Impacts of Future Non-Growing-Season Vegetation Increase on Winter Snow Cover over the Tibetan Plateau and the Regional Climate

Vegetation and snow cover over the Tibetan Plateau (TP) strongly regulate land–atmosphere energy and water exchanges and alter the thermal forcing of the TP and thereby influence regional climate. Against the background of climate change, pronounced vegetation greening has been observed and is projected to continue in the future. However, the potential impacts of increased non-growing-season vegetation, particularly exposed withered grass stems, on winter snow cover and regional climate remain poorly understood. Using numerical experiments conducted with the Community Earth System Model version 2.2 (CESM2.2), we investigate the effects of increased withered grass stems (WGS) over the TP on winter snow cover fraction, the surface energy budget, atmospheric circulation, precipitation, and near-surface air temperature. The results show that increased WGS reduces the winter-mean snow cover fraction by approximately 11.1%, with the largest decrease occurring in December (16.7%), primarily over the central and eastern TP. Greater exposure of WGS above the snow-pack lowers surface albedo and increases net shortwave radiation, thereby enhancing surface diabatic heating. The enhanced thermal forcing induces warming in the low-er-to-middle troposphere over the TP and its southern flank, but cooling to the north. Correspondingly, at 500 hPa, anticyclonic circulation anomalies develop to the south of the TP, while cyclonic anomalies occur to the north. These circulation changes further modify large-scale horizontal heat and moisture transport. Enhanced moisture convergence leads to increased winter precipitation over the Yangtze River basin, regions to its south, and the western TP, whereas enhanced moisture divergence contributes to reduced precipitation over the eastern Indochina Peninsula and adjacent regions. The cooling north of the TP is mainly associated with anomalous horizontal heat transport, whereas warming over the TP is directly related to the WGS-induced reduction in surface albedo and increase in net surface radiation. These results highlight that changes in non-growing-season vegetation can substantially modify wintertime thermal forcing over the TP through vegetation–snow–surface energy interactions and, in turn, exert considerable impacts on atmospheric circulation and regional climate over surrounding and downstream regions.