Abstract:Decadal variations of the East Asian summer monsoon and the summer precipitation are investigated by using the NCAR/NCEP and ERA 40 reanalysis dataset and the monthly rainfall data at 160 Chinese stations during 1951-2010. For better illustration, an East Asian summer meridional circulation index (EASMCI) is defined in this paper and its relationship with the summer precipitation over the Haihe River Basin is analyzed as well. Results indicate that the westerly winds at both the lower and the higher troposphere in East Asia show anti phase variations since the 1980s. Results also indicate the meridional wind speed at lower troposphere and the EASMCI have decreased in recent 30 years, weakening the summer monsoon and blocking the northward movement of the summer rain belt. Statistical analysis shows a significant positive correlation between the summer precipitation over the Haihe River Basin and the EASMCI. Under the positive phase of the index, the East Asian summer meridional circulation is strengthened and accelerates the south wind to blow more northward, favoring the summer precipitation over the basin. However, under the negative phase of the index, the precipitation is less. In a word, the results in this paper demonstrate that the decadal weakening of the EASMCI is possibly a direct reason for the decrease of the precipitation over the Haihe River Basin in recent 30 years.