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

Abstract

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°S and east of 25°E. Cases are selected when the zonal 850 hPa wind anomaly in the area 5° to 20°S, 40 to 90°E exceeds ± 1 standard deviation. During wet (dry) summers, lowlevel 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° to 20°S band. Convection is reduced over the tropical South Indian Ocean whilst increased over SE Africa.

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