STUDY ON MODAL VARIATION OF SUBTROPICAL HIGH AND ITS MECHANISM DURING SEASONAL TRANSITION PART Ⅱ:SEASONAL TRANSITION INDEX OVER ASIAN MONSOON REGION

The relationships between Asian summer monsoon onset and meridional displacement of the local temperature ridge in the middle and upper troposphere are further explored based on the previous part Ⅰ.850 hPa zonal wind and deep convection display opposite charact eristics before and after the seasonal transition over the Bay of Bengal,South China Sea and South Asia,respectively.The most significantly reversed elementis the meridional gradient of middle-upper temperature(200-500hP a)in the vicinity of the boundary surface between westerly to north and easterly to south(WEB in brief).The reversal of meridional temperature gradient near WEB reflects the common charact eristics of summer monsoon onset for the different Asian monsoon region.According to the thermodynamic foundation of seasonal transition,a new and uniform index,which is define as the time when the meridional gradient of area-averaged middle-upper temperature in the vicinity of WEB changes direction,is proposed for measuring the seasonal transition and monsoon onset over Asian monsoon region.The time series of seasonal transition date is then given by using this index.Correlation analyses indicate that 850hPa zonal wind is only regional index,but the meridional temperature gradient is an appropriate and uniform index.