中文摘要：

利用上海台风研究所整理的1949--2007年CMA-STI热带气旋最佳路径资料，对1949—1975年、1976—1987年、1988--2007年这三个时段7—10月强度达到热带风暴及以上的热带气旋（简称为TC）强度的年际变化特征进行了分析比较，对TC活动强、弱年的大气环流背景场做了合成分析，并对各年TC强度与对应的西北太平洋副热带高压（简称为西太副高）、东亚副热带西风急流、热带东风急流、季风槽各特征指数做了相关分析，揭示了西北太平洋TC强度年际变化的机理。结果表明：（1）这三个时段TC强度都有明显的年际变化特征；（2）当TC强度偏大时，西太副高减弱东退，东亚副热带西风急流位置偏南，热带东风急流强度偏大，位置偏东，南半球冷空气活动偏强，越赤道气流偏强，季风槽较活跃、位置偏东，倾斜指数偏小；反之亦然；（3）当越赤道气流偏强，季风槽较活跃时，有利于TC生成源区热带低压扰动的发生发展；而西太副高减弱东退，热带东风急流位置偏东时，TC生成源区850hPa低空为正相对涡度异常，200hPa高空为正散度异常。这样，低层水平辐合强、高层水平辐散强的配置使得该地区辐合上升运动、高层气流的辐散流出得以维持，且TC生成源区垂直风切变偏小，从而有利于TC的发生发展，TC强度偏大；反之亦然。

英文摘要：

Based on the CMA-STI Tropical Cyclone Best Track Data from 1949 to 2007, the inter-annual variation characteristics of the tropical storm or typhoon （TC） intensity in the Western North Pacific between July and October in 1949--1975, 1976---1987, 1988--2007 are respectively investigated. And by way of composite analysis and correlation analysis, the mechanism of the inter-annual variation characteristics of the TC intensity in the Western North Pacific is investigated. The results show as follows： （1） There is an obvious inter-annual variation of the TC intensity in 1949--1975, 1976--1987, 1988--2007 respectively; （2） During the strong TC years, the Western North Pacific Subtropical High （WNPSH） becomes weaker and is more northward and eastward than normal. Meanwhile, the 200 hPa Eastern Asian Subtropical Westerly Jet（EASWJ） locates more southward than normal. Besides, the 200 hPa Tropical Easterly Jet（TEJ） becomes stronger and locates more eastward than normal. Furthermore, the cold air in the Southern Hemisphere becomes stronger and the cross-equatorial flow strengths so that the monsoon trough （MT） becomes more active and locates more eastward, and vice versa; （3） When the corss-equatorial flow strengths and the MT becomes more active, it is in favor of the development of the tropical depression in the TC generation area. Besides, the WNPSH becomes weaker and locates eastward, and the TEJ locates easterward. In this way, tlie low level is convergent while the high level is divergent so that the convergent updraft in the TC generation area and the divergent outflow in the high level are maintained, and the vertical wind shear is less, which is in favor of the development of the TC; and vice versa.