A New Monsoon Index,Its Interannual Variability and Relation with Monsoon Precipitation

The normalized seasonality of winds δ presented firstly by Zeng is calculated again by use of new datasets.To consist with the traditional and known definitions from many researchers,we take the region where δ~*=δ-20,i.e,the difference in wind direction between winter and summer is lager than π/2 as monsoon region.The geographical extent of the global monsoons form the method here covers all monsoon regions over the world known so far(however,the domain here is slightly smaller than that from Zeng),especially the tropical monsoons are just included in the region between the positions of the Intertropical Convergence Zone or ITCZ in winter and summer.Then,the interannual variations of areal average δ~*_m of some key monsoons as the broad scale monsoon index proposed by Li and Zeng are analyzed by virtue of the dynamical normalized seasonality(DNS) of winds δ~*_m=δ_m-2(δ_m is similar to δ but depends on year m).The long-term decreasing trends with different extent can be found in the intensities of the South Asian summer monsoon(SASM) and East Asian summer monsoon(EASM) since the mid-1970s,the South China Sea summer monsoon(SCSSM) since 1980s,and the West African summer monsoon(WASM) since 1967,especially the most remarkably weakening in the WASM.Both the West African summer monsoon index(WASMI) and the South Asian summer monsoon index(SASMI) are significant positively correlated with the summer precipitation over the local monsoon regions,respectively,and there is statistical structure in the correlation map between the East Asian summer monsoon index(EASMI) and summer rainfall in China and East Asia,and there exist better statistical correlations over large regions in every Oceans between the South China Sea summer monsoon index(SCSSMI) and summer global rainfall and sea level pressure anomalies.