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 29^oE to 34^oE.

Keywords: Rainfall, Seasonality, Annual, Semi-annual, East Africa