Easterly flow in the tropical Indian Ocean and climate variability over south-east Africa

The relationship between African rainfall and zonal wind anomalies in the adjacent tropical Indian Ocean is investigated. Comparisons are made using a December to March rainfall index for Africa south of 20 degrees S and east of 25 degrees E. Cases are selected when the zonal 850 hPa wind anomaly in the area 5 degrees to 20 degrees S, 40 to 90 degrees E exceeds +/- 1 standard deviation. During wet (dry) summers, low-level winds are more easterly (westerly). The easterly flows surge at certain times, bringing intra-seasonal 'pulses' of convection from the tropical Indian Ocean to south-east (SE) Africa during austral summer. National Center for Environmental Prediction (NCEP) reanalysis composites are constructed and structural features are analysed by subtracting the dry westerly cases from the wet easterly cases. Composite differences of zonal easterly flow extend through the troposphere in the 5 degrees to 20 degrees S band. Convection is reduced over the tropical South Indian Ocean whilst increased over SE Africa. Differences between east and west regimes are evident in the velocity potential and outgoing longwave radiation (OLR), indicating a convective sink near 15 degrees S, 75 degrees E over the South Indian Ocean. Composite sea surface temperatures (SST) are significantly above normal in the latitude band 25 degrees to 35 degrees S, and below normal in the tropics to the north. Precursor patterns are investigated to determine the origins of this climate mode.