A Unified Monsoon Index for South China

Abstract

A unified index for both the summer and winter monsoons over south China (SC) is proposed for the purpose of studying their interannual variability. By examining the monthly distribution of the meridional flow ? over the Asia?Pacific region from 20 yr (1976?95) of the reanalysis data of the National Centers 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 ? fields at 1000 and 200 hPa over the SCS show the most significant reversal in direction between summer and winter. The summer rainfall over SC is found to correlate well with these two fields as well as their differences averaged over the northern part of the SCS (7.5°?20°N, 107.5°?120°E). Winter temperatures over SC are, however, only related to the ? field at 1000 hPa within the same region. It is therefore proposed to define a unified monsoon index for SC as the value of ? at 1000 hPa averaged over this region within the period of June through August for summer and November through February for winter.