Interannual Variations of Summer Rainfall and Their Causes in the Mid-latitude Arid/Semi-Arid Areas of East Asia

Based on the precipitation data on land (PREC/L) from NOAA during 1948-2003 (56 years), the monthly mean NCEP/NCAR reanalysis data, and the sea surface temperature (SST) data from Meteorology Office Hadley Centre of the UK, the authors choose the arid/semi-arid areas from the climate average chart of precipitation. The regional characteristics of the mean rainfall from June to August are revealed by the empirical orthogonal function (EOF) analysis, the first EOF mode showing a consistent change in the region, while the second EOF mode showing an east-west contrast pattern. The authors divide the arid/semi-arid areas by the 100°E meridian. The circulation anomalous features associated with the three areas are then investigated through composite analyses. In the circulation field, corresponding to the more rainfall years of East Asian arid/semi-arid areas, at the lower troposphere the anomalous circulation showing a west-east wave train structure, a strong positive anomaly locates to the east of the Ural ridge of high, and a low pressure trough controls the region to the east of Lake Baikal. Differently, the wave train moves eastward and the western Pacific subtropical high moves northward and becomes stronger corresponding to the more rainfall years of the arid/semi-arid areas in the western part of 100°E. Corresponding to the more rainfall years of the arid/semi-arid areas in the eastern part of 100°E, the anomalous circulation is in essence the same as the wave train, except the negative anomalous located at the Caspian Sea. In the 200-hPa zonal wind field, when the West Asian westerly jet stream (ASWJS) axis moves southward than normal, there is more rainfall in the west part of arid/semi-arid areas. When the westerly jet stream strengthens significantly and moves northward, there is more rainfall in the east part of arid/semi-arid areas, which is related to the secondary circulation inspired by ASWJS. Further analyses on the SST shows that SST has different influences on the two regions. During preceding winter (December to next February) and preceding spring (March to May), the equatorial eastern Pacific SST anomalies are positive, while there are negative SST anomalies in summer, and meanwhile the south Pacific SST anomalies are negative, the entire arid/semi-arid region has more summer precipitation. When the central and eastern equatorial Pacific SST in preceding winter and preceding spring shows a warm anomaly and be weakened with the time goes on, but maintains to the summer, and the North Indian Ocean SST shows similar anomalies, the western part of arid/semi-arid area seems to have more rainfall in summer. When there is a warmer anomaly in central and eastern Pacific and a colder anomaly to the south, during the preceding spring the anomalies are weakened, a wide range of weak cold anomaly appeared in the eastern pacific, the eastern part of arid/semi-arid area seems to have more rainfall. Corresponding to the more summer rainfall years in the arid/semi-arid areas and the eastern part, the SST shows the same anomalous pattern except a small difference at the Southeast Pacific Ocean, but the anomalous pattern of the western part is quite different.