The Dominant Modes of Precipitation Anomalies over Eastern China during the Northern China Rainy Season and the Possible Causes

Based on the daily mean rainfall data at 316 China meteorological stations and the NCEP-NCAR reanalysis dataset for 1958-2011, the dominant modes of precipitation anomalies over eastern China (east of 105°E) during the northern China rainy season (11 July to 31 August) and the accompanying circulation as well as precursors, are investigated with EOF and composite analysis, along with other methods. The results show that there are three dominant modes-the first EOF mode (EOF1) depicting a meridional tripole pattern, the second EOF mode (EOF2) depicting a seesaw precipitation anomaly between northern China and the Yangtze River valley, and the third EOF mode (EOF3) depicting a seesaw precipitation anomaly between areas from Northeast China to the upper reaches of the Yangtze River and coastal areas in East China. The major circulation characteristics resulting in the three modes are different. EOF1 is attributed to the meridional displacement of the West Asian subtropical westerly jet stream (WAJS), EOF2 is attributed to the East Asian-Pacific (EAP) teleconnection pattern, and EOF3 is attributed to the anomalous East Asian summer monsoon (EASM) due to an anomalous sea level pressure gradient between East Asia and the western North Pacific. Possible causes of the above anomalous circulation are also analyzed. For EOF1, the 200 hPa wave train from the North Atlantic causes the seesaw geopotential height anomalies between the Ural mountains and Balkhash Lake in early July, leading to the meridional displacement of the WAJS. For EOF2, the EAP teleconnection pattern is closely associated with the geopotential height anomaly over the North Atlantic and corresponding wave train dating from early July. For EOF3, the anomalous cyclonic/anticyclonic circulation over Mongolia beginning from March is the precursor of stronger/weaker EASM.