西北太平洋区域纬向风垂直切变的气候特征研究

西北太平洋区域纬向风垂直切变的变化是影响西北太平洋热带气旋生成和发展的一个重要的动力因子,弱的纬向风切变有利于热带气旋的发生、发展。文中将西北太平洋区域纬向风垂直切变幅度(MWS)定义为850与200 hPa的纬向风之差的绝对值,以研究MWS的气候特征。结果表明,西北太平洋区域的MWS有两个主要空间模态,第1空间模态表现为在15°N以南的热带西太平洋存在MWS东西向变化相反的两个区域,20°N附近的热带西太平洋MWS的变化与其以北海区的MWS的变化相反。第2空间模态表现为在热带太平洋140°E东、西的变化相反。研究了两个模态相关的大气环流特征,发现去掉强ENSO信号后,第1模态不但与低纬度大气环流有关,而且还与南、北半球中高纬度的大气环流有关,第2模态主要与热带西太平洋和北太平洋局地大气环流有关。另外,第1模态的时间系数与赤道东太平洋海温、西北太平洋台风生成频次有着密切联系;第2模态时间系数与西北太平洋台风活动频次联系密切。

An investigation into the climatic characteris tics of vertical shear of zonal wind in the western North Pacific.

The change of magnitude of the vertical zonal wind shear (MWS) in the western North Pacific is a very important dynamic condition for the genesis and development of the tropical cyclones with the weak MWS identified with a favourable condition. In this article, MWS in the western North Pacific is defined as the absolute value of the difference between 850 hPa zonal wind and 200 hPa zonal wind. The climatic characteristics of the MWS are analyzed. The results show that there exist two major spatial patterns of MWS in the western North Pacific. The first EOF mode shows an organized longitudinal pattern from the tropical Pacific to the subtropical Pacific. The spatial pattern of the second EOF mode displays opposite variations in the east part of to the west part of 140°E. We also investigate the circulation associated with the two spatial patterns. It is found that the first EOF is not only correlated with the low latitude, but also with the high latitude circulation in both hemispheres. The second EOF is mainly correlated with the local circulation in the western tropical Pacific and North Pacific. In addition, the time series of the first EOF mode is related with the equatorial sea surface temperature in the East Pacific and the tropical cyclone frequency in the western North Pacific and the time series of the second EOF mode is related mainly with the tropical cyclones frequency in the western North Pacific.