The Fluctuation Of The Monsoon Region Boundary In China And The Relationship With East Asian General Circulation

Based on the monthly mean temperature and precipitation data at 155 stations covering the year 1961-1999 in northern china(the area is within the north of 34°N and the west of 105°E), and using the Thornthwaite formula, the water requirement and the moisture index in our study area are calculated in this paper. The -10 moisture index isoline is defined to be the boundary of the monsoon region in the east of China. and based on it, we defined the summer monsoon boundary index (SMBI). Then the relationship between SMBI and the summer precipitation of Eastern China is investigated by composites, and also the relationship between SMBI and the general circulation abnormity over Eastern Asia by using the NCEP/NCAR reanalysis data. The main results are:(1) The SMBI over northern China has a clear inter-annual variation and a period about 15 years. There is 3 oscillation phase during the year 1961-1999, they are the year covering 1961-1974,1975-1983,1984-1999. The SMBI of the North China and the Northeast China have basically unanimous inter-decadal period with the SMBI of the northern China.(2) The correlation coefficients between the SMBI and summer precipitation of each region in northern China show significant, positive inter-decadal relationship. The highly correlated region between them all located north of 34°N. For the scale of annual and decade, when SMBI is high(low), the precipitation anomaly percent in northern China is positive(negative) and it is negative(positive) in southern China.(3) The possible mechanism that the East Asian general circulation influence the boundary of summer monsoon is: when SMBI is high(low), the geopotential height in low and middle stratosphere displays a positive(negative) anomaly in the high-latitude over East Asian land, it also displays a negative(positive) anomaly in the middle-latitude, trough in Northeast China is strengthened(weakened) and eastward(westward), subtropical high of the western Pacific is strengthened(weakened) and northwestward(southeastward), aleutian low is weakened(strengthened), the divergence(convergence) in high stratosphere and convergence(divergence) in low stratosphere are strengthened too. These lead south(north) anomaly wind compiling and blowing to the north(south) in east China, the tropical and subtropical summer monsoon strengthening(weakening), the boundary of summer monsoon's northward(southward) displacement.