Global Increase in Tropical Cyclone Rapid Slowdown Events in Offshore Regions

Abstract Rapid slowdown (RS) of tropical cyclone (TC) motion in offshore regions poses an increased threat to coastal populations and economies due to its unexpected nature and prolonged exposure. However, the variations in global RS events in offshore regions remain poorly understood. Here, we show that, unlike open oceans where RS counts show no significant changes, offshore areas within 400 km of the coastline have experienced a fourfold increase in RS events from 1982 to 2023. Both the global sea surface temperature and the Pacific decadal oscillation contribute to the RS trend. RS variation is significantly correlated with the rapid intensification (RI) change within the coastal regions. The steering flow becomes more favorable for the RS variations in the open oceans but plays a minor role within the coastal regions. The RI-favorable environmental conditions—namely, reduced vertical wind shear and increased potential intensity—are the main factors leading to the increase in RS events in the offshore regions. Climate model simulations suggest that global ocean warming has intensified these changes. These findings underscore the increasing threat of RS in coastal regions over the recent decades, which may continue under a warming climate. Significance Statement The characteristics and the possible mechanisms of rapid slowdown (RS) of tropical cyclone (TC) motion in offshore regions remain poorly understood. Here, we find that unlike open oceans where RS counts show no significant changes, offshore areas within 400 km of the coastline have experienced a fourfold increase in RS events from 1982 to 2023. Moreover, our research shows that RS variation is significantly correlated with the rapid intensification (RI) change within the coastal regions, and the RI-favorable environmental conditions are the main factors leading to the increase in RS events in the offshore regions, whereas the steering flow plays a minor role. These findings underscore the increasing threat of RS in coastal regions over the recent decades.