The recent decades-long decline in East African rainfall suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the instrumental record implicates both Indian and Pacific ocean sea surface temperatures (SSTs) as possible drivers of East African multidecadal climate variability, but the short length of the instrumental record precludes a full elucidation of the underlying physical mechanisms. Here we show that on timescales beyond the decadal, the Indian Ocean drives East African rainfall variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal. Our results, based on proxy indicators of relative moisture balance for the past millennium paired with long control simulations from coupled climate models, reveal that moist conditions in coastal East Africa are associated with cool SSTs (and related descending circulation) in the eastern Indian Ocean and ascending circulation over East Africa. The most prominent event identified in the proxy record-a coastal pluvial from 1680 to 1765-occurred when Indo-Pacific warm pool SSTs reached their minimum values of the past millennium. Taken together, the proxy and model evidence suggests that Indian Ocean SSTs are the primary influence on East African rainfall over multidecadal and perhaps longer timescales.
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Acknowledgements J.E.T. acknowledges the US NOAA Climate and Global Change Postdoctoral Fellowship and NSF OCE-1203892 for support. J.E.S. and R.S. were supported by the NOAA award Global Decadal Hydroclimate Variability and Change (NA10OAR431037). We thank the National Center for Atmospheric Research, who made the CCSM4 control simulation available through the US Earth System Grid (ESG) Center. Support for the ESG is provided by the Office of Science, US Department of Energy, with co-sponsorship from the US NSF. Thanks also to NOAA GFDL for providing the multimillennial control simulation output from the GFDL CM2.1 model, to N. Naik for her on-sitesupportatLDEOandtoJ.JungclausoftheMaxPlanckInstituteforprovidingoutput from the MPI COSMOS control simulation. This is LDEO contribution number 7641.
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