Tropical cyclones are among the most deadly and costly natural disasters that affect the United States and many other countries each year. This has led the scientific community to prioritize improving tropical cyclone prediction and the understanding of how tropical cyclone activity has changed and will change in the future. In a new study published in Science Advances, NOAA and partner researchers project an increase in the variability of Atlantic tropical cyclone activity, leading to more active and inactive hurricane seasons and less near-normal seasons.

The study investigates how the frequency and accumulated cyclone energy (ACE) in the North Atlantic might change in the future. ACE is the measure of total seasonal activity used by NOAA scientists to look into past and future changes in tropical cyclone activity, focusing on changes that occur between or over two or more years and the impact of a changing climate. Researchers examine why there have been so many active seasons in recent years. The results show both unusually active and inactive hurricane seasons have become more common since the 1990s, and computer models predict that by the middle of this century, the variability could increase by an additional 36%, with the biggest increase expected to occur in the central tropical North Atlantic, which refers to the mid-section of the North Atlantic Ocean where tropical storms and hurricanes most frequently form.

These changes are linked to shifts in wind patterns and ocean temperatures. In order for a tropical cyclone to form, several atmospheric and oceanic conditions must be met: a pre-existing weather disturbance, warm ocean waters (greater than 79 degrees Fahrenheit), thunderstorm activity, and low vertical wind shear which are wind velocity differences between the top and the bottom of the hurricane. Higher wind shear serves to knock down and discourage the development of a hurricane.

Changes in vertical wind shear and atmospheric stability (the ability of strong thunderstorms to develop), driven by ocean temperature differences between the Pacific and Atlantic Oceans are responsible for the shift in variability. These changes are expected to become more evident in the future, further intensifying the variability in Atlantic hurricane seasons. 

“The historical year-to-year variability in hurricane seasons is much larger than any projected future trends in hurricane activity,” said Hosmay Lopez, Ph.D., NOAA oceanographer and lead author of the new study. “While the scientific consensus is for a future reduction of 2 storms per year, the number of named storms in the North Atlantic revealed much larger swings between 28 named storms in 2005 and  8 in 2014. Despite these large year-to-year fluctuations, future changes in the interannual variability of hurricane activity are rarely investigated in detail, resulting in significant uncertainty in future projections and impacts. Therefore, there is an urgent need to better understand not only the mean activity changes but also the interannual variability changes.” 

The study also found that while the total number of hurricanes in a given season, averaged over many years, should not change much, the likelihood of extremely active seasons will increase due to a projected increase in the year-to-year variations. An amplified variation will increase the number of inactive and active seasons at the expense of a reduction in the near-normal seasons. Note that the seasonal hurricane outlook currently issued by NOAA and others are based on tercile categories (i.e., above normal, near normal, and below normal seasons).

Larger year-to-year swings between busy and quiet hurricane seasons in the North Atlantic pose significant challenges for forecasters and emergency decision makers. The variability from one season to the next makes it more difficult for scientists to predict the severity of any given hurricane season. Additionally, the unpredictability of active vs. inactive seasons creates challenges for disaster preparedness and response since some seasons will be extremely dangerous while others will be relatively quiet. This means that communities in hurricane-prone areas will need to adapt to seasons that have the possibility of being extremely active, like we saw in 2005 and 2020.

Citation: Hosmay Lopez et al., Projected increase in the frequency of extremely active Atlantic hurricane seasons.Sci. Adv.10,eadq7856(2024).DOI:10.1126/sciadv.adq7856

Authors: Hosmay Lopez (NOAA AOML), Sang-Ki Lee (NOAA AOML), Robert West  (NOAA AOML/ UM CIMAS), Dongmin Kim (NOAA AOML/ UM CIMAS), Greg Foltz (NOAA AOML), Gus Alaka (NOAA AOML), Hiroyuki Murakami (NOAA GFDL)