南海夏季风爆发与大气对流低频振荡的年际变化

根据1980～1991年云顶黑体温度（TBB）相位和强度的变化确定了南海夏季风爆发的时间，分析研究了夏季风爆发期间TBB场和850hPa风场的变化过程及其与海温的关系。结果表明:南海夏季风爆发平均时间是5月第4候，它爆发的时间和强度有显著的年际变化，并与大气的低频振荡及前期海洋的热力状况有密切关系。南海夏季风爆发早年（4月第6候），副热带高压较弱，撤离南海较快，从赤道东印度洋到赤道西太平洋，大气对流活动较强，夏季风爆发南海早于孟加拉湾，季风爆发时90～100°E区域过赤道气流显著加强。夏季风爆发晚年（6月第1候）情况相反。南海夏季风爆发早晚与大气30～60天振荡到达南海的位相有关，前冬和早春南海海温的高低和4月中旬至5月中南半岛强对流区的出现时间，是南海夏季风爆发年际变化的前期征兆。根据前冬南海海温预测1998年南海夏季风爆发的时间和强度与实际相符。

Interannual Variations of the Onset of the South China Sea Summer Monsoon and Low-Frequency Oscillation of Convective Activities in the Atmosphere

The onset time of the South China Sea Summer Monsoon (SCSSM) is defined by the changes in phase and strength of temperatures of block body (TBB) at cloud top, and the process of change of TBB and wind field at 850 hPa, as well as its association with sea surface temperatures during SCSSM onset time is analysed. The main results are as follows. The mean onset time of SCSSM is the fourth pentad of May for the period of 1980 to 1991. There exists a significant interannual variation in the onset time and strength of SCSSM which is closely related to low-frequency oscillation of the atmosphere and the heat states of ocean before the SCSSM onset time. In the years of early onset of SCSSM (the sixth pentad of April) deep convective activities are observed over the region from the eastern equatorial Indian ocean to the western equatorial Pacific ocean, in association with the fast eastwards retreat of the weak western Pacific subtropical high, and the onset time of summer monsoon is earlier over the South China Sea than over the Bay of Bengal. The stronger cross-equatorial flow over the 90～100oE region during the onset time is observed, and vice versa in the years of late onset of SCSSM (the first pentad of June). The onset time of SCSSM is associated with the phase of 30～60 day oscillation of atmosphere arriving over SCS. High or low SST of SCS during the prior winter and early spring and the time of the strongly convective weather occurrence over the Indo-China from middle of April to May are indicators of interannual variations of onset of SCSSM. Prediction of the onset of 1998' SCSSM based on SST during the prior period in SCS is consistent with the fact.