1. El Nino, Indian Ocean dynamics and extremely rainy years in East Africa Emily Black, Julia Slingo and Ken Sperber emily@met.rdg.ac.uk Introduction Rainfall in East Africa is, economically and socially the most important part of the climate system. This study focuses on the boreal autumn rainy season (the short rains) in equatorial and southern tropical East Africa (see Figure 1). Much has been published about the association between El Nino and high rainfall in tropical, coastal East Africa. Figure 2, however, shows that precipitation during some El Ninos (for example 1986-1987) is average or low implying that the El Nino- rainfall relationship is modulated by some other factor. Analysis of time series and composites (Figures 2, 3 and 4) suggests that this factor is the Indian Ocean dipole or zonal mode (IOZM). We propose a dynamic scenario in which the wind anomalies that characterise IOZM events drive above average rainfall in East Africa (see Figures 5 and 6). We further suggest that the IOZM can, in certain circumstances, be associated with El Nino and this relationship explains the observed teleconnection between East African rainfall and ENSO. A paradigm for very strong short rains In the following conditions, the short rains in coastal tropical East Africa are likely to be above average: El Nino is sufficiently strong during the boreal summer to perturb the convection and circulation in the vicinity of the Maritime Continent The perturbation of the climate in this region is strong enough to generate a persistent change in the Hadley circulation with enhanced southerly winds in the eastern Indian Ocean The enhanced southerlies are so long-lasting that the cooling of the eastern Indian Ocean reverses the zonal gradient in SST and triggers an IOZM The easterly wind anomalies resulting from the IOZM extend over the Indian Ocean and significantly suppress the mean westerly flow Short rains Figure 1: The seasonal cycle in East African rainfall (mean rainfall and one standard deviation either side). Note that the largest interannual variability is experienced during the short rains -200 400 200 0 Rainfall anomaly El Nino year zero IOZM year Figure 2: A time series of the short rains in coastal, equatorial East Africa highlighting IOZM events and El Nino year zeros Seasonal and interannual variability in East African rainfall Figure 3: SST anomaly composite for SON of very rainy years in coastal equatorial East Africa. Figure 4: Seasonal cycle in SST difference between the tropical eastern and western Indian Ocean during rainy years (dots). The solid lines show the mean seasonal cycle and one standard deviation above and below. Note that the reversal of the climatological SST gradient during rainy boreal autumns is characteristic of IOZM events. SST anomaly patterns during rainy years Figure 5: Wind during IOZM years compared to climatology Climatology Average vector wind during IOZM years Average zonal wind anomaly during IOZM years El Nino High short rains in East Africa Reversed SST gradient for several months Easterly zonal wind anomalies in the northern central Indian Ocean Weakening of the climatological moderately strong westerlies Less transport of moisture away from the African continent Cooler SST in the eastern Indian Ocean if the El Nino is very strong or if the gradient in SST in the Indian Ocean is vulnerable to reversal… ??? (some other factor) Weakening of the Indonesian through flow Anomalous along shore southerly winds near Sumatra Figure 6: The anomalous winds shown in Figure 5 and the paradigm for strong short rains summarised above are combined into a dynamic scenario for strong East African short rains A dynamic scenario for high autumn rainfall in East Africa