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.