Abstract: In this paper,the similarity of day-to-day wind during the period of onset of Asia tropic summer monsoon (ATSM) was analyzed using 30-year averaged daily wind data on standard isobaric surface with the spatial resolution of 2.5°×2.5° during 1968-1997 derived from NCAR/NCEP reanalysis dataset.The results show that the similarity can reflect the onset of the ATSM better.Specially,in the lower troposphere,the level grads of contour line of wind similarity are very large and regarded as "frontal zone of the similarity".The change of atmospheric circulation is larger relative to that in the middle troposphere when South China Sea monsoon (SCSM) and South Asia summer monsoon (SAM) set up.The monsoon moves along the normal of the grads.Climatic SCSM sets up early in most cases in western Pacific warm pool and then covers the whole South China Sea after several days,which is caused by saltation of lower atmospheric circulation.Climatic SAM sets up early in most cases in tropical Eastern and Western Indian Ocean and then covers the whole South Asian Subcontinent and its saltation is weaker than that of the SCSM.Besides monsoon area and its neighboring districts,dramatic adjustment and even abrupt changes in the atmospheric circulation occur in some areas in global when the ATSM starts to set up.

Key words: South China Sea summer monsoon, South Asia summer monsoon, Wind, Similarity, Atmospheric circulation