MESOSCALE CYCLONE ACTIVITY OVER ANTARCTICA DURING 1991 .2. NEAR THE ANTARCTIC PENINSULA

A survey of mesoscale cyclogenesis near the Antarctic peninsula has be en carried out for 1991 based on digital satellite imagery collected a t Palmer Station. Slightly higher cyclonic activity was found over the Bellingshausen Sea sector (BSS) than in the Weddell Sea sector (WSS). Both sides were characterized by a pronounced formation maximum in su mmer and a minimum in winter. Comma clouds were the dominant vortex ty pe, and their average diameter was around 370 km. Trajectories show th at many vortices in the BSS move northeastward toward the Drake Passag e, with some of the most well developed moving north-northeastward to the southern tip of South America, bringing severe weather conditions to that area; little systematic motion was noted over the WSS. The fra ction of mesoscale cyclones with middle and/or high cloud on the west side of the Antarctic peninsula was much higher than on the east side. These numerous and deep mesoscale cyclones are a consequence of the f requent outbreaks of cold air associated with synoptic-scale cyclones that move cold air northward over the relatively warm Bellingshausen S ea, with convective instability resulting. This happens much less ofte n in the WSS, where low-level baroclinic instability and vortex stretc hing are the dominant formation mechanisms and air-sea interaction is usually much less vigorous due to the stabilizing influence of the ext ended sea-ice cover.