The Initial Mesoscale Vortexes Leading to the Formation of Tropical Cyclones in the Western North Pacific

Abstract

A statistical analysis of the initial vortexes leading to tropical cyclone (TC) formation in the western North Pacific (WNP) is conducted with the ECMWF ERA5 reanalysis data from 1999 to 2018. It is found that TCs in the WNP basically originate from three kinds of vortexes, i.e., a mid-level vortex (MV), a low-level vortex (LV), and a relatively deep vortex with notable vorticity in both the lower and middle troposphere (DV). Among them, LV and DV account for 47.9% and 24.2% of tropical cyclogenesis events, respectively, while only 27.9% of TCs develop from the MV, which is much lower than that which occurs in the North Atlantic and eastern Pacific. Such a difference might be ascribed to the active monsoon systems in the WNP all year round. Due to the nearly upright structure of mid-level convergence in the early pre-genesis stage, TC genesis efficiency is the highest in DV. Compared with MV, LV generally takes a shorter time to intensify to a TC because of the higher humidity and the stronger low-level cyclonic circulation, which is related to air-sea interaction and boundary-layer convergence. Further examination of the relationship between tropical cyclogenesis and large-scale flow patterns indicate that the TC genesis events associated with LV are primarily related to the monsoon shear line, monsoon confluence region, and monsoon gyre, while those associated with MV are frequently connected with easterly waves and wave energy dispersion of preexisting TC. Compared with other flow patterns, tropical cyclones usually form and intensify faster in the monsoon confluence region.

摘 要

本文利用 1999–2018 年欧洲中心 ERA5 再分析资料, 统计分析了西北太平洋地区诱发热带气旋生成的初始中尺度涡旋类型, 将其分为三类: 源于中层涡旋 (MV), 源于低层涡旋 (LV) 和中低层深厚涡旋 (DV). 统计结果显示 47.9% 和 24.2% 的热带气旋是源于低层涡旋和中低层深厚涡旋, 仅 27.9% 的热带气旋生成个例是源于中层涡旋, 远低于北大西洋和东太平洋, 这种差异可能归因于西北太平洋常年盛行的季风系统. 由于生成早期几近垂直的结构和中层辐合, 热带气旋生成效率在中低层深厚涡旋中是最快的. 而与中层涡旋相比, 低层涡旋通常会增强发展地更快些, 这主要是由于更饱和的水汽条件和更强的低层气旋性环流, 这与海气相互作用以及边界层辐合有关. 在此基础上, 热带气旋生成类型与大尺度背景环流的关系表明, 源于低层涡旋的热带气旋大多与季风切变、 季风辐合区以及季风低压有关, 而源于中层涡旋的热带气旋通常与东风波和先前存在热带气旋的波能频散相关. 与其他大尺度背景流型相比, 热带气旋的生成效率在季风辐合区是更高的.