2004年与2006年7～9月西北太平洋上空大尺度环流场与天气尺度波动的差别及其对热带气旋生成的影响

Differences between the Large-Scale Circulations and Synoptic-Scale Waves in July–September 2004 and Those in 2006 and Their Impacts on Tropical Cyclogenesis over the Western North Pacific

2004年和2006年是西北太平洋热带气旋（TC）活动具有明显差别的两年,2004年TC的生成位置主要位于西北太平洋的中东部上空,而2006年的TC主要生成在西北太平洋西部与中国南海。本文利用JTWC（Joint Typhoon Warning Center）热带气旋最佳路径数据、NCEP-DOE AMIP-II再分析资料和NOAA的OLR资料分析并比较了2004年与2006年7~9月西北太平洋上空大尺度环境要素场及天气尺度波动对TC生成的作用。分析结果表明：2004年7~9月与2006年7~9月西北太平洋上空季风槽的平均位置没有明显的区别,但是形态有着显著的差异。2004年季风槽的槽线不明显,在西北太平洋中部呈现一显著的气旋式环流;而2006年季风槽的槽线非常明显,槽线南北两侧呈现平直的水平气流,具有明显的水平切变特征。2004年和2006年对流层低层的相对涡度、高层辐散和垂直风切变具有明显的纬向分布差异,这是这两年TC生成的位置具有明显差异的重要原因之一。并且,本文还分析对比了2004年以及2006年7~9月西北太平洋上空3~8 d周期的天气尺度波动的活动,其结果表明：2004年和2006年TC的生成大多数与天气尺度波列的活动有关。2004年的天气尺度波列强度比较强,其活动的位置位于西北太平洋中、东侧上空;而2006年西北太平洋上空的天气尺度波列相对较弱,主要活动于中国南海和西北太平洋西部。纬向基本气流的切变与辐合所引起的瞬变扰动动能倾向的水平分布差异是天气尺度波动活动具有以上差异的重要原因。因此,西北太平洋大尺度环境场与天气尺度波动活动的区别共同造成了2004年7~9月的TC生成位置偏东、而2006年7~9月TC的生成位置偏西。

There are significant differences in the tropical cyclone（TC） activities between the year 2004 and 2006 over the western North Pacific, in that TC formations occurred over the central and eastern part of the western North Pacific during July and September 2004, but over the South China Sea and the western part of the western North Pacific during July and September 2006. In this paper, the impacts of large-scale atmospheric factors and tropical synoptic-scale perturbation on tropical cyclogenesis over the western North Pacific during July–September 2004 and those in 2006 are analyzed and compared by using Joint Typhoon Warning Center（JTWC） best track data, National Centers for Environmental Prediction – Department of Energy（NCEP-DOE） Atmospheric Model Intercomparison Project Phase-II（AMIP-II） reanalysis data, and National Oceanic and Atmospheric Administration（NOAA） outgoing longwave radiation（OLR） data. The results show that there is no remarkable difference between the average position of the monsoon trough in 2004 and that in 2006. However, the shapes of the monsoon trough are quite different. In 2004, the trough line is not noticeable but a cyclonic vortex is located over the central western North Pacific as the prominent feature. However, in 2006, the principle feature of the monsoon circulation is the distinct trough line, and horizontal wind shear is characterized by straight horizontal flow to the north and south of the trough line. Besides, an important reason for the distribution of TC events is the zonal differences of low-level relative vorticity, high-level divergence and vertical wind shear between 2004 and 2006. Moreover, the evolutions of synoptic-scale wave trains with a 3–8-day period over the western North Pacific during July to September in the years of 2004 and 2006 are analyzed and compared. The results show a close relationship between the tropical cyclogenesis and synoptic-scale wave trains in both 2004 and 2006. A stronger synoptic-scale wave train is located over the central and eastern part of the western North Pacific in 2004, and a weaker synoptic-scale wave train is located over the South China Sea and western part of the western North Pacific in 2006, which could be ascribed to the horizontal differences of the transient eddy kinetic energy tendency through the shear and confluence of the zonal mean flow. Therefore, the differences between the large-scale environment and synoptic-scale waves during July to September 2004 and those in 2006 led to different TC activities, in that TCs tended to form over the eastern area in 2004 and over the western area in 2006.