Late-spring vertical profiles in atmospheric water vapor stable isotopes reveal westerlies moisture transport across the Himalaya into the Asian Water Towers.

Moisture carried by the mid-latitude westerlies is an important source of water for the Asian Water Towers (AWTs), but how this moisture crosses the high Himalaya has remained unclear. Here, we use a tethered balloon to measure atmospheric water vapor and its isotopic compositions (δDv and d-excessv) from the surface to 9050 m above sea level in late spring (May 2022) on the northern side of Mt. Qomolangma. These measurements show that the westerlies can transport moisture from the south of the Himalaya across the high-altitude barrier, particularly above 7000 m above sea level, into the AWTs. By combining our observations with atmospheric model simulations, we show that the water vapor becomes strongly depleted in heavy isotopes during transport, primarily due to rapid cooling and ice formation high above the mountains. These results provide the first direct evidence, for late spring over the Mt. Qomolangma region, of how moisture from distant ocean regions can reach the interior of the AWTs. Our findings provide an unprecedented understanding of the westerlies advection across the Himalayas in late spring and a benchmark for future climate projections. They also highlight that the interpretation of isotopic records in Himalayan ice cores should consider the influence of multi-sources and kinetic effects aloft in the AWTs.