Synergistic Effect of Monsoon and Westerlies Dominate Temporospatial Variation of May Precipitation over Tibetan Plateau and Its Southern Vicinity

Although the influence of South Asian summer monsoon (SASM) and westerlies on precipitation has been widely investigated, the synergistic effect of the dominant configuration of the SASM and westerlies on precipitation over Tibetan Plateau and its southern vicinity (VTP) is unclear. Given the peculiarities of May precipitation, this study investigates the synergistic mechanism of SASM and westerlies affecting May precipitation over the VTP on an interannual scale by using the latest reanalysis data. The results show that the east-west dipole pattern of May precipitation over the VTP is dominated by the synergistic effect of SASM and westerlies, rather than by either system alone. This synergistic effect accounts for over 69% of the interannual precipitation anomaly in May compared to the SASM (47%) or westerlies (22%) alone. Mechanistically, the strengthened SASM and weakened westerlies jointly induce an anomalous anticyclone over the western VTP and a cyclone over the southeastern VTP, which favors more water vapor transport to the southeastern VTP and less water vapor output, leading to moisture convergence. Moreover, the weakened westerlies and strengthened SASM enhance vorticity advection and temperature advection, resulting in ascending motions over the southeastern VTP. These circulation patterns favor descending motions and moisture divergence over the western VTP. Consequently, more precipitation occurs over the southeastern VTP and less precipitation over the western VTP. The moisture budget analysis further substantiates that increased precipitation over southeastern VTP is primarily due to the dynamic process. This study could provide some clues for understanding the variation mechanism of VTP precipitation.