Intraseasonal Variability of Satellite-Derived Rainfall and Vegetation over Southern Africa

Abstract

Satellite-derived rainfall and vegetation [normalized difference vegetation index (NDVI)] data for the period 1981?2000 are used to reveal spatial and temporal interrelationships via principal component analysis. The unimodal seasonal cycle peaks in austral summer over the Zambezi Valley and interior plateau. Spectral analysis indicates major cycles of intraseasonal rainfall events at ?20 and 40 days in response to atmospheric wave forcing that is partially phase locked to the annual cycle. The spatial loadings exhibit a ?center of action? along the eastern edge of the Kalahari (±23°E), extending from the western Zambezi toward central South Africa, spanning 20° of latitude. The loading pattern is consistent with tropical?temperate troughs and associated northwest (NW) cloud bands. Twenty-day rainfall oscillations occur over the Agulhas Current and Angola, hence NW cloud bands are a slower terrestrial harmonic of the faster modes at either end. NDVI also exhibits intraseasonal cycles operating near 40 days with spatial loadings collocated with the slow rainfall mode at a 10?20-day lag. It is postulated that an earlier rainfall event and subsequent ?greening? results in a moisture flux that promotes the next rainfall event. This is reflected in a composite analysis of low-level velocity potential. The vegetation draws airflow toward itself in a self-sustaining way.