Increased extreme precipitation in May over southwestern Xinjiang in relation to Eurasian snow cover in recent years

In this study, the interdecadal variation of extreme precipitation in May over southwestern Xinjiang (SWX) and related mechanisms were investigated. The extreme precipitation in May over SWX exhibited a decadal shift in the 1990s (negative phase during 1970–86 and positive phase during 2003–2018). The decadal shift corresponded to strengthened moist airflow from the Indian Ocean and an anomalous cyclone over SWX during 2003–2018. It is found that the interdecadal change of the wave trains in Eurasia might account for the differences in atmospheric circulation between the above two periods. Further analyses reveal that spring snow cover over Eurasia is closely linked to extreme precipitation over SWX during 2003–2018. Increased snow cover in western Europe (WE) from February to March is accompanied by more snowmelt. This resulted in less local snow cover and lower albedo which lead to warm temperature over WE in May. The changes in temperatures increase the local 1000–500-hPa thickness over WE. These factors provide favorable conditions for the enhancement of the Eurasian wave trains which significantly influence extreme precipitation over SWX. On the other hand, corresponding to decreased albedo caused by the reduction of northern Eurasia (NE) snow cover in May, anomalous surface warming occurs over NE. The anomalous warming result in positive geopotential height anomalies which intensifies the meridional geopotential height gradient over Eurasia and causes an acceleration of the westerly jet in May. Anomalous upper-level divergence in SWX induced by the enhanced westerly jet provides a favorable dynamical condition for increased extreme precipitation.