Abstract:

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.

Key words: West Asian subtropical westerly jet, summer precipitation, annual variation, Three River Source region