Interdecadal Variations od East Asia Summer Monsoon Northward Progressing and Its Influences on Summer Precipitation Over East China

The intedecadal variations of the northward propagation of East Asian summer monsoon (EASM) and summer precipitation over east China have been investigated using the 1951-2001 National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis daily datasets and the 1957-2001 daily rainfall from 366 stations of China. Results show that the 1.5 contour of the unified precipitation index (UPI) clearly depicts the characteristics of summer rain belt (SRB) activities over east China, in terms of jumps and durations during its northward propagation. The interdecadal variation is a prominent feature during the EASM northward movement. The EASM in 1960's had a quick northward advance, and a north edge beyond its normal position. As the results, both rain over north China and Mei-Yu in Hui river basin are above normal conditions. In contrast, the EASM in 1980's had a slow northward movement, and a north edge located in the south of its normal position. Therefore, precipitation mainly located in the Yangtze River basin while rain over north China is not evident. The characteristics of EASM northward propagation in 1970's is similar to that in 1980's while the 1990's is alike the 1960's. There was a dramatic shift of the EASM northernmost position (ENP) in the mid-1960's. The ENP has stayed consistently south of its normal position since then. By same token, the EASM northernmost intensity (ENI) weakened abruptly at the end of 1970's. The ENP and ENI have a close relationship with the activities of EASM, especially at interdecadal timescale. In addition, the variability of ENP and ENI has a large influence on interdecadal variations of large-scale precipitation anomaly over east China. For example, there is a significantly negative correlation between the interdecadal variation of summer rainfall over the Yangtze rive basin and the quasi-15-year oscillation of ENI, while interdecadal variations of summer precipitation over north china and south China are positively related with the interdecadal variability of both ENP and ENI.