The Importance of the Montreal Protocol in Mitigating the Potential Intensity of Tropical Cyclones

The impact of the Montreal Protocol on the potential intensity of tropical cyclones over the next 50 years is investigated with the Whole Atmosphere Community Climate Model (WACCM), a state-of-the-art, stratosphere-resolving atmospheric model, coupled to land, ocean, and sea ice components, with interactive stratospheric chemistry. An ensemble of WACCM runs from 2006 to 2065 forced with a standard future scenario is compared to a second ensemble in which ozone-depleting substances (ODS) are not regulated (the so-called World Avoided). It is found that by the year 2065, changes in the potential intensity of tropical cyclones in theWorldAvoided are nearly 3 times as large as for the standard scenario. TheMontreal Protocol thus provides a strong mitigation of the adverse effects of intensifying tropical cyclones. The relative importance of warmer sea surface temperatures (ozone-depleting substances are important greenhouse gases) and cooler lower-stratospheric temperatures (accompanying themassive destruction of the ozone layer) is carefully examined. It is found that the former are largely responsible for the increase in potential intensity in the World Avoided, whereas temperatures above the 70-hPa level—which plunge by nearly 15K in 2065 in the World Avoided—have no discernible effect on potential intensity. This finding suggests that themodest (compared to theWorldAvoided) tropical ozone depletion of recent decades has not been amajor player in determining the intensity of tropical cyclones, and neither will ozone recovery be in the coming half century.