An examination of the recent droughts in the Horn of Africa and their predictability

The rainfall regime in East Africa is exceedingly complex. To the north, mainly in Ethiopia and the southern Sudan, is a summer rainfall regime. To the south is an equatorial regime with two rainy seasons during the transition months. It prevails in most of Kenya, Somalia, Tanzania, Uganda, and southern Ethiopia. Recent rainfall variability and its seasonal predictability have been examined for all three seasons. The analysis suggests that drought has become more frequent and more intense. They have also become more persistent, extending through all three rainy seasons. While drought has prevailed during the last decade, this interval has also included extreme flood conditions that likewise have devastating consequences. “Drought”, i.e., widespread conditions of well below average rainfall, occurred during 1998, 2000, 2005/06, 2007, 2008, 2009 and 2011. The years 2008 to 2011 were particularly severe: rainfall was 30% to 75% below average over the Horn of Africa, Kenya and most of the Sudan. Notably, all of these episodes affected both the summer and equatorial rainfall regions. The year 2010 was also abnormally dry in the summer rainfall region. The drought situation became much more severe during late 2010 and early 2011; rainfall was at least 50% to 75% below normal in roughly half of the drought-stricken region. Within months record floods ensued. The predictability of each of the three rainy seasons affecting the Horn of Africa is also examined, using multiple linear regression and cross-validation. In contrast to most other empirical forecast models, atmospheric dynamics are emphasized. Excellent predictability is noted up to five months in advance for both the summer and equatorial rainfall regions during the boreal autumn and two months in advance for the boreal summer in the summer rainfall region. There is also excellent predictability for the short rains of the equatorial region up to two months in advance. Two notable findings are that atmospheric variables generally provide higher forecast skill than surface variables, such as sea-surface temperatures and sea-level pressure, and that ENSO and the Indian Ocean Dipole provide less forecast skill than variables associated with them. As in other studies, the results showed that the spring predictability barrier limits the lead-time for the forecasting of spring and summer rainfall.