西亚副热带西风急流年际变化与三江源区夏季降水的关系

Annual relationship between the West Asian subtropical westerly jet and summer precipitation over the Three River Source region

利用1979—2018年中国地面逐月降水量网格数据集和ERA-interim逐月再分析资料,通过定义夏季西亚副热带西风急流(以下简称西亚急流)位置和强度指数,采用统计和动力诊断方法研究了西亚急流位置和强度年际变化与三江源区夏季降水异常的关系,揭示了急流位置变化对三江源区夏季降水的可能影响机制。结果表明:夏季西亚急流南北位置和强度具有明显的年际和年代际变化特征,急流位置在2000s之前表现为南北振荡变化特征,在2000s及其以后以偏南为主。在年际时间尺度上,西亚急流位置对三江源区夏季降水的影响比急流强度更加明显。当急流位置偏南(北)时,三江源区夏季降水偏多(少)。进一步研究发现,在西亚急流偏南年,三江源区位于高层正高度异常和中层负高度异常控制区,全区存在显著的上升运动异常,有利于高原低值系统活动。配合中低层异常气旋性环流东部的偏南风,有利于将热带海洋的暖湿气流输送到源区,导致降水偏多,反之亦然。

Based on the monthly gridded precipitation in China and monthly ERA-interim reanalysis dataset from 1979 to 2018, the annual relationship between the position and intensity of West Asian subtropical westerly jet(WASWJ) and summer precipitation over the Three River Source(TRS) region and their corresponding physical mechanism are discussed. The results show that the position and intensity of WASWJ have evident interannual and interdecadal variability. The position of WASWJ is characterized by north-south oscillation before 2000s and dominated by southerlies during and after 2000s. On the interannual time scale, the position of WASWJ is more important than its strength in influencing summer precipitation over the TRS region. When the WASWJ locates at latitudes more southern than normal(more northern than normal), the summer precipitation over the TRS is higher(less). Further analysis shows that when the WASWJ moves southward, the anomalous upper-level positive and middle-level negative geopotential height and significant ascending movement over the TRS region favor the development of the plateau low-value system. The combination of the anomalous southerly flow in the east of anomalous cyclonic circulation at the middle to lower troposphere, which is beneficial for the warm and humid air flow from tropical ocean to the TRS region, increases precipitation and vice versa.