The "Asian Water Towers" (AWTs), a high-altitude region with a mean elevation exceeding 4,000 meters, serve as the primary freshwater source for nearly two billion people. While the Indian Summer Monsoon is a well-known controller of seasonal rain, a team supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) has finally solved a key knowledge gap: how the mid-latitude Westerlies, which dominate the region for three-quarters of the year, integrate their moisture into the local water cycle under non-precipitating conditions.

A study led by Professor Gao Jing and Yao Tandong from the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, in collaboration with international scientists, has identified a "vertical conveyor" atmospheric mechanism. This finding reveals how moisture from high-altitude winds is delivered to the plateau through a complex process of nocturnal "decoupling". It is published online in Proceedings of the National Academy of Sciences (PNAS) under the title, “Vertical conveyor driving the integration of moisture transported by the Westerlies into the Asian Water Towers’ atmospheric water cycle" (https://www.pnas.org/doi/10.1073/pnas.2529749123).

To understand these dynamics, the team combined in-situ vertical observations with the state-of-the-art ECHAM6-wiso isotope-enabled atmospheric model, providing the first unified, process-based picture of how the AWT’s atmospheric water was supplied. Using specialized helium-tethered "Jimu Balloons," the team captured 32 unprecedented vertical profiles of atmospheric water vapor stable isotopes (δDᵥ and d-excessᵥ) and meteorological parameters at Lulang, a forested moisture corridor, and Nam Co, a high-altitude inland lake. These isotopes allowed the team to identify a highly stratified atmospheric structure: Atmospheric Boundary Layer, located at roughly 600–900 meters, where locally sourced moisture is shaped by diurnal cycles; The Mixed Layer, an intermediate zone between 600 and 1,600 meters characterized by minimal isotopic variance; Free Troposphere, found above 1,600–1,800 meters, where large-scale Westerlies transport moisture across the Himalayan barrier.

The study reveals that the remote-source atmospheric water vapor undergoes Westerlies subsidence, which causes large-scale advection to descend toward the AWT’s atmospheric boundary layer (Fig. 1). As this moisture sinks, it interacts with local air, creating two distinct thermal inversion layers. These layers act as physical "caps" that suppress vertical mixing and decouple atmospheric water vapor in distinct layers. This decoupling isolates the cold, dry Westerlies moisture aloft from the relatively moist, local air trapped within the atmospheric boundary layer. This process constitutes a primary pathway for integrating westerlies-advected moisture into the local moisture budget without precipitation, sustaining near-surface moisture accumulation. Even without precipitation, approximately 30% of the moisture flux transported by the Westerlies is integrated into the local cycle through phase transitions at night.

These findings arrive at a critical juncture as anthropogenic warming drives rapid hydrological transitions, including accelerated glacier retreat and altered runoff patterns. The findings of this study provide critical benchmarks for improving atmospheric models, optimizing climate projections of the accelerating water cycle in the AWTs, and advancing the climatic interpretation of regional isotopic records, such as those from ice cores.

Fig. 1. Schematic illustration of the two decoupled conveyor mechanisms driving the vertical integration of moisture advected by the westerlies into the atmospheric water cycle on the AWTs.