The tropical Atlantic is as hot right now as it usually is in mid-August — peak hurricane season. That makes it about 2.5 months ahead of schedule. And there are some little-known reasons as to why.
Yes, climate change is a factor, say experts, but only one of them. Global air temperatures have risen by about 2 degrees F since the late 1800s and oceans have absorbed 90% of the warming in recent decades, according to NASA. Additionally, the top few yards of the ocean store as much heat as Earth’s entire atmosphere.
“You have a background increase (in sea-surface temperature) due to global warming,” said University of Miami climatologist Brian McNoldy.
That’s not enough to explain the August-like heat content in the “main development region,” where Atlantic hurricanes form and strengthen. The area runs from the west coast of Africa to the Gulf of Mexico, and if the water’s hot, it means more storms.
Two little-known forces are adding to the Atlantic’s sea-surface temperature this year.
The temperature pendulum
When forecasters looked at sea-surface temperatures over time, they noticed that there was a reliable oscillation that cools and then warms the ocean for decades at a time. It’s called the Atlantic Multidecadal Oscillation. “The sea-surface temperature of the entire Atlantic will be warm for three decades,” said Molly Baringer, of the National Oceanic and Atmospheric Administration.
She and others at NOAA use statistical models to assess how active hurricane seasons might be. “Right now we’re still in a warm phase,” she said. “The AMO is one of the predictors that these statistical models evaluate.”
The oscillations are natural and have been occurring for at least the last 1,000 years, said NOAA, and only swing about 1 degree F. That might not seem like a big deal, but twice as many tropical storms grow into severe hurricanes during warm phases like the one we’re in now, which began around 1995, as compared to cool phases.
It’s not clear when this warm phase will end, but it’s adding heat to the already high warm water produced by climate change.
A mini El Niño?
Another factor is the Atlantic Niño. “It’s like a mini El Niño,” said Baringer. The more famous El Niño occurs naturally when east-to-west trade winds slack over the Pacific and hot water piles up off South America. Those warm seas cause a cascade of atmospheric reactions, including wet winters in Florida, and summertime wind shear over the Atlantic, which can topple hurricanes.
The same thing can happen in the eastern Atlantic off the coast of Africa, usually a year after the Pacific El Niño. That’s what’s happening this summer. Like El Niño, Atlantic Niño is characterized by warmer-than-average sea-surface temperatures along the equator near West Africa, and weaker-than-average trade winds farther east along the equator, said NOAA.
Weak trade winds can cause downwelling off Africa, when hot surface water pushed downward and warms the depths. The downwellings make it more difficult for cool deep water to hinder tropical storm formation above.
A feedback loop forms, where the hot surface water off Africa weakens trade winds even more, which in turn causes downwelling.
Unusual confidence
The forces making the Atlantic hot, combined with other factors, have given NOAA very high confidence in its active hurricane season forecast. Forecasters said in their May hurricane outlook that there’s an 85% chance of an above-normal season.
That level of confidence might seem out of character for hurricane forecasters — they’re really not hedging their bets. By contrast, NOAA’s 2023 outlook had a 40% chance of a near-normal season, a 30% chance of an above-normal season and a 30% chance of a below-normal season.
2022 had a 65% chance of an above-normal season, a 25% chance of a near-normal season and a 10% chance of a below-normal season.
It’s not just heat. Another hurricane-friendly force is what’s turning out to be an active African monsoon season. These storms don’t add heat, but as wave after wave of huge thunderstorms roll off the continent into the tropical Atlantic, they often grow into tropical storms and eventually hurricanes.
Right now, Baringer said there’s still enough remnant Pacific El Niño wind shear to limit the monsoon’s impacts, but that will likely change later this summer as El Niño fades. A La Niña is likely to follow in August or later, dropping wind shear even more.
“Those features are all aligned right now, and not in a good way,” Baringer said. “We have unusual confidence in this forecast.”
Bill Kearney covers the environment, the outdoors and tropical weather. He can be reached atbkearney@sunsentinel.com. Follow him on Instagram@billkearneyor on X@billkearney6.
