Increasing summer precipitation in arid Central Asia linked to the weakening of the East Asian summer monsoon in the recent decades

It is generally believed that water vapour from the North Atlantic transported by the mid-latitude westerlies dominates precipitation variations in arid Central Asia, where monsoonal water vapour source is beyond the reach. This study reexamines water vapour sources for arid Central Asia based on daily reanalysis data. The results reveal two water vapour sources for arid Central Asia: the westerly water vapour transport from the North Atlantic and the monsoonal water vapour transport from the tropical Indian Ocean and South China Sea. The monsoonal transport includes two pathways: lower tropospheric water vapour along the eastern and northern periphery of the Tibetan Plateau from the tropical Indian Ocean and South China Sea, and middle-upper tropospheric water vapour via the Tibetan Plateau from the tropical Indian Ocean. Precipitation variability during the monsoonal transport events explains 81% of the total variance. The monsoonal transport is related to a weakened East Asian summer monsoon (EASM) that is accompanied by persistent westward shift of the western Pacific subtropical high and increasing frequency of Mongolian anticyclone activities. The EASM has experienced a persistent weakening trend since 1958, causing an increasing contribution of the monsoonal water vapour transport and thereby enhancing summer precipitation in arid Central Asia. In particular, the monsoonal easterly water vapour has become the dominant source for arid Central Asia in the lower troposphere during the last decade. This study highlights the important role of the EASM in the wetting trend of summers in arid Central Asia during the past six decades.