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Water mass transformation (WMT) around the Antarctic margin controls Antarctica Bottom Water formation and the abyssal limb of the global meridional overturning circulation, besides mediating ocean-ice shelf exchange, ice sheet stability and its contribution to sea level rise. However, the mechanisms controlling the rate of WMT in the Antarctic shelf are poorly understood due to the lack of observations and the inability of climate models to simulate those mechanisms, in particular beneath the floating ice shelves. We used a circum-Antarctic ocean-ice shelf model to assess the contribution of surface fluxes, mixing, and ocean-ice shelf interaction to the WMT on the continental shelf. The salt budget dominates the WMT rates, with only a secondary contribution from the heat budget. Basal melt of ice shelves drives buoyancy gain at lighter density classes (27.2 27.6). We found a large sensitivity of the WMT rates to model horizontal resolution, tides and topography within the Filchner-Ronne, East and West Antarctica ice shelf cavities. In the Filchner-Ronne Ice Shelf, an anticyclonic circulation in front of the Ronne Depression regulates the rates of basal melting/refreezing and WMT and is substantially affected by tides and model resolution. Model resolution is also found to affect the Antarctic Slope Current in both East and West Antarctica, impacting the on-shelf heat delivery, basal melt and WMT. Moreover, the representation of the ice shelf draft associated with model resolution impacts the freezing temperature and thus basal melt and WMT rates in the East Antarctica. These results highlight the importance of resolving small-scale features of the flow and topography, and to include the effects of tidal forcing, to adequately represent water mass transformations on the shelf that directly influence the abyssal global overturning circulation.

Strong offshore wind events (SOWEs) occur frequently near the Antarctic coast during austral winter. These wind events are typically associated with passage of synoptic- or meso-scale cyclones, which interact with the katabatic wind field and affect sea ice and oceanic processes in coastal polynyas. Based on numerical simulations from the coupled Finite Element Sea-ice Ocean Model (FESOM) driven by the CORE-II forcing, two coastal polynyas along the East Antarctica coast––the Prydz Bay Polynya and the Shackleton Polynya are selected to examine the response of sea ice and oceanic properties to SOWEs. In these polynyas, the southern or western flanks of cyclones play a crucial role in increasing the offshore winds depending on the local topography. Case studies for both polynyas show that during SOWEs, when the wind speed is 2–3 times higher than normal values, the offshore component of sea ice velocity can increase by 3–4 times. Sea ice concentration can decrease by 20–40%, and sea ice production can increase up to two to four folds. SOWEs increase surface salinity variability and mixed layer depth, and such effects may persist for 5–10 days. Formation of high salinity shelf water (HSSW) is detected in the coastal regions from surface to 800 m after 10–15 days of the SOWEs, while the HSSW features in deep layers exhibit weak response on the synoptic time scale. HSSW formation averaged over winter is notably greater in years with longer duration of SOWEs. [ABSTRACT FROM AUTHOR]

Polynyas are an important factor in the Antarctic and Arctic climate, and their changes are related to the ecosystems in the polar regions. The phenomenon of polynyas is influenced by the combination of inherent persistence and dynamic factors. The dynamics of polynyas are greatly affected by temporal dynamical factors, and it is difficult to objectively reflect the internal characteristics of their formation. Separating the two factors effectively is necessary in order to explore their essence. The Special Sensor Microwave/Imager (SSM/I) passive microwave sensor has been making observations of Antarctica for more than 20 years, but it is difficult for existing current sea ice concentration (SIC) products to objectively reflect how the inherent persistence factors affect the formation of polynyas. In this paper, we proposed a long-term multiple spatial smoothing method to remove the influence of dynamic factors and obtain stable annual SIC products. A halo located on the border of areas of low and high ice concentration around the Antarctic coast, which has a strong similarity with the local seabed in outline, was found using the spatially smoothed SIC products and seabed. The relationship of the polynya location to the wind and topography is a long-understood relationship; here, we quantify that where there is an abrupt slope and wind transitions, new polynyas are best generated. A combination of image expansion and threshold segmentation was used to extract the extent of sea ice and coastal polynyas. The adjusted record of changes in the extent of coastal polynyas and sea ice in the Southern Ocean indicate that there is a negative correlation between them.

An accurate representation of Antarctic coastal sea ice in Earth System Models is important as it affects the rate of Antarctic Bottom Water formation. Here we investigate the effect of using relatively high-resolution (∼20 km) near-surface wind fields of two different operational analyses (from the European Centre for Medium-Range Weather Forecasts (ECMWF), and the Antarctic Mesoscale Prediction System (AMPS)) to force a sea-ice - ocean model. We focus on the wind impact on coastal polynya formation in winter. Even though the horizontal resolution of both analyses is about the same, striking differences emerge in simulated coastal sea ice and thus the representation of coastal polynyas and net freezing rates. In AMPS, offshore winds are up to 10 m s −1 stronger than those of ECMWF, and mostly overestimated compared to weather station data. This leads to major differences in simulated coastal ice drift, concentration, and thickness, as well as discrepancies in the annual net freezing rate of up to 5 m per year. To identify the reasons for the discrepancies in the coastal wind fields of the analyses we scrutinized their orography, boundary-layer stratification, sea-ice boundary conditions, large-scale atmospheric circulations, data assimilation and initialization schemes, and their parameterization of subgrid-scale orographic effects. The final factor explained most of the discrepancies along steep coastlines, whereas the differences in boundary-layer stratification are mainly a consequence of the differences in surface winds. The data assimilation and initialization procedure biased the AMPS wind fields toward those of the NCEP analyses, which probably overestimate offshore winds as a result of coarse-resolution orography. The sea-ice boundary conditions and the large-scale circulations turned out to have a minimal effect. [ABSTRACT FROM AUTHOR]

Polynyas are an important factor in the Antarctic and Arctic climate, and their changes are related to the ecosystems in the polar regions. The phenomenon of polynyas is influenced by the combination of inherent persistence and dynamic factors. The dynamics of polynyas are greatly affected by temporal dynamical factors, and it is difficult to objectively reflect the internal characteristics of their formation. Separating the two factors effectively is necessary in order to explore their essence. The Special Sensor Microwave/Imager (SSM/I) passive microwave sensor has been making observations of Antarctica for more than 20 years, but it is difficult for existing current sea ice concentration (SIC) products to objectively reflect how the inherent persistence factors affect the formation of polynyas. In this paper, we proposed a long-term multiple spatial smoothing method to remove the influence of dynamic factors and obtain stable annual SIC products. A halo located on the border of areas of low and high ice concentration around the Antarctic coast, which has a strong similarity with the local seabed in outline, was found using the spatially smoothed SIC products and seabed. The relationship of the polynya location to the wind and topography is a long-understood relationship; here, we quantify that where there is an abrupt slope and wind transitions, new polynyas are best generated. A combination of image expansion and threshold segmentation was used to extract the extent of sea ice and coastal polynyas. The adjusted record of changes in the extent of coastal polynyas and sea ice in the Southern Ocean indicate that there is a negative correlation between them. [ABSTRACT FROM AUTHOR]