Antarctic Offshore Leads and Polynyas and Oceanographic Effects

Extensive areas of open water are located within the Antarctic sea ice pack in a near-shore zone of several hundred kilometers, as well as the marginal ice zone near the sea ice edge. The time-series of satellite passive microwave observations during 1974 provides quantitative values of the area of open water in the coastal zone and details per thousand during the winter months. Although uncertainties remain regarding the amount of ice exported from the shelves and the shelf-slope exchange of water required to maintain steady state shelf water salinity, it is concluded that ice formation in leads and polynyas over the shelf is likely to be a primary factor in the production of saline shelf of the opening and closing of polynyas on daiwater and ultimately in bottom water formaly time scales. Along the coast, numerous potion. lynya areas are observed. Sixteen study areas located over the continental shelf are analyzed to provide time series of the maximum brightness temperature (TB) , minimum TB, average T B, standard deviation of T B within each study area, and the derived area of open water. Examination of the synoptic pressure Introduction The Antarctic sea ice pack is known to conrain substantial areas of open water during all seasons (e.g., Priestley [1913] (see Bromwich and Kurtz [1982]); Zwally and Gloermaps in the Ross Sea indicates that the intersen [1977]; Bromwich and Kurtz [1982]; Carsey mittent formation of a polyuya near the ice [1980]; and Zwally et al. [1983]). The size, shelf front is strongly influenced by the syndistribution, and duration of the open water optic winds. Other po!yuya areas appear to be areas within the ice pack are highly variable located offshore of major outlet glaciers that as the sea ice grows, deforms, and melts in are locations of enhanced katabatic winds. In response to atmospheric, oceanic, and radiaall cases, the intermittent increases in open tire forcings. Principal factors influencing water during the polynya events are superirathe Antarctic sea ice dynamics are the absence posed on a significant background of nearof geographical constraints to the northern shore open water, which averages about 19% edge of the ice pack and the strong near-shore during the winter period from March 17 through drainage wind•s from the ice sheet [Kurtz and November 11 (days 76-315). In some locations, Bromwich, 1983; Parish, 1982]. The windmore open water is observed during the winter induced divergence of the sea ice substantialperiod than in the summer days November 12 ly reduces the time-averaged sea ice concenthrough March 16 (316-75). The principal heat trations, particularly in the near-shore (insource in the near-shore zone during the winher) and marginal (outer) zones of the ice ter is deduced to be latent heat from ice propack. During much of the year, ice production duction as ice divergence continually produces is rapid in the open water and thin ice areas new areas of open water. The oceanic heat that are continually formed near the coast. from cooling of the shelf water column by itNet divergence of the near-shore ice is generself would maintain open water for a period ally accompanied by a northerly transport of less than 30 days. The resulting ice producice, which contributes to the expansion of the tion within the latent-heat polynyas and leads ice edge during the growth season and provides significantly contributes to the sea ice coyice for melting at the outer edge [Hibler and er, which is generally driven from the contiAckley, 1983]. Following the period of maxinental shelf toward the deep ocean. Estimates mum winter ice extent, for example, the nearof the rate of ice production along with the shore zone is expected to be a region of net observed values of open water area are used to ice production, and the marginal zone a region calculate the resultant salinization of the of net ice dissipation. Antarctic shelf waters, giving increases in The recent realization of the extensive shelf water salinity of many tenths of parts distribution of open water within the ice pack 203 Antarctic Research Series Oceanology of the Antarctic Continental Shelf Vol. 43 Copyright American Geophysical Union 204 OCEANOLOGY OF THE ANTARCTIC CONTINENTAL SHELF