西北太平洋夏季风的变化对台风生成的影响

INFLUENCE OF WESTERN NORTH PACIFIC SUMMER MONSOON CHANGES ON TYPHOON GENESIS

研究了西北太平洋夏季风特征及其季风槽结构对台风生成的影响。当西北太平洋季风槽增强并向东扩展使季风加强时,西北太平洋的风速垂直切变、高低空辐散风、湿度和海温等都对台风的生成产生有利的影响,台风数明显比季风槽弱时多。而且对台风生成的位置也有很大的影响,即季风槽强时,台风的生成位置偏东,季风槽弱时台风的位置偏西。这表明西北太平洋夏季风主要是通过季风槽活动影响台风的生成。而夏季风的强弱对台风也有影响,在西北太平洋夏季风的活跃阶段,西北太平洋夏季风强时,台风生成的比较多,夏季风中断时台风生成的比较少。西北太平洋夏季风通过季风的季节内振荡对西北太平洋台风也有显著的影响。季节内振荡对台风生成的影响主要以30—60 d振荡为主。在这种低频振荡对流活动的湿位相时期台风生成个数明显多,干位相时期台风生成的少。而且低频振荡的西风位相也有利于台风生成,在东风位相时生成的台风少。另外,还研究了多台风期西北太平洋夏季的特征(群发性),发现在这些时期,存在强的季风槽,弱的垂直切变与充足的水汽供应。这表明西北太平洋台风时空的群发性与夏季风活动的异常密切相关。

The influence of Northwest Pacific summer monsoon （NWPSM） characteristics and the structure of monsoon trough on typhoon genesis is studied. When the NW Pacific monsoon trough strengthens and stretches eastwards, causing the enhancement of NWPSM, the vertical wind shear, divergent winds in the upper and lower atmosphere, humidity and sea surface temperature are all favorable to the typhoon genesis in the NW Pacific, and the number of typhoon genesis is evidently more than that when the NW Pacific monsoon trough is weak. Furthermore, the strength of the NW Pacific monsoon trough also has great impact on the locations of typhoon genesis. Typhoon geneses mostly occur in the western Northwest Pacific in the strong phase of the monsoon trough, whereas in the eastern Northwest Pacific in the weak phase. These show that the NWPSM system impacts typhoon genesis mainly through the activities of the monsoon trough. And the strength of the summer monsoon also has influence on typhoons. When the NWPSM strengthens in the active phase, the number of typhoon geneses is more than that in the break phase. On the other hand, the NWPSM has impact on typhoon activities through intraseasonal oscillations, and the 30 - 60 day oscillation is the dominant component of intraseasonal oscillations. The number of typhoon genesis is obviously more in the wet-phase of low frequency convection, and less in the dry-phase. The west wind phase of the low frequency zonal wind is favorable to typhoon genesis, and the typhoon genesis is less in the east wind phase. The cases on 4 and 7 September 1987 also clearly reveal that the typhoon genesis is influenced by intraseasonal oscillations, especially by the 30 - 60 day oscillations. In addition, the features of NW Pacific summer circulation in the excessive typhoon （group occurrence） period are analyzed. In these periods, there are generally strong monsoon trough, weak vertical wind shear and sufficient water vapor supply, and vice versa in deficient typhoon periods, The above indicates that the temporal-spatial group occurrence of typhoon genesis in the NW Pacific has a close correlation with the activities of the NWPSM.