This paper examines the density, distribution and characteristics of mid-latitude cyclones in the oceans south of Africa in the GENESIS general circulation model (GCM). The latest version of the GENESIS GCM (version 2.0.a), as well as its predecessor (version 1.02), are evaluated to assess whether version 2.0.a is an improvement over version 1.02 in terms of midlatitude cyclones. An automated cyclone finding program was used to identify cyclone centres. This program was applied to 5 years of twice daily GENESIS (versions 1.02 and 2.0.a) sea-level pressure data, as well as to 10 years of a gridded assimilation of observed data for the winter season (June, July, August).
The results show that version 1.02 does not simulate the full meridional sea-level pressure range over the analysis window, whereas version 2.0.a is closer to that of the observed data. The circumpolar trough in version 1.02 is between 10 and 15 hPa too weak and it extends too far equatorward. The trough is captured better by version 2.0.a, probably due to its finer grid resolution (T31) compared with that of version 1.02 (R15 resolution). In both versions, cyclone densities north of 55 S are higher than observed and the high cyclone density band around the pole in version 1.02 is poorly defined. Discrepancies between the two versions, and the model and observed data, have been related to factors such as grid resolutions, topography, heat transport and sea-ice extent. Version 1.02 does not simulate the full meridional gradient of cyclone central pressures, whereas version 2.0.a is a better representation of the observed data. Both versions exhibit greater variability in the sea-level pressure field compared with the observed data. GENESIS version 2.0.a is a considerable improvement over its predecessor in terms of mid-latitude cyclones in the African region of the southern oceans. #1997 by the Royal Meteorological Society. Int.