Divergent and non-divergent water vapour transport over southern Africa during wet and dry conditions

Inter-seasonal and inter-annual variations of low-level tropospheric water vapour transport over southern Africa and adjacent oceans have been examined using the ECMWF III-B global analysis data set. Velocity potential and stream function analyses have been used to investigate differences between the large-scale circulation fields and the sources and sinks of water vapour over subtropical southern Africa in wet and dry early (October) and late (January) summer months. Water vapour transported over the region in early summer has been found to originate from both the Atlantic and Indian oceans. Transport takes place in a southerly direction over the subcontinent with eddy transport playing an important role only in early summer. During late summer transport is effected mainly by the mean circulation fields. At this time the tropical Indian Ocean becomes the most important source for water vapour. Variations in the intensity and position of the Walker and Hadley cells are important controls of moisture availability over southern Africa. Wet late summers are characterized by an anomalous Hadley cell over tropical 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 locates over the south-western Indian Ocean and diminished vapour transport takes place across southern Africa from the south east.