Influence of Indian Ocean SSTs on the East African Short Rains

Precipitation over equatorial East Africa (30°-40°E, 5°S-5°N) exhibits pronounced regional variation and complicated seasonality. Past studies link interannual variability of the East African short rains during boreal fall with SST variability in the Indian Ocean. The goals of this study are to identify which regions of the Indian Ocean influence the equatorial East African short rains most strongly on interannual timescales, and to investigate the mechanisms of that influence. Three “hot spot” regions are identified, located in the western, central, and eastern Indian Ocean. A regional climate model with 30-km horizontal resolution is used to conduct simulations in which 20-member ensembles represent a control climate and climates with idealized SSTAs. The control simulations use climatological (1998-2017) SSTs. In the idealized simulations, various Gaussian-shaped SSTAs derived from observed correlations are imposed. One additional ensemble uses all three SSTAs together to evaluate the potential for interference among the individually-forced responses.

The control simulations capture the observed patterns and seasonality of East African precipitation and circulation systems with reasonable accuracy, providing confidence in the simulations. The simulated processes that support regional rainfall as revealed by the atmospheric moisture budget also agree well with reanalyses. This analysis shows that the mechanisms that support precipitation in the western and eastern parts of the equatorial East Africa domain are fundamentally different due to the presence of complex topography. The short rains over the western half of the domain are mainly supported by wind convergence in a moist environment, while orographic uplift plays a dominant role farther east.

SSTAs in the western Indian Ocean are shown to exert a stronger influence on the equatorial East African short rains than central and eastern SSTAs in terms of the coverage of significantly-changed precipitation and the magnitude of the precipitation response. For example, positive western Indian Ocean SSTAs significantly increase the short rains over 95% of the equatorial East Africa domain, while only 30% of the region responds to central and eastern Indian Ocean SSTAs. The maximum precipitation anomaly associated with western Indian Ocean SSTAs is about three times larger than the anomalies due to central and eastern SSTAs in the simulation.

The mechanisms of this influence are identified using moisture budget and moist static energy analyses, with reference to Kelvin and Rossby wave generation similar to the Gill model. Wind convergence anomalies in a moist environment mostly support precipitation anomalies in all three cases. Specifically, the increased precipitation over the western domain due to western Indian Ocean SSTAs is mainly supported by zonal wind convergence anomalies associated with anomalous westerlies from the Congo basin. The meridional wind convergence anomaly contributes to increased precipitation over the eastern domain most strongly, in association with meridional perturbations of the Rossby wave response to equatorial heating. Increases in atmospheric instability are provided by increases in low-level moisture due to western Indian Ocean SSTAs. Central and eastern Indian Ocean SSTAs change circulations and precipitation locally over the ocean, but the influence on East Africa is less than for the western Indian Ocean SSTAs.