中文摘要：

利用1958--1997年的NCEP／NCAR再分析资料，以南海季风爆发日为临界日期，计算了40年合成的季风爆发前月平均带状基流；在该基流上，计算了球面正压涡度方程中Rossby波的稳定性；并用谱函数展开法定义和计算了发展型波包的演变。结果显示：南海夏季风爆发前气候平均场上有球面Rossby波的正压不稳定，该不稳定主要由南半球的西风急流所激发，且不稳定扰动的最大振幅均出现在南半球西风急流以南。球面Rossby波发展型波包的最大振幅随时间会由两个半球的中高纬度向低纬扩展，虽然不能越过赤道，却激发了热带地区的积云对流，积云对流的爆发并向季风区传播，加速了大气环流的调整，其结果造成了南海夏季风的爆发。可见，南海夏季风的爆发虽是局地现象，但其爆发原因却是全球性的。

英文摘要：

By using the NCEP/NCAR reanalysis data of 1958 to 1997, we first looked into the atmospheric flow conditions in the one month immediately prior to the onset of the South China Sea summer monsoon （SCSSM） each year. A monthly-averaged zonal basic flow of 40-year composite was then calculated. The stability of Rossby wave in the basic flow was studied based on the spherical barotropic vorticity equation. Furthermore, the spectral function expansion method was adopted to define and compute the evolvement of a developing wave packet. The results indicate that there exists barotropic instability of spherical Rossby wave in the climatically- averaged flow field before the SCSSM onset. The instability is triggered by the westerly jet stream in the southern hemisphere, and the strongest instable perturbation lies to the south of the westerly jet stream. The peak of the developing spherical Rossby wave pack- et propagates from mid and high latitude to low latitude, though not crossing the equator, spurring the cumulus convection in the tropical zones. The eruption of the cumulus convection and its spread to monsoon regions help to speed up the adjustment of the gen- eral circulation and the SCSSM onset. It is concluded that elements that contribute to the SCSSM onset are at global scale, albeit the onset itself looks a local phenomenon.