The westerlies control the zonal migration of rainy season over the Tibetan Plateau

Abstract

Precipitation over the Tibetan Plateau (TP) is modulated by both the South Asian summer monsoon and the midlatitude westerlies. However, their dissimilar impacts on the seasonal evolution of precipitation are not well explored. This study reveals that the onset and retreat of rainy season over the TP, presenting a west-east migration pattern, are mostly controlled by the westerlies. The annual cycle of precipitation amount over the TP is out-of-phase with that of westerly wind speed. The weakening of westerlies favors more precipitation over the TP by reducing input of dry and cold air at the west and output of moist and warm air at the east. Due to the northward shift of westerly jet before July, the weakening of westerlies over the TP leads to a westward shift of the low-level warm air center and associated low-pressure center, and a westward extension of moist air convergence. Consequently, rainy season advances westward. In contrast, the southward shift of westerly jet after August leads to a strengthening of westerlies and an eastward retreat of rainy season.