Using seasonal rainfall clusters to explain the interannual variability of the rain belt over the Greater Horn of Africa

Larisa S Seregina,Joaquim G Pinto,Chris Funk,Andreas H Fink,Roderick Van Der Linden

doi:10.1002/joc.6802

Larisa S Seregina, Joaquim G Pinto + Show 3 more

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https://doi.org/10.1002/joc.6802

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Abstract

AbstractThe seasonal cycle of rainfall over the Greater Horn of Africa (GHA) is dominated by the latitudinal migration and activity of the tropical rain belt (TRB). The TRB exhibits high interannual variability in the GHA and the reasons for the recent dry period in the Long Rains (March–May) are poorly understood. In addition, few studies have addressed the rainfall fluctuations during the Msimu Rains (Dec.–Mar.) in the southern GHA region. Interannual variations of the seasonal cycle of the TRB between 1981 and 2018 were analysed 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 TRB. The SLI and RCI depict the full seasonal cycle of the TRB supporting interpretations of the interannual variations and trends. The Msimu Rains are dominated by two clusters with opposite rainfall characteristics between the Congo Basin and Tanzania. The associated anomalies in moisture flux and divergence indicate variations in the location of the TRB originating from an interplay between low‐level air flows from the Atlantic and Indian Oceans and tropical and subtropical teleconnections. The peak period of the Long Rains shows a complex composition of five clusters, which is tightly connected to intraseasonal and interannual variability of latitudinal locations of the TRB. A persistent location of the TRB near the equator, evidenced in a frequent occurrence of a cluster related to an anomalously weak Walker circulation, is associated with wet conditions over East Africa. Dry Long Rains are associated with strong and frequent latitudinal variations of the TRB position with a late onset and intermittent rainfall. These results offer new opportunities to understand recent variability and trends in the GHA region.

Highlights

The seasonal cycle of rainfall over the Greater Horn of Africa (GHA, cf., Figure 1) is dominated by the latitudinal migration of the tropical rain band (TRB), which follows the seasonal variations of solar insolation and is strongly modulated by topography (Figure 1)
To account for this, we looked into non-overlapping 14-day windows of the Msimu Rains and determined if the occurrence of a cluster had positive or negative anomalies with respect to the climatology
Our results suggest that abundant rainfall in April is characterized by frequent Equatorial East (EE) clusters and related low Seasonal Location Index (SLI) variability, whereas poor rainfall is FIGURE 1 0 Temporal and spatial patterns of rainfall anomalies for April: (a) standardized anomalies of cluster occurrence, (b–f) composite maps of difference between seasonal rainfall anomalies across the region associated with cluster occurrence

Summary

Introduction

The seasonal cycle of rainfall over the Greater Horn of Africa (GHA, cf., Figure 1) is dominated by the latitudinal migration of the tropical rain band (TRB), which follows the seasonal variations of solar insolation and is strongly modulated by topography (Figure 1). To understand how anomalous frequencies of clusters are related to regional circulation anomalies, composite differences of vertically integrated moisture flux, flux divergence, and geopotential at the 700-hPa level between days in a particular cluster and seasonal climatology were calculated (Figure 7).

Results

Conclusion