An observational study of the relationship between excessively strong short rains in coastal East Africa and Indian Ocean SST

Composites of SST, wind, rainfall, and humidity have been constructed for years of high rainfall during September, October, and November (SON) in equatorial and southern-central East Africa. These show that extreme East African short rains are associated with large-scale SST anomalies in the Indian Ocean that closely resemble those that develop during Indian Ocean dipole or zonal mode (IOZM) events. This is corroborated by the observation that strong IOZM events produce enhanced East African rainfall. However, it is also shown that the relationship between the IOZM and East African rainfall is nonlinear, with only IOZM events that reverse the zonal SST gradient for several months (extreme events) triggering high rainfall. Comparison of the wind anomalies that develop during extreme IOZM events with those that develop during weaker (moderate) events shows that strong easterly anomalies in the northern-central Indian Ocean are a persistent feature of extreme, but not of moderate, IOZM years. It is suggested that these anomalies weaken the westerly flow that normally transports moisture away from the African continent, out over the Indian Ocean. Thus, during extreme IOZM years, rainfall is enhanced over East Africa and reduced in the central and eastern Indian Ocean basin. It is also shown that the IOZM cannot be viewed in isolation from the El Nino-Southern Oscillation (ENSO). Instead it is postulated that in some years, a strong ENSO forcing can predispose the Indian Ocean coupled system to an IOZM event and is therefore a contributory factor in extreme East African rainfall. The results of this study imply that the relationship between El Nino and the IOZM explains the previously described association between El Nino and high East African rainfall. Thus, understanding the way that ENSO drives Indian Ocean dynamics may aid the development of predictive scenarios for East African climate that could have significant economic implications.