Eastern Africa exhibits bimodal rainfall consisting of long rains (March-May) and short rains (October-December), changes in which have profound socioeconomic and environmental impacts. In this Review, we examine the drivers and corresponding impacts of Eastern African rainfall variability. Remote teleconnections, namely the El Niño-Southern Oscillation and the Indian Ocean Dipole, exert a dominant influence on interannual variability. From the mid-1980s to 2010, the long rains have tended toward a drier state (trends of −0.65 to −2.95 mm season −1 year −1 ), with some recovery thereafter, while the short rains have become wetter since the mid 1980s (1.44 to 2.36 mm season −1 year −1 ). These trends, overlain by substantial year-to-year variations, affect the severity and frequency of extreme flooding and droughts, the stability of food and energy systems, the susceptibility to water-borne and vectorborne diseases, and ecosystem stability. Climate model projections of rainfall changes differ, but there is some consensus that the short rains will deliver more rainfall than the long rains by 2030-2040, with implications for sustaining agricultural yields and triggering climaterelated public health emergencies. Mitigating the impacts of future Eastern African climate requires continued investments in agriculture, clean water, medical and emergency infrastructures, and development and adoption of adaptation strategies, as well as targeted early-warning systems driven by improved meteorological observations. Sections 2015 coincided with a weaker IOD, producing anomalies ~50% above the climatological mean 18 . However, these relationships are nonlinear, as demonstrated by extreme 2019/2020 rainfall that occurred during an anomalously positive phase of the IOD but neutral ENSO conditions 20 .The IOD and ENSO physically influence Eastern African short rains by modifying regional atmospheric circulation features (Fig. 2a,b).Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author selfarchiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Interannual variability of precipitation over eastern Africa (Somalia, Ethiopia, and Kenya) is impacted by the Indian Ocean dipole (IOD), which has its maximum amplitude during autumn. During northern spring, sea‐surface temperature (SST) in the tropical Indian Ocean is nearly always sufficiently high to sustain convection, but it exhibits no clear pattern of variability. Thus, precipitation variability over Eastern Africa during spring is not connected to any Indian Ocean interannual mode sustained by SST anomalies. Yet, seasonal variability in atmospheric convection might sustain interannual modes during spring independent of SST. We construct an index for the IOD based on outgoing long‐wave radiation anomaly (OLRA) instead of SST anomaly. During northern spring, analysis of this index shows that interannual precipitation over Eastern Africa is correlated with interannual variability similar to the IOD, as measured by OLR. The largest part of that relationship originates from the western Indian Ocean, with smaller contributions from the eastern Indian Ocean. Results indicate that atmospheric convection over the tropical Indian Ocean couples the atmospheric circulation over the eastern and western Indian basin irrespective of SST anomalies. Results from constructed analogue analysis show that positive SST anomaly during northern winter caused by persistence of subsidence and cloud–radiation–SST negative feedback over the southeastern Indian Ocean from the previous positive IOD is associated with formation of negative IOD later during the following autumn.

Stratospheric Kelvin waves are often understood as plane gravity waves, yet tropospheric Kelvin waves have been interpreted as a superposition between the baroclinic modes. Fourier filtering is used to decompose the ECMWF‐Interim reanalysis dynamical fields into upward and downward propagating components. Then wavelet regression is used to isolate the propagating Kelvin waves over the Indian Ocean across different speeds at zonal wavenumber 4. Results for fast waves show dry upward‐phase signal in the troposphere, while downward‐phase Kelvin waves occupy most of the stratosphere. The presence of upward‐phase tilted waves in the troposphere suggests that the tropospheric Kelvin wave is not a superposition of the upward and downward components, as one might expect in a normal mode. We found that propagating Kelvin waves in the troposphere obey gravity wave dynamics with geopotential height in phase with the zonal wind, the vertical velocity out of phase with the zonal wind, and the temperature in quadrature with the zonal wind. Both dry and moist tropospheric Kelvin waves show a westward vertical tilt, suggesting that tilt probably cannot be a superposition between baroclinic modes coupled to convective and stratiform heating. In the context of radiating gravity waves, results suggest that faster tropospheric Kelvin waves appear to be associated with higher Brunt–Väisälä frequencies, and waves maintain similar vertical tilt across a wide range of phase speeds.

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