OS1.7

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.

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.

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.

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.

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.

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.

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.

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.

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.

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, 2021.

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.

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.