DIVERGENT AND NON-DIVERGENT WATER-VAPOR TRANSPORT OVER SOUTHERN AFRICA DURING WET AND DRY CONDITIONS

Inter-seasonal and inter-annual variations of low-level tropospheric w ater vapour transport over southern Africa and adjacent oceans have be en examined using the ECMWF III-B global analysis data set. Velocity p otential and stream function analyses have been used to investigate di fferences between the large-scale circulation fields and the sources a nd sinks of water vapour over subtropical southern Africa in wet and d ry early (October) and late (January) summer months. Water vapour tran sported over the region in early summer has been found to originate fr om both the Atlantic and Indian oceans. Transport takes place in a sou therly direction over the subcontinent with eddy transport playing an important role only in early summer. During late summer transport is e ffected mainly by the mean circulation fields. At this time the tropic al Indian Ocean becomes the most important source for water vapour. Va riations in the intensity and position of the Walker and Hadley cells are important controls of moisture availability over southern Africa. We late summers are characterized by an anomalous Hadley cell over tro pical and subtropical Africa such that excess vapour transport occurs across southern Africa from the north in the second half of the summer rainfall season. During dry late summers the vapour source region loc ates over the south-western Indian Ocean and diminished vapour transpo rt takes place across southern Africa from the south east.