Signatures of supercooling: McMurdo Sound platelet ice

Near ice shelves around Antarctica the ocean becomes supercooled and has been observed to carry small suspended ice crystals. Our measurements demonstrate that these small crystals are persistently present in the water column beneath the winter fast ice, and when incorporated in sea ice they reduce the mean grain size of the sea-ice cover. By midwinter, larger ice crystals below the ice/water interface are observed to form a porous sub-ice platelet layer with an ice volume fraction of 0.25 +/- 0.06. The magnitude and direction of the oceanic heat flux varied between (5 +/- 6) W m(-2) (upwards) and (-15 +/- 10) W m(-2) (downwards) in May, but by September it settled between (-6 +/- 2) and (-11 +/- 2) W m(-2). The negative values imply that the ocean acts as a heat sink which is responsible for the growth of 12% of the ice thickness between June and September. This oceanic contribution should not be ignored in models of Antarctic sea-ice thickness close to an ice shelf.