Two University of Arizona researchers accurately predicted an abnormally busy and intense Atlantic hurricane season, while other forecasting centers predicted otherwise.
“We were really nervous,” said one of the researchers, Xubin Zeng. Other prediction centers expected an average year. “Months later, we didn’t change,” but other centers moved their predictions slightly closer to Zeng and his colleague’s.
Zeng, director of the UA’s Climate Dynamics and Hydrometeorology Center and a professor of atmospheric sciences, former graduate student Kyle Davis, and former professor Elizabeth Ritchie developed the forecasting model in 2015.
Thomas Galarneau, assistant professor in the UA’s department of hydrology and atmospheric science, collaborates each season with Zeng on algorithms that go into the model to forecast that year’s season.
In early June, the team predicted 11 total hurricanes in the Atlantic hurricane season stretching between June 1 and Nov. 30. Six were expected to become major hurricanes — reaching Category 3 or higher.
The team’s predictions were almost spot on. A total of 10 hurricanes developed in the Atlantic this year; six were major.
“It’s pretty amazing how good our prediction was,” Zeng said.
Midway through the season in early August, the Climate Prediction Center, Colorado State University and Tropical Storm Risk adjusted their predictions to only a slightly-above-average year — about seven or eight total hurricanes including three or four major.
The UA team also forecast an intense season, which is measured using an Accumulated Cyclone Energy index. The index uses a number to describe the strength and duration of each hurricane. A value of 92 is average.
UA researchers predicted an index of 181 this season, and while this is lower than the actual 223 observed this season, it is more accurate than the other centers’ midseason predictions, which hovered around 125.
September stood out as the most active month on record for Atlantic hurricanes as measured by the cyclone energy index, surpassing the previous record set in September 2004 and reaching 3½ times the normal intensity for that month, according to the National Oceanic and Atmospheric Administration.
Five hurricanes formed that month. Four were major.
One of the biggest reasons that the UA team’s predictions were so accurate was because of the ingredients they decided to use when coming up with their model, Zeng said.
For example, Galarneau said, many global forecasting centers expected this to be an El Niño year, so other modelers used the El Niño variable to make their forecasts.
El Niño years are characterized by high temperatures in the Pacific Ocean surface. La Niña is the opposite. Conditions created by either La Niña or El Niño impact climate throughout the year.
During hurricane season specifically, they act like a seesaw between the Pacific and Atlantic oceans, according to NOAA, “strengthening hurricane activity in one region while weakening it in the other.”
If it was an El Niño year, like many predicted, it would have reduced activity in the Atlantic and strengthened it in the Pacific.
The UA team had less confidence in global predictions, so instead it used its own observations of the months just before hurricane season to determine that it might be a neutral, or even slightly La Niña year.
That ended up being right. “This was a hyperactive year” in the Atlantic, Zeng said.
Atlantic Ocean conditions were perfect. The warm sea surface temperature and the weak wind shear — which, when strong can rip hurricanes apart even before they form — allowed hurricanes to grow and thrive.
The team also relied on historical data stretching back more than a century to determine the variables they put into the model, Zeng said, as well as physical insights that come from experience.
On Aug. 25, Harvey was the first hurricane to make landfall this year on the United States, where it brought historic rain and flooding to the Texas coast.
But as the storm finally dissipated, Hurricane Irma pounded the Caribbean and then swept through Florida.
Hurricanes Jose and Katia formed soon after. Katia made landfall in Mexico, while Jose has stayed out at sea.
Hurricane Maria then devastated Puerto Rico, where thousands are still without electricity, basic necessities and aid from the U.S. government.