Extreme rainfall events and lake level changes in East Africa: recent events and historical precedents

Towards the end of 1961 an extreme rainfall event occurred that extended over much of East Africa stretching across the Indian Ocean to India. This event caused widespread flooding, rapid and prolonged increases in the levels of many lakes in East Africa and significant economic disruption (Odingo, 1962; Morth, 1967). During the last few months of 1997, in a similar fashion to 1961, heavy rainfall caused flooding across East Africa (FAO/GIEW, 1998; Birkett et al.,1999). The principal driving mechanism of these extreme events has recently been established, a dipole reversal in atmospheric circulation and Indian Ocean sea surface temperatures (SSTs, Webster et al., 1999; Saji et al.,1999). Rapid progress is being made in understanding the dynamics of the event, for instance Latif et al. (1999) have directly related rainfall anomalies with Indian Ocean SSTs in ensemble general circulation model (GCM) experiments. Little is known, however, about the spatial and temporal characteristics of the hydrometeorological anomalies associated with these and other dipole events in the Indian Ocean. For instance, over India, rainfall in 1961 was the highest on record (Grove, 1996; Saji et al.,1999) but in 1997 totals were close to normal (Janowiak and Xie, 1999). Other major SST gradient reversals apart from 1961 and 1997 have occurred (for instance 1967 and 1994) but it is not clear to what extent other SST gradient reversals have been associated with the type of anomalies that occurred in 1961 and 1997.