Effect of air-sea-ice interaction on winter 1996 Southern Ocean subpolar storm distribution

Air-sea-ice interaction processes in the Southern Ocean are investigated utilizing space-observed surface winds, sea ice concentration, and sea surface temperature (SST) from September through December, 1996. The sea ice edge (SIE) shows three ice-extent maxima around the Antarctic during September and October when sea ice coverage is maximum. They are located in the central Indian Ocean, east of the Ross Sea, and in the eastern Weddell Gyre. During September and October, most of the strong and long lasting storms initiate northeast of the three sea ice maxima. Such spatial distributions of storms and sea ice reflect coupling processes of the air-sea-ice interaction. A relatively stable, wave number 3 atmospheric circulation pattern that is believed to be fixed by the land-ocean distribution prevails during the ice maximum season. The ice-extent maxima coincide with strong southerlies and divergent wind fields associated with this pattern, which suggests that the mean atmospheric circulation determines the ice distribution. The ice-extent maxima can enhance the regional meridional surface pressure gradient and therefore strengthen the westerly winds north of the ice edge. The decreasing ice extent east of the ice maxima creates a local zonal thermal gradient which enhances local southerlies. This positive feedback between the wave pattern in the mean atmospheric circulation and ice distribution partially causes the eastward propagation of the ice maxima and also provides a favorable condition for cyclogenesis northeast of the ice-extent maxima. The mechanism of the cyclogenesis is the baroclinic instability caused by the cold air blown from the ice pack to the warm open-ocean waters. Where the SST is warmest off the SIE and the southerlies are the strongest, the potential for cyclogenesis is most likely. This is consistent with the observations.