Explosive Cyclone Development in the Southern Hemisphere and a Comparison with Northern Hemisphere Events

Abstract

A compilation of Southern Hemisphere (SH) explosively developing cyclones (or ?bombs?) has been assembled based on the National Centers for Environmental Prediction?Department of Energy reanalysis-2 data over the 21-yr period from 1979 to 1999. The identification of these features was undertaken with an objective automated cyclone finding and tracking scheme. The procedure allows for the confounding influence of spatial variations of climatological mean pressure on the pressure deepening of explosive cyclones, a perspective of particular importance in the SH. On average, 26 explosive cyclones occur per year in the SH. They are more prevalent in winter although their seasonality is more modest than that seen in the Northern Hemisphere (NH). The distribution of SH explosive cyclones has a close association with that of strong baroclinicity, although the relationship is not one to one. It is found that many of these cyclones occurring south of 50°S show equatorward movement, in contrast with the poleward motion of most NH bombs. The explosive cyclones exhibit greater mean intensity and depth than does the entire population of cyclonic systems. Aspects of NH explosive cyclones revealed in the reanalysis-2 set are briefly examined, with a view to comparing them with the details revealed about SH events. The authors' analysis detects, on average, 45 explosive cyclones per year in that hemisphere. It is found that over the last 21 yr the number of these systems has increased globally and in both hemispheres, and that positive trends of global and SH systems are statistically significant.