Terrestrial moisture sources dominate summer precipitation fluctuations in Northwest China

Abstract Northwest China (NWC), is characterized by its arid and semi-arid environment, and exhibits high sensitivity to precipitation variations. Recent research indicates a wetting tendency over NWC, yet quantifying its moisture source remains challenging. Here, employing a 40 year simulation with Community Atmosphere Model version 5.1 (CAM5.1) coupled to an atmospheric water tracer algorithm, we ascertain that the dominant source of summer moisture over NWC are from terrestrial sources (82% of vapor and 77% of precipitation), i.e. from local evaporation, North Asia (NA), Europe (EUP), the southern Tibetan Plateau (STP), and southeastern China (SEC), rather than oceanic sources. Favorable synoptic patterns over NWC enhance the precipitation-conversion efficiency from the southeasterly airflow transport (STP and SEC) compared to northwesterly airflow (NA and EUP). We also find that the fluctuations in precipitation over NWC, manifesting as alternating dry and wet summers, are primarily driven by increased moisture contributions from direct inputs from NA and re-evaporation transport from STP. Our study indicates that moisture variability in inland is predominantly driven by nearby terrestrial sources and underscores the complex mechanisms of terrestrial moisture transport.