1998和1999年西北太平洋热带气旋的异常特征及其大尺度条件

ANOMALOUS ACTIVITIES OF TROPICAL CYCLONE OVER THE WESTERN NORTH PACIFIC AND THE RELATED LARGE-SCALE CIRCULATION FEATURES DURING 1998 AND 1999

中文关键词:  热带气旋  季风槽  垂直切变  异常热源

英文关键词:Tropical cyclone  Monsoon trough  Vertical wind shear  Anom alous heat sources

中文摘要:

      应用联合台风警报中心(JTWC)的资料,研究了1998和1999年西北太平洋(WNP)热带气旋(TC)的活动,发现这两年具有较为相似的异常特征:台风个数明显偏少,台风的生成源地显著偏西。对导致这种异常的大尺度条件场的分析表明,1998和1999年的台风季季风槽、垂直风切变、海平面气压等因子的分布都不利于台风在WNP东部海域的生成,因此是导致这两年台风偏少和生成源地异常偏西的重要原因。其中,季风槽的异常偏西及在盛夏的异常偏北对TC的异常活动产生了主要影响。对低纬大气环流异常的进一步分析表明,WNP地区的环流异常是导致季风槽和垂直风切变场异常的主要原因,而1999年异常环流的出现可能是由于低纬大气对印尼附近异常热源强迫的响应所致。由于该热源的存在,在其东侧(WNP地区)激发出东传的Kelvin波,而相应的异常风场则表现为低层盛行异常东风,而高层盛行异常西风,因此造成了WNP东部和西部海域垂直切变场的近乎相反的变化。同时,由于低层异常强的东风不利于季风西风的向东延伸,从而使季风槽明显偏西,未到达菲律宾以东的热带气旋频发区的位置。

英文摘要:

      Based on the data from the Joint Typhoon Warning Center (JTWC), the activities of tropical cyclone (TC) in the western North Pacific (WNP) have been studied. The results showed that the TC anomalous activities in these two years were very similar in such a way that the typhoon number was far less than normal and the location was westward off the mean. The analyses of the main factors resulting in these anomalies showed that during the period from April 1998 to 1999, the patterns of the monsoon trough, vertical wind shear and sea-level pressure were all not favorable to the genesis of typhoon in the eastern WNP, which was an important reason causing the decrease of T Y number and westward movement of genesis location. And in which the anom alous westward and northward movements of monsoon trough in midsummer exerted a main effect on the TC anoma lous activities. The further analyses of the tropical large-scale anomalous circulation have shown that the anomalies of the circulation over the WNP were the main reason leading to the anomalies of the monsoon trough and vertical wind shear. And the 1999 anomalous circulation may be caused by the response of the tropical atmosphere to the anomalous heat sources around Indonesia. Because of the anomalous heating, the eastward propagating Kelvin waves were exited in its eastern side (eastern WNP) and the corresponding anomalous wind field displayed the pattern that the anomalous easterlies prevailed in the high troposphere and the anomalous westerlies in the low level. This led to the almost reverse change of vertical wind shear in the eastern and western WNP. On the other hand, the anomalous strong easterlies in the low troposphere were not favorable to the eastward extension of the monsoon westerlies, which led to the anomalous westward movement of the monsoon trough so that it can not reach the region east of Philippine where the TC frequently generate clim atologically.