Winter Observations of Platelet Ice Growth and Oceanographic Conditions in McMurdo Sound, Antarctica

Almost all that is known about sea ice growing attached to the Antarctic landmass has been measured in the spring and summer when the sea ice has grown to over 1~m thick. Platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth in the sea water or loosely attached to the ice water interface. It appears to form in sea water cooled below its freezing point, and was first observed in McMurdo Sound by scientists of Scott's Discovery Expedition of 1901{\textendash}1904. It is known that the formation of platelet ice is linked to the proximity of ice shelves, however the exact freezing mechanism is a matter of some conjecture. In this study we have spent the winter months (February{\textendash}September) 2003 on the sea ice in McMurdo Sound simultaneously measuring ice and oceanographic conditions in order to increase our understanding of platelet ice formation. Early results indicate the presence of a relatively warm water layer in April which has been replaced by a stable mixed layer by June, with periodic episodes of cold water intrusions into the mixed layer. The water near the ice - water interface was found to be consistently supercooled from July onwards. Platelet growth was observed to be episodic and correlated with times of low tidal flow. Ice coring data reveals that the first layer of platelet ice incorporated into the sea ice sheet did not appear until the ice was around 0.7~m thick, even though platelet ice was observed on foreign bodies in the water column when the ice was significantly thinner.