An Examination of Drought in Eastern Africa and its Predictability

During the last decade severe droughts have affected much of East Africa, including both the summer rainfall region to the north and the equatorial region to the south, where rainfall tends to occur during the boreal spring and autumn and summer is dry.  This paper examines two aspects of the droughts in this region: predictability for individual seasons and causal factors for the “short rains” of the boreal autumn.  Predictability was shown to be strong for all three rainy seasons, but only two months in advance for the boreal spring and summer, but five months in advance for the boreal autumn.  In contrast to most other predictive models, these were based primarily on atmospheric circulation variables, as opposed to SSTs and other surface parameters.  For the short rains, the causal factors examined were ENSO, the Indian Ocean Zonal Mode, and zonal winds at the surface and 200 mb over the central equatorial Indian Ocean.  While all variables showed a strong link to interannual variability of October-November rainfall, that with surface winds was by far the strongest.  However, the relationship was much stronger for wet years that for drought.  This is particularly true for the last thirty years: the relationship to ENSO predicted twice as many droughts as actually occurred.