CLIMATOLOGICAL FEATURES OF THE ESTABLISHMENT OFSOUTH CHINA SEA SUMMER MONSOON I——40-YEAR MEAN

By using 40-yearly NCEP reanalysis daily data (1958-1997), we have analyzed the climatic characteristics of summer monsoon onset in the South China Sea (105~120 °E,5~ 20 °N) pentad by pentad (5 days). According to our new definition, the monsoon area in the SCS should satisfy the two conditions: (1) At 850 hPa, the southwesterlies should be greater than2m/s. (2) At850hPa, θse should be greater than 335 °K. The new definition means that the summer monsoon is the southwesterlies with high temperature and high moisture. The onset of the SCS summer monsoon is defined as the start time when one half of the SCS area (105~ 120 °E,5~20 °N) is controlled by the summer monsoon.The results revealed that: (1) The summer monsoon onset in the SCS happens abruptly on the 4th pentad in May. (2) The summer monsoon onset in the SCS is resulted from the development and intensification of southwesterly monsoon in Bengal Bay. (3) The onset of the summer monsoon and establishment of the summer monsoon rainfall season in the SCS occur simultaneously. (4) During the summer monsoon onset in the SCS, in the lower troposphere of the Indian-Burman trough deepen and widen quickly; in the middle troposphere, the subtropical high in the Western Pacific moves eastward off from the SCS; in the upper troposphere the easterlies advance northward over the SCS.