East African rainfall seasonality based on modulated annual cycle (MAC)

The spatial and temporal variability of East African rainfall seasonal cycle is investigated based on modulated annual cycle (MAC). Ensemble empirical mode decomposition (EEMD) is used to extract MAC from monthly data for the period 1900 to 2008.  The MAC is given by year-to-year changes in annual and semi-annual components. Results show that MAC account for between 45 to 80% proportion of rainfall variability. Spatial structure of MAC shows that it captures the north-south (equatorial) pattern for annual (semi-annual) mode with explained variance of 54% (22%). The spatial structure of MAC further reveals that the seasonality of rainfall over the whole region does not follow the north/south classical scheme where the semi-annual component dominates the equatorial while the annual component dominates areas relatively far from the equator. MAC amplitude tends to be high during anomalous rainfall indicating that the impact of major climate processes that produce seasonal cycle are amplified or suppressed by interannual to longer timescale climate variability modes. This study has shown variability in the seasonal cycle of East African rainfall on interannual-to-longer time scales, which should be considered in studies on variability of rainfall and in seasonal- to-longer time scale prediction of rainfall over the region.   Key words: East Africa, rainfall variability, modulated annual cycle, empirical mode decomposition.