Enhanced Internal Moisture Recycling from Eastern to Western Tibetan Plateau Sustains the Recent Increase in Inner Plateau Precipitation

Summer precipitation over the inner Tibetan Plateau (TP) has increased markedly since the 1990s, leading to widespread lake expansion and exerting profound impacts on local ecosystems and infrastructure. While previous studies focused on external moisture transport associated with weakened westerlies, our study identifies a critical yet overlooked internal driver. To quantify the spatial origins of this increase, we employed an Eulerian moisture back-tracking model (Water Accounting Model: WAM-2layers) that decomposes the regional water vapor budget into specific source contributions. Using this framework combined with circulation diagnostics for 1979–2021, we reveal that internal moisture recycling within the TP is essential for sustaining the observed wetting trend. Specifically, the eastern TP (ETP), contributes an amount of moisture to western TP (WTP) precipitation comparable to major external sources. Moreover, the ETP’s contribution has increased by more than 23% compared to the earlier period, surpassing the growth from western and southern external sources. Our analysis bridges the gap between regional moisture budget equations and quantitative source attribution by demonstrating that this enhanced ETP contribution is driven not by local evapotranspiration, but by anomalous easterly winds. These anomalies, associated with a westward shift of the westerly jet core, intensified east-to-west moisture transport, suppressed vertical kinetic energy exchange, and increased lower-tropospheric moisture retention. These results highlight that intensified internal water recycling is a primary mechanism reshaping the regional hydrological balance and accelerating lake expansion in the western TP.