On Typhoon–Terrain Interaction: The Looping Motion of Typhoon Gaemi (2024) before Its Landfall in Taiwan

Abstract At midnight of 25 July 2024, Typhoon Gaemi made landfall in Taiwan and became the first tropical cyclone (TC) at the severe typhoon intensity level to hit the island directly since Typhoon Nepartak in 2016. What is so special about Gaemi is its unusual looping motion occurring near the coast of Taiwan just a few hours before its final landfall. Such a deflection of track significantly prolonged the length of time in which Gaemi impacted Taiwan, leading to unexpected and devastating damages. Particularly, the central and southern parts of Taiwan suffered badly from sustained heavy rainfalls which subsequently led to serious floods and landslides. Here, we demonstrate that this rare looping phenomenon can be explained by the channeling effect as the TC approaches the mountainous terrains of Taiwan, which creates a low- to midlevel northerly jet at the western side of the TC that induces it to move southward. In the later stage of the looping track, a southwesterly corner flow develops, contributing to Gaemi’s subsequent northward turning. The potential vorticity budget further supports that both the southward deflection and northward turning are both dominated by the horizontal advection term associated with these terrain-induced asymmetric flows, providing a clear dynamical explanation for the entire looping process.