| A 45-year climatology (1957--2002) of North Atlantic subtropical cyclones is presented, using the ECMWF ERA40 reanalyses at a 1.24° grid resolution, in order to deduce the basin-scale characteristics necessary for subtropical cyclogenesis. A survey of 18 subtropical storms from the last 5 years (1999--2004) is also presented, using NCEP GFS operational analyses, at a 1° 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.; Results indicate that subtropical storms in the North Atlantic have a warm lower core and a cold upper core structure. On average, the storms in this study preferentially form in the western North Atlantic, over warm sea surface temperatures and in moderate vertical wind shear, associated with an upper trough or cut-off low. Further partitioning of subtropical storm formation environments is also presented. Comparisons of this 45 year climatology are made with a recent survey of subtropical storm candidates and with the operational hurricane database, HURDAT. Finally, a proposed subtropical cyclone genesis parameter is introduced and discussed. |
