Using Seasonal Rainfall Clusters to Explain the Interannual Variability of the Rain Belt over the Greater Horn of Africa

Interannual rainfall in the Greater Horn of Africa (GHA) region is highly variable and the reasons for the decline in the Long Rains (Mar.–May) are not well understood. In addition, few studies have addressed the rainfall fluctuations during the austral summer rainy season (Dec.–Mar.) in the southern GHA region. The interannual variability of the seasonal cycle over the GHA for both seasons is diagnosed for the period 1981-2018 using two statistical indices. The Rainfall Cluster Index (RCI) describes the seasonal cycle as a succession of six characteristic rainfall patterns, while the Seasonal Location Index (SLI) captures the latitudinal location of the tropical rain band. Both indices are based on a principal component analysis of daily rainfall. Wet conditions during April mostly occur in years when the SLI frequently attains its climatological value for this period of the year. These conditions are accompanied by high frequencies of one particular RCI pattern. Drought conditions are associated with high variations and jumps of SLI values and RCI patterns during the MAM season leading to less or intermitted rainfall. Similarly, rainfall anomalies in the austral summer rainy season can be explained through varying occurrences of RCI patterns that enhance or suppress rainfall over the southern GHA. The circulation anomalies associated with the RCI values show relations to sea surface temperatures in the tropical and subtropical oceans that are linked to global and local atmospheric anomalies in kinematic and moisture fields. The results offer new approaches to understand recent rainfall trends in the GHA region.