An unforgettable parade of deadly and destructive hurricanes ripped across the Atlantic in 2017, making it, by one measure, the seventh-strongest season on record. Early projections are that another active year looms in 2018.
An analysis released Wednesday by Colorado State University’s Philip Klotzbach outlined five scenarios for next year, the two most likely of which would produce an above-normal season.
A key question is what happens to the La Niña — cooler than normal water in the tropical Pacific that reduces wind shear in the Atlantic. One reason this year’s hurricane season was so hectic was that La Niña conditions were brewing, which relaxed Atlantic wind shear and allowed storms such as hurricanes Harvey, Irma and Maria to power up.
An El Niño in the Pacific — above-average water temperatures — would increase wind shear in the Atlantic and limit development next year, but that’s not in the long-range outlook, at least for now.
Official National Oceanic and Atmospheric Administration forecasts call for neutral El Niño conditions in late spring and summer.
In that case, Atlantic Ocean temperatures — impacted by another phenomenon called the Atlantic Multi-Decadal Oscillation (AMO) — become more important.
In the first two CSU scenarios, a strong or above-average AMO combined with no El Niño would likely produce an accumulated cyclone energy (the strength and duration of all storms combined) of between 130-170. They gave those scenarios a 35 percent and a 25 percent chance of occurring, respectively.
A season with an accumulated cyclone energy of 170 usually has 14-17 named storms; nine to 11 hurricanes; and four to five major hurricanes, Category 3 and higher, according to the CSU forecast.
But Klotzbach and other experts caution that El Niño conditions are notoriously difficult to predict beyond the winter and spring. That’s one reason CSU won’t issue a definitive prediction, with projected numbers of named storms and hurricanes, until April 5.
Forecasters were surprised in 2017 by a “La Niña double dip” — La Niña conditions last winter looked like they would give way to an El Niño in summer. Instead, another La Nina developed.
“The waters did warm in the spring and early summer, but then they cooled quickly back down,” said David Zierden, a climatologist and a research associate with the Center for Ocean-Atmospheric Prediction Studies at Florida State University.
How rare was that double dip? Not that rare, says Zierden. A third return to La Niña wouldn’t be that uncommon either, he said.
“It’s the El Niño phase that tends to be one year, whereas the La Niñas are often two- or three-year events,” he said. “The cycle seems to be that it goes into neutral during the summer months and then it seems to re-energize again late summer or fall. That’s what happened from 1999 through 2001 and what happened last year.”
In the United Kingdom, meanwhile, Tropical Storm Risk — a consortium of climate researchers and insurance risk management experts — is calling for a slightly above-normal Atlantic season in 2018 due in large part to anticipated lower-than-usual wind shear in the tropics.
“TSR anticipates that July-September trade wind speed will be slightly weaker than normal and thus have an enhancing effect on Atlantic hurricane activity in 2018,” Mark Saunders and Adam Lea of the Department of Space and Climate Physics at University College London said in the TSR analysis issued Dec. 7.
They also see slightly warmer than average sea surface temperatures in the Atlantic.
TSR forecasters predict 15 named storms, seven hurricanes and three intense hurricanes with an accumulated cyclone energy index of 117. That projection compares with an average over the last 10 hurricane seasons of 115. The 2017 hurricane season accumulated cyclone energy index was 226, the seventh-strongest season on record.