5.1 Ocean Circulation

As with the Northern Hemisphere, the distribution of sea ice around the Antarctic continent is influenced by the winds, ocean currents, and temperatures in the southern polar region (Zwally et al., 1983; Gloersen et al., 1992). The mean surface wind fields consist of clockwise wind patterns in the Weddell and Ross Seas, strong westerlies (i.e., winds from the west, blowing eastward) in the midlatitudes, and weaker easterlies near Antarctica (Zwally et al., 1983). The ocean circulation near Antarctica is dominated by the Antarctic circumpolar current, a strong eastward flowing current whose position is correlated with the maximum eastward wind stress associated with the midlatitude westerlies (Zwally et al., 1983; Gloersen et al., 1992). Closer to the continent, the ocean circulation is dominated by the Antarctic divergence, a region of westward flowing coastal currents and oceanic upwelling driven by the prevailing easterly winds near the continent and the midlatitude prevailing westerlies further north. The distribution of temperature is governed by the location of the Antarctic convergence, an oceanic front that nominally lies between 50° and 60°S.

The distribution of sea ice is again strongly affected by these atmospheric and oceanic features.

5.2 Seasonal Sea Ice Cycle

The seasonal sea ice cycle is driven primarily by the annual cycle of solar energy received at Earth's surface and atmospheric conditions. Like that of the Northern Hemisphere, the sea ice cycle in the Southern Hemisphere lags the solar cycle by about 3 months; for example, in the Southern Hemisphere the maximum solar input occurs in late December while the minimum sea ice extent occurs in March. The distribution of sea ice is also heavily influenced by the ocean and atmospheric circulation (Gloersen et al., 1992).

On "average," the ice cover in the Southern Hemisphere varies from a maximum sea ice extent of 18-20.2 x 106 km2 in August-October to a minimum sea ice extent of 3.4-4.3 x 106 km2 in February (Figure 1.08). Only 13% of the winter maximum ice covered area is retained in summer in the Southern Hemisphere (Gloersen et al., 1992). This is in contrast with the Arctic where the area covered by the permanent ice cover in the late summer-early fall minimum is 60% of the winter maximum. This difference is again due to the fact that the Arctic Ocean has no land over the pole so ice can retreat to a region of minimum solar input in summer. In the Southern Hemisphere, however, the Antarctic continent prevents sea ice from existing poleward of 70-75°S. There are also higher surface salinities and greater divergent forces in the Southern Hemisphere than in the Northern Hemisphere.

In February Southern Hemisphere ice is concentrated in the Weddell, Bellingshausen, Amundsen, and eastern Ross seas, with only a narrow fringe of ice existing around most of the rest of the Antarctic continent (Figure 1.09).

Late summer expansion of the ice cover is generally most noticeable in the eastern Ross Sea, and early autumn expansion is most noticeable in the entire Ross Sea and the central and eastern Weddell Sea. By May, the ice cover has expanded northward to about 65°S in the Ross Sea and around much of the eastern portion of the continent, while lagging somewhat in the Bellingshausen and Amundsen Seas and extending outward to near 60°S in the western Weddell Sea. Growth continues around the continent for the next 3-4 months, but with temporary intervals of decay in individual localities. At maximum ice coverage, the ice surrounds the Antarctic continent and at most latitudes extends equatorward to between 55°S and 65°S (Figure 1.10). At this time, the ice edge is generally farthest north in the eastern Weddell Sea near the Greenwich Meridian and farthest south in the western Bellingshausen Sea.

Decay of the ice proceeds slowly in the early spring, but then increases very rapidly from October to January. A primary anomaly in the generally southward retreat of the ice edge occurs in the Ross Sea, where a large polynya consistently opens off the coast of the Ross Ice Shelf in November or December and then expands northward, contributing to a local decay of the ice pack from south to north. By January, the western Ross Sea is nearly free of ice, although considerable ice remains to the east.

The asymmetry in the typical Antarctic growth and decay cycle, with the spring-summer decay occurring more rapidly (September to February) than the fall-winter growth (February to September), contrasts with the more symmetrical growth and decay cycle typical in the Arctic, where ice extent minima and maxima tend to be about 6 months apart, occurring in September and March. Gordon (1981) suggests that the contrast between the Northern and Southern Hemispheres reflects fundamental differences in the heat budgets of the sea ice in the two regions. In particular, the rapid decay of the Antarctic ice is likely related to significantly greater upwelling of relatively warm, deep water that occurs in the Antarctic, as the atmosphere-to-ocean heat flux alone appears insufficient to account for the observed rate of melting, especially during the period of most rapid melt, from mid-November to mid-January.

Another interesting contrast between the Arctic and Antarctic sea ice distributions lies in the amount of open water area within the sea ice pack. Figures 1.11 and 1.12 show the areal sea ice extent and the amount of open water area within this ice covered area for the Southern Hemisphere from 1979 through 1987. Again, the sea ice extent is defined as the area covered with at least 15% sea ice (or no more than 85% open water). Comparing these two figures we see that the maximum open water area coincides with the maximum sea ice extent. This pattern is a marked contrast to the Northern Hemisphere, where the periods of maximum sea ice extent in March and minimum extent in September have the least amount of open water area within the sea ice pack. In other words, the Southern Hemisphere sea ice cover is not as consolidated and has more open water area within the sea ice pack than does the Northern Hemisphere's. Again, this difference is because the Northern Hemisphere polar region is bounded by land and has a lower surface salinity, while the Southern Hemisphere polar region is bounded by the ocean. The Southern Hemisphere sea ice cover experiences more divergent forces from winds and currents, thus spreading the sea ice cover out and creating areas of open water within the sea ice cover. Also, the lower surface salinities in the Arctic Ocean are more conducive to forming a consolidated sea ice cover.

These results were determined from the Scanning Multichannel Microwave Radiometer (SMMR) that operated onboard the Nimbus-7 satellite from 1978 to 1987 and was the predecessor of the Special Sensor Microwave Imager (SSM/I) currently operating onboard the DMSP polar orbiting satellites.

5.3 Location of Polynyas

See Figure 1.13. Latent heat polynyas occur along the Antarctic coastline, particularly near the Wilkes Land Coast, when strong offshore winds sweep the ice away from the coastline (Smith et al., 1990).

Sensible heat polynyas occur in the Cosmonaut Sea and over the Maud Rise because of upwelling of warmer subsurface waters. The seasonal sea ice melt and breakup typically begins in these regions (Smith et al., 1990).