Mesocyclone activity over the North-East Atlantic. Part 1: vortex distribution and variability

Polar mesocyclones are subsynoptic-scale low pressure systems that form over high-latitude oceans in a cold air mass. It is known that these systems can be very vigorous but only relatively short time periods have been studied in the past to estimate their characteristics. Here satellite imagery is used to study mesocyclones observed over the North-East Atlantic and Nordic Seas during the 2-year period October 1993–September 1995. An unexpectedly large number (4054) of mesocyclones were found, occurring throughout the year, although winter was the most active season. Most mesocyclones were observed in the northern regions of the study area, near the ice edge, and were small, with 86% of cyclones having diameters of <400 km. Most mesocyclones form, and decay, within 1 day and travel only short distances. The large-scale circulation plays a role in determining mesocyclone numbers as the two separate 12-month periods in the study exhibited very different characteristics.