Characteristics of North Atlantic subtropical storms

A 45 year climatology (1957-2002) of North Atlantic subtropical cyclones is presented, using the ECMWF ERA40 reanalyses at a 1.24º grid resolution, in order to deduce the basin-scale characteristics necessary for subtropical cyclogenesis. Comparisons are made with a recent survey of subtropical storm candidates from the period 1951-2002 (Roth, 2002). A closer examination of 18 subtropical storms from the last 5 years (1999-2004) is also presented, using NCEP GFS operational analyses, at a 1º grid resolution. The synoptic and smaller scale features associated with the development and evolution of North Atlantic subtropical storms are elucidated and discussed, utilizing the following tools: Cyclone Phase Space analysis (Hart, 2003), satellite data and imagery, surface and upper observations, and storm-centred composites. Finally, a proposed subtropical cyclone genesis parameter is introduced and discussed.