南海夏季风建立的气候特征Ⅰ——40年平均

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

使用NCEP 1958～1997年40年逐日12层再分析的格点气象资料,对南海地区（105～120°E,5～20°N）夏季风建立的气候特征进行分析,分析的时间尺度为5天（候）。将南海上空850hPa上暖湿的西南风定义为它的夏季风,这暖湿的西南风之θse必须大于335°K,风速须大于2m/s。南海夏季风的建立被定义为南海海域一半以上面积为夏季风所控制的初始时刻。分析结果发现:（1）南海夏季风的建立具有爆发性,爆发时间是5月第4候。（2） 南海夏季风的建立与孟加拉湾东部西南季风有密切关系,它是孟加拉湾西南季风爆发性的发展和东移的结果。（3）南海夏季风雨季是随着夏季风建立而建立的。（4） 在南海夏季风建立期间,南海及其邻近地区高低空环流都有急速的变化,在对流层低层表现为印度低槽的南扩和加深,在对流层中层表现为西太平洋副热带高压迅速东移撤出南海地区,在对流层高层最明显的变化是东风带迅速从南海中部向北扩展到整个南海地区。

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.