Centre for Ecology and Hydrology, Wallingford OX10 8BB, UK.
National Centre for Earth Observation, Wallingford OX10 8BB, UK.
Nature. 2017 Apr 26;544(7651):475-478. doi: 10.1038/nature22069.
The hydrological cycle is expected to intensify under global warming, with studies reporting more frequent extreme rain events in many regions of the world, and predicting increases in future flood frequency. Such early, predominantly mid-latitude observations are essential because of shortcomings within climate models in their depiction of convective rainfall. A globally important group of intense storms-mesoscale convective systems (MCSs)-poses a particular challenge, because they organize dynamically on spatial scales that cannot be resolved by conventional climate models. Here, we use 35 years of satellite observations from the West African Sahel to reveal a persistent increase in the frequency of the most intense MCSs. Sahelian storms are some of the most powerful on the planet, and rain gauges in this region have recorded a rise in 'extreme' daily rainfall totals. We find that intense MCS frequency is only weakly related to the multidecadal recovery of Sahel annual rainfall, but is highly correlated with global land temperatures. Analysis of trends across Africa reveals that MCS intensification is limited to a narrow band south of the Sahara desert. During this period, wet-season Sahelian temperatures have not risen, ruling out the possibility that rainfall has intensified in response to locally warmer conditions. On the other hand, the meridional temperature gradient spanning the Sahel has increased in recent decades, consistent with anthropogenic forcing driving enhanced Saharan warming. We argue that Saharan warming intensifies convection within Sahelian MCSs through increased wind shear and changes to the Saharan air layer. The meridional gradient is projected to strengthen throughout the twenty-first century, suggesting that the Sahel will experience particularly marked increases in extreme rain. The remarkably rapid intensification of Sahelian MCSs since the 1980s sheds new light on the response of organized tropical convection to global warming, and challenges conventional projections made by general circulation models.
水文循环预计将在全球变暖的情况下加剧，研究报告显示，世界许多地区的极端降雨事件更加频繁，并预测未来洪水频率会增加。由于气候模型在描述对流降雨方面存在缺陷，因此这种主要发生在中纬度地区的早期观测结果至关重要。一类具有全球重要意义的强风暴——中尺度对流系统（MCS）——构成了一个特殊的挑战，因为它们在传统气候模型无法解决的空间尺度上动态组织。在这里，我们利用来自西非萨赫勒地区的 35 年卫星观测数据，揭示了最强烈的 MCS 发生频率持续增加的现象。萨赫勒风暴是地球上最强大的风暴之一，该地区的雨量计记录到“极端”日降雨量总和的上升。我们发现，强烈的 MCS 频率与萨赫勒年降雨量的多十年恢复仅呈弱相关，但与全球陆地温度高度相关。对非洲各地趋势的分析表明，MCS 加剧仅限于撒哈拉沙漠以南的狭窄地带。在此期间，雨季萨赫勒地区的气温并未升高，排除了降雨因当地条件变暖而加剧的可能性。另一方面，横跨萨赫勒地区的干湿季温度梯度在最近几十年有所增加，这与人为驱动的撒哈拉变暖增强一致。我们认为，撒哈拉变暖通过增加风切变和撒哈拉气层的变化，加剧了萨赫勒 MCS 中的对流。预计在整个 21 世纪，南北梯度将增强，这表明萨赫勒地区将经历特别明显的强降雨增加。自 20 世纪 80 年代以来，萨赫勒 MCS 迅速加剧，这为有组织的热带对流对全球变暖的响应提供了新的认识，并对通用环流模型做出的传统预测提出了挑战。
