Southern Ocean sea-ice distributions and extents

Abstract

Southern Ocean sea-ice coverage undergoes a large seasonal cycle, with a nearly fivefold increase in areal ice extents from the minimum in February to the maximum in September, and significant interannual variations. Results presented here show in a variety of forms some of the variability that occurred in Southern Ocean sea-ice distributions and extents over the 1970s and 1980s. Interannual variability is examined by identifying changes in three measures: sea-ice extents, sea-ice distributions, and the length of the sea-ice season. Regarding these three: (i) Maximum ice extents varied by approximately 12%, decreasing during the mid-1970s, followed by increases over the next few years and a levelling off for much of the 1980s. (ii) The area of interannual variability in monthly average sea-ice distributions in summer far exceeds the summertime area of consistent ice coverage, in sharp contrast to the wintertime situation, when the area of consistent ice coverage is considerably larger. In winter, the ice-distribution variability is largely confined to two regions: a relatively narrow outer band (generally 2-5° of latitude) surrounding the region of consistent ice coverage, and, for the mid-1970s, the region of an occasional large open water area within the ice pack of the Weddell Sea, termed the Weddell polynya. The Weddell and other polynyas within the ice cover allow intensified heat, mass, and momentum exchanges between ocean and atmosphere, thereby affecting regional oceanic and atmospheric circulations, (iii) The length of the sea-ice season, calculated for the years 1979-1986, with satellite passive-microwave data coverage through all months of the year, showed increases over that period in the Ross Sea but decreases in the Weddell and Bellingshausen seas. In both cases it appears, through comparisons with data from 1973-1976, that the 1979-1986 changes more likely reflect a fluctuating behaviour of the ice cover than a long-term trend. The changes in the ice cover have influences not only on the ocean and the atmosphere but on aquatic plant and animal life as well.