The timing of the rainy season is essential for several climate-sensitive sectors in Eastern Africa.

This is particularly true for the agricultural sector, where most activities depend on both the spatial and temporal distribution of rainfall throughout the season. Using a combination of observational and reanalysis datasets, the present study investigates the atmospheric and oceanic conditions associated with the early and late onset of Eastern Africa's short rain season (October–December).

Our results indicate enhanced rainfall in October and November during years with early onset and a rainfall deficit in years with late onset for the same months. Early onset years are found to be associated with warmer sea surface temperatures (SSTs) in the western Indian Ocean, and an enhanced moisture flux and anomalous low-level flow into Eastern Africa as early as the first decade of September. The late onset years are characterized by cooler SSTs in the western Indian Ocean, anomalous westerly moisture flux, and zonal flow limiting moisture supply to the region.

We split the changes in the onset date into two groups: those that happen every year and those that happen every decade. For each group, we look at the links with SSTs and low-level circulation over the Indian Ocean basin separately. Significant correlations are found between the interannual variability of the onset and the Indian Ocean dipole mode index.

On decadal timescales, the onset is shown to be partly driven by the variability of the SSTs over the Indian Ocean.

Understanding the influence of these potentially predictable SST and moisture patterns on onset variability has huge potential to improve forecasts of the East African short rains. Improved prediction of rainy season onset variability has huge implications for improving key strategic decisions and preparedness actions in many sectors, including agriculture.