The observed evolution of Antarctic sea ice extent is marked by an abrupt decrease in 2016/2017. After several years of gradual increase culminated in an all-time record high in 2014/2015, a rapid decline in 2016 led to an unprecedented minimum, and unusual low extents have been observed since then. Even though this record has now been beaten, the sudden drop from extreme high values to a minimum in less than two years is unique to this event, whose dynamics are still uncertain. While it was likely triggered by anomalous atmospheric conditions in the prior months, the contribution of the ocean conditions, as a preconditioning which amplified the response of the sea ice or helped to maintain the anomalies for a longer period, still needs to be quantified. 

To evaluate the respective influences of the atmosphere and ocean on this 2016 event, we have performed sensitivity experiments using the circum-Antarctic fully coupled model (ice-sheet–ocean–sea-ice–atmosphere) PARASO. First, a control experiment with the model forced by lateral boundary conditions derived from observations (ERA5 in the atmosphere, ORAS5 in the ocean) is performed over the period 1985-2018. In such a set-up, the model correlates well with the observations and is able to capture the 2016 drop. Then, the model is integrated again between 2016 and 2018 with the same atmospheric boundary forcing, but with different initial conditions in the ocean: namely, ocean conditions from previous years in the control run are used as initial state in 2016 in the sensitivity experiments, producing an ensemble of 5 members.

Preliminary results indicate that the 2016 drop is captured by all members, suggesting the atmospheric boundary forcing as the dominant driver and confirming that the event is induced by large-scale atmospheric dynamics. However, some variability is present in the amplitude and timing of the drop, as well as in the evolution and recovery of the sea ice in the following months, which may be influenced by the different states of the ocean. Related processes are further investigated by examining different oceanic and atmospheric fields, focussing on the role of ocean preconditioning by identifying the differences between the members and their impact. 

How to cite: Mezzina, B., Goosse, H., Huot, P.-V., Marchi, S., and Van Lipzig, N.: Contributions of atmospheric forcing and ocean preconditioning in the 2016 Antarctic sea ice extent drop, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-419, https://doi.org/10.5194/egusphere-egu23-419, 2023.

The impacts of upper-ocean mixing on primary productivity are complex and range from an entrainment of nutrients to modulating light limitations. Sea ice in turn plays an important role in determining mixing conditions through its cycles of formation and melt, and by moderating wind forcing. With sea ice conditions in the Southern Ocean projected to undergo large changes over the course of the century, understanding the relationship between sea ice and upper-ocean mixing is crucial for understanding the impacts of climate change on biological production in this region. Due to the inaccessibility of sea ice-covered waters however, mixed layer depth observations are often not available at a high temporal and spatial resolution. Here we present an analysis of sea ice-mixed layer depth relationships during a 40-year regional ocean-sea ice simulation of the  West Antarctic Peninsula (WAP) and Bellingshausen Sea, a highly biologically productive region of global importance. The relationship between winter sea ice and spring mixed layer depth shows clear differences on and off the WAP continental shelf, with decadal variations in the location of the boundary between negative and positive correlations. Potential mechanisms causing this effect are considered in detail, including the nonlinear relationship between sea ice cover and turbulent mixing, the transport of sea ice within the region, and a difference in the timing of the sea ice seasonal cycle between the two regions. The transport of warm Circumpolar Deep Water onto the shelf is also discussed. The presented findings have implications for the spatial distribution of primary producers in a more ice-free future WAP.

How to cite: Bischof, M., Goldberg, D., Henley, S., and Fraser, N.: Spatial variations in the sea ice-mixed layer depth relationship in the West Antarctic Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-480, https://doi.org/10.5194/egusphere-egu23-480, 2023.

The local temperarturecannot explain the inter-annual variation in δ¹⁸O_precip in the coastal Antarctic in past few decades. To understand this enigmatic variation, we have used long-term modern δ¹⁸O_precip value of three coastal Antarctic sites. Using the δ¹⁸O-d-excess relationship and modelled δ¹⁸O value of vapor at source, we have shown that δ¹⁸O_precip inherits the signature of moisture source parameters (MSPs). Furthermore, the wavelet analysis suggests that the variation in the MSPs impacts the seasonal cycle of δ¹⁸O_precipwhich lead to disparity in the seasonal isotope-temperature relationship. The Southern Ocean surface stratification, due to increase in the freshwater flux by glacier melting, led to alignment of MSPs in such a manner that altogether significantly lowered the isotopic composition of initially formed vapor, which is reflected in δ¹⁸O_precip at inter-annual scale.Our observations suggest that the palaeothermometry will underestimate the Antarctic temperature change for the periods characterized by warming and high glacier-melt.

How to cite: Sanyal, P. and ajay, A.: The Imprint of Southern Ocean Stratification on the Isotopic Composition of Antarctic Precipitation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12482, https://doi.org/10.5194/egusphere-egu23-12482, 2023.

Deep convection from dense water formation in the Southern Ocean drives the lower limb of the global overturning circulation, sequesters anthropogenic heat and carbon from the atmosphere and ventilates the abyssal ocean. The rate and location of dense water formation and its trajectory to the deep ocean is determined by changes in ocean density and stratification and influenced by ocean-ice-atmosphere interactions such as polynya openings (both open-ocean and coastal), sea ice formation and ice shelf collapse.

Signatures of deep convection are logistically difficult to measure. The highest-quality observations of water column density are currently provided by in-situ moorings and profiles from Argo floats or CTDs mounted on elephant seals (MEOP data[1]), but these data are spatially and temporally sparse. Satellite products providing complete coverage of high latitudes at regular repeat periods are becoming more readily available and offer an alternative method for capturing changes the extent and variability of deep-water formation in polar regions.

We compute steric height anomalies in the Southern Ocean from 2002-2018 using a novel method combining satellite altimetry and gravimetry data. We use these to explore density changes, focussing on deep water formation regions including the Weddell and Ross seas, the Adelie coastline and Amery shelf region, and infer multi-decadal changes in deep convective processes. Long term changes in the steric height anomalies can be linked to recorded ocean-ice events, such as the 2010 collapse of the Mertz glacier, the 2017 Maud Rise polynya and recent recovery of Ross Sea Bottom Water. The satellite-derived steric height anomalies have been validated against in-situ Argo and MEOP profiles and show good agreement in regions with a high data density.

How to cite: Cocks, J., Silvano, A., Marzocchi, A., Naveira-Garabato, A., and Hogg, A.: Multi-decadal trends in Antarctic deep convection from satellite-derived steric height, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5080, https://doi.org/10.5194/egusphere-egu23-5080, 2023.

The Antarctic Slope Front and the associated Antarctic Slope Current shield the continental shelves in East Antarctica from offshore warm water that holds the potential for considerable ice shelf melting and, consequently, sea level rise. Here, we present two-year-long records of temperature, salinity, and velocity (2019-2020), obtained from two oceanographic moorings located within the slope front/current over bathymetries of around 1000m and 2000m slightly east of the prime meridian. The two-year data record reveals clear differences in the seasonality of the thermocline depth and the baroclinicity of the current between the deep and shallow mooring locations. In combination with climatologies of hydrography and satellite-derived surface geostrophic currents, we use the new data to refine the baroclinic seasonality of the ASF. The results highlight the role of surface buoyancy fluxes via seasonal sea ice melt and freeze. Finally, the slope current is shown to control flow into and out of the cavity of the close-by Fimbulisen Ice Shelf on seasonal time scales depending on the orientation of the entrances of the cavity. Our findings contribute to a better understanding of the processes controlling the slope front/current seasonality and resulting inflow into the East-Antarctic ice shelf cavities.

How to cite: Lauber, J., de Steur, L., Hattermann, T., and Darelius, E.: Observed Seasonal Evolution of the Antarctic Slope Current System at the Coast of Dronning Maud Land, East Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12791, https://doi.org/10.5194/egusphere-egu23-12791, 2023.

The Filchner Trough on the continental shelf in the southern Weddell Sea is a region of great importance for the water mass exchange between the open ocean and the Filchner Ronne Ice Shelf cavity. Observations of the last 20 years and modelling studies show seasonal variations and longer lasting pulses of warm water intruding into the trough and reaching the Filchner Ice Shelf front. In this study, we evaluate the evolution of these intrusions in four climate scenarios defined for CMIP6 and simulated with the AWI Climate Model. We show that a warming climate will lead to more frequent pulses in the mitigation scenarios SSP1-2.6 and SSP2-4.5. For the high emission scenarios SSP3-7.0 and SSP5-8.5, hydrography in Filchner Trough will shift to a substantially warmer state during the second half of the 21st century with a temperature rise of 2°C in the trough until 2100. We demonstrate that the system‘s tipping into a warmer state is primarily caused by changes in the local sea ice formation and the depth of the Antarctic Slope Front. Our results show that a regime shift can be avoided by reaching the 2°C climate goal.

How to cite: Teske, V., Timmermann, R., and Semmler, T.: Evolution of warm water intrusions in the Filchner Trough, Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11864, https://doi.org/10.5194/egusphere-egu23-11864, 2023.

Ocean circulation patterns along the continental shelf in the Australian Antarctic Basin remain poorly understood due to the scarcity of in-situ observations and limited modeling studies. In this dynamically complex and climatically important region, the Ross Gyre, Antarctic Slope Current, and Antarctic Circumpolar Current converge just offshore of the George V Land continental shelf. If warm deep water could access the continental shelf and increase basal melt rates along the George V Land coastline, marine-terminating glaciers in the region could retreat and threaten the stability of the vast Wilkes Subglacial Basin. Here, we explore potential pathways for warm deep water to access the shelf along the George V Land coastline using output from the Southern Ocean State Estimate (SOSE) model. We use the SOSE output to map bottom temperatures and identify where warm bottom water could come close to the grounding line due to bathymetric steering. While SOSE provides observationally constrained hydrographic estimates along the George V Land continental shelf, there are substantial discrepancies between the model’s estimates and observations. Most notably, SOSE does not reproduce the dense, high salinity shelf waters observed in the region. SOSE is a model-generated best fit to Southern Ocean observations, so biases could be present in sparsely sampled regions like this one. To further examine the dynamics of this region, we also present preliminary results from an idealized ocean circulation model that explores the sensitivity of cross-shelf heat transport to changes in local heat and wind forcing.

How to cite: Dawson, E. and Wilson, E.: Exploring oceanic heat pathways along the George V Land continental shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10081, https://doi.org/10.5194/egusphere-egu23-10081, 2023.

Relatively warm modified Circumpolar Deep Water accesses the southern Ross Sea steered by bathymetric troughs. There it provides nutrients to support phytoplankton blooms in spring, and heat to melt the Ross Ice Shelf.  Here we present new observations collected by two ocean gliders during December 2022 and January 2023, in the Ross Sea polynya adjacent to the Ross Ice Shelf.  The gliders surveyed the full depth of the water column (about 700 m depth) carrying sensors measuring temperature, salinity, dissolved oxygen, chlorophyll fluorescence and optical backscatter, and also yielded estimates of the dive-average-current which we use to reference geostrophic shear.  Repeated quasi-meridional high resolution (profiles approximately every 1.5 km) sections along the sea ice edge allow analysis of the spatial and temporal variability, as well capturing the dynamic field of eddies, tides and coastal current. We discuss the influence of the sea ice and the atmospheric forcing on the water properties. One glider made an unauthorised foray beneath the Ross Ice Shelf, surveying the upper 200 m of the water column in high resolution beneath an ice shelf base at about 80 m depth. We observe solar-warmed water penetrating beneath the ice shelf with significant signatures of elevated chlorophyll fluorescence and optical backscatter, and low oxygen and salinity. We discuss the likely mechanisms involved in advecting this water beneath the ice shelf and its importance for physical and biogeochemical processes of ocean-ice interaction.

How to cite: Heywood, K. J., Portela, E., Smith, W., Damerell, G., Sheehan, P., and Meyer, M.: Spatial and temporal variability of water masses in the Southern Ross Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9657, https://doi.org/10.5194/egusphere-egu23-9657, 2023.

The melting of the Antarctic ice shelves becomes critical in a warming climate. However, the ocean component of climate models do not consider the effect of the Antarctic ice-shelf cavities. Here, we implement ice-shelf cavity features into the new AWI Earth system model (AWI-ESM2) based on unstructured meshes allowing for varying resolution in a multi-scale approach. We create a global mesh explicitly resolving the Antarctic ice-shelf cavities and evaluate the effect of the cavities under global warming scenarios. The new mesh provides a more realistic freshwater input into the Antarctic coast and the Southern Ocean. In an extreme warming climate scenario, the melting of the Antarctic ice shelves gets stronger by a factor of ~3, affecting the North Atlantic salinity and the overturning circulation. We conclude that the incorporation of ice-cavity feedback is essential to study the past, present, and future. Our approach might be seen as a prototype for the next phase of the Coupled Model Intercomparison Project.

How to cite: Song, P., Scholz, P., Knorr, G., Sidorenko, D., Timmermann, R., and Lohmann, G.: The ice-cavity feedback in an Earth system model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6731, https://doi.org/10.5194/egusphere-egu23-6731, 2023.