the Amundsen Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1761, https://doi.org/10.5194/egusphere-egu25-1761, 2025.
Dissolved organic carbon and nitrogen (DOC and DON, respectively) are important components of the carbon and nutrient cycle in marine systems. However, there are still significant gaps in understanding the role of these compounds in the biogeochemical cycles of polar environments, due to the limitations of spatiotemporal sampling. Here, we present an overview of surface distributions of DOC and DON along the Northern Antarctic Peninsula (NAP) and Atlantic Southern Ocean (ASO), using the dataset available between 1992 and 2022, collected during different seasons. We used DOC data collected from RV Almirante Maximiano cruises by the Brazilian High Latitude Oceanography Group (GOAL), along the NAP, DOC and DON data collected from RV Polarstern cruises (Alfred Wegener Institute, Germany), along the ASO, as well as DOC and DON datasets from international repositories. DOC and total dissolved nitrogen were analyzed primarily with a Shimadzu TOC-L® Series. DON was calculated by subtracting dissolved inorganic nitrogen concentrations from total dissolved nitrogen. Excess DOC and DON were calculated by subtracting the respective deep concentrations. Surface DOC concentrations ranged from 42.0 to 127.0 μmol kg–1, while surface DON concentrations ranged from 1.0 μmol kg–1 to 11.3 μmol kg–1. At the surface, the highest concentrations of DOC and DON were observed mainly in the western sector of the Southern Atlantic (longitudes > 20° W), due to the proximity of coastal areas such as in the Gerlache Strait in the NAP and South Georgia and the Falklands Islands in the ASO. Increase in the surface concentrations of both DOC and DON were also associated to frontal systems. The accumulation of DOC and DON along the NAP and western sector of the southern Atlantic seem to confirm the link between the production of organic matter and the proximity of iron-supplying land-masses leading to enhanced primary production and plankton biomass (chlorophyll-a concentrations and particulate organic carbon). The production of nitrogen-rich organic matter by zooplankton seemed to be the main factor determining DON distributions. The wide spatial coverage of DOC and DON made it possible to identify significant differences in DOC and DON distributions between different regions, as well as interannual and seasonal differences from data collected in the same regions. DOC and DON can be considered important indicators for evaluating the coupling between physical, biogeochemical, and climate processes over time.
How to cite: Avelina, R., Klaas, C., Hamacher, C., O. Farias, C., Ludwichowski, K.-U., Burau, C., Kerr, R., P. Koch, B., M. Mata, M., and C. da Cunha, L.: Drivers of surface distribution of dissolved organic carbon and nitrogen along the northern Antarctic Peninsula and the Atlantic Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10829, https://doi.org/10.5194/egusphere-egu25-10829, 2025.
Ocean export production is the backbone of marine ecosystems, and it is crucial to carefully track their changes under the global warming. This study analyzes the irreversibility of ocean export production and the major factors affecting it by performing CO2 reduction experiments with the LOVECLIM, a medium complexity model. After a 1,500-year spin-up to the present-day CO2 level of 367 ppm, it was increased by 1% per year for 140 years and then decreased again for the same period. The present-day CO2 level was then held for 5,000 years. Ocean export production decreased at low latitudes and increased at mid- and high-latitudes, with the largest changes occurring in the equatorial and Antarctic Circumpolar Current regions. Nutrient concentrations in the euphotic zone decreased as the global ocean circulation weakened and ocean stratification intensified. Nevertheless, ocean export production has increased at high latitudes because of deepen mixed layer depth due to strong westerlies of both hemisphere and the creation of sea ice melting, which has led to the widen euphotic zones, high nutrient concentrations despite decreased nutrient concentrations, and increased ocean temperatures. The irreversibility of ocean export production with CO2 concentration is seen in 84.3% of the world's oceans. It will take more than 1,300 years for global ocean export production to return to its initial concentration after a CO2 reduction experiment, so it will need to be carefully monitored over a long period of time.
Acknowledgements: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Government of Korea (MSIT) (No. 2022R1A2C1008858) and Global - Learning & Academic research institution for Master’s·PhD students, and Postdocs (LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2024-00443714).
How to cite: Wie, J. and Moon, B.-K.: Analyzing Factors that Influence the Irreversibility of Ocean Export Production, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8145, https://doi.org/10.5194/egusphere-egu25-8145, 2025.
The dense waters formed on the broad continental shelves of the western Weddell Sea are the source of Weddell Sea Bottom Water (WSBW) found along the slope and at the bottom of the basin. WSBW is considered to be an effective conduit for carbon sequestration, eventually contributing to the redistribution of carbon around the global oceans. To quantify the efficiency of this carbon sequestration, it is necessary to have a good understanding of the processes that transform carbon within the ocean surface layer, as well as those occurring along the slope. Lack of biogeochemical data in the southern and western Weddell Sea is hindering progress. In situ observations are particularly necessary in the wintertime, when dense shelf waters and WSBW are formed and thus CO2 uptake is constrained. We present a method that reconstructs the wintertime fugacity (i.e., the adjusted partial pressure) of CO2 (fCO2) on the southwestern continental shelves from the dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements made in the WSBW, but collected in the summertime, when most expeditions take place. The method relies on relationships between the contributions of different water masses to WSBW, and potential temperatures, as found in previous work. Results for reconstructed surface wintertime fCO2 are comparable to the very few other wintertime observations elsewhere in the Weddell Sea. Without in situ wintertime observations, validation of the results is challenging. The results suggest a negligible role for biogeochemical processes transforming DIC between wintertime shelf water and WSBW. Assumptions in the methodology need to be tested against in situ biogeochemical measurements on the shelves and along the slope. We applied our method to recent data and relate findings to other biogeochemical variables to narrow down the uncertainty in our assumptions.
How to cite: Hoppema, M., Droste, E. S., Bakker, D. C. E., and Huhn, O.: Reconstructing wintertime surface layer fCO2 in the western Weddell Sea using summertime observations of Weddell Sea Bottom Water, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10954, https://doi.org/10.5194/egusphere-egu25-10954, 2025.
The Southern Ocean plays a crucial role in the global carbon cycle, uptaking about 43% of the ocean’s CO₂ uptake. However, significant uncertainties remain regarding the processes governing CO₂ fluxes in this region. Mesoscale eddies have been identified as a potential source of these uncertainties. In this study, we analyze 30 years of daily, high-resolution (10 km) global simulations using the ICON-O ocean model coupled with the HAMOCC biogeochemistry model. The Okubo-Weiss parameter and vorticity is used to classify four flow regimes—anticyclonic eddy cores, cyclonic eddy cores, eddy core peripheries, and quiescent background—allowing us to generate composites for each. Our results show that CO₂ flux is directed into the ocean across all four regimes, with the magnitude of CO₂ uptake varying by regime. Anticyclones have a greater capacity for CO₂ uptake compared to cyclones. In certain regions (i.e. Agulhas retroflection and the Brazil-Malvinas confluence) anticyclones exhibit the highest CO₂ uptake capacity among the four regimes, a pattern linked to the greater eddy intensity of these areas. An analysis of the CO₂ flux terms shows that wind speed and ∆pCO₂ are the primary contributors to flux magnitud and variability. As atmospheric pCO₂ is prescribed, the main changes in ∆pCO₂ are related to changes in oceanic pCO₂. These variations are primarily driven by dissolved inorganic carbon (DIC) and sea surface temperature, which tend to compensate for each other, with DIC having a stronger influence. To explore DIC changes, we analyzed DIC budgets in the first 300 m. In the surface layer, the total DIC tendency is predominantly driven by vertical diffusion, which causes a net loss of DIC to deeper layers and induces atmospheric CO₂ flux into the ocean. Vertical diffusion is particularly stronger in anticyclonic eddies, explaining their enhanced ability to absorb CO₂. In deeper layers, the total DIC tendency is primarily controlled by the divergence of advective fluxes, while changes in DIC from sources and sinks (i.e. biogeochemical processes) are almost entirely balanced by vertical diffusion. These findings highlight the dominant role of mesoscale eddies in oceanic carbon uptake and underscore the need for more refined models to accurately represent their impact on the global carbon cycle.
How to cite: Salinas Matus, M., Serra, N., Chegini, F., and Ilyina, T.: Impact of Mesoscale Eddies on CO₂ Fluxes in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5183, https://doi.org/10.5194/egusphere-egu25-5183, 2025.
The Southern Ocean (SO) plays a crucial role in regulating the Earth’s climate by absorbing atmospheric carbon and storing it in the deep ocean. (Brovkin et al., 2012). Past studies reported a reduction in this uptake capacity over the last few decades. However, the temporal and spatial variability of the Southern Ocean’s carbon pump appears to be more complex (Landschützer et al., 2015), with the Indian sector arguably being the least explored and understood region. A key objective of this PhD is to use radiocarbon signatures of dissolved inorganic carbon (DI14C), combined with stable carbon isotopes to quantify exchange processes between the ocean, atmosphere and sediments along the East Antarctic margin. A comprehensive sample set of sea water, sediment pore water and sediment surface samples was collected during three expeditions (2022 - 2024) on R/V Polarstern from more than 50 stations. Several sites from key locations will be selected to study ocean ventilation, water mass alterations and exchanges, and bottom water formation in the region. In addition, we will perform analyses in high spatial resolution to target more specific research questions, in particular ocean – ice shelf interactions. The East Antarctic coast from Prydz Bay to Vincennes Bay lends itself as case study area for tracing the inflow of Circumpolar Deep Water onto the continental shelf and its interaction with the marine-based portions of the Amery and Shackleton Ice Shelves, as well as Totten Glacier. Sections sampled along meridional transects further offshore and to the west allow to differentiate between regional varieties of Antarctic Bottom Water formation. Comparing water column data with 14C signals recorded in surface sediment benthic foraminifera will contribute to an improved understanding of the use of radiocarbon as a ventilation age proxy. Initial analyses are being conducted at the radiocarbon laboratory at Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, equipped with an accelerator mass spectrometer (AMS) Mini Carbon Dating System (MICADAS). Next steps involve method development, focusing on optimizing the graphitization process of DIC samples prior to radiocarbon analysis.
Brovkin, V., Ganopolski, A., Archer, D., & Munhoven, G. (2012). Glacial CO 2 cycle as a succession of key physical and biogeochemical processes. Climate of the Past, 8(1), 251-264.
Landschützer, P., Gruber, N., Haumann, F. A., Rödenbeck, C., Bakker, D. C., Van Heuven, S., ... & Wanninkhof, R. (2015). The reinvigoration of the Southern Ocean carbon sink. Science, 349(6253), 1221-1224.
How to cite: Rosmann, M. L., Grotheer, H., Lembke-Jene, L., Haumann, A., and Mollenhauer, G.: Carbon Exchange and Ocean Ventilation along the East Antarctic Margin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8065, https://doi.org/10.5194/egusphere-egu25-8065, 2025.
Under a warming climate ice sheets are releasing freshwater to the ocean and affecting the global ocean circulation. One important region where freshwater is released and impacts the global ocean is the Antarctic continental shelf, where Antarctic Bottom Water (AABW) is formed. The response of AABW formation and circulation to increased glacial melt is uncertain, because global ocean models struggle to capture AABW formation, sinking and spreading across the abyss. Here, we present an equilibrated 1º degree global ocean-ice configuration with AABW formation on the shelves and realistic abyssal ventilation. This configuration also includes dye and age tracers that track ventilation pathways and timescales. Using this model, we perform an idealized ‘antwater’ hosing experiment, releasing exactly 0.1 Sv of freshwater uniformly around the Antarctic coast for a century (without offset from surface salinity restoring). The response of Southern Ocean circulation and ventilation is analysed and discussed in the context of previous model studies.
How to cite: Gülk, B., de Lavergne, C., Sallée, J.-B., Madec, G., and Rousset, C.: The response of the Southern Ocean to freshwater hosing in an equilibrated 1º NEMO configuration with realistic ventilation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11251, https://doi.org/10.5194/egusphere-egu25-11251, 2025.
The Southern Ocean is known as the most prominent oceanic sink of anthropogenic carbon and heat (Khatiwala et al. 2009, Frölicher et al. 2015). Among the processes involved in the region's carbon and heat cycle, ventilation–defined as the transfer of water and tracers from the mixed layer to the stratified pycnocline–is recognised as being of prime significance (Morrison et al. 2022). Recent and future trends in Southern Ocean surface conditions (wind stress and sea ice cover) are likely to impact ventilation patterns and timescales, with concurrent effects on the ocean’s role as a climate mitigator.
Sea ice has been shown to be an important conveyor of heat and salinity throughout the Southern Ocean (Abernathey et al. 2016, Haumann et al. 2016). Recent modelling (Waugh 2014, Waugh et al. 2019) and data analysis (Cerovečki et al. 2019) studies have revealed the impact of changing surface conditions on ventilation characteristics; however, the contribution of sea ice dynamics and thermodynamics, and their coupled interaction with wind stress changes, have not been specifically addressed.
In this study, we focus on this mechanism by making use of a coupled ocean/sea ice (MOM6/SIS2, Adcroft et al. 2019) reentrant channel model, with generic continental shelf and meridional ridge topography. Quasi-realistic seasonally varying, zonally invariant surface forcing fields of wind stress, heat flux and fresh water are imposed. A meridional overturning circulation is sustained by a sponge region at the domain’s northern boundary. Several diagnostics, including the ideal age, are used to quantify changes in ventilation intensity.
To investigate the impact on ocean ventilation of changing surface conditions and their interactions with sea ice dynamics, we run two configurations of the model: with and without an interactive sea ice component. In both configurations, the model is brought to statistical equilibrium and then perturbed by mimicking a Southern Annular Mode anomaly. By comparing the ventilation characteristics in the two configurations, we isolate and quantify the contribution of the freezing/melting dynamics and their interaction with changes in wind stress forcing.
How to cite: Poumaëre, N., MacGilchrist, G., and Khatri, H.: Changes in ideal age distribution in response to coupled wind/sea ice perturbations in a Southern Ocean channel model , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13255, https://doi.org/10.5194/egusphere-egu25-13255, 2025.
The Southern Ocean plays a critical role in global climate regulation, acting as a key pathway for heat, momentum, and biochemical transport. Using expanded hydrographic observations from the WOCE/CLIVAR SR03 section (1991–2018), a choke point between Tasmania and Antarctica, along with high-resolution GLORYS12 reanalysis data (1993–2018), we investigate long-term changes in the Antarctic Circumpolar Current (ACC) and Tasman Outflow (TO) and their relationship with the southward migration of the Subtropical Front (STF). Our results reveal a significant southward migration of the STF, while the Subantarctic Front remains meridionally stable. This STF migration is associated with an intensified TO and strengthened Ekman convergence, leading to enhanced subduction of cold, low-salinity intermediate water in the Subantarctic Zone and increased subsurface warming (0.4–0.8°C per decade) north of the ACC. The TO has strengthened significantly (+3.1 Sv per decade), while the ACC’s geostrophic transport shows an increasing trend (+3.2 Sv per decade), indicating the poleward expansion of subtropical circulation towards Antarctica. Additionally, the correlation between TO transport and the Southern Annular Mode (SAM) index (r = 0.48, p < 0.05) suggests that westerly wind variability plays a key role in modulating ocean transport in this region. Our findings emphasize the importance of STF migration and TO expansion in shaping Southern Ocean circulation under climate change. The poleward shift of westerlies and subtropical systems underscores the ongoing sensitivity of transport processes to global warming. Further research with extended observations and high-resolution climate models is needed to refine projections of circulation changes and their impact on oceanic heat transport in the Southern Ocean.
How to cite: Park, T., Kim, Y. S., and Park, J.: Poleward Migration of the Subtropical Front and Its Implications for the Tasman Outflow and Antarctic Circumpolar Current in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7897, https://doi.org/10.5194/egusphere-egu25-7897, 2025.
The Southern Ocean (SO), dominated by the Antarctic Circumpolar Current (ACC), plays a key role in the global uptake and transport of heat and carbon. Interactions between the ACC and topographic features form standing meanders, hotspots of small-scale motions that enhance cross-frontal exchanges. In-situ observations are challenging and remain sparse in the SO, and hence progress in understanding the dynamics of eddies and their role in tracer exchanges readily relies on satellite altimetry data. An accurate representation of velocity and kinetic energy toward the smaller scales is thus needed to better understand the Southern Ocean's role in the global climate system.
Launched in December 2022, the SWOT (Surface Water and Ocean Topography) satellite mission offers an unprecedented view of ocean dynamics at scales down to 15 km. Ocean currents and kinetic energy budget are typically inferred by applying geostrophic balance to sea surface height (SSH) observations. However, at the small spatial scales resolved by SWOT, this balance may not hold anymore or shift to higher-order equilibrium, and validation steps are crucial before exploiting these observations in climatic studies. Using surface drifters deployed during the SWOT validation campaign FOCUS, we present the first analysis of velocities and dispersion derived from SWOT SSH, in an energetic meander of the ACC.
Introducing a fitting kernel method tailored to derive velocities from SWOT observations, we show that SWOT SSH remain primarily balanced and valid for inferring surface velocities at scales as small as 10 km in this region. At these scales, geostrophic balance alone becomes insufficient and leads to a 10-20% bias compared to drifter velocities in cyclonic eddies, which is effectively corrected by applying cyclogeostrophy to SWOT SSH. Then, we compute distance-averaged pair statistics from real drifter pairs and virtual particles and show that SWOT accurately captures dispersion properties over the 5-200 km range, unveiling distinct dispersion patterns between large and small separation scales. This suggests that balanced dynamics resolved by SWOT are still the main driver for ocean dispersion in this range. By capturing balanced dynamics with unprecedented accuracy, SWOT offers new opportunities to understand the impact of small scales on tracer exchange and better quantify the transport of heat and carbon in the Southern Ocean.
How to cite: Tranchant, Y.-T., Legresy, B., Foppert, A., Pena-Molino, B., and Phillips, H.: SWOT reveals fine-scale balanced motions and dispersion properties in an energetic meander of the Antarctic Circumpolar Current, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15046, https://doi.org/10.5194/egusphere-egu25-15046, 2025.
Antarctic Intermediate Water (AAIW), occupying a vast area in the intermediate depths of the southern hemisphere oceans, is important for heat and freshwater redistribution in the global oceans. However, questions remain regarding its sources at the surface and pathways to the intermediate depths. To answer these questions, a recently-defined distance metric, which can distinguish similar water parcels, is applied to Argo data. Results show that the Pacific and Atlantic AAIW originates from only limited regions at the surface, specifically near the Subantarctic Front in the southeast Pacific and the Falkland Plateau in the southwest Atlantic. Further investigation indicates that through subduction in the deep mixed layer in these regions, low-salinity water penetrates into the intermediate depths of the Pacific and Atlantic Ocean. In the Indian Ocean, such a fast pathway for low-salinity to reach the intermediate depths is absent. Instead, the AAIW here, characterized by lower oxygen concentrations, consists of a mixture of locally slow downward spreading low-salinity water and AAIW inflow carried by the Antarctic Circumpolar Current from the Atlantic Ocean. The AAIW in the three southern hemisphere oceans exhibits slightly different physical properties, reflecting their different origins. Additionally, results indicate that mixing with surrounding saline waters is crucial in the transformation of surface waters into the AAIW cores. Our findings confirm the reliability of the distance metric and emphasize the importance of localized physical processes in the penetration of AAIW into the ocean interior.
How to cite: Hong, Y.: Origin of the Antarctic Intermediate Water in the Southern Ocean identified by a distance metric, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15079, https://doi.org/10.5194/egusphere-egu25-15079, 2025.
Antarctic Bottom Water (AABW) plays a crucial role in the intensity and variability of the Global Overturning Circulation (GOC). AABW formation sustains the lower cell of the GOC, fundamentally regulating the storage and transport of heat and carbon, key properties influencing Earth’s climate. Changes in AABW have far-reaching implications for the stability of the GOC. However, the mechanisms governing the variability and potential changes in the strengthening of the AABW cell remains uncertain. Understanding the variability of AABW is crucial for projecting changes in ocean circulation and assessing associated climate impacts. This study aims to use the outputs of the high-resolution Community Earth System Model (CESM-HR) pre-industrial run to explore AABW natural variability and water mass transformation processes governing its formation. By analyzing the pre-industrial run, we aim to characterize baseline dynamics in the absence of anthropogenic forcing, focusing on how atmospheric variability drives changes in AABW properties and transport. Additionally, we investigate the role of surface cooling and salinization in shaping AABW characteristics through a water mass transformation framework. While small-scale processes are not fully resolved, CESM-HR offers an improved perspective on large-scale patterns and variability compared to coarser-resolution models. This study aims to provide insights into the baseline state of AABW dynamics under pre-industrial conditions. The results are expected to provide an enhanced understanding of AABW variability, offering insights into anthropogenic impacts and underscoring the need for complementary observational and modeling efforts to refine our understanding of AABW formation.
How to cite: Noro, M., Wainer, I., Dotto, T., and Marcello, F.: Natural Variability of Antarctic Bottom Water in the Pre-Industrial CESM-HR simulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20050, https://doi.org/10.5194/egusphere-egu25-20050, 2025.
Temperature reconstructions from Greenland and Antarctica during glacial times show anti-phased oscillations which are assumed to be connected to changes in ocean circulation patterns. However, the research focus of most studies is set to the Northern Hemisphere, connecting Dansgaard-Oeschger (DO) events to changes of the Atlantic Meridional Overturning Circulation (AMOC). Meanwhile, in the Southern Ocean (SO), millennial-scale oscillations, driven by changes in the formation of deep water, have been found in different climate model simulations, yet the exact mechanism leading to these changes is still not fully understood. These oscillations, diagnosed by the strength of Antarctic Bottom Water (AABW) formation, have been simulated under warmer climate conditions and also in experiments with additional freshwater input to the North Atlantic. Here we present results of multi-millennial experiments with the fast Earth system model CLIMBER-X and the coarse resolution General Circulation Model (GCM) CM2Mc in which the AMOC is collapsed by freshwater forcing in the north Atlantic and convection is eventually triggered in the SO. We aim to find the drivers of convection onset in the SO and the subsequent strengthening of AABW by analysing the changes of temperature and salinities in both models. Between the two models, we compare how the dynamics of features such as sea-ice, wind stress and thermodynamic ocean variables contribute to changes of SO convection and AABW formation. We also analyze additional sensitivity experiments with CLIMBER-X to explore which conditions lead to oscillations.
How to cite: Höse, A., Willeit, M., Feulner, G., and Robinson, A.: Understanding multi-millennial variability in the Southern Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11096, https://doi.org/10.5194/egusphere-egu25-11096, 2025.
Dense waters formed in the North Atlantic (NADW) propagate southward into the Southern Ocean. There, a portion upwells to the surface in a south-eastward spiral (Tamsitt et al. 2017), a portion is entrained into the northward flowing Antarctic Bottom water (AABW), and a portion propagates northward at mid-depths of the Indian and Pacific Oceans. Combined, these processes result in a gradual eastward dilution of NADW at its core density around the Southern Ocean. However, no consensus has been reached on the dilution rate of these waters and how it impacts global ocean ventilation. Here, we use historical observations to track the dilution of NADW around the Southern Ocean. We find a persistent maximum at mid-densities, corresponding to the NADW core, which erodes eastward from the Atlantic sector. Furthermore, we evaluate the dilution of an artificial North Atlantic dye 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). The erosion of the North Atlantic dye maximum around the Southern Ocean varies markedly across models, and is largest in TMI. The available data point to an overly rapid erosion in TMI, but remain too scarce in the Pacific sector to place strong constraints on the dilution rate of the NADW core. A zonal circumpolar transect of salinity and measurements, together with characterisation of inter-laboratory offsets, would greatly help to constrain the fate of NADW.
How to cite: Millet, B., Gray, W., de Lavergne, C., Waelbroeck, C., Reverdin, G., Pöppelmeier, F., and Roche, D.: On the fate of North Atlantic deep waters in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9664, https://doi.org/10.5194/egusphere-egu25-9664, 2025.
Walter Munk’s seminal work, Abyssal Recipes, has established a foundational framework for comprehending abyssal water upwelling for almost 60 years. While it has profoundly influenced theoretical, laboratory, and observational studies of deep-ocean processes, discrepancies arise when compared with long-term observational data. One prominent paradox is known as the interior downwelling conundrum: When Munk's framework is applied to conditions of bottom-intensified mixing, it predicts downwelling rather than upwelling, which contradicts the mass balance in the abyssal ocean. This study revisits this challenge by investigating the unsteady dynamics of abyssal isopycnals. We demonstrate that under a cooling regime in the abyssal ocean, which is linked with the formation of the Antarctic Bottom Water (AABW) in the last little Ice Age more than 1000 years ago, rising water parcels can co-exist with downward diapycnal velocities. This reconciliation aligns Munk’s theory with the observed mass balance and resolves the longstanding paradox. These findings provide fresh insights into the conundrum and contribute to advancing our understanding of the closure of the global thermohaline and overturning circulations.
How to cite: Han, L.: Revisiting the Conundrum of Interior Downwelling in Munk’s Abyssal Recipes: An Unsteady Perspective Linked to a Cooling AABW, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2364, https://doi.org/10.5194/egusphere-egu25-2364, 2025.
