Weakened Subtropical Westerlies and Their Deflection by the Tibetan Plateau Contribute to Drying Southeastern China in Early Spring.

An obvious long‐term drying trend in recent early springs (February–March–April) is observed over southeastern China (SEC). Here, we attribute this drying to the weakened subtropical westerlies and deflected by the Tibetan Plateau (TP). Climatologically, the low‐level southwesterlies at the southeastern margin of the TP, a branch of the upstream subtropical westerly jet deflected by the TP terrain, bring water vapor to SEC and the southerlies move upward over SEC mainly through isentropic gliding mechanism, inducing persistent precipitation in early spring. However, the subtropical westerlies weakened significantly in recent decades due potentially to the decreased Eurasian snow cover. Consequently, an easterly trend appears along the southern margin of the TP with anomalous northeasterlies over SEC. These northeasterlies suppress both moisture supply and upward motions over SEC, and reduce regional early spring precipitation. Our results highlight the interaction between the TP terrain and the weakened subtropical westerlies that leads to the drying SEC. Plain Language Summary: Spring precipitation in southeastern China (SEC) is a major rainband during the pre‐flood season in East Asia, which is significant for agricultural production and social economy. However, in the recent few decades, a robust long‐term drying trend has occurred over SEC in early spring. In this study, we propose a new mechanism for the decreased SEC precipitation and highlight the important influence of the weakened subtropical westerlies and their interaction with the Tibetan Plateau (TP). Deflected by the TP large terrain, the upstream weakened subtropical westerlies induce weakened westerlies and southwesterlies along the southern and southeastern margins of the TP, respectively. As a result, the weakened southwesterlies at the southeastern TP not only reduce the moisture transport downstream, but also suppress the ascending motions over SEC through the isentropic gliding mechanism. Both the water vapor and atmospheric circulation conditions finally induce the drying SEC in recent early springs. Key Points: An early spring drying trend has occurred in southeastern China (SEC), much of this can be attributed to the weakened subtropical westerliesDeflected by the Tibetan Plateau (TP), the weakened subtropical westerlies decelerate downstream westerlies along the TP's southern marginThe decelerated westerlies at the southeastern TP suppress moisture supply and rising motions over SEC, both processes cause the drying SEC [ABSTRACT FROM AUTHOR]