Moisture and Precipitation Sources in Northwest China: A Humidified Context Analysis (1982-2021)

Northwest China (NWC) is situated in an inland arid or semi-arid region, rendering its ecosystem highly susceptible to precipitation changes. This study employs the CAM5.1-based atmospheric water tracer approach to investigate the characteristics and patterns of long-term moisture and precipitation sources in the NWC from 1982 to 2021, within a humidified context. Our findings indicate that, in addition to local evaporation contributing to 18.0% (0.37 mm d-1) of precipitation, terrestrial sources in North Asia (NA), Europe (EUP), the Qinghai-Tibet Plateau (TP), and Southeastern China (SEC) represent the primary moisture source regions in the NWC, contributing 19.6% (0.93 g kg-1), 16.0% (0.75 g kg-1), 5.0% (0.24 g kg-1), and 4.5% (0.21 g kg-1) of moisture, respectively. However, the local precipitation efficiency of some source areas varies due to the synoptic pattern of the precipitation source and the local atmospheric vertical movement. Specifically, the conversion rate of precipitation from EUP and NA is lower than that of TP and SEC. Furthermore, although humidification in the NWC displays an overall upward trend, it has become significantly stronger since 2000, primarily due to the increase in local evaporation and TP's greater contribution to precipitation in the NWC, which has increased by 20.4% (0.07 mm d-1) and 19.6% (0.05 mm d-1), respectively. Our analysis also reveals that, despite humidification, there are still dry and wet years in the NWC, with substantial interannual variations in precipitation. This phenomenon may be attributable to the increased moisture transport from direct land sources (NA & TP) in wet years, which increases precipitation by 81.1% (0.13 mm d-1) and 52.8% (0.08 mm d-1), respectively. Additionally, some of the increased moisture from oceanic sources, mainly in the North Indian Ocean (NIO) and the Pacific Northwest (NWP), is transported directly to the NWC, while the remainder is indirectly transported to the NWC by local evaporation, contributing to the increase in annual wet precipitation in the NWC as an indirect oceanic source.