Platelet ice distribution in Antarctic sea ice and its implications for ocean - ice shelf interaction

Abstract:

Ice shelf basal melting freshens and cools the fluid in the ice shelf-ocean boundary layer, producing Ice Shelf Water (ISW). The potential temperature of ISW is below the surface freezing point. Antarctic sea ice that has been affected by supercooled Ice Shelf Water (ISW) has a unique crystallographic structure and is called platelet ice. We have synthesized platelet ice observations to construct a continent-wide map of the winter presence of ISW at the ocean surface. Where suitable observations exist, oceanic heat fluxes are derived from sub-ice platelet layer measurements and these are shown in the figure. They demonstrate that in some regions of coastal Antarctica, supercooled ISW drives a negative oceanic heat flux of approximately -30 Wm^-2 that persists for several months during winter. This heat flux from the sea ice to the ocean significantly increases the sea ice thickness close to an ice shelf. In other regions, particularly where the thinning of ice shelves is believed to be greatest, platelet ice is not observed. The most extensive dataset, which includes our new results, dates back to 1902 and extends north of the combined Ross and McMurdo Ice Shelf front in the southern Ross Sea. Here the surface water is held just below its freezing point as it enters McMurdo Sound from beneath the McMurdo Ice Shelf. Despite a more recent decrease in ocean salinity consistent with observations in the south-western Ross Sea, there has been no detectable change in the volume or temperature of this supercooled ISW under sea ice since the early twentieth century. The inclusion of platelet ice into first year sea ice is an annual process. Hence it will respond immediately to changes in the sub-ice shelf circulation pattern and its export of supercooled water, emphasizing the urgent need for careful, Antarctic-wide monitoring.