In this study, we define a new summer monsoon index for Vietnam region (VSMI) by averaging 850hPa zonal wind (U850hPa) over the area of 5-15o N and 100-110o E using the 1981-2010 CFSR reanalysis data. The evaluation results show that VSMI index can reflect well the interannual variability of both low and high broad-scale circulation of the summer monsoon. On the intraseasonal timescale, the pentad-to-pentad VSMI index has a close relationship to the local summer monsoon rainfall observed. The correlation coefficients between VSMI index and rainfall are at 95% confidence level and show clearly effects of summer monsoon on the local rainfall due to terrain. The comparison results among three indices show that the VSMI index is the most suitable index in the term of describing summer monsoon for Vietnam region. In addition, features of summer monsoon as intensity, onset and withdrawal date as well as break event can be defined by the VSMI index.
