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.

从1980年代到2000年代后期，东非长时间降雨出现了下降，这似乎与未来气候变化下的预计增长相反。这种“东非气候悖论”混淆了将气候预测用于整个东非的适应计划。在这里，我们显示下降与长时间降雨的开始和停止有关，面积平均日降雨量具有相似的季节性最大值。先前的研究已经探索了远程远程连接的作用，但是这些机制不足以解释季节性的下降或新近发现的变化。我们使用大量的观测，再分析和大气模拟集合，提出了一种解释所观测到的下降和近期部分恢复的区域机制。阿拉伯和北阿拉伯海温暖的地表压力的下降与南风的增强和长时间降雨的提前停止有关。在许多仅大气模式的模型中，类似的表面压力信号也支持了这一点，从而降低了五月的降雨并提前停止了降雨。东非南部异常温暖的海面推迟了热带雨带的北移，使以后的发作更加缓慢。这些结果是理解悖论的关键。现在，建立导致这些趋势的机制之间的平衡是当务之急，这些趋势在仅大气模拟中会部分体现出来。东非南部异常温暖的海面推迟了热带雨带的北移，使以后的发作更加缓慢。这些结果是理解悖论的关键。现在，建立导致这些趋势的机制之间的平衡是当务之急，这些趋势在仅大气模拟中会部分体现出来。东非南部异常温暖的海面推迟了热带雨带的北移，使以后的发作更加缓慢。这些结果是理解悖论的关键。现在，建立导致这些趋势的机制之间的平衡是当务之急，这些趋势在仅大气模拟中会部分体现出来。