Contrasting Terrestrial Moisture Controls on Precipitation and Interannual Variability across the Southern and Northern Tibetan Plateau

The Tibetan Plateau (TP) plays a crucial role in the Asian hydrological cycle, yet the dominant moisture sources and their governing physical processes remain debated. Here, we employ the Eulerian water tracer technique embedded in Community Earth System Model version 1 (CESM1), together with Lagrangian tracking and isotopic analysis, to quantify moisture sources of summer and winter precipitation over the TP during 2003–2021. We show that, within the Eulerian AWT source-attribution framework, terrestrial sources dominate both climatological mean and interannual variability of TP precipitation, with much stronger terrestrial control over the northern TP (NTP) than over the southern TP. Terrestrial sources account for 78.9 ± 12.1% of summer and 68.4 ± 10.0% of winter precipitation over the NTP, mainly from Western Europe (WEU), Central Asia (CA), Northern Africa (NAF), and TP evapotranspiration. This terrestrial dominance arises because most of the moisture originating from remote oceans is depleted before directly reaching the TP. Specifically, remote ocean-derived moisture (e.g., North Atlantic Ocean) is first deposited as precipitation over intermediate continental regions (WEU–CA and NAF), then re-enters the atmosphere through evapotranspiration, and is subsequently transported farther downstream to the TP. We identify this process as a terrestrial “moisture relay” mechanism. This relay mechanism also regulates interannual precipitation variability over the NTP, favoring enhanced convective precipitation during wet NTP summers and increased large-scale precipitation during wet NTP winters. Our results suggest that precipitation over the TP, especially over the NTP, is primarily controlled by the partitioning between precipitation loss and re-evaporation over upstream continental regions.