Isotope Profiles in Atmospheric Water Vapor Reveal Vertical Moisture Transport process to the Asian Water Towers

While moisture transported by the mid-latitude westerlies constitutes a critical hydrological source for the Asian Water Towers (AWTs), the mechanisms governing its transport across the Himalayan barrier have remained elusive. In this study, we utilized a tethered balloon system to conduct high-altitude vertical profiling of atmospheric water vapor and its stable isotopic compositions (δD and d-excess) from the surface up to 9,050 meters above sea level (asl) on the northern slope of Mt. Qomolangma. Our measurements reveal that the westerlies can effectively facilitate the trans-Himalayan transport of moisture sourced from south regions. By integrating these observations with atmospheric model simulations, we demonstrate that water vapor undergoes significant isotopic depletion during transit. These findings provide the first direct empirical evidence of the pathways through which moisture from oceanic basins transports into the AWT interior. Furthermore, our results offer an unprecedented understanding of westerly advection across the Himalayas, establishing a crucial benchmark for future climate projections.