Although climatologically dry, the western slopes of the southern Andes of Peru (WSA) can experience precipitation extremes (PEs) during the summer (December–February) resulting in great economic and human losses. Generally, WSA has a positive upslope gradient in precipitation, meaning more rain falls at higher elevations, but observations have shown this gradient can become negative with higher rainfall near the coastal cities. In this study we analyse 2000–2019 regional atmospheric patterns associated with different upslope precipitation gradients and PEs in WSA using principal component analysis methods and surface station observations. Results show important changes in the atmospheric circulation patterns during the occurrence of PE events. A prevailing pattern of negative southerly wind anomalies and regional warming of the southeastern Pacific Ocean leads to significant increases in moisture along the coast of WSA. Eastern moisture flows associated with the presence of the Bolivian High are observed at upper levels of the atmosphere and transport water vapour from the Amazon to the western side of the Andes. Additionally, there is a blocking effect aloft in response to an intense gradient of geopotential height that attenuates the easterly circulations. These large-scale mechanisms act to concentrate high precipitable water amounts and high levels of convective available potential energy in the troposphere which favours the vertical velocities essential to trigger PEs. These results increase our knowledge of the large-scale characteristics of PEs to help with forecasting these impactful events and protecting the more than 1.8 million people living in WSA.

中文翻译：

---

夏季极端降水和大气流动对秘鲁南安第斯山脉西坡的影响

尽管气候干燥，但秘鲁南部安第斯山脉 (WSA) 的西坡在夏季（12 月至 2 月）可能会出现极端降水 (PE)，从而造成巨大的经济和人员损失。一般来说，WSA 的降水量呈正上升坡度，这意味着海拔较高的地区降雨量较多，但观察表明，随着沿海城市附近降雨量增加，这种梯度可能会变为负值。在这项研究中，我们使用主成分分析方法和地面站观测分析了 2000-2019 年与 WSA 中不同上坡降水梯度和 PE 相关的区域大气模式。结果表明，在 PE 事件发生期间，大气环流模式发生了重要变化。负向南风异常和东南太平洋区域变暖的普遍模式导致 WSA 沿岸的水分显着增加。在大气层上层观察到与玻利维亚高压存在相关的东部水分流，并将水蒸气从亚马逊河输送到安第斯山脉的西侧。此外，在高空存在阻挡效应以响应减弱东风环流的强烈位势高度梯度。这些大规模机制的作用是将高可降水量和高水平的对流可用势能集中在对流层中，这有利于触发 PE 所必需的垂直速度。