‘Eastern African Paradox’ rainfall decline due to shorter not less intense Long Rains

An observed decline in the Eastern African Long Rains from the 1980s to late 2000s appears contrary to the projected increase under future climate change. This “Eastern African climate paradox” confounds use of climate projections for adaptation planning across Eastern Africa. Here we show the decline corresponds to a later onset and earlier cessation of the long rains, with a similar seasonal maximum in area-averaged daily rainfall. Previous studies have explored the role of remote teleconnections, but those mechanisms do not sufficiently explain the decline or the newly identified change in seasonality. Using a large ensemble of observations, reanalyses and atmospheric simulations, we propose a regional mechanism that explains both the observed decline and the recent partial recovery. A decrease in surface pressure over Arabia and warmer north Arabian Sea is associated with enhanced southerlies and an earlier cessation of the long rains. This is supported by a similar signal in surface pressure in many atmosphere-only models giving lower May rainfall and an earlier cessation. Anomalously warm seas south of Eastern Africa delay the northward movement of the tropical rain-band, giving a later onset. These results are key in understanding the paradox. It is now a priority to establish the balance of mechanisms that have led to these trends, which are partially captured in atmosphere-only simulations. Springtime drying trends in East Africa can be explained by shifts in rainfall seasonality and not reductions in daily rainfall. The East African “Long Rains” — which fall during March, April and May — deliver the majority of precipitation to the region. Since the mid-1980s, observations reveal a reduction in rainfall which some climate models fail to capture. Using a suite of observations, reanalyses, and atmospheric simulations, Caroline Wainwright from the University of Reading, UK, and colleagues investigate the mechanisms behind this change. The reduction in Long Rain precipitation is characterised by a later onset and earlier cessation — and thereby a shortening — of the rainy season, attributed to a more rapid movement of the rain band associated with warmer SSTs off East Africa. Understanding these mechanisms will benefit climate projections and adaptation measures in an already-vulnerable region.