Interannual and Interdecadal Variations of Precipitation over Eastern China during Meiyu Season and Their Relationships with the Atmospheric Circulation and SST

Based on the monthly mean rainfall data at 160 China stations, the National Centers for Environmental Prediction /National Centers for Atmospheric Research (NCEP/NCAR) reanalysis dataset, and HadISST1 dataset for 1951-2009, the temporal evolution and spatial distribution of precipitation over eastern China (15°N-40°N, east of 100°E) during the Meiyu season (June and July) are investigated with the empirical orthogonal function (EOF). It shows that there are two dominant modes, the first EOF mode depicting a precipitation oscillation between the Yangtze River valley and South China, and the second EOF mode describing a consistent precipitation anomaly over the south of the Yangtze River. In this study two time scales are separated as: interannual component whose period is shorter than eight years and interdecadal component whose period is longer than eight years. After analyzing the precipitation at these two time scales during Meiyu season, it is found that for both of the annual scale and the decadal scale, the first two leading modes are a precipitation oscillation between the Yangtze River valley and South China, and a consistent precipitation anomaly over the south of the Yangtze River. The result of the comparison between these two time scales shows that the pattern for the precipitation oscillation between the Yangtze River valley and South China has clear interannual and interdecadal variations, and the pattern for the consistent precipitation anomaly over the south of the Yangtze River is mainly characterized by interdecadal variation. The atmospheric circulation and the sea surface temperature (SST) patterns associated with these two modes are investigated through the regression / correlation method. It shows that although the leading modes of interannual component are similar to those at the decadal scale, the associated circulation and SST anomalies are quite different. The atmospheric circulation anomalies associated with the pattern which shows the precipitation oscillation between the Yangtze River valley and South China for the interannual component include the double blockings at mid-high latitudes and the meridional wave train over Eurasia, and the East Asian subtropical jet shows significant positive anomaly. The atmospheric circulation anomalies associated with this pattern of the interdecadal component include the consistent positive geopotential height anomaly over Eurasia, and the subtropical high over East Asia strengthens and shifts northward. The SST anomalies associated with this pattern of the interannual component is mainly represented by the positive SST anomaly over the ocean around Japan and the Indian Ocean during the preceding winter and spring, and over the South China Sea during the simultaneous Meiyu period. The SST anomalies associated with this pattern of the interdecadal component is mainly represented by the negative horseshoe SST anomaly over the North Pacific during the simultaneous Meiyu period. The atmospheric circulation anomalies associated with the pattern which shows the consistent precipitation anomaly over the south of the Yangtze River for the interannual component are characterized by the Okhotsk blocking high and the meridional wave train in East Asia. And the atmospheric circulation anomalies associated with this pattern of the interdecadal component include the positive geopotential height anomaly over Eurasia and the East Asian subtropical jet weakens and shifts southward. The SST anomalies associated with this pattern of the interannual component mainly are represented by the negative SST anomaly over the North Pacific and the positive SST anomaly over the east coast of Australia during the preceding winter and spring, and the positive SST anomaly over the South China Sea during the simultaneous Meiyu season. The positive SST anomalies over the coast seas of East Asia during the preceding winter and spring, and those over the South China Sea and the tropical western Pacific during the simultaneous Meiyu period are highly correlated with this pattern of the interdecadal component.