AN EAST ASIAN SUB-TROPIC SUMMER MONSOON INDEX DEFINED BY MOISTURE TRANSPORT

Using daily NCEP/NCAR reanalysis dataset and observation rainfall data in China for the 1971～ 2000 period,a sub-tropical summer monsoon index is defined by meridional moisture transport of the total atmosphere based on the discussion about climate characteristics of moisture transport.Results show that the advance and retreat of east Asia summer monsoon can be reflected by the intraseasonal variation of moisture transport averaged over 110～120 °E.The sub-tropical summer monsoon index defined by difference of meridional moisture transport between South China and North China can be used to describe the intensity of the subtropical summer monsoon.High(low) index is corresponding to strong(weak) sub-tropical summer monsoon.And the new index is well related to the summer rainfall over the middle and lower reaches of the Yangtze River.In addition,the convergence of moisture transport from the west Pacific via the South China Sea and that from the North China may be responsible for the anomalously excessive summer rainfall over the middle and lower reaches of the Yangtze River.