An extended climatology of extratropical cyclones over the Southern Hemisphere | The Meteorological Society of New Zealand

An extended climatology of extratropical cyclones over the Southern Hemisphere

M. Sinclair

This paper surveys the behaviour of extratropical cyclones in the Southern Hemisphere from 15 years of ECMWF analyses, extending a previous study by this author. Cyclones were identified as local maxima of cyclonic geostrophic vorticity and tracked by an automated method. This use of vorticity avoids missing mobile circulation centres for which a pressure minimum does not exist. Stationary orographic features were not considered. The overall occurrence of cyclones at all stages of development is quite uniform in the east-west direction, with largest numbers south of 55ºS, and a second lower-latitude maximum spanning the Pacifc Ocean near 40ºS in winter. Cyclone formation and intensification locations are more localised, occurring near eastern seaboards of Australia and South America, leeward of the Andes, near the climatological positions of the polar and subtropical upper-tropospheric jet streams and near SST gradients. The region south of 60ºS is a favoured region for cyclone dissipation. Explosive cyclogenesis is most frequent in a band extending southeast from South America, in the Indian Ocean, east of Australia and in a zone spanning much of the Pacific from northeast of NZ. There are marked year-to-year changes in the paths that cyclones take. Variations related to the Southern Oscillation were found in the South Pacific subtropics. Results from the 15-year data series suggest an increase (decrease) in cyclone numbers in the eastern Pacific during negative (positive) SOI an increase in cyclonic activity to the north of NZ during La Nina years, and a decrease over Australia during El Ninos.

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