1998年南海夏季风爆发过程中副热带高压减弱的诊断分析

A DIAGNOSTIC ANALYSIS OF THE WEAKENING OF WEST PACIFIC SUBTROPICAL ANTICYCLONE DURING THE PERIOD OF SOUTH CHINA SEA SUMMER MONSOON ONSET IN 1998

1998年南海夏季风爆发于5月21日。利用NCEP再分析资料,用Zwack-Okossi涡度方程诊断分析引起季风爆发前后南海-西太平洋副热带反气旋脊迅速减弱的物理因子,结果表明:在夏季风爆发前,华东-华南沿岸附近高空槽前的正涡度平流区随高空槽的东南移动向南海和菲律宾中部附近地区推进;正涡度平流是引起上述地区850 hPa气旋性涡度增大的最主要的物理因子,其次是暖平流。在夏季风爆发后,潜热加热项的作用迅速增大至与正涡度平流项大小相当,这两项是引起区域平均气旋性涡度增加的最主要的物理因子;它们和暖平流项一起抵偿了辐散项和绝热项的负贡献,令季风区的气旋性涡度得以发展/维持。因此,正涡度平流是直接导致南海-西太平洋副热带反气旋脊迅速减弱最重要的物理因子。

The onset of South China Sea（SCS） summer monsoon in 1998 occurred over 21 May.Using the NCEP reanalysis data,this paper examined the physical process of the weakening of West Pacific Subtropical Anticyclone during the onset period using the Zwack-Okossi vorticity equation.The result shows that during the pre-onset period,the positive vorticity advection in front of the trough at the upper troposphere was the most dominant physical mechanism for the increase of the cyclonic vorticity on the 850 hPa layer over the SCS and its nearby region.The secondary contribution to the increase of the cyclonic vorticity was the warm-air advection.After the onset,the magnitude of latent heating term rapidly increased and its effect on the increase of the cyclonic vorticity was about the same as the positive vorticity advection.The adiabatic term and divergence term contributed negatively to the increase of the cyclonic vorticity most of the time.Thus,the positive vorticity advection is the most important physical mechanism for the weakening of West Pacific Subtropical Anticyclone over the SCS during the onset period.