A Comprehensive Classification of Anomalous Circulation Patterns Responsible for Persistent Precipitation Extremes in South China

Based on observational precipitation at 63 stations in South China and NCEP-NCAR reanalysis data during 1951-2010, a cluster analysis is performed to classify large-scale circulation patterns responsible for persistent precipitation extremes (PPEs) that are independent of the influence of tropical cyclones (TCs). Conceptual schematics depicting configurations among planetary-scale systems at different levels are estab-lished for each type. The PPEs free from TCs account for 38.6% of total events, and they tend to occur during April-August and October, with the highest frequency observed in June. Corresponding circulation patterns during June-August can be mainly categorized into two types, i.e., summer-I type and summer-Ⅱ type. In summer-I type, the South Asian high takes the form of a zonal-belt type. The axis of upstream westerly jets is northwest-oriented. At the middle level, the westerly jets at midlatitudes extend zonally. Along the southern edge of the westerly jet, synoptic eddies steer cold air to penetrate southward; the Bay of Bengal (BOB) trough is located to the north; a shallow trough resides over coastal areas of western South China; and an intensified western Pacific subtropical high (WPSH) extends westward. The anomalous moisture is mainly contributed by horizontal advection via southwesterlies around 20N and southeasterlies from the southern flange of the WPSH. Moisture convergence maximizes in coastal regions of eastern South China, which is the very place recording extreme precipitation. In summer-Ⅱ type, the South Asian high behaves as a western-center type. The BOB trough is much deeper, accompanied by a cyclone to its north; and a lower-level trough appears in northwestern parts of South China. Different to summer-I type, moisture transport via southwesterlies is mostly responsible for the anomalous moisture in this type. The moisture convergence zones cover Guangdong, Guangxi, and Hainan, matching well with the areas of flooding. It is these set combinations among different systems at different levels that trigger PPEs in South China.