A groundbreaking study has unveiled how massive, high-altitude wind currents — known as the westerlies — deliver essential moisture to the Qinghai-Tibet Plateau. Often called the "Asian Water Towers", this region serves as a critical freshwater source for nearly two billion people downstream.

The research, led by Gao Jing and Yao Tandong from the Chinese Academy of Sciences' Institute of Tibetan Plateau Research, alongside international scientists, solves a long-standing puzzle: how these high-altitude winds pump moisture into the local environment even when it is not raining or snowing.

The findings were published in the journal Proceedings of the National Academy of Sciences on May 6.

As rising temperatures accelerate glacier retreat and change river flows, understanding how the Qinghai-Tibet Plateau gets its water is vital for predicting future water supplies.

To map this hidden water cycle, the team combined real-world field data with advanced computer weather models. They launched specialized, helium-filled "Jimu balloons" to collect 32 vertical atmospheric profiles at two key locations on the plateau. These sites included Lulang, a forested valley that acts as a moisture pathway, and Nam Co, a high-altitude saltwater lake.

These balloons measured temperature, humidity, and the unique chemical "fingerprints" of water molecules, known as stable isotopes. By tracking these isotopes, scientists can trace exactly where water vapor came from and how it has changed over time.

At the heart of the discovery is a nighttime process called atmospheric "decoupling".

During the day, solar heating mixes the air near the ground with the air high above. But at night, the ground cools rapidly, creating a highly stratified, three-layer atmospheric sandwich over the plateau.

This structure is divided into the boundary layer closest to the ground, a middle buffer zone called the mixed layer, and the free troposphere, which acts as the high-altitude highway where the westerlies blow.

As the ground cools, these air layers separate, or "decouple", allowing the moisture carried by the high-altitude westerlies to sink toward the plateau.

"The results showed that moisture carried by the westerlies undergoes significant phase changes — turning from invisible vapor into tiny water droplets — as it sinks," said Gao Jing, deputy director of the national key laboratory of earth system and resource environment on the Qinghai-Tibet Plateau.

As this sinking moisture cools, it forms a "thermal inversion cap" — a layer of warm air trapping cooler air beneath it. This cap stops the air from mixing aggressively, allowing the water vapor to quietly condense into the lower atmosphere instead.

"Remarkably, around 30 percent of the moisture transported by the westerlies is integrated into the local cycle through these nighttime phase transitions," Gao said.

The study provides a crucial breakthrough for climate science. By revealing exactly how the westerlies feed the Qinghai-Tibet Plateau, the research gives scientists a more accurate benchmark to improve weather forecasting models and optimize climate projections for the region.

Furthermore, understanding this modern process helps scientists better interpret historical climate data locked away inside ancient ice cores, offering a clearer picture of how Asia's water supply has fluctuated over centuries and how it might change in the future.

Contact the writers at palden_nyima@chinadaily.com.cn