Recurring polynyas over the Cosmonaut Sea and the Maud Rise

Abstract

Two polynyas over the deep ocean were observed in the Antarctic region during the winter of 1980: one near 43°E, 66°S (Cosmonaut polynya) and another near 2°E, 64°S (Maud Rise polynya). The time history of these two polynyas was examined on an alternate day basis using ice concentration maps from the Nimbus 7 scanning multichannel microwave radiometer (SMMR). A quantitative analysis of a study area around it shows that the totally enclosed Cosmonaut polynya attained a maximum size on July 25, 1980, with an open water area of as much as 137,700 km2. This polynya lasted for a few weeks, disappeared on August 16, 1980, and was not observed for the rest of the winter. Similar polynyas in the same region have occurred for several years, including 1973, 1975, 1979, 1982, and 1986. The Maud Rise polynya, on the other hand, was observed as a reduction in ice concentration to about 37% within the SMMR resolution of about 900 km2. However, the open water area in the region amounted to 92,800 km2 on July 20, and the polynya recurred several times during the same winter period. It is proposed that both polynyas are products of deep‐reaching convection which introduces warmer deep water into the surface layer. In this way, they are viewed as sensible heat polynyas in that they are maintained by oceanic heat. The oceanographic settings of these two polynyas are similar. The hydrographic data at both sites indicate the existence of localized doming of the pycnocline. This brings warmer, saltier deep water closer to the sea surface, which has been demonstrated to be an effective preconditioner for deep‐reaching convection. It is probable that the polynyas are terminated by “invasion” of sea ice from the sides, which attenuates convection. The capability of a polynya to survive an entire winter period may be related to its size; the larger polynyas are better protected from convergence of the surrounding sea ice. For example, a simple model shows that a polynya with a diameter of about 100 km or greater is much more likely to survive a winter season than a smaller one.