The relationship of the El Nino Southern oscillation to African rainfall

This is a comprehensive study of the rainfall response over Africa to ENSO episodes in the Pacific. The harmonic method utilized by Ropelewski and Halpert is applied to 90 regionally averaged rainfall time series for the period 1901-1990. The analysis was a composite of 20 episodes within this period. Seasons of maximum positive anomalies and maximum negative anomalies in the composite were identified. The method identifies 15 multiregion sectors where ENSO appears to modulate rainfall. The strongest signals are in eastern equatorial and south-eastern Africa. A continental-scale signal is also apparent. The magnitude, seasonal timing and duration, and consistency of the rainfall response to ENSO vary among the sectors and from episode to episode. The rainfall response is clearly seasonally specific. In general, the onset of the ENSO signal in rainfall commences far to the south and propagates latitudinally northward. For this reason, the equatorial regions are out-of-phase with the continental pattern. The ENSO mechanism is probably responsible for many of the well-established rainfall teleconnections over the continent, including the strong tendency for opposite anomalies in equatorial and southern Africa. There is a strong tendency for positive anomalies to occur during the first half of the ENSO cycle, negative during the second half. This corresponds to 'cold' and 'warm' phases in the adjacent Atlantic and Indian Oceans; continentally, rainfall tends to be enhanced during the cold phase, reduced during the warm phase. The northward propagation is most pronounced during the cold phase; a similar propagation and phase shift occurs at this time in the Atlantic. The rainfall anomalies of the warm phase are nearly constant in phase, as are the SST anomalies in the Indian Ocean. This suggests that, in general, the Atlantic Ocean controls rainfall during the cold phase, the Indian Ocean during the warm phase.