El Niño could spell quick end to hurricane season

The formation of Tropical Storm Debby last weekend in the Gulf of Mexico brought the tally of Atlantic storms to four this season, the earliest that's ever happened.

But despite the quick beginning, scientists say this season may have a much quicker end, with an El Niño system likely to ride to the rescue later this summer.

"I'm becoming fairly confident that we will have a weak to moderate El Niño by the peak of this year's hurricane season in September," said Phil Klotzbach, a seasonal hurricane forecaster at Colorado State University.

Scientists have long understood that El Niño, a natural warming of the tropical Pacific Ocean, tends to suppress hurricane activity in the Atlantic.

But now, through a combination of research techniques, they're beginning to gain a much deeper understanding of not only why but how El Niño changes the tracks of Atlantic storms and, crucially for Houston, how it may affect activity in the Gulf of Mexico.

The primary way in which El Niño affects hurricanes is through its action on winds in the upper and lower levels of the atmosphere.

At the surface of the Atlantic Ocean, trade winds generally prevail out of the east-northeast. Higher up in the atmosphere, the more dominant winds flow from the west. The difference between these two is called wind shear. When the trades and upper-level winds are at great variance, shear is high, and hurricanes are simply blown apart.

Picking up speed

An El Niño pattern in the Pacific Ocean generally enhances the speed of the lower-level trade winds and upper-level westerlies over the tropical Atlantic, said Anthony Barnston, chief forecaster at Columbia University's International Research Institute for Climate and Society.

"So there's more shear, and you can understand how a high shear would rip apart the vertical structure of a hurricane," he said.

Studies have found that, during the last 60 years, when an El Niño pattern prevailed during the Atlantic hurricane season, only one-quarter of those seasons had more activity than normal. During La Niña years, when Pacific tropical temperatures are cooler, two-thirds of years had more activity than normal.

El Niño years also produced fewer than average major hurricanes as well as fewer landfalling hurricanes.

In addition to reducing wind shear, new research provides an explanation for why there have been markedly fewer land- falls.

Bermuda high

University of Miami researcher Angela Colbert found that the presence of an El Niño often leads to a weaker area of high pressure over the middle Atlantic - commonly known as the Bermuda high - during the summer months.

With a smaller, weaker high-pressure system less likely to block them, storms that develop in the tropical Atlantic often have the opportunity to curve northward into the northern Atlantic before crossing its entire extent and reaching the United States.

This is good for the U.S. East Coast, but Colbert said her research also found that while fewer storms reach there, the number of hurricanes that follow a track like Hurricane Ike, moving almost due west across the Atlantic before reaching the Gulf of Mexico or Caribbean Sea, didn't change much during El Niño years.

Fewer of these storms would mean fewer threats to Texas from storms that develop deep in the tropics.

In fact, Colbert said, during some El Niño years there were considerably more of these "straight-moving" storms, and she believes she knows why.

A number of climate scientists now believe there are actually two kinds of El Niños, one in which temperatures in the central Pacific show more pronounced warming and another in which the warming is closer to South America.

During the central Pacific warming events, Colbert noted, the limited data available show a significant increase in "straight-moving" storms that reach the Gulf of Mexico.

But during the El Niños in which the warming is in the eastern Pacific, there's a large drop in hurricanes moving straight across the Atlantic and into the Caribbean and Gulf.

"We think we have a pretty good hypothesis, but there's not enough data to fully back that up yet," she said. "We need a few more years to gather more data."

Uncertainty remains

As it's not definitive whether an El Niño will develop this year, it's not possible to know for sure which kind it will be, she said.

However, signs indicate it will be the more favorable kind, suppressing the straight moving, Ike-like storms.

And as hectic as the early hurricane season has been, we might be thankful for that in August, when the activity tends to really heat up out there.

eric.berger@chron.com