Empirical predictability study of October-December East African rainfall

Relationships between September atmospheric dynamics over the global tropics and the East Africa short rains over the domain 4degreesS-4degreesN, 30-39degreesE were investigated to define a seasonal rainfall forecasting scheme. From total and partial correlation analyses over the period 1968-97, three major and useful signals appear in September. Precursor components of the October-December east-west circulation cell appear over the equatorial Indian Ocean (EIO). They were discovered through various indices of sea-surface temperature (SST), zonal wind, vertical velocity and moist static energy computed over the western, central and eastern parts of the EIO. Principal component analyses were applied to these indices to form two synthetic indices which discriminate ocean-atmosphere variability in the eastern and western parts of the basin. The Indian monsoon dynamics control the SSTs off the Somalia coast which are known to play an important role in the variability of the short rains. Ridge-trough systems develop off South Africa which, when persistent enough, drive SST anomalies in the eastern tropical and equatorial Atlantic; these in turn are associated with anomalies in the middle levels over an equatorial band ranging from the Guinea Gulf to the western Indian Ocean. The three September signals used as predictors in a multiple linear regression model explain 64% of the rainfall variance; a comparable efficiency is also achieved with a linear discriminant analysis model.