Local Evapotranspiration and Atlantic Decadal Variability Dominate the Humidification of Northwest China

Over recent decades, Northwest China (NWC) has experienced significant precipitation enhancements under global warming. However, the moisture sources and mechanisms of the increased precipitation remain controversial, a critical issue for projecting the persistence of the humidification trend. Previous studies have generally attributed the increased precipitation to enhanced external moisture transport. Using the dynamic precipitation recycling model (DRM) combined with dynamic circulation diagnostic methods, this study systematically examined the physical mechanisms of summer precipitation changes in NWC during 1961–2020. The results reveal a precipitation trend shift that occurred around 1997, followed by a significant, 9.18% increase in precipitation after 1997. Although external moisture transport remains the dominant source of climatological precipitation, the decadal increase has been governed by local evapotranspiration, whose growth outpaced net external inflow changes. This change was primarily modulated by zonal planetary waves driven by the Atlantic Multidecadal Oscillation (AMO), placing NWC under anomalous ascent and moisture convergence. During 1998–2020, the AMO-induced circulation simultaneously reduced eastward moisture export from NWC and favored local recycling by enhancing evapotranspiration (+9.12%) and recycled precipitation (+10.51%). Temperature-induced cryospheric melt and vegetation recovery amplified the local evapotranspiration, allowing the recycled moisture component to become the primary contributor to the precipitation increase (78%), surpassing the contribution from advected moisture (22%). Despite recent wetting, water budget constraints acting in conjunction with a likely AMO phase transition imply that the humidification may not persist in the future.