| Summary: | A cold water mass, termed Ice Shelf Water, appears to exist for much of the year beneath the sea ice cover in western McMurdo Sound, Antarctica, yet it is present for only a few months in the east. In an east–west transect taken 3km in front of the McMurdo Ice Shelf edge during spring tide in late November 2011 and repeated during neap tide in early December, this water mass was observed throughout the entire water column at the two, of four, westernmost sites. In situ supercooling was observed at all sites and, at the coldest site, was measured to depths of 60–73 m. Ice Shelf Water alters the sea ice fabric through the introduction of millimetre-sized ice crystals, termed frazil ice, that grow in supercooled water. Four first-year sea ice cores from the transect are analysed to determine the extent of the altered sea ice fabric, platelet ice, to provide a time-history of oceanographic conditions during the 2011 austral winter. The onset of platelet ice is delayed to greater depths in the core with distance eastward along the transect, which suggests that the lateral extent of Ice Shelf Water flowing into McMurdo Sound from beneath the ice shelf expands from the west throughout winter. A steady-state, one-dimensional Ice Shelf Water plume model is adapted for McMurdo Sound to predict the evolution of this supercooled water emerging from beneath the McMurdo Ice Shelf at the site where the coldest water was observed. A third oceanographic transect following the likely direction of this supercooled water provides initial model conditions and, in conjunction with historical data, downstream validation. Application of the plume model under sea ice is reliant on the addition of an ambient current, which moves parallel to the plume and accounts for currents that are not driven by thermohaline processes within the ice shelf cavity. The RMS tidal velocity, the ambient current velocity, the drag coefficient and parameters affecting the nucleation of frazil ice each affect the size distribution of suspended frazil ice crystals. These parameters are the key physical controls on the survival of in situ supercooled water as it travels northwards away from the ice shelf. This survival is predicted from the average of 26 different model runs along the approximately 250km path between the McMurdo Ice Shelf and the Drygalski Ice Tongue. Starting at 65m, the thickness of the in situ supercooled layer beneath the ice-ocean interface decreases to 11–6m and 4–3m at distances from the ice shelf of 100km and 200 km, respectively.
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