Dominant Effect of Relative Tropical Atlantic Warming on Major Hurricane Occurrence in the North Atlantic: 2017 and the Future

In the 2017 boreal summer, there were six Major Hurricanes (MHs) generated in the North Atlantic – including three that made landfall (Hurricanes Harvey, Irma, and Maria). Various media and scientific stories discuss the potential major factors responsible for the active MH season (e.g., anthropogenic forcing or La Niña conditions), along with any implications for potential future change; however, there is no clear consensus on the dominant driving factors.

Here we propose a new approach to address the cause of active 2017 MH season. We have undertaken a systematic assessment of the mechanisms controlling this extreme MH season, even as the hurricane season is underway – i.e., real-time attribution. We leverage real-time seasonal forecasts with a state-of-the-art high-resolution dynamical model (HiFLOR) developed at the Geophysical Fluid Dynamics Laboratory that correctly predicted active 2017 MH season when initialized before the 2017 hurricane season.

We find that the enhanced MH activity in the Atlantic in 2017 was mainly caused by unusually warm upper ocean conditions in the tropical Atlantic relative to the rest of the global oceans. In contrast, the moderate La Niña conditions in the tropical Pacific did not play a role. Moreover, we have conducted projections of hurricane activity toward the end of this century as the climate system warms in response to anthropogenic forcing. We find that a repeat of the 2017 sea surface temperature anomalies, relative to a future warmed state, would also lead an unusually active MH season in the North Atlantic. Therefore, the pattern of upper ocean warming in the Atlantic that was observed in 2017 would lead to an even greater risk of such active MHs years in the end of this century, thereby amplifying the risk of MHs in the coastal regions in the North Atlantic, with corresponding socio-economic implications.