Decadal and Interannual Variability of Persistent Heavy Rainfall Events over the Middle and Lower Reaches of the Yangtze River Valley

This study investigates the relationship between the anomalous atmospheric circulation pattern and summertime persistent heavy rainfall(PHR) over the middle and lower reaches of the Yangtze River valley(MLYRV) on the decadal and interannual timescales. Based on the gridded daily rainfall data of the US Climate Prediction Center, the PHR events on grid-point and the regional PHR events considering both the area of PHR and regionally averaged rainfall intensity are identified over the MLYRV during the summers of 1979–2017. A PHR index(PHRI) is defined,to describe the variability of summertime PHR event number over the MLYRV. The PHRI is then divided into the decadal and interannual components. Further analysis reveals that the decadal PHR events are closely related to anomalous precipitation, intensified vertical motion, and strengthened upper-level divergence over southern China, as well as an anomalous anticyclone over the western Pacific transporting more water vapor from the South China Sea(SCS) to southern China. As for the interannual component, the above pattern still co-exists but over a narrow region around the MLYRV. By choosing the years in which the decadal and interannual components of the PHRI are simultaneously positive(SP) or negative(SN), the features of regional PHR events in SP and SN years are analyzed, respectively. The results show that there are more regional PHR events in SP years with enhanced intensity and larger affected areas compared with those in SN years. Meanwhile, the zonal oscillations of the South Asian high(SAH)and the western Pacific subtropical high(WPSH) during the regional PHR events demonstrate a better regularity in SP years than those in SN years.