New insights into why the severity of the 2017 Atlantic hurricane season was a surprise

Research published in Nature Communications today may help forecast severe hurricane seasons by identifying the common ocean factors in three of the most active hurricane seasons since 1980.

The 2017 Atlantic hurricane season was the most costly to date, with estimates of damage at £275bn. Yet this high level of hurricane activity came as a surprise, with most early predictions from forecasting systems suggesting it would be an unremarkable hurricane season.

This paper shows that ocean heat build-up in the main hurricane formation area was a dominant factor in controlling hurricane intensity during the seasons of 2017, 2010 and 2005.

In 2010 and 2005 this heat build-up in the tropical Atlantic was caused by a slowdown in ocean currents. In contrast, in 2017 it was caused by a significant reduction in the north-east trade winds off the coast of Africa, which usually ‘blow’ cold dry air over the ocean removing heat from the Atlantic.

Samantha Hallam, the lead author of this paper from the University of Southampton and the National Oceanography Centre (NOC), said “A slowdown of the Atlantic Meridional Overturning system of currents during the preceding winter months can cause the sort of heat build-up that increases the severity of the subsequent hurricane season, giving us some warning.

That wasn’t the case in 2017, with ocean temperatures in the main hurricane formation area looking broadly ‘normal’ until around April, when the trade winds dropped. Even then unusual levels of heat didn’t built up until around June, leaving very little warning that the impending hurricane season would be a strong one.

We hope these factors will be taken into account in future hurricane season forecasting models.”

These findings are the result of a statistical analysis on hurricane paths, sea surface temperatures and winds between 1980 and 2017. The research also used observations of the Atlantic Meridional Overturning Circulation from the RAPID programme, managed by the NOC.

2005, 2010 and 2017 were chosen for analysis because they were very active hurricane seasons and had the highest sea surface temperatures in the Atlantic since 1980, the date after which satellite and oceanographic data is more accurate.

This research was funded by the Natural Environmental Research Council (NERC) , and involved a collaboration between the University of Southampton, the National Oceanography Centre and the Met Office.

To learn more about the response of NOC scientists to the 2017 hurricane season, see this article on the NOC website.