On 25 May, NOAA forecasters at the Climate Prediction Center released their outlook for the 2023 Atlantic hurricane season, which spans 1 June to 30 November. They predict 5–9 hurricanes this season, which is lower than the predictions of the last three years.
The reduction is due in large part to what’s happening in the Pacific Ocean. The last three hurricane seasons have occurred under La Niña conditions, an ocean–atmosphere state characterized by cool sea surface temperatures off the west coast of South America. But in May, above-average ocean temperatures emerged in the equatorial Pacific. So this year climate scientists are predicting a transition to El Niño conditions, which heat up the Pacific Ocean and often suppress Atlantic hurricane activity because of a drying and stabilization of the atmosphere in the Atlantic. (See the article by David Neelin and Mojib Latif, Physics Today, December 1998, page 32.)
Like the Pacific, the Atlantic basin also has El Niño and La Niña events, often referred to as Atlantic Niño and Atlantic Niña, albeit smaller and less influential on a global scale. To better determine the influence of Atlantic Niño on the genesis of tropical cyclones, Dongmin Kim, of the University of Miami and NOAA’s Atlantic Oceanographic & Meteorological Laboratory, and colleagues analyzed the spatial patterns of tropical cyclones over 74 years.
The data show that, unlike its Pacific counterpart, Atlantic Niño is associated with increases in the strength and severity of hurricanes, particularly those that form off the coast of West Africa and travel to the Caribbean and North America. Known as Cape Verde hurricanes, they’re the most intense and destructive type of tropical cyclone.
Despite years of study, much about Atlantic Niño remains unknown. Many events happened in the 1990s, and then Atlantic Niña conditions persisted throughout the 2000s until 2016, when Atlantic Niño became active again. So to try to capture as many events of the phenomena as possible, Kim and his colleagues worked with a data set that ranges from 1948 to 2021. It shows the trajectories taken by tropical cyclones, which are based on a hurricane’s position, intensity, and other time-dependent parameters.
The researchers selected all the years from the data set when Atlantic Niño events occurred and analyzed the patterns of tropical cyclones and did the same for Atlantic Niña conditions too. In the tropical Atlantic region north of the equator, Kim and his colleagues found many locations where more tropical cyclones form during Atlantic Niño conditions compared with the average. The tropical North Atlantic, for example, saw a statistically significant addition of two tropical cyclones during years with Atlantic Niño conditions compared with years with Atlantic Niña conditions.
Kim and his colleagues hypothesize that during Atlantic Niño conditions, the tropical Atlantic rainband—known as the intertropical convergence zone—strengthens, which enhances not only westward traveling atmospheric waves that originate near West Africa but also the strength of low-level eastward winds across the area. Those changes in the atmosphere induce rotational motion that helps produce tropical storms.
Together, those large-scale atmospheric dynamics create conditions more favorable for the development of Cape Verde hurricanes off the coast of West Africa. If that hypothesis holds, Kim and his colleagues suspect that it may help forecasters better understand how Atlantic Niño affects predictions of the hurricane season. (D. Kim et al., Nat. Commun. 14, 3704, 2023.)
