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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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