Synergistic Effect of Monsoon and Westerlies Dominates Spatiotemporal Variation of May Precipitation over Tibetan Plateau and Its Southern Vicinity

Abstract Although the influence of the 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 the Tibetan Plateau and its southern vicinity (VTP) is unclear. Given the peculiarities of May precipitation, this study investigates the synergistic mechanism of the 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 the 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 the increased precipitation over the southeastern VTP is primarily due to the dynamic process. This study could provide some clues for understanding the variation mechanism of VTP precipitation. Significance Statement The Tibetan Plateau (TP) feeds many rivers that provide precious freshwater resource for human life, agriculture, industry, and ecosystem in Asia. Because precipitation is a major supplement to water resources over the TP and its southern vicinity (VTP), its changes affect the supply of water resource to Asia. Precipitation variability over the VTP is influenced by multiple factors, including the South Asian summer monsoon (SASM) and westerlies. Using the latest reanalysis data, this study reports that the synergistic impacts of the SASM and westerlies dominate May precipitation over the VTP on an interannual scale, rather than the influence of either the SASM or the westerlies alone. When SASM is strengthened and the westerlies are weakened in May, their synergistic effect induces an anomalous anticyclone over the western VTP and an anomalous cyclone over the southeastern VTP. These anomalous atmosphere circulations favor enhanced ascending motion and moisture transport convergence over the southeastern VTP, while inducing descending motion and moisture divergence over the western VTP. Consequently, more precipitation occurs over the southeastern VTP and less precipitation over the western VTP. Therefore, it is necessary to consider the synergistic impacts of the SASM and westerlies when studying the variation mechanism of precipitation over the VTP.