Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctica - Sea Ice

The area of sea ice around Antarctica covers about twice the area of the Antarctic Ice Sheet, and the sea ice acre increases 5-fold annually (Mullan & Hickman, 1990), and this variability of the area of sea ice cover has been found to be one of he most significant factors in the regulation of the energy balance of the ocean and atmosphere of the Southern Hemisphere (Mullan & Hickman, 1990; Martinson & Iannuzzi, 1998). The large difference between the albedo of sea ice and that of the ocean surface allows the sea ice to act as a barrier to energy exchange between the atmosphere and the ocean. Weather patterns and cyclogenesis around Antarctica are affected by the location of the edge of the sea ice in any particular year, though the relationship between sea ice and weather patterns is not well understood (Schwerdtferger & Kachelhoffer, 1973; Streten &Pike, 1980; Mullan & Hickman, 1990). The extent of Antarctic sea ice regulates the penetration of light in surface waters, the radiance flux, so regulates primary productivity.

Antarctic sea ice differs from Artic sea ice in physical and behavioural characteristics, such as the Arctic sea ice being landlocked, the Arctic basin generally confining the sea ice, which favours the formation of pressure ridges and sea ice that is multilayered (Wadhams, 1986). Antarctic sea ice is divergent in its general behaviour around all parts of Antarctica (Wadhams, 1986). Wind also affects the development of sea ice, having a tendency to impede its development.

There is a very large change in the area of Antarctic Sea Ice in the southern spring from about 3 million km² to 20 million km² in the southern autumn (Mullan & Hickman, 1990). As about 85 % of sea ice melts annually most of the winter ice is first-year ice of roughly relatively uniform thickness ranging from 1 - 3 m, averaging about 1.5 m. Ice in sheet that is permanently attached firmly to the coast may reach thicknesses of several metres.

There is a tendency for the dispersal of sea ice and the opening of new leads, cracks in the sea ice cover, by the action of winds, currents and storms, with polynyas, areas of open water in the midst of pack ice, that are believed to possibly be maintained by katabatic winds or upwelling of currents that inhibit the formation of ice. The presence of leads and polynyas make the pack ice more navigable for ships that have been strengthened to withstand the pressure imposed by the ice, though the these same factors also make the passage through sea ice more hazardous to shipping. Sea ice is capable of moving quickly, and often unpredictably, as it is driven by winds and surface currents. Several ships have been trapped and crushed by this moving sea ice, Deutschland, Endurance, Aurora, Antarctic, and Southern Quest. Antarctic sea ice has a large degree of variability on a seasonal and a yearly basis.

The amount of sea ice at any particular location can vary over a wide range, though a decrease is balanced by an increase in another area, with the result that the overall amount of ice may not vary greatly between years (Mullan & Hickman, 1990; Parkinson, 1998). The extent of the area covered by sea ice from the southern summer to the southern winter increases by a factor of at least 3 times up to 30 times. The Ross Sea and the Weddell Sea are the places where the largest annual fluctuations occur, the sea ice factories of Antarctica (Lemke et al., 1981; Ropelewski, 1983).

In the Bellingshausen Sea, in the South Indian Ocean, and in the seas around the Antarctic Peninsula, the smallest changes of sea ice cover occur. Sea ice movement generally appears to correspond with prevailing winds and currents, indicating that in the dynamics of sea ice on a large scale around Antarctica advection of sea ice has a major role (Lemke et al., 1981).

The interannual fluctuations of Antarctic maximum sea-ice coverage are small, about 20 %, though the minimum of Antarctic sea ice can vary by more than 100 % (Ropelewski, 1983). In the Ross Sea the minimum sea ice cover is well known for its extreme variability from one southern summer to the next (Keys, 1990). There is a tendency for extremes to occur at the same time every year, in February for the annual minimum sea ice cover and the maximum sea ice cover occurs in either August or September

Sources & Further reading

1. Anderson, John B., 1999, Antarctic Marine Geology, Cambridge University Press