Interannual variability of East African rainfall: role of seasonal transitions of the low-level cross-equatorial flow

During boreal spring and fall, the low-level cross-equatorial flow along the East African coast weakens and reverses. These transition periods are embedded within the East African rainy seasons—the spring long rains and the fall short rains. ERAI, ERA5, JRA-55, and MERRA2 atmospheric reanalyses and CHIRPS2 and ARC2 rainfall observations are examined to improve our understanding of how the transition periods vary on interannual timescales, and how these variations are related to regional rainfall. Transition period length is positively correlated with spring and fall East African rainfall regionally. Variations in transition period length are associated with the seasonal development and variability of three low-level circulation features, namely, the South Indian Convergence Zone, the Mascarene High over the South Indian Ocean, and the Arabian High over the Arabian Sea. In addition to affecting the low-level moisture transport and convergence over East Africa, these circulation features interact to influence the start and end dates that define the transition period. Composites are constructed and analyzed for different transition period types based on length, start date, and end date to distinguish differences among event types as well as seasonal differences. Air–sea coupling over the western Indian Ocean is stronger for the spring transition than for the fall, modifying precipitation through variations in specific humidity. In the fall transition period, rainfall variations are primarily controlled by circulation variations. These results have implications for climate change prediction over East Africa, for example, as the Indian Ocean warms.