Spatial distribution of rainfall seasonality over East Africa

In this study, we have investigated the geographical variations of rainfall seasonality components over East Africa on the basis of pentad rainfall data at 36 stations during the period 1962-2006. Harmonic analysis was used to model the annual and semi-annual modes of rainfall seasonality at each station. The ratio of the modeled semi-annual range (R2) to annual range (R1) was used as an objective index to measure the degree of bimodal or unimodal behavior of rainfall at a given station. Areas of unimodal/bimodal regime were qualitatively delineated based on the difference of the fractional variance (V1-V2) explained by each of the modes. Results show that negative values of V1-V2, signifying bimodal regime, compares well to R2/R1 exceeding 0.3. Stations in areas of transition between annual and semi-annual regimes have values of V1-V2 near zero indicating that neither of the two modes dominates. Results further show that the seasonality of rainfall over the whole region does not follow the classical scheme of north-south pattern with bimodal (unimodal) dominance in areas in the vicinity (north/south) of the equator. This scenario is only true for the western strip of the region between longitudes 29oE to 34oE.   Keywords: Rainfall, Seasonality, Annual, Semi-annual, East Africa