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 (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 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 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 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.

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

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

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

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 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 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 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.

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.

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 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.

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

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

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.