OS1.11

Ocean warming around Antarctica has the potential to trigger marine ice-sheet instabilities. It has been suggested that abrupt and irreversible cold-to-warm ocean tipping points may exist, with possible domino effect from ocean to ice-sheet tipping points (Hellmer et al. 2017). Here we investigate the existence of drivers of ocean tipping points in the Amundsen Sea. This sector is currently relatively warm, but a cold-to-warm tipping point may have occurred in the past. The conditions for an hypothetic abrupt return to a cold state are also investigated. A 1/4° ocean model configuration of the Amundsen Sea, representing interactions with sea-ice and ice-shelves, is used to characterize warm-to-cold and cold-to-warm oceanic transitions induced by perturbations of the atmospheric forcing and their influence on ice-shelf basal melt. We apply idealized perturbations of heat, momentum and freshwater fluxes to identify the key physical processes at play. We find that the Amundsen Sea switches permanently to a cold state for an air cooling of 2.5°C and intermittently for either an air cooling of 0.5°C, precipitations decreased by 30% or a 2° northward shift of the winds. All simulated transitions are reversible, i.e. restoring the forcing to its state before the tipping point is sufficient to restore the ocean to its original state although the recovery time is correlated to the amplitude of the perturbations. Perturbations of the heat and freshwater fluxes modify the properties of the ocean by impacting the buoyancy flux, either through their impact on the sea-ice or, directly, to a lesser extent. Perturbations of the momentum flux involve more complex mechanisms as it combines both an Ekman effect and an indirect effect on the buoyancy flux related to changes in sea-ice advection.

How to cite: Caillet, J., Jourdain, N., and Mathiot, P.: Drivers and reversibility of abrupt ocean cold-to-warm and warm-to-cold transitions in the Amundsen Sea, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3444, https://doi.org/10.5194/egusphere-egu22-3444, 2022.

The marine-terminating glaciers of the Amundsen Sea are experiencing increased basal melting associated with an inflow of warm and salty water from the deep ocean onto the shelf via submarine glacial troughs. Modelling work suggests that variability in the transport of this source of heat across the shelf-break and onto the Dotson Trough in the western Amundsen Sea is regulated by wind-driven changes in an eastward undercurrent that flows along the continental slope.

What controls the strength and variability of the undercurrent, however, is not well documented due to a lack of observations in the region. Here, we use a 5-year mooring record of undercurrent velocity in the Dotson Trough in conjunction with a novel 16-year altimetric sea level product that includes measurements in regions of near-perennial ice cover to describe the connection between undercurrent variability and climate modes on seasonal to interannual time scales.

We find a robust signature of the undercurrent variability that is linked to both a circumpolar coastal sea level signal as well as to the sea level in an offshore region in the Amundsen Sea. We discuss the implications of this undercurrent-sea level covariability in the context of atmospheric climate modes and we further explore what this link conveys about the undercurrent variability on interannual timescales by using of our full altimetry record.

How to cite: Dragomir, O., Silvano, A., Hogg, A., Meredith, M., Nurser, G., and Naveira Garabato, A.: Coastal and offshore controls on the variability of the Undercurrent in the Amundsen Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10635, https://doi.org/10.5194/egusphere-egu22-10635, 2022.

Basal melting of the Amundsen Sea ice shelves is caused by relatively warm waters accessing the ice base through turbulent processes at the ice-ocean boundary layer. Here we report hydrographic variability in Thwaites Eastern Ice Shelf (TEIS) from January 2020 to March 2021 using novel subglacial mooring measurements and ocean modelling. The layers ~100 m beneath the ice base warmed considerably (~1˚C) in this period. The meltwater fraction doubled associated with basal melting due to the higher heat, leading to a freshening in the upper layers. The lighter layer contributed to the acceleration of the under-ice circulation, which led to higher basal melting through intensified temperature flux, creating positive feedback beneath the ice. The interannual variability of the water masses in the TEIS cavity is linked to the seasonal strengthening and weakening of the Pine Island Bay gyre. During periods that the sea-ice covers the bay, such as winter 2020 and the 2020-2021 summer season, the momentum transfer from the wind to the ocean surface is less effective and the gyre weakens. The deceleration of the gyre leads to relaxation and shoaling of the isopycnals beneath the TEIS, which brings warmer water upwards closer to the ice base. The results discussed in this work shows that the fate of the Amundsen Sea ice sheet is tightly controlled by adjacent small-scale gyres, which could prolongate warming periods beneath ice shelf cavities and lead to high basal melting rates.

How to cite: Dotto, T., Heywood, K., Hall, R., Scambos, T., Zheng, Y., Nakayama, Y., Snow, T., Wåhlin, A., Wild, C., Truffer, M., Muto, A., and Pettit, E.: Interannual hydrographic variability beneath Thwaites Eastern Ice Shelf, West Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-817, https://doi.org/10.5194/egusphere-egu22-817, 2022.

The upper-ocean processes near ice shelves play crucial roles in the local freshwater budget, carbon take-up, surface albedo, and ice-shelf melting via controlling the air-sea heat exchange and thermocline depth. The upper-ocean processes are particularly complex during the austral autumn when both the air temperature and solar radiation flux drop dramatically, which result in an intense sea-ice formation and further influence the air-sea-ice interactions. However, in regions near the ice shelves like the Dotson Ice Shelf, where sea ice covers the ocean ten months a year, the lack of high-resolution and long-period observations limit our understanding of the upper-ocean processes in this sea-ice formation season. Here we present a dataset of high-frequency (1 Hz) temperature and salinity measurements collected by a recovered seal’s tag. This tag recorded the ocean properties during late summer to autumn (mid-February to mid-April 2014) in a small region (within a 15-km radius circle) in front of the Dotson Ice Shelf, when sea ice formed and mixed-layer depth deepened. During those two months, mixed-layer depth increased from about 25 m to 125 m. The mixed-layer water temperature was always near the freezing point, while the salinity increased from 33.35 to 34.25 g per kg, equivalent to a sea ice formation of about 3.26 cm per day. We compare the changes of the upper-ocean properties with ERA-5 reanalysis atmospheric data and find that the upper-ocean heat content can be largely explained by the air-temperature changes. We run a 1-D upper-ocean model with and without sea-ice formation to explore the effect of sea-ice formation on the processes on the salinification and deepening of the mixed layer during autumn.

How to cite: Zheng, Y., Webber, B., Heywood, K., and Stevens, D.: Upper-ocean processes in sea-ice formation season in front of Dotson Ice Shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-342, https://doi.org/10.5194/egusphere-egu22-342, 2022.

The Ross Ice Shelf (RIS) is one of the biggest Antarctic ice shelves and buttresses ice streams draining both the West and East Antarctic ice sheets. Recent  observations indicate that the melting of Antarctic ice-shelves is accelerating with great spatial heterogeneity. However, estimates of basal melting, which rely on indirect methods, are affected by large uncertainties: as for the RIS, the literature includes basal melt rates from 48 to 123 Gt/yr. To improve basal melting predictions we must understand what causes its spatio-temporal variability. Here, we use a regional configuration of the MIT general circulation model (MITgcm) to analyze the interactions between various water masses and the ice shelf, and their connection to local and global climate. The model simulates the ocean circulation in the Ross Sea and inside the RIS cavity from 1993 to 2018. In the actual configuration it does not account for tidal forcing. Basal melting of the RIS is parameterized by the three-equation formulation. The simulated RIS basal averaged melt rate is 78.6 ± 13.3 Gt/yr averaged over 1993-2018.

To better understand which local water mass causes basal melting, we developed a new methodology based on mixing ratios of endpoint-water masses. The endpoints are defined by: the High and Low Salinity Shelf Water (HSSW/LSSW), characterized by high and low salinity respectively and a near-freezing temperature; warm and salty modified Shelf Waters (mSW); warm and fresh Antarctic Surface Water (AASW); and cold and fresh Ice Shelf Water (ISW).

Our analyses show that in the long-term, HSSW causes ~45% of the total basal melting and is found mostly in the Western half of the RIS cavity. It shows a long-term trend due to the increase in the volume of cavity occupied by HSSW at the expense of LSSW. LSSW yields ~20% of the total basal melting and is mostly found in the Eastern half of the RIS cavity. As expected, melting due to mSW (~15% of the basal melting) and AASW (~7% of the basal melting) shows a strong seasonal cycle. Simulated mSW mostly reaches the Central-Eastern RIS during summer. Similarly, AASW intrudes below the RIS near Ross Island exclusively in summer. Melting attributed to ISW is only ~2%. About 11% of the simulated basal melting cannot be clearly attributed to any of the main water masses due to local mixing.

Finally, RIS basal melting and Ross Sea water masses variability inside the cavity are likely driven by a combination of local forcing (katabatic wind), large-scale wind/pressure systems (Amundsen Sea Low, Southern Annular Mode) and teleconnections (El-Niño Southern Oscillation, Pacific Decadal Oscillation), mediated by ocean-sea ice interactions, in particular by sea ice production in Western Ross Sea polynyas, and sea ice import in the Eastern Ross Sea. Identifying such climatic connections can inform which melting mode will be more important in the future climate and which region of the RIS will be more affected.

How to cite: Pochini, E., Colleoni, F., Bergamasco, A., Bensi, M., Budillon, G., Castagno, P., Dinniman, M., Falco, P., Farneti, R., Forte, E., Kovačević, V., and Mack, S.: Characterizing the Basal Melting Spatio-Temporal Variability of the Ross Ice Shelf using a Regional Ocean Model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4275, https://doi.org/10.5194/egusphere-egu22-4275, 2022.

The complete length of Parker Ice Tongue (18 km or 41 km2) calved in March 2020. This event occurred at the same time as repeated full summer break-outs of surrounding land-fast sea ice. Our results showed that periods of continuous ice tongue growth coincided with extended periods of land-fast sea ice coverage for at least the past 60 years. We also found that seasonal variations in the ice tongue dynamics were linked to variations in the local land-fast sea ice extent. A complete Antarctic ice tongue calving right at the grounding line has not been reported before.

Based on the analysis of satellite images and aerial photographs we determined Parker Ice Tongue length variations for the last 65 years. We found that the average growth of Parker Ice Tongue has been ~193 m/y-1. If we assume a constant growth rate, a break-off event of the magnitude observed has not occurred in the last 169 years.

We used a Sentinel-1 SAR image sequence to create a 2017-2020 time series of surface ice velocities. We found a significant inverse correlation between fast ice extent and ice tongue velocities (R= -0.62; R2=0.39). The short summer period, characterized by decreased land-fast sea ice extent, showed around 11% higher velocities compared to winter. This supports the idea that fast-ice extent can influence ice tongue dynamics seasonally.

Here we showcase the vulnerability of Parker Ice Tongue once left exposed to oceanic processes, which poses questions about the fate of other ice tongues if land-fast sea ice decreases more broadly in the future.

How to cite: Gomez Fell, R., Rack, W., Purdie, H., and Marsh, O.: Antarctic ice tongue collapse triggered by loss of stabilizing land-fast sea ice, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8960, https://doi.org/10.5194/egusphere-egu22-8960, 2022.

Coastal polynyas are key regions of Dense Shelf Water (DSW) formation that ultimately contributes to the ventilation of the ocean abyss. However, not all polynyas form DSW. In this study, we analyse the main drivers of DSW formation in four East Antarctic polynyas: Mackenzie, Barrier, Shackelton and Vincennes Bay from west to east. Mackenzie and Barrier (in lesser extent) were the only two polynyas where DSW formation was observed while it is absent in Shackelton and Vincennes Bay in the particular years when they were best sampled. We analysed the role of Bathymetry, water-mass distribution and transformation, stratification of the water column, sea-ice production rate and associated salt advection. We found that sea ice production was highest in Mackenzie, particularly in early winter, which likely contributed to reach higher salinity than the other polynyas at the beginning of the sea ice formation season. From April to September, the total salinity change in Mackenzie polynya was lower than in the other polynyas, and the strong contribution of the brine rejection was partly offset by freshwater advection. Overall, the preconditioning in early winter in Mackenzie polynya, likely due to strong SIP in February and March was the main driver determining DSW formation in MAckenzie in contrast with the other East Antarctic polynyas.

How to cite: Portela Rodriguez, E., Rintoul, S. R., Herraiz-Borreguero, L., Roquet, F., Tamura, T., van Wijk, E., Bestley, S., McMahon, C., and Hindell, M.: Drivers of Dense Shelf water formation in East Antarctic polynyas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1558, https://doi.org/10.5194/egusphere-egu22-1558, 2022.

     Extensive studies have addressed the characteristics and mechanisms of open-ocean polynyas in the Weddell and Cosmonaut Seas. Here, we show that more persistent open-ocean polynyas occur in the Cooperation Sea (CS) (60°E-90°E),  a sector of the Southern Ocean off the Prydz Bay continental shelf,  between 2002 and 2019. Polynyas are formed annually mainly within the 62°S-65°S band, as identified by sea ice concentrations less than 0.7. The polynyas usually began to emerge in April and expanded to large sizes during July-October, with sizes often larger than those of the Maud Rise polynya in 2017. The annual maximum size of polynyas ranged from 115.3 × 10³ km² in 2013 to 312.4 × 10³ km² in 2010, with an average value of 188.9 × 10³ km². The Antarctic Circumpolar Current (ACC) travels closer to the continental shelf and brings the upper circumpolar deep water to much higher latitudes in the CS than in most other sectors; cyclonic ocean circulations often develop between the ACC and the Antarctic Slope Current, with many of them being associated with local topographic features and dense water cascading. These oceanic preconditions, along with cyclonic wind forcing in the Antarctic Divergence zone, generated polynyas in the CS. These findings offer a more complete circumpolar view of open-ocean polynyas in the Southern Ocean and have implications for physical, biological, and biogeochemical studies of the Southern Ocean. Future efforts should be particularly devoted to more extensively observing the ocean circulation to understand the variability of open-ocean polynyas in the CS.

How to cite: Qin, Q., Wang, Z., Liu, C., and Chen, C.: Open-Ocean Polynyas in the Cooperation Sea, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1259, https://doi.org/10.5194/egusphere-egu22-1259, 2022.

Dense water is formed when sea ice around Antarctica drifts apart leaving open-water areas called polynyas. Both the processes of cooling sea water in contact with the atmosphere and salt accumulation in sea water during sea ice formation, lead to the sea water getting denser. The dense water formation in the oceans surrounding the Antarctic continent contributes to meridional overturning circulation, making it crucial to understand the changes in the Antarctic sea ice and oceans to improve model predictions. Using NEMO output from both a regional configuration and a coupled global configuration we ask how well are polynyas and deep water formation represented in the models? How do regional trends in sea ice affect the polynyas and deep water formation? In the model we find several types of polynya; including the open-water Great Weddell Sea Polynya and coastal polynyas. We have developed and applied an algorithm for classifying coastal polynyas based on sea ice concentration to identify and separate these from the open water polynya areas, in addition, we include sea ice thickness in the classification of coastal polynyas to select areas where the mixed-layer is deep, and surface salt flux is present. In the coastal polynyas the mixed-layer is deep and densification of the upper ocean is strong due to the surface salt flux. The Great Weddell Sea Polynya is also found to deepen the mixed-layer but the strong salt flux, found along the coast, is not present in the open-water polynya suggesting an alternative mechanism is taking place. The favourable ice divergence in the Weddell Sea builds over several years in both models but the Great Polynya itself does not reoccur after the 1980s. Coastal polynyas make up the largest area of the polynyas but show a negative trend in total area, possibly suggesting a diminishing role of these polynyas in future dense water formation. The study asserts different contributions of the two types of polynyas to deep water production.

How to cite: Barton, B., Nurser, G., and Aksenov, Y.: Model Based Polynya: Deep water formation in the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4235, https://doi.org/10.5194/egusphere-egu22-4235, 2022.

Antarctic Bottom Water (AABW) is one of the most important deep water masses contributing to the lower limb of the global overturning circulation, which modulates the deep ocean ventilation and oceanic heat/carbon exchanges on multidecadal to millennial timescales. Weddell Sea Bottom Water (WSBW) is a key precursor of the AABW exported from the Weddell Sea. Its formation involves intense air-sea-ice interaction on the continental shelf that releases brine from sea ice formation, and occurs mostly in the austral winter. Here we report a distinct long-term volume decline of WSBW revealed by data collected along repeat occupations of World Ocean Circulation Experiment (WOCE) hydrographic sections. We estimate a >20% reduction of WSBW volume since the early 1990s and a resultant widespread deep Weddell Sea warming associated with a basin-scale deepening of isopycnal surfaces. With the most significant volume reduction concentrating within the densest classes of WSBW and a concurrent decline of sea ice formation rate (>30%) over the southwestern Weddell continental shelf inferred from remote-sensed sea ice concentration data, we propose that the observed WSBW volume reduction is likely to be driven by a multidecadal weakening of dense shelf water production due to the sea ice changes. Reanalysis atmospheric data and ice drift data suggest that the reduction of sea ice formation rate is predominantly linked to changes in wind-driven sea ice convergence in front of Ronne Ice Shelf and Berkner Bank, as a response to a vigorous Amundsen Sea Low deepening that is teleconnected to tropical Pacific SST variability, and associated with the local radiative forcing from long-term ozone depletion.

How to cite: Zhou, S., Meijers, A., Meredith, M., Abrahamsen, P., Silvano, A., Holland, P., Sallée, J.-B., and Østerhus, S.: A multidecadal decline of Weddell Sea Bottom Water volume forced by wind-driven sea ice changes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7897, https://doi.org/10.5194/egusphere-egu22-7897, 2022.

Tipping of an ice shelf cavity from a cold to a warm state happens when a sustained inflow of warm Circumpolar Deep Water (CDW) or a modified variant of it replaces High Salinity Shelf Water (HSSW) and Ice Shelf Water (ISW) in a cold-water cavity. HSSW and ISW with temperatures close to or even below the surface freezing point provide little heat for melting glacial ice. CDW derivatives, however, can cause a substantial multiplication of the ice shelf basal melt rates. The increased melt water release may trigger a positive feedback loop that stabilizes the warm state. Therefore, if the outside circumstances  turned back to previous conditions, a reversal from warm to cold would not occur under the same conditions as the switch from cold to warm.

A warm tipping has been found possible for the Filchner-Ronne Ice Shelf (FRIS) cavity in previous studies. In the framework of the EU project TiPACCs, we now reinforce our focus on the conditions which can cause a tipping for the Filchner Ronne and other Antarctic ice shelf cavities. We conducted a series of FESOM-1.4 simulations with different manipulations of the atmospheric forcing variables in order to analyse the common factors of tipping events, opposed to more stable results.

We found that for the Filchner Trough region in a warming world, the crucial balance is between the different rates of warming and freshening of (a) the continental shelf waters in front of the ice shelf and (b) the waters transported with the slope current. While other studies identified an uplift of the pycnocline at the continental shelf break as a necessary condition for warm onshore flow, we deem a tipping more likely to hinge on the density loss of the shelf waters. When density on the continental shelf decreases more rapidly than in the slope current at sill depth, the ice shelf cavity is prone to tip. Reversibility of the tipping is possible within three decades under ERA Interim atmospheric forcing (1979-2017), but our study also confirms that hysteresis effects can cause a bistability of warm and cold state in the FRIS cavity under the 20th century HadCM3 forcing.

How to cite: Haid, V., Timmermann, R., and Hellmer, H.: Tipping of the Filchner-Ronne and other Antarctic ice shelf cavities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6053, https://doi.org/10.5194/egusphere-egu22-6053, 2022.

Sea-ice advance is a key moment to the Antarctic climate and ecosystem. Over the last 4 decades, sea-ice advance has been occurring earlier in the Weddell and Ross Seas and later west of the Antarctic Peninsula and in the Amundsen Sea. However, not much is known on the drivers of the observed changes nor on the physical processes determining the date of advance in the Southern Ocean. To progress understanding, we investigate the respective roles of ocean-sea ice processes in controlling the timing of sea-ice advance using observational and reanalysis data. Based on the satellite-based sea-ice concentration budget at the time of advance, we identify two regions with distinct processes. In the outermost ice-covered region, a few degrees of latitude within the winter ice-edge, no ice growth is observed and the ice advance date can only occur by transport of ice from higher latitudes. This is consistent with above freezing reanalysis sea surface temperature (SST) at the time of sea-ice advance. Elsewhere in the seasonal ice zone, ice import is a minor contributor to the sea-ice concentration budget hence sea-ice advance must be due to freezing only. In situ hydrographic observations show that the date of advance is more strongly linked to the seasonal maximum of the mixed layer heat content (MLH) than to the seasonal maximum SST — which reflects that the need for the full mixed layer to approach freezing before sea ice can appear. The relationship is stronger in regions with no contribution of sea-ice transport. Based on these considerations, we suggest that upper ocean hydrographic properties and sea ice drift are key features to determine the timing of sea-ice advance.

How to cite: Himmich, K., Vancoppenolle, M., Madec, G., Sallee, J.-B., De Lavergne, C., Lebrun, M., and Holland, P.: Drivers of Antarctic sea-ice advance date, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7257, https://doi.org/10.5194/egusphere-egu22-7257, 2022.

The Antarctic Climate is characterized by strong interactions between the Southern Ocean, its sea ice cover, and the overlying atmosphere taking place over a wide range of spatio-temporal scales. This coupling constrains our ability to isolate the role of specific components of the climate system on the dynamics of the Antarctic Climate, especially with stand-alone approaches neglecting the feedbacks at play. Based on coupled model simulations, we explore how the ocean can drive the interactions with the cryosphere and atmosphere at two distinct spatio-temporal scales. First, the role of ocean mesoscale eddies is investigated. We describe the imprint of mesoscale eddies on the sea ice and atmosphere in a high-resolution simulation of the Adélie Land sector (East Antarctica) performed with a regional coupled ocean--sea ice--atmosphere model (NEMO-MAR). Specific attention is given to the role of the sea ice in the modulation of the air-sea interactions at mesoscale and to the influence of eddy-driven fluxes on the ocean and sea ice. We show that mesoscale eddies affect near-surface winds and air temperature both in ice-free and ice-covered conditions due to their imprint on the sea ice cover. In addition, eddies promote northward sea ice transport and decrease momentum transfer by surface stress to the ocean. In a second section, we move to larger spatial and temporal scales and delve into the influence of the ocean on the seasonal to interannual variability of the sea ice, atmosphere, and ice shelves basal melt at the scale of the Southern Ocean. This work is based on early results from a new coupled ocean–sea ice--atmosphere--ice sheet configuration with explicit under-ice shelf cavities called PARASO. We focus on subsurface heat content variability and its influence on the interactions between the ocean, the sea ice, the atmosphere, and the Antarctic Ice Sheet.

How to cite: Huot, P.-V., Kittel, C., Fichefet, T., Marchi, S., Van Lipzig, N., Fettweis, X., Verfaillie, D., Klein, F., and Jourdain, N.: Oceanic drivers of air-sea-ice interactions: the imprint of mesoscale eddies and ocean heat content on the sea ice, atmosphere, and ice sheet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8256, https://doi.org/10.5194/egusphere-egu22-8256, 2022.

Over the last three decades there have been two catastrophic disintegrations events on the Antarctic peninsula, the Larsen A ice shelf in 1995 and the Larsen B in 2002, alongside the Wilkins ice shelf which underwent multiple partial disintegrations between 1998—2009.  Previous research into these events indicated that there had been prolonged periods where the Larsen and Wilkins Ice Shelves were without a sea-ice buffer to protect them from ocean swell in the leadup to their respective disintegrations. Swell potentially acted as a trigger mechanism to each shelf to disintegrated, as they had already been destabilised by surface flooding, fracturing, thinning and other glaciological factors.

This study will focus on the algorithm we developed which calculates the time where an ice shelf is without a local sea ice buffer (“exposure”), the size of the ocean which could directly propagate waves into the shelf (“corridor”) and the maximum wave height of swell which is directed towards the shelf in the corridor. An analysis of the last forty-one years showed that there was a large variation over individual ice shelves for both exposure and the available swell, due to the impact of polynyas, ice tongues and fast-ice growth which can protect the ice shelf. On a regional scale, the East Antarctic Ice Shelf and West Antarctic Ice Shelf had opposing trends, with the West Antarctic Ice Shelf recording a weak increasing trend of exposure and available swell.

How to cite: Teder, N., Bennetts, L., Massom, R., and Reid, P.: Antarctic ice shelf open ocean corridors with large swell available, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3373, https://doi.org/10.5194/egusphere-egu22-3373, 2022.

How to cite: Ong, Q. Y. E., England, M., Hogg, A., Constantinou, N., and Doddridge, E.: Investigation Into Antarctic Slope Front Regimes Using an Idealised Isopycnal Model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13276, https://doi.org/10.5194/egusphere-egu22-13276, 2022.

Antarctic Bottom Water (AABW) forms around Antarctica, sinks to the ocean’s abyss and fills more than 30% of the ocean’s volume. The formation of AABW includes mixing of distinct water masses, such as High Salinity Shelf Water (HSSW), Ice Shelf Water (ISW) and Circumpolar Deep Water on the continental shelf. Despite its climatic importance, the mechanisms of AABW formation are poorly known due to the lack of observations and the inability of climate models to simulate those mechanisms. We applied the Water Mass Transformation (WMT) framework in density space to simulations from a circumpolar ocean-ice shelf model (WAOM, with horizontal resolution ranging from 10 to 2 km) to understand the role of surface fluxes and oceanic processes to water mass formation and mixing on the Antarctic continental shelf, including the ice shelf cavities. The salt budget dominates the water mass transformation rates, with only secondary contribution from the heat budget. The buoyancy gain at relatively light density classes (27.2 < σ_Θ < 27.5 kg/m³) is dominated by basal melting. At heavier densities (σ_Θ > 27.5), salt input associated with sea-ice growth in coastal polynyas drives buoyancy loss. The formation of HSSW occurs via diffusion of the surface fluxes, but it is advected towards the cavities of large ice shelves (e.g., Ross, Ronne-Filchner), where it interacts with ice shelf through melting and refreezing and forms ISW. The sensibility of those mechanisms to the model horizontal resolution was evaluated. The basal melting and associated buoyancy gain rates largely decrease with increased resolution, while buoyancy loss associated with coastal polynyas are less sensible to resolution as surface fluxes are estimated from sea ice concentration observations. These results highlight the importance of high resolution to accurately simulate AABW formation, where mixing processes occurring below ice shelf cavities play an important role in WMT.

How to cite: Boeira Dias, F., Uotila, P., Galton-Fenzi, B., Ritcher, O., Rintoul, S., Pellichero, V., and Nie, Y.: Water Mass Transformation in the Antarctic shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11231, https://doi.org/10.5194/egusphere-egu22-11231, 2022.

The processes that govern freshwater flux from the Antarctic Ice Sheet (AIS)—ice-shelf basal melting and iceberg calving—are generally poorly represented in current Earth System Models (ESMs). The processes governing ocean flows onto the Antarctic continental and into ice-shelf cavities can only be captured accurately at resolutions significantly higher than those in typical CMIP-class ESMs. The Energy Exascale Earth System Model (E3SM) from the US Department of Energy supports regional refinement in all components, allowing global modeling with high resolution in regions of interest. Here, we present fully coupled results from an ocean/sea-ice mesh that has high resolution (12 km) on the Antarctic continental shelf and much of the Southern Ocean and low resolution (~30 to 60 km) over the rest of the globe. E3SM includes Antarctic ice-shelf cavities with fixed geometry and calculates ice-shelf basal melt rates from the heat and freshwater fluxes computed by the ocean component. In addition, E3SM permits prescribed forcing from a climatology of iceberg melt, providing a more realistic representation of these freshwater fluxes than found in many ESMs. With these new capabilities, E3SM version 2 produces realistic and stable ice-shelf basal melt rates across the continent. We show preliminary results of modeled ice-shelf basal melt rates across a range of Antarctic ice-shelves under pre-industrial and historical climate forcing, as well as the impacts of these added capabilities on the region’s climate. We show that the use of a mesoscale eddy parameterization, tapered with the mesh resolution, reduces biases even in the 12-km region where some eddies are resolved.  The accurate representation of these processes within a coupled ESM is an important step towards reducing uncertainties in projections of the Antarctic response to climate change and Antarctica's contribution to global sea-level rise.

How to cite: Asay-Davis, X., Barthel, A., Begeman, C., Comeau, D., Hoffman, M., Lin, W., Petersen, M., Price, S., Roberts, A., Veneziani, M., Van Roekel, L., and Wolfe, J.: Antarctic ice-shelf basal melting in a variable resolution Earth System Model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11368, https://doi.org/10.5194/egusphere-egu22-11368, 2022.

Increased sub-shelf melting and ice discharge from the Antarctic Ice sheet has both regional and global impacts on the ocean and the overall climate system. Additional meltwater, for example, can reduce the formation of Antarctic Bottom Water, potentially affecting the global thermohaline circulation. Similarly, increased input of fresh and cold water around the Antarctic margin can lead to a stronger stratification of coastal waters, and a potential increase in sea-ice formation, trapping warmer water masses below the surface, which in turn can lead to increased basal melting of the ice shelves.

So far these processes have mainly been analysed in simple unidirectional cause-and-effect experiments, possibly neglecting important interactions and feedbacks. To study the long-term and global effects of these interactions, we have developed a bidirectional offline coupled ice-ocean model framework. It consists of the global ocean and sea-ice model MOM5/SIS and an Antarctic instance of the Parallel Ice Sheet Model PISM, with the ice-shelf cavity module PICO representing the ice-ocean boundary layer physics. With this setup we are analysing the aforementioned interactions and feedbacks between the Antarctic Ice Sheet and the global ocean system on multi-millenial time scales.

How to cite: Kreuzer, M., Huiskamp, W., Albrecht, T., Petri, S., Reese, R., Feulner, G., and Winkelmann, R.: Millennial-scale interactions of the Antarctic Ice Sheet and the global ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11440, https://doi.org/10.5194/egusphere-egu22-11440, 2022.

The West Antarctic Peninsula (WAP) has warmed rapidly due to global climate change and there is large interannual variability in winter conditions, especially sea ice duration. Sea ice driven changes in the water column stability and marine biogeochemistry are impacting the CO₂ uptake in this highly productive region. This work extends the Rothera Oceanographic and Biological Time Series (RaTS) to a decade of year-round observations of surface water carbonate chemistry (2010-2020). This spans considerable sea ice variability, allowing assessment of the air/ice/ocean system across a wide range of conditions, including low sea ice cover as is predicted for the region. It includes rare winter-time data that show an unbiased view of annual carbonate processes and how they might be seasonally interconnected and coupled to sea ice dynamics. Even though the coastal region at Marguerite Bay is a net sink of CO₂, the time series is characterised by strong seasonal variability, indicating that this coastal region is a source of CO₂ to the atmosphere during the austral winter and a strong CO₂ sink in the summer. Additionally, we see differences in the net CO₂ uptake between different years. Net annual CO₂ uptake increased between 2014 and 2017 compared to previous years due to longer durations of heavier sea ice cover. Annual CO₂ uptake decreased again between 2017 and 2020, which are years associated to lower sea ice concentration and shorter duration of sea ice cover. We focus on the interannual differences in sea ice concentration and extent and how they are linked to differences in the water column structure, biogeochemical properties, and air-sea CO₂ exchange.

How to cite: Droste, E., Bakker, D., Venables, H., Hoppema, M., Dall'Olmo, G., and Queste, B.: Interannual variability in the ocean CO2 uptake along the West Antarctic Peninsula: A decade of year-round observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-572, https://doi.org/10.5194/egusphere-egu22-572, 2022.