BEIJING, May 16 (Xinhua) -- Scientists have discovered how a major wind system, known as the mid-latitude westerlies, delivers moisture to the Asian Water Towers, the high mountain regions that supply fresh water to nearly two billion people.
The study, led by researchers from the Institute of Tibetan Plateau Research (ITP) of the Chinese Academy of Sciences, together with international partners, was recently published in the journal Proceedings of the National Academy of Sciences (PNAS).
The Asian Water Towers, including the Qinghai-Tibet Plateau, have an average elevation of over 4,000 meters. While the Indian summer monsoon is known to bring seasonal rainfall to the region, the role of the westerlies -- which dominate local weather for three-quarters of the year -- has remained unclear until now.
In the study, the research team found that the westerlies, except for bringing rain, operate like a vertical conveyor to add moisture to the Asian Water Towers. Moisture carried by high-altitude winds slowly sinks toward the plateau. As it descends, it becomes trapped in separate layers of the atmosphere.
This process happens mostly at night. The researchers described it as nocturnal decoupling. The sinking moisture interacts with local air, creating two inversion layers that act like lids. These lids prevent the moisture from mixing with the air below. As a result, the moisture brought by the westerlies stays separate from the local, humid air near the ground. The condensation below these inversion layers during the decoupling integrates the moisture brought by the westerlies into the local moisture budget. This process integrates the westerlies' moisture into the local water cycle without any rainfall.
The researchers discovered that even without precipitation, about 30 percent of the moisture transported by the westerlies is absorbed into the local environment through this nighttime process.
To make this discovery, the team used special helium-filled balloons called Jimu balloons. These balloons collected detailed measurements of water vapor and weather conditions at two locations on the Qinghai-Tibet Plateau. The data allowed the scientists to see the three distinct layers of the atmosphere and track how the moisture moved.
The findings are especially important because human-induced warming is causing faster water cycle change in the region, said Gao Jing, a researcher from the ITP.
"Understanding how the westerlies supply moisture will help scientists build better climate models and make more accurate predictions about the future of Asia's water supply," Gao added. ■
