Exploring the predictability of the ‘Short Rains’ at the coast of East Africa

The boreal autumn ‘Short Rains’ at the coast of East Africa are deficient when there is weak development of a zonal circulation cell along the Indian Ocean equator, an anomalously low sea‐surface temperature in the western portion of the basin, and in the high phase of the southern oscillation. Such large‐scale circulation departures and their precursors are described by compact indices. September values of these indices for the period 1958–96 are used to explore the predictability of an index (RON) of October–November rainfall at the coast of East Africa. Regressions with cross‐validation over the entire 1958–96 period are evaluated for the early (1958–77) and late (1978–96) halves of the record. In complementary experiments, the entire record is separated into 1958–77 as a training period and 1978–96 as a verification period. In all experiments, correlation of calculated versus observed rainfall is high for the early record and low for the late half of the record, a behaviour not noted in cross‐validation over the entire 39 year time span. The 11‐year sliding correlations of the indicated circulation indices with RON all reveal a drastic deterioration of relationships from the early to the late half of the record, although the equatorial zonal circulation cell appears to remain strong throughout.