Sea ice

Sea ice
          The ice-covered continent of Antarctica is itself surrounded by an ice-covered sea for most of the year, which more than doubles the area of Antarctica in the winter. The sea ice reaches its maximum extent in September, when it covers 20 million km² of the Southern Ocean and recedes to a minimum of 4 million km² at the end of February. Freezing is most rapid in May and June when the ice edge advances northwards by as much as 4km a day. As it spreads the ice also thickens, by the freezing of water underneath and the addition of snow on top. By midwinter it can reach a thickness of 3-4m and form floes many kilometres across.

The sea ice varies in density. Within the total area there may be as much as 20% open water in pools known as polynyas, and in channels between floes. The ice is constantly moving because of storms and ocean currents. Floes can be rafted up on top of each other, or edge to edge in pressure ridges, making it very dangerous for ships.

Sea ice influence the climate

          The formation of sea ice has a major effect on Antarctica’s climate, though the relationship between them is not yet well understood. Like the continental ice sheet, sea ice reflects most of the sun’s energy back into the atmosphere. So its presence in spring delays the warming effect of the returning sun and its re-formation in autumn hastens the cooling of the continent. It also makes the air much drier by preventing the exchange of water vapour between the ocean and the atmosphere. This is one reason why so little moisture reaches the continent, and so little snow falls in the centre. On a broader scale, changes in sea ice influence ocean and weather conditions, and these in turn affect the distribution of ice. The amount of sea ice varies greatly from year to year and area to area as a result.

Icebergs

Dry-valleys