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Under cover: The Southern Ocean’s connection to sea ice and ice shelves

The interaction between the ocean and the cryosphere in the Southern Ocean has become a major focus in climate research. Antarctic climate change has captured public attention, which has spawned a number of research questions, such as: Is Antarctic sea ice becoming more vulnerable in a changing climate? Where and when will melting of ice shelves by warm ocean waters yield a tipping point in Antarctic climate? What role do ice-related processes play in nutrient upwelling on the continental shelf and in triggering carbon export to deep waters? Recent advances in observational technology, data coverage, and modeling provide scientists with a better understanding of the mechanisms involving ice-ocean interactions in the far South. Processes on the Antarctic continental shelf have been identified as missing links between the cryosphere, the global atmosphere and the deep open ocean that need to be captured in large-scale and global model simulations.

This session calls for studies on physical and biogeochemical interactions between ice shelves, sea ice and the ocean. The ice-covered Southern Ocean and its role in the greater Antarctic climate system are of major interest. This includes work on all scales, from local to basin-scale to circumpolar. Studies based on in-situ observations and remote sensing as well as regional to global models are welcome. We particularly invite cross-disciplinary topics involving physical and biological oceanography, glaciology or biogeochemistry.

Co-organized by BG4/CL4/CR6
Convener: Torge Martin | Co-conveners: Xylar Asay-Davis, Alice BarthelECSECS, Ralph Timmermann
| Mon, 23 May, 13:20–14:50 (CEST), 15:10–18:30 (CEST)
Room N2

Mon, 23 May, 13:20–14:50

Chairpersons: Torge Martin, Verena Haid

Southern Ocean: Pacific sector

Karen J. Heywood et al.

The circulation of the Bellingshausen Sea has not attracted as much attention as that of its neighbours, the Amundsen Sea and the West Antarctic Peninsula.  Like them, it hosts a wide variety of vulnerable ice shelves, and exhibits inflows of warm deep water onto the continental shelf, and outflows of resulting ice shelf meltwater. Quantifying heat and freshwater transport, and understanding their temporal and spatial variability, is important for understanding the impact of a warming, melting Antarctica on ocean circulation.

First, we identify processes influencing interannual variability in warm deep water on the southern Bellingshausen Sea continental shelf using the GLORYS12V1 1/12° reanalysis from 1993 to 2018. EOFs of potential temperature below 300 m allow separation into warm and cold regimes. The Amundsen Sea Low is more intense and extends further to the east during warm regimes than during cold regimes. Increased Ekman transport results in a stronger frontal jet and Antarctic Coastal Current (AACC) in the cold regime. The warm and cold regimes are also linked to different temperature tendencies.  In the warm regime, a wind-induced reduction of sea ice results in increased heat loss to the atmosphere, convection, and formation of cold dense water in winter associated with a cooling of the southern Bellingshausen Sea and a net northward heat transport. In contrast, conditions of the cold regime favour a gradual warming of the southern Bellingshausen, consistent with a net southward heat transport.

Second, we use high-resolution sections collected from two ocean gliders deployed in the Bellingshausen Sea between January and March 2020 to quantify the distribution of meltwater. We observe a cyclonic circulation in Belgica Trough, whose western limb transports a meltwater flux of 0.46 mSv northwards and whose eastern limb transports a newly-identified meltwater re-circulation (0.88 mSv) southwards. Peak meltwater concentration is located into two layers (~150 m and ~200 m) associated with different density surfaces (27.4 and 27.6 kg m-3). The deeper layer is characterised by elevated turbidity. The shallower layer is less turbid, and is more prominent closer to the shelf break and in the eastern part of Belgica Trough. We hypothesise that these different meltwater layers emanate from different ice shelves that abut the Bellingshausen Sea.

To test the hypothesis of multiple source regions, we perform experiments using a regional set-up of MITgcm (approx. 3 km resolution), in which tracers released beneath ice shelves are used as a proxy for meltwater to diagnose transport pathways. Meltwater at the glider study site originates from ice shelves in the eastern Bellingshausen, particularly from George VI. Meltwater is primarily transported westward in the AACC; a small proportion detaches from the AACC via eddies and lateral mixing and, from the west, enters the cyclonic circulation within Belgica Trough, consistent with the glider-observed northward meltwater flow in the west and the southward re-circulation in the east. Very little meltwater from ice shelves immediately south of Belgica Trough enters this in-trough circulation.

How to cite: Heywood, K. J., Oelerich, R., Sheehan, P., Damerell, G., Thompson, A., Schodlok, M., and Flexas, M.: Circulation and water masses on the Bellingshausen Sea continental shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2561, https://doi.org/10.5194/egusphere-egu22-2561, 2022.

Alessandro Silvano et al.

The West Antarctic Ice Sheet is losing mass at an accelerating rate, contributing to sea level rise. Ocean forcing is considered to be the main driver of this mass loss, associated with warm intrusions of Circumpolar Deep Water onto the continental shelf. Here we describe these intrusions, focussing on the role of the Amundsen Undercurrent. The Amundsen Undercurrent is an eastward, bottom-intensified current located at the shelf break/upper slope that transports warm Circumpolar Deep Water. This current enters the continental shelf through deep canyons that connect the shelf break with ice shelf cavities, bringing oceanic heat to the base of the ice shelves. We use a regional ocean model to introduce the forcing mechanisms of the Amundsen Undercurrent and the drivers of its temporal variability. We conclude by discussing how this variability ultimately influences melting of ice shelves in the Amundsen Sea.

How to cite: Silvano, A., Holland, P., Naughten, K., Dragomir, O., Dutrieux, P., Jenkins, A., Si, Y., Stewart, A., Peña-Molino, B., and Naveira Garabato, A.: Simulated warm water access to the Amundsen Sea continental shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3067, https://doi.org/10.5194/egusphere-egu22-3067, 2022.

Justine Caillet et al.

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.

Nicolas Jourdain et al.

Approximately 10% of the global mean sea level rise over 2005–2010 was attributed to the glaciers flowing into the Amundsen Sea. This was mostly driven by changes in intrusions of Circumpolar Deep Water and subsequent ice shelf melt. Yet, projecting future ice shelf melt remains challenging because of large biases of CMIP models near Antarctica and because resolving the ocean circulation below the relatively small ice shelves in this sector requires a relatively high model resolution. Previously, we built atmospheric projections of the Amundsen sector at 10km resolution constrained by the rcp85 CMIP5 multi-model mean (Donat-Magnin et al. 2021). Here we use this atmospheric forcing to drive an ensemble of three 1/12° NEMO projections of the Amundsen Sea circulation and ice shelf melting. We find that melt rates are typically increased by 50% to 100% at the end of the 21st century compared to present day. Approximately half of this increase is explained by remote ocean changes transmitted through the model boundaries, while increased iceberg discharge does not have a significant effect. We describe the mechanisms at play through the terms of the ocean heat budget equations. We then use these projections to re-discuss some of the ISMIP6 projections (Seroussi et al. 2020, Edwards et al. 2021).

How to cite: Jourdain, N., Mathiot, P., Caillet, J., and Burgard, C.: Twenty-first century projections of ice-shelf melt in the Amundsen Sea, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10311, https://doi.org/10.5194/egusphere-egu22-10311, 2022.

Oana Dragomir et al.

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.

Q&A and Discussion

Tiago Dotto et al.

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.

Yixi Zheng et al.

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.

Vår Dundas et al.

As the melt rates of Getz Ice Shelf (GIS) increase, its grounding line is retreating, possibly destabilizing GIS. Detailed oceanographic observations from all the GIS frontal regions are needed to describe its drivers of basal melt and obtain an accurate projection of its melt rates. We present the first mooring observations from the bathymetrically sheltered trough between Siple and Carney Islands - one of the remaining GIS fronts to be described in detail. Although the ocean is colder in this central trough compared to what is observed in adjacent troughs, temperatures more than 1° above freezing are present throughout the mooring period, with a positive mean heat transport directed towards the ice shelf. Output from a high-resolution regional model indicates that heat is advected to the trough from both the eastern Amundsen Sea and from the continental shelf break in the north. The variability in heat content and heat transport are both affected by ocean surface stress, but while westward stress drives increased heat transport towards the ice shelf, eastward stress drives enhanced heat content. These relationships are most prominent in winter. Anomalously low summertime sea ice concentration and weak winds during the mooring period appear to suppress the effect of a strong positive anomaly in cumulative Ekman pumping, causing relatively low heat content during the mooring period compared to long-term estimates from the regional model.

How to cite: Dundas, V., Darelius, E., Daae, K., Steiger, N., Nakayama, Y., and Kim, T.-W.: Hydrography, circulation and warm inflow toward the central Getz Ice Shelf: two years of mooring observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9080, https://doi.org/10.5194/egusphere-egu22-9080, 2022.

Enrico Pochini et al.

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.

Rodrigo Gomez Fell et al.

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.

Jinyeong Kim and Hyangsun Han

Terra Nova Bay Polynya (TNBP) is one of the representative coastal polynya in East Antarctica. TNBP plays a major role of sea ice producers in the Antarctica, and it influences the regional current circulation and the surrounding marine environment. Therefore, it is important to investigate the influencing factors of TNBP. In this study, time series of TNBP area was estimated from Landsat-8 OLI/TIRS (2013-2016) and Sentinel-1 SAR (2017-2021) images by visually analyzing the boundary of polynya. To analyze the environmental factors influencing the area of ​​TNBP, wind speed, temperature, air pressure, and humidity measured at an automatic weather station installed near the polynya, and sea surface temperature, salinity and heat fluxes predicted by a reanalysis data were compared to the time series TNBP area. The area of TNBP showed a moderate correlation with the wind speed, but it was statistically low correlated with all other environmental factors. Meanwhile, a multiple linear regression between the time series area and all environmental factors showed a much higher correlation coefficient than between the polynya area and wind speed. However, the polynya areas predicted by the multiple linear regression model were largely deviated from those estimated from the satellite images. In future work, we intend to develop a model that retrieve more accurate TNBP area by selecting environmental factors suitable for polynya area estimation and applying them to machine learning techniques.

How to cite: Kim, J. and Han, H.: A study on the influencing factors of Terra Nova Bay Polynya using satellite imagery, AWS, and reanalysis data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11096, https://doi.org/10.5194/egusphere-egu22-11096, 2022.

Chiara Pambianco et al.

Here we present preliminary results from the Joides Basin, one of the depressions placed on the continental shelf adjacent to the Ross Ice Shelf (RIS) edge during the Last Glacial Maximum (LGM). We studied a south west – north east transect composed of four gravity cores and one piston core collected along the axis of the Joides Basin in order to reconstruct the past-LGM glacial sedimentary facies and provide new stratigraphic information. A suite of organic biomarkers were used to reconstruct sea-ice conditions and retreat of the RIS during the last termination.

The last glacial termination has been broadly targeted as a potential analogue to current/future global warming, and many studies on this timeframe have been conducted in the RIS, which, with its buttressing effect on continental ice, and its connection to the surrounding marine environment, represents a key element in bridging atmosphere and ocean. The RIS balance and behavior, during rapid climate change, however, is still poorly understood. Many questions are still open regarding the RIS retreat and warming effects on both the atmosphere and ocean, and concerns remain about the reliability of the chronology of marine sediments recovered from this region.

Based on radiocarbon dates of bulk organic carbon and foraminifera, our proposed age model provides new results on the paleo-environmental changes in the Joides Basin as the system moved from an ice-sheet dominated environment to a distal ice-sheet-system. Our preliminary results provide new information to better improve our understanding of the RIS modalities of retreat and the related effects to the surrounding marine and glacio-marine environment during the last deglaciation and Holocene.

How to cite: Pambianco, C., Capotondi, L., Giglio, F., Di Roberto, A., Belt, S., Mollenhauer, G., Nogarotto, A., and Tesi, T.: Last Glacial Maximum ice shelf retreat and sea-ice dynamics in the Joides Basin, Ross Sea, Antarctica , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7243, https://doi.org/10.5194/egusphere-egu22-7243, 2022.

Q&A and Discussion

Mon, 23 May, 15:10–16:40

Chairpersons: Stefanie Arndt, Moritz Kreuzer

Southern Ocean Atlantic & Indian sectors

Esther Portela Rodriguez et al.

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.

Qing Qin et al.

     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.

Benjamin Barton et al.

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.

Martin Mohrmann et al.

Maud Rise is a seamount in the eastern Weddell Sea and the location of the Maud Rise halo of reduced sea ice and polynyas. In this region, we present novel in situ data from two profiling floats with up to daily-resolved hydrographic profiles. Over Maud Rise, the mixed layer is especially deep during winter (150-200 m), leaving a thick layer of winter water after re-stratification that persists throughout the year and increases the rate of autumn mixed layer deepening. In contrast, the halo around Maud Rise is characterized by a shallow mixed layer depth and only a thin layer of winter water. Below the mixed layer, the water properties in the Maud Rise region are significantly correlated with bathymetric depth; thus, the Maud Rise flank defines the fronts between the Warm Deep Water of the abyssal ocean and the colder, less stratified Maud Rise Deep Water characteristic of the Taylor cap over Maud Rise. We analyse the curvature of spiciness in density space to quantify observed interleaving, which is substantially higher over and along the flanks of Maud Rise than in the surrounding deeper waters. These intrusions are indicative of enhanced lateral and vertical mixing along heavily sloping isopycnals, creating favorable conditions for thermobaric and double diffusive convection that facilitate the Maud Rise halo and may contribute to the formation of polynyas.

How to cite: Mohrmann, M., Swart, S., and Heuzé, C.: Observed mixing at the flanks of Maud Rise in the Weddell Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-346, https://doi.org/10.5194/egusphere-egu22-346, 2022.

Shenjie Zhou et al.

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.

Stefanie Arndt

The sensitivity of sea ice to the contrasting seasonal and perennial snow properties in the southeastern and northwestern Weddell Sea is not yet considered in sea ice model and satellite remote sensing applications. However, the analysis of physical snowpack properties in late summer in recent years reveal a high fraction of melt-freeze forms resulting in significant higher snow densities in the northwestern than in the eastern Weddell Sea. The resulting lower thermal conductivity of the snowpack, which is only half of what has been previously assumed in models in the eastern Weddell Sea, reduces the sea ice bottom growth by 18 cm. In the northwest, however, the potentially formed snow ice thickness of 12 cm at the snow/ice interface contributes to an additional 2 cm of thermodynamic ice growth at the bottom. This emphasizes the enormous impact of unappreciated regional differences in snowpack properties on the thermodynamic ice growth.

How to cite: Arndt, S.: Sensitivity of sea ice growth to snow properties in opposing regions of the Weddell Sea in late summer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2870, https://doi.org/10.5194/egusphere-egu22-2870, 2022.

Nadine Steiger and Jean-Baptiste Sallée

The Filchner Trough on the continental shelf in the southern Weddell Sea is the gateway for warm water from off the continental shelf to flow towards the Filchner Ice Shelf. The warm water is steered southward along the eastern slope of the trough, potentially increasing basal melt rates of the ice shelf and leading to the formation of cold and dense Ice Shelf Water that overflows and contributes to the Antarctic Bottom Water. We present mooring time series from 2017 to 2021 in key inflow regions of modified Warm Deep Water onto the eastern continental shelf. Three moorings were placed across the eastern flank of the Filchner Trough close to the shelf break and captured the changes in the thickness of the northward-flowing Ice Shelf Water as well as the overlying southward warmer water. Another mooring was placed over the shallower eastern shelf and allowed a comparison between the two pathways of warm water onto the continental shelf. The four-year-long observations provide a better understanding of the processes that influence the seasonal and interannual variability in temperatures and circulation and possible changes in the flow of warm water towards the ice shelf.

How to cite: Steiger, N. and Sallée, J.-B.: Four year-long observations from a key inflow region onto the southern Weddell Sea continental shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12887, https://doi.org/10.5194/egusphere-egu22-12887, 2022.

Verena Haid et al.

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.

Q&A and Discussion

David Ferreira and Jonathan Day

Since satellite records began in the 1970s, a small expansion of sea ice area around Antarctica has been observed, in stark contrast with the large decrease seen in the Arctic region. This expansion is difficult to reconcile with the observed rise in global temperatures and appears at odds with the ice loss simulated by climate models over the same period. Efforts to elucidate the driving mechanism are hampered by a short observational record, with little information available prior to the advent of satellite observations. Here we use direct observations recovered from logbooks of early explorers and routine shipping reports (1900 to 1953) to shed new light on the position of the ice edge. The data reveals that the early 20th century sea ice extended 3.1$^\circ$ (2.6$^\circ$-3.3$^\circ$ for 5-95\% confidence interval) further north ($\sim$100\% more extensive) than the present day. This finding re-frames the 20th century as a period of overall long-term sea ice loss in the Antarctic. The extensive sea ice cover, compared to present, goes hand-in-hand with cooler sea surface temperatures and reduced zonal wind speed in the region, consistent with reduced concentrations of anthropogenic forcing agents (greenhouse gas, ozone depletion) in the early 20th century, and may reflect the unperturbed state of Antarctic sea ice.


How to cite: Ferreira, D. and Day, J.: Direct evidence for a 20th Century decline in Southern Ocean sea ice, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13422, https://doi.org/10.5194/egusphere-egu22-13422, 2022.

Kenza Himmich et al.

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.

Pierre-Vincent Huot et al.

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.

Q&A and Discussion

Mon, 23 May, 17:00–18:30

Chairpersons: Tiago Dotto, Nadine Steiger

Southern Ocean: circumpolar, atmosphere, biogeochemistry

Michael Meredith et al.

Ocean mixing around Antarctica is a key process that influences the vertical distributions of heat and nutrients, affecting glacier and ice shelf retreats, sea ice formation and marine productivity, with implications for regional ecosystems, global sea level and climate. Here we show that collapsing glacier fronts associated with calving events trigger internal tsunamis, the propagation and breaking of which can lead to significant mixing. Observations of one such event at the West Antarctic Peninsula, during which 3-20 megatonnes of ice were discharged to the ocean, reveal rapidly-elevated internal wave kinetic energy and upper-ocean shear, with strong homogenisation of the water column. Scaling arguments indicate that, at the West Antarctic Peninsula, just a few such events per summer would make this process comparable in magnitude to winds, and much more significant than tides, in driving shelf mixing. We postulate that this process is likely relevant to all regions with calving marine-terminating glaciers, including also Greenland and the Arctic. Glacier calving is expected to increase in a warming climate, likely strengthening internal tsunamigenesis and mixing in these regions in the coming decades.

How to cite: Meredith, M., Inall, M., Brearley, A., Munday, D., Ehmen, T., Sheen, K., Retallick, K., Annett, A., Jones, R., Carvalho, F., Van Landeghem, K., Naveira Garabato, A., Gerrish, L., Scourse, J., Cook, A., and Bull, C.: Internal tsunamigenesis and mixing driven by glacier calving in Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1163, https://doi.org/10.5194/egusphere-egu22-1163, 2022.

Nathan Teder et al.

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.

Qing Yee Ellie Ong et al.
The Antarctic Slope Current is a current that flows westward around Antarctica and lies close to the coast on the continental shelf. The slope current region features steeply sloping isopycnals at the continental shelf, characterising the Antarctic Slope Front (ASF). The ASF serves as a barrier between warm Circumpolar Deep Water and the continental shelf. Depending on the local structure of the ASF, Circumpolar Deep Water can flood on to the continental shelf and induce basal melt, with implications for sea level rise globally. Observations in these regions of the ocean are scarce, or even non-existent, and eddy-resolving modelling studies of the ASF are also limited. We have developed a set of idealised configurations with an isopycnal model that can emulate the conditions in different ASF regimes. We investigate how the different ASF regimes are affected by variations in wind forcing, topography and stratification. This aims to identify the different dynamics and the sensitivity of forcings and boundary conditions that allow warm water to reach the shelf in different ASF regimes.

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.

Fabio Boeira Dias et al.

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 < σΘ < 27.5 kg/m3) is dominated by basal melting. At heavier densities (σΘ > 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.

Kyriaki M. Lekakou et al.

The ice shelves of the Amundsen Sea are rapidly thinning, and this can be largely explained by basal melting driven by the ocean. However, sparse observational data and poorly known bathymetry contribute to the difficulty of quantifying the key ocean mechanisms that transport warm water onto the Amundsen Sea continental shelf and their variability. These processes should be represented in coupled climate models such as those used for CMIP6. Previously, we leveraged recent observational campaigns and gains in process understanding to assess how well four models, UKESM1 and the HadGEM-GC3.1 family of models, represent the ocean processes forcing warm water onto the Amundsen Sea continental shelf. We identified the medium resolution (1/4°) HadGEM-GC3.1-MM model’s inability to represent warm water intrusion on the continental shelf, revealing substantial biases in sea ice, SST, salinity and circulation in the Southern Ocean. It is important to understand the processes that are driving these biases, to guide the improvement of this and similar models. Here, we study model behaviour during the spin-up, control and historical runs, to identify what is causing this unrealistic behaviour. A key result is the rapid development of biases in temperature and salinity on the Amundsen’s Sea continental shelf, after only 15 years in the spin-up run, entering a state which persists throughout the following runs. By calculating the differences in sea ice concentration between years 0-5 and 10-15 of the spin up-run, we found significant changes across multiple regions of the Southern Ocean and continental shelf, with most of the East Antarctic sector and Bellingshausen Sea showing a considerable decline that exceeds 20% in some places. The differences between years 0-5 and 10-15 Notable freshening takes place in the whole West Antarctic sector and a strong westward slope current appears, which encircles Antarctica. While strong biases in sea ice and salinity develop later in the Weddell Sea, during the first 15 years the largest biases occur in Drake Passage and the west Antarctic sector. We analyse tendencies and the freshwater budget from the spin-up run to quantify the key processes that drive the development of these biases in selected regions.

How to cite: Lekakou, K. M., Webber, B. G. M., Heywood, K. J., Stevens, D. P., Hyder, P., and Hewitt, H.: Development of persistent Southern Ocean biases in HadGEM-GC3.1-MM and implications for modelled ocean-ice interaction in West Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11967, https://doi.org/10.5194/egusphere-egu22-11967, 2022.

Q&A and Discussion

Xylar Asay-Davis et al.

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.

Moritz Kreuzer et al.

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.

Deborah Verfaillie et al.

The climate of the polar regions is characterized by large fluctuations and has experienced dramatic changes over the past decades. In particular, the patterns of changes in sea ice and ice sheet mass are complex in the Southern Hemisphere. The Antarctic Ice Sheet has also lost mass in the past decades, especially in Western Antarctica, with a spectacular thinning and weakening of ice shelves, i.e., the floating extensions of the grounded ice sheet. Despite recent advances in observing and modelling the Antarctic climate, the mechanisms behind this long-term mass loss remain poorly understood because of the limited amount of observations and the large biases of climate models in polar regions, in concert with the large internal variability prevailing in the Antarctic. Among all the processes involved in the mass variability, changes in the general atmospheric circulation of the Southern Hemisphere may have played a substantial role. One of the most important atmospheric modes of climate variability in the Southern Ocean is the Southern Annular Mode (SAM), which represents the position and the strength of the westerly winds. During years with a positive SAM index, lower sea level pressure at high latitudes and higher sea level pressure at low latitudes occur, resulting in a stronger pressure gradient and intensified Westerlies. However, the current knowledge of the impact of these fluctuations of the Westerlies on the Antarctic cryosphere is still limited. Over the past few years, some efforts investigated the impact of the SAM on the Antarctic sea ice and the surface mass balance of the ice sheet from an atmosphere-only perspective. Recently, a few oceanic studies have focused on the local impact of SAM-related fluctuations on the ice-shelf basal melt in specific regions of Antarctica, particularly Western Antarctica. However, to our knowledge, there is no such study at the scale of the whole Antarctic continent. In this study, we performed idealized experiments with a pan-Antarctic regional ice-shelf cavity-resolving ocean - sea-ice model for different phases of the SAM. We show that positive (negative) phases lead to increased (decreased) upwelling and subsurface ocean temperature and salinity close to ice shelves. A one-standard-deviation increase of the SAM leads to a net basal mass loss of 40 Gt yr-1, with strong regional contrasts: increased melt in the Western Pacific and Amundsen-Bellingshausen sectors and the opposite response in the Ross sector. Taking these as a baseline sensitivity, we estimate last millennium and end-of-21st-century ice-shelf basal melt changes due to SAM of -60.7 Gt yr-1 and 1.8 to 26.8 Gt yr-1 (depending on the emission scenario considered), respectively, compared to the present.

How to cite: Verfaillie, D., Pelletier, C., Goosse, H., Jourdain, N. C., Bull, C. Y. S., Dalaiden, Q., Favier, V., Fichefet, T., and Wille, J.: How does the Southern Annular Mode impact ice-shelf basal melt around Antarctica?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5388, https://doi.org/10.5194/egusphere-egu22-5388, 2022.

Paul Holland et al.

Ocean-driven ice loss from the West Antarctic Ice Sheet (WAIS) is a significant contributor to sea-level rise. In the 20th century, modelled wind trends over the Amundsen Sea imply an ocean warming that could explain this ice loss. In this presentation, climate model simulations are used to separate internal and anthropogenic influences on these wind trends. Tropical Pacific variability is found to be most influential in winter and over the Amundsen Sea continental shelf, while greenhouse gases and ozone depletion are dominant in summer and north of the shelf. Model projections feature strong wind trends that imply future ocean warming. In these projections, moderate greenhouse-gas mitigation has no influence on wind trends near the Amundsen Sea shelf. Internal climate variability creates a large and irreducible uncertainty in winds over the shelf. This complex regional and seasonal interplay between anthropogenic forcing and internal variability may determine the attribution and projection of ice loss from the WAIS.

How to cite: Holland, P., Bracegirdle, T., Dutrieux, P., Naughten, K., Schneider, D., O'Connor, G., Steig, E., and Jenkins, A.: Influence of anthropogenic forcing and internal climate variability on winds over the Amundsen Sea shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6237, https://doi.org/10.5194/egusphere-egu22-6237, 2022.

Mike Dinniman et al.

Previous studies showed that correlations of satellite-derived estimates of chlorophyll a in coastal polynyas over the Antarctic continental shelf with the basal melt rate of adjacent ice shelves are a result of upward advection or mixing of iron-rich deep waters due to circulation changes driven by ice shelf melt, rather than a direct influence of iron released from melting ice shelves.  In this study, the effects of projected changes in winds, precipitation, and atmospheric temperatures on this relationship were examined with a 5-km resolution ocean/sea ice/ice shelf model of the Southern Ocean.  The atmospheric changes are added as idealized increments to the forcing.  Inclusion of a poleward shift and strengthening of the winds, increased precipitation, and warmer atmospheric temperatures resulted in an 83% increase in the total Antarctic ice shelf basal melt, with changes being heterogeneously distributed around the continent.  The total dissolved iron supply to the surface waters over the continental shelf increased by 62%, while the surface iron supply due just to basal melt driven overturning increased by 48%.  However, even though the total increase in iron supply is greater than the increase due to changes in the ice shelf melt, the ice shelf driven supply becomes relatively even more important in some locations, such as the Amundsen and Bellingshausen Seas.  The modified atmospheric conditions also produced a reduction in summer sea ice extent and a shoaling of the summer mixed layers.  These simulated responses to projected changes suggest relief of light and nutrient limitation for phytoplankton blooms over the Antarctic continental shelf and perhaps an increase in annual production in years to come.

How to cite: Dinniman, M., St-Laurent, P., Arrigo, K., Hofmann, E., and van Dijken, G.: Sensitivity of the relationship between Antarctic ice shelves and iron supply to projected changes in the atmospheric forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3041, https://doi.org/10.5194/egusphere-egu22-3041, 2022.

Elise Droste et al.

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.

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