Seasonal asymmetry of equatorial East African rainfall projections: understanding differences between the response of the long rains and the short rains to increased greenhouse gases

Ensemble simulations with a regional climate model on a large domain and 30-km resolution are used to understand why projected precipitation changes under greenhouse gas-forcing are asymmetric across seasons in equatorial East Africa, with rainfall increasing during the short rains (October through December) but not during the long rains (March through May). The model captures an accurate simulation of observed East African precipitation improving over coupled GCM simulations. Future simulations are generated by increasing atmospheric CO₂ according to the RCP8.5 scenario and adding anomalies to observed SSTs as well as initial and lateral boundary conditions derived from coupled GCM simulations. In November, simulated rainfall rates increase by approximately one-third over much of equatorial East Africa by the mid-twenty-first century, and double by the end of the century. The long rains are not significantly increased. The difference in the seasonal response is attributed to differences in the background state. The East African short rains are greatest in November, more than 1 month after the autumnal equinox, when the climatological basic state is in a solstitial pattern. The well-defined heat low over southern Africa and the South Indian Ocean subtropical high to its east are intensified, leading to enhanced moisture convergence over equatorial East Africa. In contrast, the long rains are near their maximum on the vernal equinox, with a continental thermal low centered near the equator that is insensitive to twenty-first century greenhouse gas-induced changes in the subtropical atmospheric hydrodynamics.