Abstract
Precipitation in the late cold season (February–April) over subtropical East Asia (STEA) exhibits a negative trend from 1979 to the present, primarily due to weakening ascent motion. This study investigates the mechanism of weakening ascent over STEA and identifies the poleward-shifting westerlies as a key driver. On an interannual time scale, a poleward shift of the westerlies leads to weakened southern branch westerlies (SBW) on the southern side of the Tibetan Plateau (TP), which further leads to a weaker downstream southerly wind component over STEA, associated with weaker ascent motion generated by isentropic upglide of the southerly flow. During the post-1979 epoch, the SBW shows a substantial weakening trend in the late cold season as a regional manifestation of the planetary-scale poleward-shifting westerlies, which suppresses the ascent motion over STEA and accounts for more than half of the decreasing precipitation over STEA in the late cold season. TP plays a key role in connecting SBW to the southerly flow and ascent motion over STEA in the cold season, and the absence of weakening SBW also explains the absence of a drying trend over STEA in the early cold season. External forcing leads to a moderate poleward shift of planetary-scale westerlies in the entire cold season based on climate model simulations, but it cannot explain the seasonality and magnitude of the observed trend in SBW. Decadal-scale internal variability may have largely boosted the poleward-shifting westerlies in the late cold season but offset it in the early cold season.
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