Two Unconventional Types of Large-scale Circulation Anomalies Inducing Heavy Rainfall over the Yangtze River Basin

Summer rainfall in the Yangtze River basin (YRB) is favored by two key factors in the lower troposphere: the tropical anticyclonic anomaly over the western North Pacific and the extratropical northeasterly anomalies to the north of the YRB. This study, however, found that approximately 46% of heavy rainfall events in the YRB occur when only one factor appears and the other is opposite signed. Accordingly, these heavy rainfall events can be categorized into two types: the extratropical northeasterly anomalies but tropical cyclonic anomaly (first unconventional type), and the tropical anticyclonic anomaly but extratropical southwesterly anomalies (second unconventional type). Anomalous water vapor convergence and upward motion exists for both types, but through different mechanisms. For the first type, the moisture convergence and upward motion are induced by a cyclonic anomaly over the YRB, which appears in the mid and lower troposphere and originates from the upstream region. For the second type, a mid-tropospheric cyclonic anomaly over Lake Baikal extends southward and results in southwesterly anomalies over the YRB, in conjunction with the tropical anticyclonic anomaly. The southwesterly anomalies transport water vapor to the YRB and lead to upward motion through warm advection. This study emphasizes the role of mid-tropospheric circulations in inducing heavy rainfall in the YRB.