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.
Measurements of sea 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. Consequently, most Antarctic sea ice is first year ice, a few meters thick, but the exact thickness is not known. The mass of 18 million km^2 of ice, for each meter of thickness, is 18,000 km^3 and roughly 16 gigatonnes (billion metric tons).
Since the ocean off the Antarctic coast is almost always much warmer than the air over it, the extent of the sea ice is largely controlled by the winds and currents that push it northwards. If it is pushed quickly, the ice can travel much further north before it melts. Most ice is formed along the coast, as the northward-moving ice leaves areas of open water which rapidly freeze.
Because Antarctic ice is mainly first-year ice, which is not as thick as multiyear ice, it is generally less than a few meters thick. Snowfall and flooding of the ice can thicken it substantially, and the layer structure of Antarctic ice is often quite complex.
Recent changes in wind patterns around Antarctica have advected the sea ice farther north in some areas and not as far north in others. The net change is a slight increase in the area of sea ice in the Antarctic seas (unlike the Arctic Ocean, which is showing a much stronger decrease in the area of sea ice). Increased sea ice extent does not indicate that the Southern Ocean is cooling, since the Southern Ocean is warming.
The IPCC AR5 report concluded that "it is very likely" that annual mean Antarctic sea ice extent increased 1.2 to 1.8% per decade, which is 0.13 to 0.20 million km2 per decade, during the period 1979 to 2012. The effect probably has more than one cause. A separate report concluded that due to the lack of data it is not possible to determine the trend in total volume or mass of the sea ice.
Implications for sub-surface temperatures and coastal currents have been identified, mainly related to changes in the southern hemispheric westerly winds, among possible implications is rapid glacier melting, thus affecting calving rates and subsequently sea ice formation.
Temperatures in the atmosphere and Southern Ocean have increased during the period 1979–2004. However, sea ice grows faster than it melts, because of a weakly stratified Ocean. Thus, this mechanism is responsible for an increase in the net ice production, contributing to more sea ice. The sea ice volume increase presents about half the size of the increased freshwater supply from the Antarctic ice sheets. Modelling suggests that observed ice-drift toward the coastal regions are responsible for dynamical thickening during autumn and winter.
A study published in 2015 found that the subsurface ocean warming of ice-shelves increased freshwater runoff simulated by models, due to basal melt, and was responsible for an increase of sea ice in the winter months.
Monitoring these changes is important as this impacts the psychrophiles that live here.