Human influence on summer wetting in Northwest China from 1961 to 2014: Roles of greenhouse gases and anthropogenic aerosols

A significant increase in summer precipitation was observed in Northwest China (NWC) from 1961 to 2014, posing opportunities and challenges for the local ecosystem and human society. Using multi-model pre-industrial control simulations, and historical and single-forcing simulations from the state-of-the-art MIROC6 model, we attributed this wetting trend and explored the relative roles of individual external forcings. Our finding indicated that external forcing is the primary driver of this observed wetting, with anthropogenic forcing—dominated by greenhouse gases (GHG) and secondarily by anthropogenic aerosols (AA)—playing a significant role than natural forcing. Moisture budget analysis indicated that intensified moisture flux convergence, driven by circulation changes, is a key mechanism behind the NWC wetting in both GHG and AA simulations. Specifically, GHG forcing alters the Asian subtropical westerly jet (ASWJ), inducing a southward shift of the western branch and a weakening of the eastern branch. These changes correspond to an anomalous cyclone (anticyclone) in the west (east) of NWC, promoting moisture convergence over NWC of anomalous southwesterly and southeasterly winds. AA forcing weakens the ASWJ, producing a meridional dipole pattern of an anomalous anticyclone (cyclone) in the north (south), along with lower-level anomalous anticyclone over Mongolia and Northeast China. This circulation pattern facilitates moisture transport from the western North Pacific into inland NWC via anomalous easterlies and intensified moisture flux convergence.