Wet and dry spells within particularly wet and dry summers in the South African summer rainfall region

Abstract

Atmospheric moisture transport over southern Africa and surrounding oceans is consid- ered during wet and dry conditions over the South African summer rainfall region. Wet and dry syn- optic spells within wet and dry austral summers are examined. A link between synoptic and seasonal timescales is investigated using seasonal statistics of wet and dry spells. Dry synoptic spells exhibit divergent moisture flux over South Africa, with inflow from the mid-latitude ocean regions to the south. Cyclonic features off the east coast may exist and attract moisture away from South Africa. For wet synoptic spells, there tends to be increased moisture flux from the tropical or subtropical south- west Indian Ocean (SWIO), either associated with ridging along the east coast or a deep low over the interior. Seasonal modulations of the intensity of the heat low over Angola/Namibia appear important for influencing early (OND) and late (JFM) summer rainfall over South Africa. This low may act as the tropical source for tropical temperate troughs and their associated cloudbands that are major synop- tic rain-producing systems. Wet (dry) summers are often associated with a southward (northward) shift and strengthening (weakening) of the ITCZ over tropical southeastern Africa. Seasonal rainfall is found to be related to the distribution of wet and dry spells within the season, such that wetter sea- sons tend to have longer or more intense wet spells rather than a greater number of wet spells.