Mesoscale cyclone activity over Antarctica during 1991 2. Near the Antarctic peninsula

A survey of mesoscale cyclogenesis near the Antarctic peninsula has been carried out for 1991 based on digital satellite imagery collected at 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 summer and a minimum in winter. Comma clouds were the dominant vortex type, and their average diameter was around 370 km. Trajectories show that many vortices in the BSS move northeastward toward the Drake Passage, 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 fraction 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 frequent outbreaks of cold air associated with synoptic-scale cyclones that move cold air northward over the relatively warm Bellingshausen Sea, with convective instability resulting. This happens much less often in the WSS, where low-level baroclinic instability and vortex stretching are the dominant formation mechanisms and air-sea interaction is usually much less vigorous due to the stabilizing influence of the extended seaice cover.