Intraseasonal and interannual variability of tropical temperate troughs over southern Africa and the southwest Indian Ocean

Tropical Temperate Troughs (TTTs) have for some time been identified as the main rainfall producing system over southern Africa. The aim of this paper is to examine the relationship between the atmospheric circulation, sea surface temperatures and TTTs on intraseasonal and interannual time scales.

Variability on intraseasonal time scales is assessed by means of Empirical Orthogonal Functions (EOFs) of a newly derived 3 hourly satellite based rainfall data set. The relationship between the leading modes of variability (which in all months October to March are TTTs) and atmospheric circulation data from the NCEP Reanalysis Project will be presented. Attention will focus on mositure fluxes, zonally asymmetric waves in the southern hemisphere westerlies and the terms of the momentum budget.

Rainfall variability on interannual time scales is studied by means of EOFs of Outgoing Long Wave Radiation (OLR) for the December to February season, the leading mode of which is a TTT dipole over southern Africa and the southwest Indian Ocean.. Links between this leading mode on the interannual variability of the atmospheric circulation are again assessed by means of NCEP data. Further analysis of model rainfall from an AGCM (HADAM2A) forced with historical sea surface temperatures from 1904-1994 indicates a that TTT dipoles are the leading mode of variability on both interannual and interdecadal time scales.