Atmospheric circulation controls and characteristics of a flood event in central South Africa

Factors contributing to the synoptic‐scale forcing of flood‐producing rainfall over central South Africa in February 1988 are identified as tropical convection over southern Africa, easterly waves and an anticyclone over the south‐west Indian Ocean and Mozambique Channel, and mid‐latitude westerly wave disturbances to the south of Africa. Tropical moisture was advected over southern Africa during this period by barotropic perturbations that originated over the south‐west Indian Ocean as easterly waves. The area of tropical convection was linked to a mid‐latitude westerly wave to the south of the subcontinent by a tropical‐temperate trough and attendant cloud band during the flood episode. The development and persistence of a lower tropospheric anticyclone over the Mozambique Channel during the rainfall event slowed the progress of westerly troughs across the region and contributed to the increased residence time of strong, organized convection over central South Africa. The thermodynamic and kinematic characteristics of the atmosphere over the subcontinent in the presence of the tropical‐temperate trough are typical of enhanced convection in a tropical environment. Distinctive features associated with the presence of the well‐developed tropical‐temperate trough during the flood event are increased westerly relative angular momentum generation around 20°S, the forcing of an anomalous Hadley‐type circulation over eastern southern Africa, strong poleward fluxes of momentum in the upper troposphere, and poleward displacement of the subtropical jet in the region of Marion Island.