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

In recent years 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? What controls the inflow of warm water into ice shelf cavities and what is the impact of enhanced meltwater outflow? What role do ice processes play in nutrient upwelling on the shelf? 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/CR4
Convener: Torge Martin | Co-conveners: Xylar Asay-Davis, Kaitlin NaughtenECSECS
vPICO presentations
| Wed, 28 Apr, 09:00–10:30 (CEST)

vPICO presentations: Wed, 28 Apr

Chairpersons: Torge Martin, Kaitlin Naughten, Xylar Asay-Davis
Amundsen and Bellingshausen Seas and Paciifc Ocean Sector
Kyriaki M. Lekakou, Ben G.M. Webber, Karen J. Heywood, David P. Stevens, and Patrick Hyder

The Amundsen Sea glaciers, in West Antarctica, are among the world’s fastest discharges of ice into the ocean. The rapid thinning of these ice shelves can be largely explained by basal melting driven by the ocean. Relatively warm water reaches the continental shelf in the Amundsen Sea and deep bathymetric troughs facilitate warm deep water flow to the base of the ice shelves. However, time sparse observational data, and even poorly known bathymetry, contribute to the difficulty of quantifying the key ocean mechanisms, and their variability, that transport warm water onto the Amundsen Sea continental shelf and guide it southward into the ice shelf cavities. Nonetheless these processes should be represented in the coupled climate models, such as those used for CMIP6, which are being used to project future sea level rise.

Here we leverage recent observational campaigns and gains in process understanding to assess how well four of these models, UKESM1 and the HadGEM-GC3.1 family of models, represent the ocean processes forcing warm water onto the Amundsen Sea continental shelf. The three HadGEM models have the same external forcing but different horizontal resolutions, 1/12, ¼ and 1 degree. The 1 degree resolution UKESM1 is based on HadGEM3.1 but includes atmospheric chemistry, aerosols and marine biogeochemistry. A key finding is the medium resolution (1/4°) HadGEM-GC3.1 model’s inability to allow warm deep water intrusion onto the continental shelf, associated with a strong westward slope current that is not present in the other models. The medium resolution model represents well the annual cycle of sea ice in the Amundsen Sea, but overall has significantly less sea ice around Antarctica, compared with the other models and satellite observations. Despite its low resolution, UKESM1 represents well all the main ocean features, including the shelf-break undercurrent, warm deep water and realistic sea ice. It captures more significant interannual variability, in contrast to the low resolution HadGEM, for which the interannual variability is more suppressed. Of the four models considered here, the best performing models are the 1/12° HadGEM and UKESM1, followed by the low resolution HadGEM model, which reasonably represents warmer deep water on the continental shelf and a shallower mixed layer. The medium resolution HadGEM, despite its better resolution is less realistic than the two low resolution models.

How to cite: Lekakou, K. M., Webber, B. G. M., Heywood, K. J., Stevens, D. P., and Hyder, P.: Evaluation of four coupled climate models in the Amundsen Sea, Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15078, https://doi.org/10.5194/egusphere-egu21-15078, 2021.

Yixi Zheng, David Stevens, Karen Heywood, Benjamin Webber, and Bastien Queste

Floating ice shelves buttress the Antarctic Ice Sheet, which is losing mass rapidly mainly due to oceanic melting and the associated disruption to glacial dynamics. The local oceanic circulation near ice shelves is therefore important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf, and the resultant melting. However, the dynamics controlling the near-coastal circulation are not fully understood, particularly relating to seasonal and interannual changes in wind stress curl and ice cover. A gyre circulation (27 km radius, cyclonic) in front of the Pine Island Ice Shelf has been identified in both numerical models and velocity observations. In 2019 in the west of Thwaites Ice Shelf, for the first time in this habitually ice-covered region, another gyre circulation rotating in a different direction (13 km, anticyclonic) was detected by velocity observations. Here we use an idealised configuration of MITgcm, with idealised forcing based on ERA-5 climatological wind fields and simplified sea ice conditions from MODIS satellite images, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf. A barotropic version of the model is able to reproduce the gyres driven solely by the wind. We show that the modelled gyre direction depends upon the angle between the wind direction and the sea ice front. Gyres generated by wind in sea-ice-free conditions have directions controlled by the wind stress curl. When sea ice is present, the wind stress exerted on the sea surface is reduced, leading to a modified wind stress curl and a resultant change in gyre direction.

How to cite: Zheng, Y., Stevens, D., Heywood, K., Webber, B., and Queste, B.: An Idealized Model of Ocean Gyres near Pine Island Ice Shelf and Thwaites Ice Shelf, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10364, https://doi.org/10.5194/egusphere-egu21-10364, 2021.

Nadine Steiger, Elin Darelius, Anna Wåhlin, and Karen Assmann

Ice shelves in West Antarctica have been thinning during the last decades due to an increased supply of ocean heat that melts the ice from below. The Getz Ice Shelf in the western Amundsen Sea has experienced an inflow of warm water during 2016-2017, but intermittent events of reduced heat content occur during this period. The processes behind the variability of heat transport towards the Antarctic ice shelves on daily to decadal time scales are not well known.
Here, we present possible drivers and implications of these events of reduced heat content. We find that they are preceded by strong easterly winds that open up a coastal polynya and depress the cold Winter Water towards the ocean floor. Simultaneously, the ocean current flowing towards the ice shelf veers to the right and aligns with the ice shelf front rather than entering the ice shelf cavity. The heat transport into the ice shelf cavity is consequently reduced by 22% in winter 2016. These events do not occur during winter 2017, possibly due to stronger stratification and weaker winds.

How to cite: Steiger, N., Darelius, E., Wåhlin, A., and Assmann, K.: Drivers of intermittent reduction in ocean heat transport into the Getz Ice Shelf cavity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5948, https://doi.org/10.5194/egusphere-egu21-5948, 2021.

Guillaume Liniger, Sebastien Moreau, Delphine Lannuzel, Fernando Paolo, and Peter Strutton

Ice shelves have been melting, thinning and retreating along the coast of West Antarctica for the past four decades, most notably in the Amundsen Sea sector. This area hosts two highly productive coastal polynyas, the Pine Island polynya and the Amundsen Sea polynya, whose opening triggers two of the largest phytoplankton blooms in the Southern Ocean. Previous work in the area suggests that ice shelf melting and thinning increases the iron content of coastal seawater, which could potentially boost ocean primary productivity locally. In this work, we use historical (1992-2017) remote sensing observations of net primary productivity, sea-ice concentration and rate of ice shelves melting to investigate the strength of this connection for these two large polynyas. We used the Abbot, Cosgrove, Pine Island, Thwaites, Dotson and Getz ice shelves for our analyses. Our initial results suggest no significant trends in net primary productivity though time but a large interannual variability for both polynyas. The basal melt rate and ice thinning seem to not be the main drivers of this interannual variability in these polynyas, but sea-ice coverage variability does seem to play a strong role, potentially allowing increased light availability and stratification. Further investigations of circumpolar deep water inputs and climate modes related to ice shelves melting such as El Niño or the southern annular mode are needed to clarify our findings. Our preliminary study points the complexity of ice-ocean systems, where several co-occurring processes influence coastal primary productivity, with consequences for carbon cycling and the climate system.

How to cite: Liniger, G., Moreau, S., Lannuzel, D., Paolo, F., and Strutton, P.: Interannual variability in primary productivity driven by sea-ice phenology in the Amundsen Sea polynyas, not ice shelves melting., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3687, https://doi.org/10.5194/egusphere-egu21-3687, 2021.

Peter Sheehan, Karen Heywood, Andrew Thompson, and Mar Flexas

Quantifying meltwater content and describing transport pathways is important for understanding the impact of a warming, melting Antarctica on ocean circulation. Meltwater fluxes can affect density-driven, on-shelf flows around the continent, and the formation of the dense water masses that ventilate abyssal regions of the world ocean. We present observations collected from two ocean gliders that were deployed in the Bellingshausen Sea for a period of 10 weeks between January and March of 2020.  Using multiple high-resolution sections, we quantify both the distribution of meltwater concentrations and lateral meltwater fluxes within the Belgica Trough in the Bellingshausen Sea. We observe a cyclonic circulation in the trough, in agreement with previous studies. A meltwater flux of 0.46 mSv is observed flowing northwards in the  western limb of the cyclonic circulation. A newly identified meltwater re-circulation (0.88 mSv) is observed flowing back towards the ice front (i.e. southwards) with the eastern limb of the cyclonic circulation. In addition, 1.16 mSv of meltwater is observed flowing northeastward, parallel to the shelf break, with the northern limb of the cyclonic circulation. Peak meltwater is concentrated into two layers associated with different density surfaces: one approximately 150 m deep (27.4 kg m-3) and one approximately 200 m deep (27.6 kg m-3}). The deeper of these layers is characterised by an elevated optical backscatter, which indicates a more turbid water mass. The shallower layer is less turbid, and is more prominent closer to the shelf break and in the eastern part of the Belgica Trough. We hypothesise that the deeper, turbid meltwater layer originates locally from the Venables Ice Shelf, whereas the shallower, less turbid meltwater layer, comprises meltwater from ice shelves in the eastern Bellingshausen Sea. The broad distribution of meltwater from multiple sources suggests the potential for remote interactions and feedbacks between the various ice shelves that abut the Bellingshausen Sea.

How to cite: Sheehan, P., Heywood, K., Thompson, A., and Flexas, M.: Sources and transport of glacial meltwater in the Bellingshausen Sea, Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2746, https://doi.org/10.5194/egusphere-egu21-2746, 2021.

Ria Oelerich, Karen J. Heywood, Gillian M. Damerell, and Andrew F. Thompson

The continental shelf of the Bellingshausen Sea, located between the West Antarctic Peninsula and the Amundsen Sea, Antarctica, is poorly investigated, compared with its neighbours. Here, the southernmost frontal jet of the Antarctic Circumpolar Current is adjacent to the continental slope which impacts the transport of warm Circumpolar Deep Water onto the shelf. This in turn can influence the transport of heat southward across the shelf and therefore the melting of vulnerable ice shelves.

We present model-based investigations using the GLORYS12V1 1/12° reanalysis monthly output (GLOBAL_REANALYSIS_PHY_001_030) over 19 years from 2000 to 2018. By connecting the location of the frontal jet to SSH contours we identify seasonal and interannual variability in this current system and demonstrate that the closer the frontal jet is to the continental slope, the greater the flow of warm deep water onto the shelf. This onshore flow is limited to specific areas closest to the frontal jet, predominantly in the eastern Bellingshausen Sea. In contrast, other areas, specifically those troughs where water flows towards the West Antarctic Peninsula and close to the coastline of Antarctica show opposite behaviour with respect to onshelf heat content. Further analyses of flow patterns also indicate the involvement of a coastal jet close to the shore that is weaker when more warm water is on the shelf. Understanding the variability in the current structures throughout the continental shelf of the Bellingshausen Sea in response to a changing frontal jet is essential to gain knowledge about the distribution of heat and therefore the melting of ice shelves.

How to cite: Oelerich, R., Heywood, K. J., Damerell, G. M., and Thompson, A. F.: Variability of warm water intrusions onto the Bellingshausen Sea continental shelf, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4788, https://doi.org/10.5194/egusphere-egu21-4788, 2021.

Weddell Sea and Atlantic Ocean Sector
Joseph Lockwood, Carolina Dufour, Stephen Griffies, and Michael Winton

This study investigates the occurrence of the Weddell Sea Polynya under an idealized

climate change scenario by evaluating simulations from climate models of different

ocean resolutions. The GFDL-CM2.6 climate model, with roughly 3.8 km

horizontal ocean grid spacing in the high latitudes, forms a Weddell Sea Polynya at

similar time and duration under idealized climate change forcing as under pre-industrial

forcing. In contrast, all convective models forming the fifth phase of the Coupled Model

Intercomparison Project (CMIP5) show either a cessation or a slowdown of Weddell

Sea Polynya events under climate warming. The representation of the Antarctic Slope

Current and related Antarctic Slope Front is found to be key in explaining the

differences between the two categories of models, with these features being more

realistic in CM2.6 than in CMIP5. In CM2.6, the freshwater input driven by sea ice melt

and enhanced runoff found under climate warming largely remains on the shelf region

since the slope front restricts the lateral spread of the freshwater. In contrast, for most

CMIP5 models, open ocean stratification is enhanced by freshening since the absence

of a slope front allows coastal freshwater anomalies to spread into the open ocean.

This enhanced freshening contributes to the slow down the occurrence of Weddell Sea

Polynyas. Hence, an improved representation of Weddell Sea shelf processes in

current climate models is desirable to increase our ability to predict the fate of the

Weddell Sea Polynyas under climate change.

How to cite: Lockwood, J., Dufour, C., Griffies, S., and Winton, M.: On the role of the Antarctic Slope Front on the occurrence of theWeddell Sea polynya under climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1468, https://doi.org/10.5194/egusphere-egu21-1468, 2021.

Elise Droste, Melchor González Dávila, Juana Magdalena Santana Casiano, Mario Hoppema, Gerd Rohardt, Bastien Queste, Hugh Venables, Giorgio Dall'Olmo, and Dorothee C. E. Bakker

Tides have a large impact on coastal polynyas around Antarctica. We investigate the effect of semi-diurnal tidal cycles on the seawater carbonate chemistry in a coastal polynya hugging the Ekström Ice Shelf in the south-eastern Weddell Sea. This region experiences some of the strongest tides in the Southern Ocean. We assess the implications for the contribution of coastal polynyas to the carbon dioxide (CO2) air-sea flux of the Weddell Sea.

Two site visits, in January 2015 and January 2019, are intercompared in terms of the dissolved inorganic carbon (DIC) concentration, total alkalinity, pH, and CO2 partial pressure (pCO2). The tides induce large variability in the carbonate chemistry of the coastal polynya in the austral summer: DIC concentrations vary between 2174 and 2223 umol kg-1.

The tidal fluctuation in the DIC concentration can swing the polynya from a sink to a source of atmospheric CO2 on a semi-diurnal timescale. We attribute these changes to the mixing of different water masses. The amount of variability induced by tides depends on – and is associated with – large scale oceanographic and biogeochemical processes that affect the characteristics and presence of the water masses being mixed, such as the rate of sea ice melt.

Sampling strategies in Antarctic coastal polynyas should always take tidal influences into account. This would help to reduce biases in our understanding of how coastal polynyas contribute to the CO2 uptake by the Southern Ocean.

How to cite: Droste, E., González Dávila, M., Santana Casiano, J. M., Hoppema, M., Rohardt, G., Queste, B., Venables, H., Dall'Olmo, G., and Bakker, D. C. E.: The role of tides on the carbonate chemistry of a coastal polynya in the south-eastern Weddell Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8978, https://doi.org/10.5194/egusphere-egu21-8978, 2021.

Stefanie Arndt, Hanno Meyer, Ilka Peeken, and Christian Haas

Summer sea ice extent in the Weddell Sea has increased overall during the last four decades, with large interannual variations. However, the underlying causes and the related ice and snow properties are still poorly known.

Here, we present results of the interdisciplinary Weddell Sea Ice (WedIce) project carried out in the northwestern Weddell Sea on board the German icebreaker R/V Polarstern in February and March 2019, i.e. at the end of the summer ablation period, focusing on 21 ice cores sampled for texture, salinity and isotope analysis.

The ice at the coring sites had an average thickness of 178 cm with an average snow depth of 13 cm and a consistently positive freeboard. Isotope and salinity analyses revealed an average meteoric ice fraction of 23%. This included about 17% (22cm) snow-ice, saline sea ice formed by flooding and refreezing of snow at the snow/ice interface. In contrast, superimposed ice, fresh sea ice formed through melting and refreezing of snow only, account for about 6% (11cm) of the sea-ice thickness. The comparison of our results with previous expeditions to the same region shows that the thickness of superimposed ice has hardly increased, indicating no dominant changes in the amount of surface summer melt/thaw, despite the observed sea ice decline in the northwestern Weddell Sea during summer in recent years.

However, we consider the evolution of snow properties, and in particular the proportion of meteoric ice in the snow cover, as a critical indicator for significant changes in the coupled atmosphere/sea ice/ocean system.

How to cite: Arndt, S., Meyer, H., Peeken, I., and Haas, C.: Sea-ice growth from the top: Meteoric ice and snow in the northwestern Weddell Sea, Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-197, https://doi.org/10.5194/egusphere-egu21-197, 2020.

East Antarctica, Indian Ocean and Australian Sectors
Jing Jin, Antony J. Payne, William Seviour, and Christopher Bull

The basal melting of the Amery Ice Shelf (AIS) in East Antarctica and its connections with the oceanic circulation are investigated by a regional ocean model. The simulated estimations of net melt rate over AIS from 1976 to 2005 vary from 1 to 2 m/yr depending primarily due to inflow of modified Circumpolar Deep Water (mCDW). Prydz Bay Eastern Costal Current (PBECC) and the eastern branch of Prydz Bay Gyre (PBG) are identified as two main mCDW intrusion pathways. The oceanic heat transport from both PBECC and PBG has significant seasonal variability, which is associated with the Antarctic Slope Current. The onshore heat transport has a long-lasting effect on basal melting. The basal melting is primarily driven by the inflowing water masses though a positive feedback mechanism. The intruding warm water masses destabilize the thermodynamic structure in the sub-ice shelf cavity therefore enhancing the overturning circulations, leading to further melting due to increasing heat transport. However, the inflowing saltier water masses due to sea-ice formation could offset the effect of temperature through stratifying the thermodynamic structure, then suppressing the overturning circulation and reducing the basal melting.

How to cite: Jin, J., Payne, A. J., Seviour, W., and Bull, C.: Modelling the ocean circulation and the basal melting in the Prydz Bay-Amery Ice Shelf system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6425, https://doi.org/10.5194/egusphere-egu21-6425, 2021.

Guillian Van Achter, Thierry Fichefet, Hugues Goosse, Charles Pelletier, Jean Sterlin, Pierre-Vincent Huot, Jean-François Lemieux, Alexander Fraser, Richard Porter-Smith, and Konstanze Haubner

The Totten Glacier in East Antarctica is of major climate interest because of the large fluctuation of its grounding line and of its potential vulnerability to climate change. The ocean above the continental shelf in front of the Totten ice shelf exhibits large extents of landfast sea ice with low interannual variability. Landfast sea ice is mostly not or sole crudely represented in current climate models. These models are potentially omitting or misrepresenting important effects related to this type of sea ice, such as its influence on coastal polynya locations. Yet, the impact of the landfast sea
ice on the ocean – ice shelf interactions is poorly understood. Using a series of high-resolution, regional NEMO-LIM-based experiments including an
explicit treatment of ocean – ice shelf interactions over the years 2001-2010, we simulate a realistic landfast sea ice extent in the area of Totten Glacier
through a combination of a sea ice tensile strength parameterisation and a grounded iceberg representation. We show that the presence of landfast sea
ice impacts seriously both the location of coastal polynyas and the ocean mixed layer depth along the coast, in addition to favouring the intrusion of
mixed Circumpolar Deep Water into the ice shelf cavities. Depending on the local bathymetry and the landfast sea ice distribution, landfast sea ice affects ice shelf cavities in different ways, either by increasing the ice melt (+28% for the Moscow University ice shelf) or by reducing its seasonal cycle
(+10% in March-May for the Totten ice shelf). This highlights the importance of including an accurate landfast sea ice representation in regional and
eventually global climate models

How to cite: Van Achter, G., Fichefet, T., Goosse, H., Pelletier, C., Sterlin, J., Huot, P.-V., Lemieux, J.-F., Fraser, A., Porter-Smith, R., and Haubner, K.: Modelling landfast sea ice and its influence on ocean-ice interactions in the area of the Totten Glacier, East Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8846, https://doi.org/10.5194/egusphere-egu21-8846, 2021.

Yoshihiro Nakayama, Chad Greene, Fernando Paolo, Vigan Mensah, Hong Zhang, Haruhiko Kashiwase, Daisuke Simizu, Jamin Greenbaum, Donald Blankenship, Ayako Abe-Ouchi, and Shigeru Aoki

The Totten Glacier in East Antarctica has received increasing attention in recent years for its ice loss and warm oceanographic conditions observed at the ice shelf front. Here, we developed satellite estimates of temporally varying Totten Ice Shelf (TIS) melt rates and a high-resolution ocean model. We show that the Antarctic Slope Current (ASC) impedes ocean heat intrusions, and on-shelf intrusions enhance when the ASC weakens. The interannually varying strength of the ASC is primarily controlled by lateral ocean boundary conditions (and thus atmosphere and ocean circulations outside of the model domain) but also likely influenced by local wind stress curl and upstream decent of shelf water. We further show that heat intrusions towards the TIS are enhanced with coastal freshening, suggesting that freshening from ice loss in West Antarcticacould start a chain reaction, leading to increased melt in East Antarctica, and further coastal freshening.

How to cite: Nakayama, Y., Greene, C., Paolo, F., Mensah, V., Zhang, H., Kashiwase, H., Simizu, D., Greenbaum, J., Blankenship, D., Abe-Ouchi, A., and Aoki, S.: Antarctic Slope Current controls warm ocean intrusions towards Totten Glacier, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1637, https://doi.org/10.5194/egusphere-egu21-1637, 2021.

Carolyn Branecky Begeman, Xylar Asay-Davis, and Luke Van Roekel

Large-eddy simulations are used to investigate boundary layer turbulence and its control on ice-shelf basal melt rates in Antarctic settings. We present simulations at relatively low thermal driving and low ice-shelf basal slopes, resulting in simulated melt rates from 10s cm/yr to several m/yr. Our results are broadly consistent with the linear relationships between far-field thermal driving and melt rate and between ice-shelf slope and melt rate reported by previous studies. The simulated thermal exchange coefficient is lower than recommended values; thermal exchange becomes less efficient as stratification increases.  In our simulations, shear production of turbulent kinetic energy outweighs buoyant production, as found below Larsen C Ice Shelf through recent microstructure measurements. We also find that turbulent intensity and melt rate vary significantly with the orientation between the ice-shelf slope and the far-field flow, even at low ice-shelf slopes. Our results suggest that numerical ocean models employing the standard ice-shelf melt parameterization will underestimate slope effects on ice-shelf melt rates even if they capture the mean buoyancy effects on boundary layer flow. The proposed slope effects would modify feedbacks between ocean circulation and ice-shelf geometry and tidal variability in ice-shelf melt rates.

How to cite: Begeman, C. B., Asay-Davis, X., and Van Roekel, L.: Turbulent dynamics and ice-shelf basal melt rates from large-eddy simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12846, https://doi.org/10.5194/egusphere-egu21-12846, 2021.

Pierre-Vincent Huot, Christoph Kittel, Thierry Fichefet, Nicolas Jourdain, and Xavier Fettweis

The mesoscale activity of the ocean – eddies and fronts with dimensions ranging from 1 to 100 km which populate the Southern Ocean – is thought to modulate air-sea interactions due to its imprint on the sea surface conditions. However, very little is known about the effects of the mesoscale activity on the exchanges between the ocean and the atmosphere of polar regions. The smaller deformation radius and the seasonal sea ice coverage suggest that air-sea interactions at the mesoscale could be very different at high latitude. In this study, we examine how mesoscale ocean eddies affect the state of the atmosphere and the air-sea interactions in polar regions. We use a regional, eddy resolving ocean-sea ice-atmosphere coupled model (NEMO-LIM 1/24° and MAR at 10 km) of the Southern Ocean off the Adélie Land sector, in East Antarctica. We describe the imprint of the eddies on the near surface atmosphere with specific attention to the effect of the sea ice. The role of feedbacks between the air, sea and ice is further investigated. A series of experiments is carried out where the signature of the mesoscale variability on the sea surface is filtered out before the exchange with the atmosphere model. We use these experiments to explore the role of the modulation of air-sea-ice interactions by the ocean mesoscale activity in the evolution of the ocean, sea ice and atmosphere near the Marginal Ice Zone on daily to seasonal time scales. This study advances our understanding of the poorly explored role of the eddies on air-sea interactions in the polar regions.

How to cite: Huot, P.-V., Kittel, C., Fichefet, T., Jourdain, N., and Fettweis, X.: Imprint of the ocean mesoscale activity on air-sea-ice interactions in a regional coupled model of the Adélie Land sector, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15670, https://doi.org/10.5194/egusphere-egu21-15670, 2021.

Ole Richter, David E. Gwyther, Matt A. King, and Ben K. Galton-Fenzi

Tides influence basal melting of individual Antarctic ice shelves, but their net impact on Antarctic-wide ice-ocean interaction has yet to be constrained. Here we quantify the impact of tides on ice shelf melting and the continental shelf seas by means of a 4 km resolution circum-Antarctic ocean model. Activating tides in the model increases the total basal mass loss by 57 Gt/yr (4 %), while decreasing continental shelf temperatures by 0.04 °C, indicating a slightly more efficient conversion of ocean heat into ice shelf melting. Regional variations can be larger, with melt rate modulations exceeding 500 % and temperatures changing by more than 0.5 °C, highlighting the importance of capturing tides for robust modelling of glacier systems and coastal oceans. Tide-induced changes around the Antarctic Peninsula have a dipolar distribution with decreased ocean temperatures and reduced melting towards the Bellingshausen Sea and warming along the continental shelf break on the Weddell Sea side. This warming extends under the Ronne Ice Shelf, which also features one of the highest increases in area-averaged basal melting (128 %) when tides are included. Further, by means of a singular spectrum analysis, we explore the processes that cause variations in melting and its drivers in the boundary layer over periods of up to one month. At most places friction velocity varies at tidal timescales (one day or faster), while thermal driving changes at slower rates (longer than one day). In some key regions under the large cold-water ice shelves, however, thermal driving varies faster than friction velocity and this can not be explained by tidal modulations in boundary layer exchange rates alone. Our results suggest that large scale ocean models aiming to predict accurate ice shelf melt rates will need to explicitly resolve tides.

How to cite: Richter, O., Gwyther, D. E., King, M. A., and Galton-Fenzi, B. K.: Tidal Modulation of Antarctic Ice Shelf Melting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8431, https://doi.org/10.5194/egusphere-egu21-8431, 2021.

Hannah Dawson, Adele Morrison, Veronica Tamsitt, and Matthew England

The Antarctic margin is surrounded by two westward flowing currents: the Antarctic Slope Current and the Antarctic Coastal Current. The former influences key processes near the Antarctic margin by regulating the flow of heat and nutrients onto and off the continental shelf, while together they advect nutrients, biological organisms, and temperature and salinity anomalies around the coastline, providing a connective link between different shelf regions. However, the extent to which these currents transport water from one sector of the continental shelf to another, and the timescales over which this occurs, remain poorly understood. Concern that crucial water formation sites around the Antarctic coastline could respond to non-local freshwater forcing from ice shelf meltwater motivates a more thorough understanding of zonal connectivity around Antarctica. In this study, we use daily velocity fields from a global high-resolution ocean-sea ice model, combined with the Lagrangian tracking software Parcels, to investigate the pathways and timescales connecting different regions of the Antarctic continental shelf with a view to understanding the timescales of meltwater transport around the continent. Virtual particles are released over the continental shelf, poleward of the 1000 metre isobath, and are tracked for 20 years. Our results show a strong seasonal cycle connecting different sectors of the Antarctic continent, with more particles arriving further downstream during winter than during summer months. Strong advective links exist between West Antarctica and the Ross Sea while shelf geometry in some other regions acts as barriers to transport. We also highlight the varying importance of the Antarctic Slope Current and Antarctic Coastal Current in connecting different sectors of the coastline. Our results help to improve our understanding of circum-Antarctic connectivity and the timescales of meltwater transport from source regions to downstream shelf locations. Furthermore, the timescales and pathways we present provide a baseline from which to assess long-term changes in Antarctic coastal circulation due to local and remote forcing.

How to cite: Dawson, H., Morrison, A., Tamsitt, V., and England, M.: Pathways and timescales of connectivity around the Antarctic continental shelf , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14011, https://doi.org/10.5194/egusphere-egu21-14011, 2021.

Wilma Huneke, Adele Morrison, and Andy Hogg

The basal melt rate of Antarctica's ice shelves is largely controlled by heat delivered from the Southern Ocean to the Antarctic continental shelf. The Antarctic Slope Current (ASC) is an almost circumpolar feature that encircles Antarctica along the continental shelf break in an anti-clockwise direction. Because the circulation is to first order oriented along the topographic slope, it inhibits exchange of water masses between the Southern Ocean and the Antarctic continental shelf and thereby impacts cross-slope heat supply. Direct observations of the ASC system are sparse, but indicate a highly variable flow field both in time and space. Given the importance of the circulation near the shelf break for cross-shelf exchange of heat, it is timely to further improve our knowledge of the ASC system. This study makes use of the global ocean-sea ice model ACCESS-OM2-01 with a 1/10 degree horizontal resolution and describes the spatial and temporal variability of the velocity field. We categorise the modelled ASC into three different regimes, similar to previous works for the associated Antarctic Slope Front: (i) A surface-intensified current found predominantly in East Antarctica, (ii) a bottom-intensified current found downstream of the dense shelf water formation sites in the Ross, Weddell, and Prydz Bay Seas, and (iii) a reversed current found in West Antarctica where the eastward flowing Antarctic Circumpolar Current impinges onto the continental shelf break. We find that the temporal variability of the Antarctic Slope Current varies between the regimes. In the bottom-intensified regions, the variability is set by the timing of the dense shelf water overflows, whereas the surface-intensified flow responds to the sub-monthly variability in the wind field.

How to cite: Huneke, W., Morrison, A., and Hogg, A.: Spatial and temporal variability of the Antarctic Slope Current in an eddying ocean-sea ice model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13947, https://doi.org/10.5194/egusphere-egu21-13947, 2021.

F. Alexander Haumann, Ruth Moorman, Stephen C. Riser, Lars H. Smedsrud, Ted Maksym, Annie P. S. Wong, Earle A. Wilson, Robert Drucker, Lynne D. Talley, Kenneth S. Johnson, Robert M. Key, and Jorge L. Sarmiento

In cold polar waters, temperatures sometimes drop below the freezing point, a process referred to as supercooling. However, observational challenges in polar regions limit our understanding of the spatial and temporal extent of this phenomenon. We here provide observational evidence that supercooled waters are much more widespread in the seasonally ice-covered Southern Ocean than previously reported. In 5.8% of all analyzed hydrographic profiles south of 55° S, we find temperatures below the surface freezing point (‘potential’ supercooling), and half of these have temperatures below the local freezing point (‘in-situ’ supercooling). Their occurrence doubles when neglecting measurement uncertainties. We attribute deep coastal-ocean supercooling to melting of Antarctic ice shelves, and surface-induced supercooling in the seasonal sea-ice region to winter-time sea-ice formation. The latter supercooling type can extend down to the permanent pycnocline due to convective sinking plumes—an important mechanism for vertical tracer transport and water-mass structure in the polar ocean.

How to cite: Haumann, F. A., Moorman, R., Riser, S. C., Smedsrud, L. H., Maksym, T., Wong, A. P. S., Wilson, E. A., Drucker, R., Talley, L. D., Johnson, K. S., Key, R. M., and Sarmiento, J. L.: Supercooled Southern Ocean Waters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-186, https://doi.org/10.5194/egusphere-egu21-186, 2020.

Matthis Auger, Jean-Baptiste Sallée, and Pierre Prandi

Subtle changes in the Southern Ocean subpolar ocean circulation patterns can lead to major changes in the global overturning circulation, as well as for floating ice-shelves with critical implications for global sea-level. It is therefore crucial to carefully understand Antarctic polar ocean circulation, but the lack of ocean observation has considerably blocked our advance in this field in the past.

In this study we benefit from a new high-resolution Sea Level Anomaly (SLA) product that has been specifically constructed to document sea-level in the ice-covered Southern Ocean. This product combines up to 3 satellite altimetry missions to map SLA data daily on an equal-area grid, including the ice-covered areas of the ocean from 2013 to 2019.

Results suggest that we can map ocean features with unprecedented resolution for the region. We characterize the main features of the subpolar Southern Ocean SLA and circulation seasonal cycle, being composed of three main modes of variability, significantly impacting the dynamics of the region. We explore how they are linked with atmospheric and sea-ice forcings. Dynamics at smaller scales are investigated, by identifying the properties of mesoscale variability where possible.

How to cite: Auger, M., Sallée, J.-B., and Prandi, P.: New insights into the ice-covered Southern Ocean circulation from multi-altimeter combination., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5615, https://doi.org/10.5194/egusphere-egu21-5615, 2021.

Kenza Himmich, Martin Vancoppenolle, Gurvan Madec, Jean-Baptiste Sallée, and Marion Lebrun

Changes in open water season duration in the Southern Ocean have been documented, with decreases in the Weddell and Ross sectors and increases west of the Antarctic Peninsula. Yet, not much is known on the mechanisms of changes. To progress understanding, we revisit Antarctic sea ice seasonality diagnostics (ice-free season duration, advance and retreat) from three satellite products. We diagnose their evolution at short and long time-scales, following the methodology of Lebrun et al (2019). We also put them in the context of oceanographic changes, as diagnosed from in situ observations. Preliminary analysis suggests that over the last decade, there was overall little change in the spatial distribution of the trends and of their magnitude, regardless of the used satellite product. Trends in all three diagnostics have slightly weakened but are still regionally significant. The ice-free season is still lengthening in the Bellingshausen and Amundsen Seas and shortening in the Weddell and Ross Seas. Where trends are significant, trends in ice advance date generally exceed those in ice retreat date. However, inter-annual variations in ice retreat date are larger than those in ice advance date. We will investigate possible reasons of this conundrum. We will also provide more analysis on possible links with water column stratification and surface energy budget. We hope such understanding will help us to better constrain the future evolution of Antarctic sea ice, and its impacts on marine biology and chemistry.

How to cite: Himmich, K., Vancoppenolle, M., Madec, G., Sallée, J.-B., and Lebrun, M.: Changes in Antarctic sea ice seasonality over the last 4 decades, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14814, https://doi.org/10.5194/egusphere-egu21-14814, 2021.