The onset and cessation of the "long rains" in eastern Africa and their interannual variability

Thirty years (1958-1987) of daily rainfall data for Kenya and north eastern Tanzania are analysed with the aim to characterize the interannual variability of the onset and cessation of the East African "long rains" (boreal spring). The leading principal component (PC1) depicts consistent rainfall variations over much of the region. Cumulative PC1 scores for each year serve to identify onset and cessation dates. The robustness of the dates derived from this method is demonstrated through the use of an independent sample of stations. Their spatial representativity is assessed by daily rainfall composites. Average onset occurs on March 25(th), and cessation on May 21(st). The interannual variability of the onset (standard deviation of 14.5 days) is larger than that of the withdrawal (10.3 days), but the onset is also spatially much more consistent. Mean dates and dates in selected anomalous years agree well with previous studies. The relationship between onset time-series and large-scale atmospheric and oceanic fields is analysed. On a monthly time-scale, interannual variations in "long rains" onset are associated with sea-surface temperature (SST) and sea-level pressure (SLP) patterns that have a different sign for the Atlantic and Indian Oceans. A warm South Atlantic and a cool Indian Ocean are associated with low and high SLP anomalies, respectively. These patterns are conducive to enhanced equatorial easterlies and surface divergence over East Africa. This maintains the meridional branch (north-south orientated) of the Intertropical Convergence Zone (ITCZ) further west, and the net result is a delayed onset of the "long rains". Some of the South Atlantic features are already present during January-February, suggesting some potential for monitoring interannual variations in the wet season onset, based on SST and SLP patterns. Additional signals are found over Europe and the Mediterranean Sea in terms of the interaction between the Northern Hemisphere extratropics and equatorial eastern Africa. A surge in the mid-tropospheric northerlies at this time induces instability that may lead to an early onset event.