A unified index for both summer and winter East Asia monsoons is proposed for the purpose of studying the interannual variability of the monsoons. By examining the global distribution of the meridional flow v from 20 years (1976-1995) of the re-analysis data of the National Center for Environmental Prediction, the area of the South China Sea (SCS) is identified as an important segment of the planetary-scale East Asia monsoon circulation. The monthly v fields at 1000 and 200hPa over the SCS form a reverse Hadley cell during summer and a Hadley cell during winter. The summer rainfall over South China (SC) is found to correlate well with these two fields as well as their differences over the northern part of the SCS (107.5-120 E/7.5-20 N). On the other hand, winter temperatures over SC are related only to the v field at 1000 hPa within the same region. It is therefore proposed to define a unified monsoon index as the value of v at 1000hPa averaged over the SCS within the period of June through August for summer and November through February for winter