Dry spell frequencies and their variability over southern Africa

Marked interannual fluctuations in rainfall are a fundamental aspect of southern African climate. This study uses dry spell frequency (DSF) to assess spatial and temporal patterns in the consistency of rainfall during the mid-summer (DJF) season and their relationships to interannual drought occurrences in southern Africa. The Climate Prediction Center Merged Analysis of Precipitation (CMAP) pentad data are used to identify dry spells, which are defined here as a pentad with mean daily rainfall less than 1 mm. It was found that DSFs over most of southern Africa are highest (lowest) during El Nino (La Nina) events and that their occurrence is associated with shifts in the location of the tropical-temperate-trough (TTT) systems that are the dominant rain-producing systems over much of southern Africa. The latter are tropical-extratropical cloudbands that link a tropical low over low latitude southern Africa with a westerly disturbance passing south of the landmass. A preferred zone of occurrence of high DSFs, within which the interannual range is also highest, lies across the 20 to 25degreesS band in southern Africa and is identified as the drought corridor. This region often experiences half or more of the season under dry spells. In addition, a tendency during 1979 to 2002 towards an increasing frequency of heavier rainfall events was observed over Angola/Namibia in the west and Tanzania/Mozambique in the east. Only a small area in central southern Africa showed the reverse tendency.