Hurricane season is still more than three months away, but in parts of the tropical Atlantic, it feels like we might as well already be in the thick of it. Across a strip of ocean where many cyclones are born, February ocean temperatures are closer to what scientists expect in July.
The ominous warmth is stirring concerns of yet another hyperactive Atlantic hurricane season. Seven of the last eight seasons have been above normal.
Last year, similarly unusual warmth fueled a storm season that was significantly more active than meteorologists might have expected given the presence of the El Niño climate pattern, which emerged last spring and creates conditions that tend to inhibit Atlantic cyclone formation.
As meteorologists look ahead to this hurricane season, which begins June 1, they see an increasing likelihood that a La Niña pattern will replace El Niño by late summer or early fall. That is another bad sign for the U.S. coastline — La Niña is associated with active patterns in the tropical Atlantic.
It’s still too early to say whether the warmth will persist into hurricane season, or when La Niña might arrive. But, especially together, the trends suggest that an active season could be difficult to avoid, said Michael Lowry, a meteorologist with WPLG-TV in Miami and a former National Hurricane Center scientist.
“There’s plenty of time ahead before we get to the meatiest part of the hurricane season,” Lowry said. “But a lot’s going to have to change … for forecasters to feel much more comfortable going into hurricane season.”
A persistent trend of record warmth
Last spring, the strongest climate signal scientists know of — El Niño — gave every indication of a slowdown in Atlantic hurricane activity in the summer and fall.
El Niño’s signature is a surge of warm waters and towering clouds in the central and eastern Pacific. It triggers changes in atmospheric circulation that, on the other side of the planet, can make it harder for tropical storms to form and strengthen: Areas of high pressure with sinking air are more common over the Atlantic, and wind shear, when wind speed and direction vary at different altitudes, increases.
The expectation of El Niño prompted National Oceanic and Atmospheric Administration forecasters to predict a mostly typical Atlantic hurricane season, a downgrade from years of increased storm activity.
But as El Niño developed, and unusual warmth appeared well beyond the Pacific zones the climate pattern is known for, forecasters warned that a quieter season was far less than certain.
By August, it became clearer: The ocean warmth was likely to counteract El Niño’s typical effect in the Atlantic, and NOAA upgraded its forecast. The season ended up with about 20 percent more activity than average, as measured by a statistic known as accumulated cyclone energy.
Now, with a new tropical weather season ahead, Atlantic temperatures are perhaps even more remarkable.
Why meteorologists have reason for concern
In a zone of the Atlantic known as the main development region for hurricanes, sea surface temperatures are running well above normal — and 1.1 degree Fahrenheit (0.6 degrees Celsius) higher than in any other year on record, said Philip Klotzbach, a tropical meteorologist at Colorado State University.
If that trend persists into hurricane season this summer, it could mean a ripe environment for tropical waves flowing from Africa to develop into cyclones.
“Basically, it is the perfect recipe for hurricanes to form and strengthen,” Alejandro Jaramillo, a meteorologist at the National Autonomous University of Mexico, said in an email. “The warmer waters provide extra fuel available for hurricanes, potentially leading to the formation of stronger storms.”
One factor behind the Atlantic warmth: weak winds over the ocean, Klotzbach said. That discourages evaporation, which would allow the waters to cool by transferring heat into the air. Models suggest weaker-than-normal winds continuing into March, Klotzbach said.
Beyond that, longer-term models predict that surface temperatures will remain elevated, and that by the heart of hurricane season, from August through October, precipitation will be above normal across the tropical Atlantic, something that suggests a strong pattern of waves flowing off Africa, Klotzbach said.
If those predictions come to pass, “I’d expect a very busy season in store,” he said in an email.
Meanwhile, climate scientists predict that La Niña is more likely than not to develop by August. While El Niño increases wind shear — which acts to disrupt hurricanes’ columns of rotating winds — La Niña tends to discourage it, clearing the way for storms to organize and strengthen.
The warm water in the tropical Atlantic is part of a global pattern of record sea-surface temperatures, fueled by both El Niño and human-caused climate change. The planet’s average sea surface temperature reached an all-time record of 70.2 degrees Fahrenheit (21.2 Celsius) on Feb. 9, according to the University of Maine’s Climate Change Institute.
Why it’s too soon to panic
Meteorologists stressed that it is far too soon to say how the hurricane season may play out. Official seasonal forecasts from NOAA, Colorado State and other sources won’t arrive for months, and even they carry plenty of uncertainty.
And there is still much scientists don’t understand about how the ocean behaves and what triggers longer-term changes in tropical weather.
For example, it wasn’t immediately clear what was behind an unusual drought of Atlantic hurricanes in the 1970s and 1980s — until scientists realized that a surge in air pollution from Europe was acting to cool the tropical Atlantic by blocking sunlight, said Kerry Emanuel, a professor at the Massachusetts Institute of Technology.
Similarly, it isn’t yet clear why the Atlantic is warming more dramatically than other oceans, or for how long it will continue, he said.
Even if scientists could predict an active hurricane season with more certainty, “that’s not what you want,” Emanuel said. “You want the number of destructive landfalling storms.”
That is outside meteorologists’ capabilities — it was just last year that NOAA extended its tropical outlooks to seven days.
But Lowry said the state of the Atlantic is such that, even if ocean temperatures trend closer to normal, there is still far more heat in the waters that could be available for storms come summer and fall.
“This is such an extreme case that it doesn’t bode well,” he said.
