The Large-Scale Circulation Patterns Responsible for Extreme Precipitation Over the North China Plain in Midsummer

Extreme precipitation events over the North China Plain (NCP) in midsummer during 1979-2016 are classified into two types using objective cluster analysis: a northern pattern with heavy precipitation and a central-southern pattern with relatively moderate precipitation. The large-scale circulation patterns responsible for the midsummer extreme precipitation are then determined. In the northern NCP type, extreme precipitation accompanies a zonal gradient between an anomalous low-pressure system at high latitudes and the westward- and northward-extended western North Pacific subtropical high (WNPSH). Anomalous southwesterlies flow is driven by a trough that extended from the high latitudes to the northern NCP, where it encounters southeasterly wind flow induced by an anomalously northward-extended WNPSH and a southern low-pressure anomaly at low latitudes. Anomalous amounts of moisture are mainly transported from the tropical western Pacific by southeasterlies. In the central-southern NCP type, remarkable anomalous low-pressure systems control all of northern China with centers over the Sichuan Basin and northeast China. The westward-extended WNPSH occupies further south than that of the northern NCP type. The southwesterly low-level jet (LLJ) is more prevalent in the central-southern NCP type than in the northern NCP type. This southwesterly LLJ plays an important role in extreme precipitation over the central-southern NCP by transporting moisture primarily from the Bay of Bengal and the South China Sea and generating convergence. In addition, the upper-level anomalous strong divergence that is anchored over the right entrance of the westerly jet makes a greater contribution to extreme precipitation in the northern type than in the central-southern type.