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 (WNP) and the extratropical northeasterly anomalies to the north of the YRB. The heavier the YRB rainfall, the necessary the cooperation between these two factors. This study, however, found that only approximately 60% of heavy rainfall events in the YRB requires the combined effect of them, while the other approximately 40% occur when only one factor appears and the other is opposite signed. Accordingly, these atypical heavy rainfall events can be categorized into two types: the extratropical northeasterly anomalies but tropical cyclonic anomaly (first type), which is characterized by a local cyclonic anomaly over the mid-latitude YRB and the tropical anticyclonic anomaly but extratropical southwesterly anomalies (second type), which is featured with the confrontation between a tropical anticyclonic anomaly over the WNP and an extratropical cyclonic anomaly that extends from Lake Baikal to the north of the YRB. Anomalous upward motion exists for both types, but through different mechanisms. For the first type, the upward motion is 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, the tropical anticyclonic anomaly remains stable throughout the event, and the emergence of upward motion is associated with the southward extension of the extratropical cyclonic anomaly in the middle troposphere. The confrontation between the extratropical cyclonic anomaly and the tropical anticyclonic anomaly results in southwesterly anomalies over the YRB. These southwesterly anomalies transport the climatological warm air toward the YRB, leading to positive temperature advection anomalies. This advection of basic temperature by southwesterly anomalies is vital for the emergence of upward motion. This study emphasizes the role of mid-tropospheric circulations in inducing heavy rainfall in the YRB and its results pose a considerable challenge to the accurate prediction of heavy rainfall. Additionally, it is interesting to investigate whether these atypical circulation patterns can be captured by climate models.

Yangtze River basin；,heavy rainfall,large-scale circulation anomalies