两类El Niño型对西北太平洋季风槽及热带气旋生成的可能影响

通过对1948~2015年不同El Ni?o事件下西北太平洋季风槽变化和热带气旋(tropical cyclone,TC)生成进行分析,初步探讨了不同El Ni?o型事件对季风槽及其对TC的可能影响。分析结果表明,较东太平洋增暖(eastern Pacific warming,EPW)年,中太平洋增暖(central Pacific warming,CPW)年季风槽偏弱,位置相对偏西、偏北。在CPW年,中(西和东)太平洋海温增暖(降低)引起了从中到西太平洋热带地区的西风异常和中太平洋地区上升运动及对流活动加强,使得季风槽加强东伸,同时西太平洋副高偏弱、偏北,季风槽向北推进;而在EPW年,赤道东(西)太平洋海温增暖(降低)使得赤道地区西风异常显著加强东扩,异常Walker环流的上升支东移至东太平洋,季风活动加强,副高偏强、偏南,这使得季风槽较CPW年相比更强、更偏东。利于TC生成的大尺度环境因子随季风槽强度和位置的变化而发生改变,在CPW年,低层气旋性涡度、高层辐散、高的中层相对湿度以及低垂直风切变区随着季风槽向北移动;而在EPW年,这些因子随季风槽向南、向东偏移。这些大尺度环境因子的变化使得西北太平洋TC生成的位置在CPW年比EPW年更加偏北、偏西。

Possible Impacts of Two Types of El Niño Events on the Western North Pacific Monsoon Trough and Tropical Cyclogenesis

The present study analyzes the monsoon trough and tropical cyclone (TC) genesis over the western North Pacific to preliminarily investigate the anomalous monsoon trough and its possible effect on TCs during different types of El Niño events for the period of 1948-2015. It is shown that, compared with that in the eastern Pacific warming (EPW) years, the monsoon trough is weaker and its position leans toward the west and north during the central Pacific warming (CPW) years. In these years, the warmer (cooler) sea surface temperature (SST) over the central (western and eastern) Pacific induces anomalous westerly winds from the central to western Pacific in the tropical region and stronger than normal ascending motions and convective activities over the central Pacific, which can induce a strengthened and eastward extending monsoon trough. Meanwhile, the western Pacific subtropical high is weaker than normal and shifted northward, which leads to the northward displacement of monsoon trough. During EPW years, however, warmer (cooler) SSTs occur over the eastern (western) equatorial Pacific; anomalous westerly winds significantly extend eastward in the tropics; the ascending branch of the anomalous Walker circulation shifts eastward to the eastern Pacific; the monsoon activity becomes stronger; the subtropical high intensifies and leans toward the south. All the above changes are favorable for a stronger monsoon trough that extends more eastward compared to that in CPW years. Further study reveals that large-scale environmental factors that are related to TCs genesis will change with variations in the intensity and position of the monsoon trough. During CPW years, the cyclonic vorticity in the lower troposphere, the divergence in the upper-level, the higher relative humidity in the middle troposphere and the lower vertical wind shear all move toward the north with the monsoon trough. In EPW years, the above factors follow the monsoon trough to move southward and westward. These changes prompt the location of TC genesis over the western North Pacific to lean northward and eastward compared to that in EPW years.