Composite winter cyclones south of Africa: Evolution during eastward transit over the Agulhas warm pool

Abstract

The structural evolution of winter low pressure systems over the Southern Ocean is studied using ECMWF (European Community Medium - Range Weather Forecasts) gridded weather data for the period 1986-1992. A limited sample size (n = 14) of mid-latitude cyclone cases meeting prescribed criteria, equally divided between two areas south-west (west) and south-east (east) of Africa, are averaged and contrasted. Comparisons of composite features for west and east cases distinguish the influence of differing sea temperature regimes. Mid-latitude cyclones in the east entrain air overlying the 'warm pool' of the Agulhas Current recirculation gyre, and exhibit increased baroclinic structure, stronger uplift, enhanced zonal winds and greater water vapor content. Analysis of structural differences may assist marine forecasts and highlights the role that SST plays in modulating the intensity of mid-latitude cyclones south of Africa.