Weakened Subtropical Westerlies Reduced Early Spring Precipitation in the Southeast Tibetan Plateau

Abstract

Abstract The Southeast Tibetan Plateau (SETP) is a major region where many low-latitude glaciers are located, with spring precipitation being a major input of the glacier mass balance. This study shows that early spring precipitation has decreased significantly since 1999, which is attributed to declined moisture contribution from the far-field sources (west of 70°E) induced by the weakened subtropical westerlies. The possible physical mechanism underlying this change has also been revealed. It is found that snow-cover extent (SCE) in March reduced in midlatitude Eurasia after 1999; meanwhile, strong solar radiation during this month may have exacerbated snow melting through snow albedo–radiation interactions. These two processes led to warming and caused a strong anticyclone over midlatitude Eurasia that weakened the subtropical westerlies near 30°N. This decadal change in the subtropical westerlies led to a decrease in moisture transport upstream. As a result, the windward slopes of large terrain along the latitudinal belt near 30°N received less precipitation, and the decrease in SETP precipitation was part of this change. A further analysis shows that the positive correlation between the westerlies and precipitation has weakened since 1999. Significance Statement The purpose of this study is to reveal the decreased early spring precipitation and explore its possible physical mechanism in the Southeast Tibetan Plateau (SETP), which is crucial to understand the shrinkage of the local glacier. Our results indicate that the reduction of snow cover in midlatitude Eurasia since 1999 and the strong solar radiation in March contributed to the weakening subtropical westerlies, which further resulted in the decreasing precipitation in the SETP and other windward slopes of large terrain along the latitudinal 30°N belt in Eurasia.