Bimodal precipitation seasonality and moisture transport in the Western Tibetan Plateau

The Western Tibetan Plateau (WTP), which serves as the source region for three major Asian rivers, the Indus River, the Amu Darya, and the Tarim River. This study comprehensively investigates the formation and changes of the bimodal precipitation seasonality in the WTP, with a focus on the behind mechanisms of moisture transport, an aspect that has been underexplored in current research. The WTP exhibits two intra-annual precipitation peaks: one in February–March and another in July–August. During February–March, enhanced moisture contributions primarily originate from the westerlies-dominated western sources and the anticyclone-influenced northwestern Arabian Sea. Although the moisture content does not peak during this period, the increased moisture input from the western boundary, coupled with the relatively lower moisture output from other boundaries, causes the region's net moisture balance to reach its first peak during the year. In the second wet season (July–August), factors such as the Indian Summer Monsoon (ISM), warming and wetting of the Eurasian continent, the northward shift of the westerlies, and intensified local recycling collectively contribute to the increased precipitation. Since the 2000s, the peak precipitation month of the first wet season has shifted from March to February, which is closely linked to changes in moisture contributions from the southwestern sources. • Precipitation seasonality in the WTP shows Bimodal pattern. • Bimodal precipitation seasonality is influenced by different moisture transport processes. • Bimodal precipitation seasonality in the WTP has undergone changes.