In early 2018, Cape Town (population ~3.7 million) was at risk of being one of the first major metropolitan areas in the world to run out of water. This was due to a severe multi-year drought that led to the levels of supply dams falling to an unprecedented low. Here we analyze rainfall data from the city catchment areas, including rare centennial records from the surrounding region, to assess the severity of the 2015–2017 drought. We find that there has been a long-term decline in the number of winter rainfall days, but this trend has been generally masked by fluctuations in rainfall intensity. The recent drought is unprecedented in the centennial record and represents a combination of the long-term decline in rainfall days and a more recent decline in rainfall intensity. Cold fronts during the winter months are responsible for most of the rainfall reaching Cape Town and our analysis shows no robust regional trend in the number of fronts over the last 40 years. Rather, the observed multidecadal decline in rainfall days, which threatens to increase the occurrence of severe drought, appears to be linked to a decrease in the duration of rainfall events associated with cold fronts. This change in rainfall characteristics associated with fronts appears to be linked to Hadley Cell expansion seen across the Southern Hemisphere and an increasing trend in post-frontal high-pressure conditions that suppress orographically enhanced rainfall.

在2018年初，开普敦（人口约370万）有可能成为世界上第一个缺水的主要大都市区之一。这是由于严重的多年干旱导致供应水坝的水位降至前所未有的低位。在这里，我们分析了城市集水区的降雨数据，包括周围地区的罕见百年记录，以评估2015-2017年干旱的严重程度。我们发现，冬季降雨天数已长期下降，但这一趋势通常被降雨强度的波动所掩盖。最近的干旱在百年记录中是前所未有的，代表着降雨天数的长期下降和降雨强度最近的下降的结合。冬季的冷锋是到达开普敦的大部分降雨的原因，我们的分析表明，过去40年中，冷锋的数量没有明显的区域性趋势。相反，观测到的降雨天数十年减少，可能会增加严重干旱的发生，这似乎与与冷锋有关的降雨事件持续时间的减少有关。与锋面相关的降雨特征的这种变化似乎与整个南半球的Hadley细胞膨胀以及抑制地形地形势的降雨的后额高压条件的增加趋势有关。可能增加严重干旱的发生，这似乎与与冷锋有关的降雨事件持续时间的减少有关。与锋面相关的降雨特征的这种变化似乎与整个南半球的Hadley细胞膨胀以及抑制地形地形势的降雨的后额高压条件的增加趋势有关。可能增加严重干旱的发生，这似乎与与冷锋有关的降雨事件持续时间的减少有关。与锋面相关的降雨特征的这种变化似乎与整个南半球的Hadley细胞膨胀以及抑制地形地形势的降雨的后额高压条件的增加趋势有关。