Slowdown of Typhoon Translation Speeds in Mid-latitudes in September Influenced by the Pacific Decadal Oscillation and Global Warming

Global warming already affects weather and climate worldwide; accordingly, various studies have been conducted to understand the effects of climate change on tropical cyclones. The translation speed of a tropical cyclone is a particularly important feature, as a slower translation speed lengthens the duration of a cyclone's impact. Here, on the basis of observational data, we report that tropical cyclone translation speeds in the middle latitudes of the western North Pacific basin have significantly decreased during September over the last 40 years. Historical model simulations with and without observational global warming trends reveal two main factors responsible for the translation speed slowdown: natural decadal climate variabilities (such as the Pacific Decadal Oscillation) and global warming. Both factors produce an anticyclonic anomaly in the westerly jet over western Japan; this anomaly relaxes the latitudinal geopotential height gradient, weakening the environmental synoptic winds by which tropical cyclones are steered. Furthermore, model simulations for a future warmer climate show that global warming further reduces the steering flows, leading to more slowly-moving tropical cyclones in autumn in the future.