SEASONAL CYCLE OF CONVECTIVE SPELLS OVER SOUTHERN AFRICA DURING AUSTRAL SUMMER

The seasonal progression of summer wet spells over southern Africa is investigated. The November to March rainy season is modulated at the i ntraseasonal scale, with major wet spells every 20-30 days and interna l evolution at higher frequencies. Corresponding circulation patterns for each wet spell illustrate the variable dynamics and physics as the season progresses. Early summer rainfall is associated with enhanced baroclinic westerly shear from the upper troposphere of the tropical A tlantic Ocean, whereas low-level moisture is advected from the central Indian)cean. Extratropical westerly wave action is an important influ ence during early summer. As the summer season progresses, heating cau ses a retreat of the subtropical westerly jet stream. A quasi-stationa ry line of confluence along 20 degrees S develops in the low levels du ring late summer, and extends from the Mozambique Channel to Angola. T his trough is supported by an influx of moisture from the Congo and so uthern Mozambique Channel. Composite differences between early and lat e summer wet spells are evaluated numerically for the first time in th is presentation. (C) 1997 by the Royal Meteorological Society. (No. of Figures: 12 No. of Tables: 0 No. of References: 15).