AN EXPLORATION ON THE DETERMINATION OF EAST ASIA SUMMER MONSOON INDEX

In this paper,NCEP/NCAR 1948-2002 daily reanalysis data and monthly rain data in 151 stations archived in NCC were used to calculate the date of est ablishment and with drawal of East Asia summer monsoon at every 5 latitude (north of 20°N) along 112.5,117.5 and 122.5°E as well.The met hod is similar to the criterion of establishment and withdrawal of East Asia summer monsoon in Northeast China defined by us.Amean date pentad map of est ablishment and withdraw al of East Asian summer monsoon is drawn,an index of East Asia summer monsoon at a certain latitude along a certain longitude,ISM,is defined,and its relationship with summer precipitation and 500 hP a height in China was calculated.the results indicate that:(1) The mean pen-tads of establishment of East Asia summer monsoon at 20,25,30,35 and 40°N along 117.5°E are 27.26,28.54,34.43,37.12 and 37.65 respectively,and the mean pentads of its with drawal are 54.44,53.69,51.85,48 and 46.76 respectively.The determined pentad of establishment of East Asia summer monsoon in north of South China Sea(117.5°E,20°N),is coincident with the 4th pentad in Mayrecog nized by China scholar.(2) The mean date of East Asia summer monsoon establishment at the same latitude is not the same along 112.5,117.5 and 122.5°E,early in east part and preceding 1-2 pentad every 5 longitude.But the mean date distribution of its withdrawal (30°N and north of it) is opposite,first withdrawal in eastpart.