Cosmonaut polynya in the Southern Ocean: Structure and variability

Along the far eastern margin of the Weddell Gyre is a persistent feature in the middle of the ice pack which we previously reported and called the Cosmonaut polynya. A study of polynya occurrences from 1973 to 1993 reveals that since 1986 the polynya has become more active with an average size of about 7.2 x 10(4) km(2) and an average location at 52 degrees E and 65 degrees S. Satellite observations indicate that the polynya has recurred several times during winter in recent years with intervals ranging from a few days to a few weeks. The centroid of the polynya varies only slightly with each formation during the year and from one year to another, suggesting a controlling influence of the ocean and bottom topography that may be initially induced by wind. The daily time series indicates two primary modes of formation: one that is initiated in the early winter during a storm at a site usually preceded by an embayment of the ice edge and another that occurs during midwinter often preceded by a coastal polynya event adjacent to Cape Ann. The Cosmonaut polynya region is characterized in this study by compression of the westward flowing coastal current and the eastward flowing southern edge of the Antarctic Circumpolar Current. Following the principle of conservation of potential vorticity, vertical stretching of the water column would ensue, enhancing upwelling. Such a process accelerates the injection of relatively warm salty deep water into the surface layer, inhibiting sea ice growth and causing the polynya formation. This theory appears to explain the general behavior of the polynya in terms of frequency, duration, size, and location.