A 1-year statistical study of mesoscale cyclonic activity over southern Marie Byrd Land, West Antarctica, was conducted using high-resolution satellite imagery collected insitu during 1991 at the U.S. McMurdo and Palmer stations. Results indicate that, on average, one or two mesoscale cyclones can be observed each week over the southernmost part of Marie Byrd Land (Siple Coast and adjacent areas), where cold katabatic winds from the high plateau of East Antarctica and relatively warm katabatic airflows from West Antarctica interact, forming a boundary layer baroclinic zone in the form of a stationary front. The 1991 annual mesoscale cyclone behavior exhibits maximum activity in March and minimum in September. Analysis of the large-scale pattern for these months suggests that the higher activity in March is associated with synoptic-scale conditions that support the cold katabatic drainage from East Antarctica and favor warm air advection into West Antarctica, a situation that can form and/or enhance the boundary layer baroclinic zone. More than half (70%) of the mesoscale vortices observed near Siple Coast were of comma cloud type with an average diameter of about 250 km. Their satellite signature characteristics show that these cyclonic perturbations were stratiform low cloud features, indicating that most of them developed within the lower troposphere, more than likely below 700 hPa, according to a cloud top temperature evaluation.¿ 1997 American Geophysical Union