A Climatology of Cyclogenesis for the Southern Hemisphere

Abstract

This study surveys life cycle characteristics of cyclones in the Southern Hemisphere (SH). An objective technique for locating instances of cyclone formation, intensification, maturation, and decay is applied to a 7-year dataset of SH cyclone positions, obtained from an automated finding and tracking scheme based on twice-daily ECMWF analyses during 1980–86. Geostrophic vorticity ζ_g at 1000 hPa is used to locate cyclones and determine their intensity. Cyclones form and develop within middle latitudes and migrate eastward and poleward during their lives. Preferred genesis and cyclogenesis regions include eastern coasts of Australia and South America in winter, and leeward of the Andes and near the baroclinic zones associated with the principal upper-tropospheric jet streams year round. Over open water, winter cyclones tend to form and intensify near zones of strongest SST gradient. Rapid cyclogenesis is most frequent east of South America, southeast of Africa, south of Australia, and near New Zealand. Large differences were found between the geographical distribution of developing cyclones as inferred from ζ_g, tendencies and those obtained from central pressure change. The latter were found to be highly dependent on cyclone motion relative to the background pressure field.