Process And Inter-annual Changes In Characteristics And Mechanism Of The Western Pacific Subtropical High Westward Extension In June

In terms of the NCEP reanalysis data, rainfall and temperature data from sixteen stations of China and the TBB data provided by the Meteorological Research Institute of Japan, the mainfeatures and basic rules of westward extension and withdrawal processes of the northwest Pacific Subtropical High (STH) and its effect on the weather of the East China are investigated in this paper , by composite analysis, theoretical analysis and dynamical diagnosis. Based on the relation between the heating and STH, the effects of the distribution of apparent heat source beside the STH on its maintenance and westward extension are also discussed, using the apparent heating to quantitative calculate the Complete Vorticity Equation (CVE). Major results are as follows:(1) the variation of 500hPa vorticity in the key area (115 -125 E,20 -30 N) is used toindicate the westward extension process of STH. Through composite analysis, results show that the increase of the negative vorticity in key area agrees well with the westward extension of STH. More studies reveal that the westward extension index can perfectly describe the position of the STH in June, according to the serial value of the STH position in June, it is founded that when the voticity value in the key area increases (decreases), the STH in June is more westerly (easterly) than normal.(2) Another interesting phenomenon is founded that the center of South Asia high in the upper level of troposphere is stabile and nearly motionless during the processes of the westward extension of western Pacific Subtropical High, but its eastern ridge strengthens and extends eastward remarkably because of the anticyclone moves westward over the southeast Japan, which may lead to the westward extension of Subtropical High in the middle level of troposphere.(3)In the low level of troposphere, the north Australia cold air activity plays an important role on the westward extension of Subtropical High. The flow propagates northward and marches across the equation, then passes through the trough of summer monsoon over SouthChina Sea and finally gets to the Yangtze River Valley, which sequentially effect the southwest airflow of the west side of Subtropical High.(4) The correlations are significant between the STH position index and the general circulation. In the westerly (easterly) year, equatorial westerly is weaker (stronger), cross-equatorial flow is weaker (stronger), South China Sea summer monsoon is weaker (stronger), the Yangtze River Valley westerly is stronger (weaker), which has the positive (negative) effect on the maintenance of rain band over the Yangtze River Valley, so the rainfall is more (less) in this area and it is hit by floods (droughts), vice versa.(5) Based on the complete form of vertical vorticity tendency equation, the effect of spatially non-uniform heating on the formation and variation of subtropical high is discussed. Results show that subtropical high in June 1998 is more intensive and westerly and than normal, leading to the floods in Yangtze River Valley. Vertical diabatic heating has stronger impacts on the variation of intensity and location of Western Pacific subtropical high. Comparing with normal, vertical variations of non-uniform heating in the north and west of subtropical high center are anomalous strong, while anomalous weak in the south and east of subtropical high center. This anomalous non-uniform heating resulted in the powerful anticyclone vorticity locating in the south of Yangtze River and Western Pacific, which lead to the intensive and westerly subtropical high.