Exchanges - Scientific Contributions - Exchanges - Scientific Contributions - * Contribution to Exchanges No. 27, June 2003 Impact of the Indian Ocean Dipole on the East African Short Rains: A CGCM Study * Swadhin Behera 1 , Jingjia Luo 1 , Sebastien Masson 1 , ToshioYamagata 1,2 ,Pascale Delecluse 3,4 , Silvio Gualdi 5 and Antonio Navarra 5 1 Frontier Research System for Global Change, Yokohama, Kanagawa, Japan 2 Department of Earth and Planetary Science, The University of Tokyo,Tokyo, Japan 3 Laboratoire d’Océanographie Dynamique et de Climatologie (LODYC), Paris, France 4 Laboratoire des Sciences du Climat et de l’Environnement (LSCE ), Orme, France 5 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Bologna, Italy corresponding e-mail: yamagata@eps.s.u-tokyo.ac.jp Introduction The seasonal rainfall in the equatorial eastern Af- rica is characterized by double peaks. This semiannual signal in the rainfall of the region is mainly attributed to the north-south migration of the intertropical conver- gence zone (ITCZ). The atmospheric convergence over East Africa becomes stronger during the two transition seasons of the monsoon winds when the ITCZ is closer to the equator. Rainfall is higher during the first peak (April-May) compared to the second peak (October-No- vember) (e.g., Hastenrath et al., 1993). The latter is known as the season of short rains. These short rains show higher interannual variability compared to the rainfall of the first season (Hastenrath et al., 1993; Black et al., 2003). Understanding causes of such interannual vari- ability will immensely contribute to regional issues of water management and disease control. Several previous studies investigated the statisti- cal relationship between the short rains and the SST vari- ability related to the El Niño/Southern Oscillation (ENSO) in the Pacific Ocean (Ropelewski and Halpert, 1987; Ogallo, 1989; Hastenrath et al., 1993; Mutai and Ward, 2000). These studies suggest the existence of a weak relation, i.e. an enhanced rainfall during warm ENSO events. The influence of the Indian Ocean on the East African rainfall variability has drawn attention recently (e.g. Goddard and Graham, 1999). In particular, the re- cent discovery of the Indian Ocean Dipole (IOD) mode has shed new light on this neighbourhood relationship (Saji et al., 1999; Yamagata et al., 2002; Saji and Yamagata, 2003b; Black et al., 2003). Saji et al. (1999) showed an east-west dipole mode in the Indian Ocean sea surface temperature (SST) anoma- lies that is coupled to the atmospheric zonal circulation. They also showed that the IOD has a significant correla- tion with the East African rains; the rainfall is increased (decreased) during a positive (negative) event. Unlike the basin-wide surface warming (cooling) that is related to warm (cold) ENSO, the dipole mode associated with sub- surface equatorial ocean dynamics is shown to be inde- pendent of the Pacific phenomenon (e.g., Rao et al., 2002; Shinoda et al., 2003). However, about 35% of the dipole events co-occur with ENSO and this raised a debate Fig. 1: Composite rainfall anomalies (mm/day) for September-November during pure IOD (left panel) and pure ENSO (right panel) events. The years considered in the composite are given in Yamagata et al. (2002). The original rainfall data are from Willmott and Matsuura (1995).