Northward Shift in Landfall Locations of Tropical Cyclones over the Western North Pacific during the Last Four Decades

Abstract

This study analyzes landfall locations of tropical cyclones (TCs) over the western North Pacific during 1979–2018. Results demonstrate that the landfall locations of TCs over this region have shifted northward during the last four decades, primarily due to the shift of landfalling TC tracks, with the decreasing/increasing proportion of westward/northward TC tracks. In particular, the northward shift of the landfalling TCs was not related to their formation locations, which have not markedly changed, whereas “no-landed” TCs have significantly shifted northward. TC movement was significantly and positively correlated to the zonal component of the steering flow, while the correlation between TC movement and the meridional component of the steering flow was relatively unobvious. The westward steering flow in the tropical central Pacific that occurred around the formation and early development of the westward TCs was significantly weakened, which was unfavorable for their westward movement, thereby, causing the higher proportions of northward moving tracks. This weakened westward flow was related to the northward shift of the subtropical high ridge, which was caused by significant weakening of the southern part of the subtropical high. The vertical wind shear, sea surface temperature, and convective available potential energy also showed that the northern region of the western North Pacific became more favorable for TC development, whereas the upper divergence, low-layer relative vorticity, and accumulated water vapor content were not obviously related to the northward shift of TCs.

摘 要

在近几十年里, 西北太平洋热带气旋对中国的影响越来越大, 本研究旨在通过分析1979年至2018年里生成在西北太平洋的登陆热带气旋活动来理解这个现象。本文使用中国气象局热带气旋中心所提供的逐6小时最佳路径资料和ERA-interim再分析资料, 系统性研究了登陆热带气旋的登陆点位置变化以及成因。结果显示在这个区域生成的热带气旋登陆点在近几十年里越来越向北移动, 主要是因为在这段时间内热带气旋路径的北移, 即西行路径热带气旋减少、西北或者转向北向路径热带气旋增多。西北太平洋登陆的热带气旋与其生成位置及热带气旋数量没有明显关系, 因为其生成位置无显著变化, 而“无登陆”热带气旋的生成位置明显向北移动。热带气旋的运动与引导气流的纬向分量有明显的正相关, 而热带气旋运动与引导气流的径向分量的相关性不明显。对于西行热带气旋形成和早期发展过程影响较大的向西引导气流明显减弱, 不利于热带气旋的西行运动, 导致热带气旋进一步向北运动。减弱的向西引导气流与西太平洋副热带高压南部有明显减弱造成的副热带高压脊的北移有关。除此之外, 垂直风切变、对流有效势能(CAPE)和海表面温度(SST)在近四十年里也有明显变化, 在西北太平洋北部(20°—25°N以北), 垂直风切变显著减少、CAPE值显著增大、SST逐渐上升, 而在西北太平洋南部的其变化则相反, 这表明西北太平洋北部地区的环境变得更有利于热带气旋发展, 但高层散度、低层相对涡度和大气水汽含量与热带气旋的北移关系不明显。