North Atlantic hurricane season: Study predicts 'busy' final month

The peak of the North Atlantic hurricane season has come and gone as temperatures cool, but at least one study shows conditions in the Caribbean “favor an active end” to the season, which ends Nov. 30.

“We forecast a busy final two months of the season in the Caribbean,” Phil Klotzbach, senior research scientist, Department of Atmospheric Science at Colorado State University, said by email.

But experts caution meteorological factors make such forecasting difficult, including global forces such as a surprising third year of a La Niña weather pattern that affects winds, warmer polar regions from climate change, and even a massive volcanic eruption in the South Pacific in January that spewed vapor and ash into the atmosphere.

Colorado State University's storm prediction is based on two main factors: what it calls “a robust La Niña event” and “slightly warmer than normal” temperatures in the “Atlantic warm pool.” That is the body of water stretching east from the Gulf of Mexico to the Caribbean and the Atlantic, where the sea surface temperatures rise to at least 83.3 degrees Fahrenheit, according to the European Geosciences Union.

Storms that spring from winds off the African coast can strengthen in the Caribbean — and make their way up the East Coast.

Specifically, the CSU researchers, who have issued such forecasts since 1984, predict that storms with power and duration of 15 on a scale called the Accumulated Cyclone Energy will hit before the Atlantic hurricane season ends.

In comparison, the late-September Hurricane Ian, whose remnants brought downpours to Long Island, reached 17.4 at one point on that yardstick, while the mid-September Hurricane Fiona packed 26.3, they say.

Specific predictions of whether such storms will travel up to the metropolitan area or Long Island was not part of the CSU outlook.

Season began slowly

This hurricane season has proved surprising so far, as not only did it begin slowly, with only three named storms in June and July, but this was the first August in 25 years with no major storms.

Yet this is the third year in a row for the La Niña weather pattern, which typically helps power hurricanes by decreasing wind shear — changes in its speed and direction — that otherwise blows storms apart.

“This year, that has not been the case; wind shear has been higher than usual,” said Jeff Masters, who co-founded Weather Underground, Ann Arbor, Michigan-based commercial forecasters, and is a meteorologist with Yale Climate Connections, which is tied to The Yale Center for Environmental Communication.

“There is a lot of head-scratching going on,” Masters said. “We expected a much more active season because it was a La Niña year.”

With the La Niña weather system, intensifying easterly trade winds around the equator drive the Pacific Ocean toward Asia, the sea’s coldest levels rise to the surface of South America’s west coast.

The jet stream then veers north, toward the U.S. West Coast, opening the door for more hurricanes, though experts say they tend to head up to the Ohio Valley and the Great Lakes, instead of along the East Coast.

La Niña’s opposite, El Niño, can curb storms, as it drives the jet stream much further south, into northern Florida and the Gulf of Mexico, according to the National Oceanic and Atmospheric Administration.

El Niño arises when easterly trade winds weaken — or even change direction. So less of the ocean’s cooler bottom waters rise to the surface off western South America. And that warmer water can, NOAA says, "slosh" toward the eastern Pacific.

South Pacific volcano a wild card

Possibly, the South Pacific's Hunga Tonga-Hunga Ha'apai massive volcanic eruption in mid-January "could be affecting global atmospheric circulation,” Masters said. That record-breaking blast released the “equivalent to around 10% of the amount of water vapor typically residing in the stratosphere,” according to a July study entitled “The Hunga Tonga-Hunga Ha’apai Hydration of the Atmosphere.”

“Unlike previous strong eruptions, this event may not cool the surface, but rather it could potentially warm the surface due to the excess water vapor,” the researchers said.

However, noted Mark Schoeberl, chief scientist, Columbia, Maryland-based Science and Technology Corporation, this particular “water vapor blob” is much higher than previous such releases — and might not impact the climate until it descends, possibly next year.

Of course, the volcano also spewed huge quantities of ash. Much like the aerosol pollution from burning coal and other fossil fuels, that ash can reflect the sunlight, cooling Earth.

Another contributing factor toward more Atlantic hurricanes is the 1967 Clean Air Act.

“Because they cleaned up our air, that allowed it to get warmer,” so it can carry more moisture, explained Matthew Rosencrans, director, NOAA Climate Testbed, and lead for the seasonal hurricane outlook at the Climate Prediction Center.

Global factors play role

Other factors driving hurricanes, experts say, have quieted: storms that circle the globe every 30 to 60 days are in a lull period and the Sahara recently has blown dry air across the Atlantic.

Sill, global warming appears to be strengthening hurricanes, as they extract more heat and water from the seas, which also are rising, experts say.

Yet this year, Masters noted, all ocean basins have had less storms, possibly partly due to this year’s unusually warm North and South poles.

“Hurricanes fundamentally form to transfer heat from the Equator to the Poles,” he explained.

“And that difference in temperature between the Equator and the Poles has been less than usual, so that means there is less of a need for storms to carry heat from the Equator to the Poles.”

Though forecasters all warn devastating storms still can strike in autumn, Kevin Reed, associate dean for research and associate professor, Stony Brook University’s School of Marine and Atmospheric Sciences, noted the cooling Atlantic helps lessen that risk toward the end of the hurricane season.

“A big part of what drives the average season,” and why the season starts to wane after September, “is really largely driven by the ocean surface temperature," Reed said.