Tropical cyclone intensification and extratropical transition under alternate climate conditions: a case study of Hurricane Ophelia (2017)

Abstract. Post-tropical cyclones can have substantial impacts on regions unaccustomed to such powerful storms. Previous studies have found that the prevalence of such storms is expected to increase in the future. Hurricane Ophelia in October 2017 presents a potential future analogue, reaching major hurricane strength far beyond current climatological boundaries and affecting Ireland as a powerful post-tropical cyclone. Here, we look at the changes in the structure, behavior, and impacts of Ophelia between current and possible future climate scenarios, with a focus on the extratropical transition phase. Using a regional model, we downscale GFS analysis data, and simulate alternate climatic conditions using a prescribed uniform temperature forcing. In warmer scenarios, Ophelia becomes a stronger and larger storm, while its path shifts westwards. These changes allow Ophelia to delay extratropical transition and maintain more of its tropical characteristics, increasing the impacts upon landfall. We also demonstrate that in the case of Ophelia, storm intensity and impact are very sensitive to initial conditions. Additionally, the tropical phase of the storm is more impacted by the change in temperature than the extratropical phase, indicating that proper tropical cyclone modelling is required for accurate predictions of Post-tropical cyclones impacts. Our simulations indicate that rare cases similar to Ophelia likely present an even larger risk to affected areas in Western Europe in a warmer future.