Stronger Waves

Most of the tropical storms that roar across the Atlantic basin are born over Africa—especially the really big ones. They begin as low-pressure systems over the Sahara Desert, and are pushed into the Atlantic Ocean by a powerful jet stream.

La Niña may boost that process. A recent study found that it may help create stronger systems over Africa, potentially leading to stronger tropical storms.

La Niña is part of a back-and-forth cycle in the eastern Pacific Ocean, from warmer to cooler waters. La Niña is the cooler phase. And it can impact climate across the globe.

The study found a link between La Niña and African easterly waves—the systems that form over Africa and head out to sea. During La Niña years, the waves are stronger, wetter, and more turbulent, so they produce more thunderstorms. That brings heavier rains to parts of Africa, the Caribbean, and the Americas, even if the systems don’t become tropical storms.

La Niña changes the way air circulates across the entire planet. Over Africa, it appears to strengthen two jet streams, and it pushes one of them northward. It also has an effect on the African monsoon season. Those changes rev up the easterly waves—and the intensity of hurricanes.

African easterly waves give birth to about 60 percent of all tropical cyclones in the Atlantic, Caribbean, and Gulf—and more than 80 percent of the major ones. So understanding the link between La Niña and the waves could improve hurricane-season forecasts.