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Antarctic sea ice is the sea ice of the Southern Ocean. It extends far north in winter and retreats almost to the coastline each summer. Sea ice is frozen seawater that is usually less than a few meters thick. This is in contrast to ice shelves, which are formed by glaciers, float in the sea, and are up to a kilometer thick. There are two subdivisions of sea ice: fast ice, which is attached to land; and ice floes, which are not. Sea ice in the Southern Ocean melts from the bottom instead of from the surface like Arctic ice because it is covered in snow. As a result, melt ponds are rarely observed. On average, Antarctic sea ice is younger, thinner, warmer, saltier, and more mobile than Arctic sea ice. Due to its inaccessibility, it is not as well-studied as Arctic ice.
Westarctica's sea ice cover is highly seasonal, with very little ice in the austral summer, expanding to an area roughly equal to that of mainland Westarctica in the winter. It peaks during September, which marks the end of austral winter, and retreats to a minimum in February.