A Methodology for Identifying Subtropical Storms in the South Atlantic

A subtropical storm is defined simply as a system that exhibits gale-force winds and displays a hybrid structure consisting of an upper-level cold core and lower-level warm core (e.g. Guishard 2006). While these criteria are straightforward to comprehend, applying this definition to build a climatology for an ocean basin is more difficult. In the compilation of a climatology for the South Atlantic basin, many candidate systems seemed to behave as a subtropical storm would be expected to, but were not represented as such according to the cyclone phase space (CPS; Hart 2003) lower tropospheric thermal wind as evaluated by the ECMWF ERA-40 and the NCEP GFS model analyses.

Due to the differing resolutions of the two models, the candidate storm structures were displayed with differing accuracy. Being a higher resolution model, the GFS displayed the potential subtropical storms more accurately with a warmer lower-atmospheric layer. Each GFS-based subtropical storm identification was confirmed by detailed synoptic and satellite analyses of the candidate system. However, the GFS only has data for the twenty-first century, while the ERA-40 data dates back to 1957. Because of this difference, a comparison of CPS biases between the two models was necessary to compile a completed and accurate climatology of subtropical storms for the South Atlantic basin.