August 4, 2005

Hurricane Ivan Generated Monster Waves

WASHINGTON -- Hurricane Ivan, which caused a swathe of destruction across the Caribbean last September before crashing into the U.S. Gulf coast, generated ocean waves more than 90 feet high, researchers said on Thursday.

They may have been the tallest waves ever measured with modern instruments, suggesting that prior estimates for maximum hurricane wave heights are too low, William Teague of the Naval Research Laboratory at the Stennin Space Center in Mississippi and colleagues reported.

"Our results suggest that waves in excess of 90 feet are not rogue waves but actually are fairly common during hurricanes," Teague said in a telephone interview.

A wave that big would snap a ship in two or dwarf a 10-floor building, Teague said. And the sensors may have missed the largest waves, which the authors estimate had crest-to-trough wave heights exceeding 40 meters or 130 feet, the researchers said.

Such giant waves disintegrated before they ever touched land, they said.

Luckily, said colleague Doug Mitchell, ships rarely ever encounter such waves.

"They know better than to be out there in those conditions," he said in a telephone interview.

But oil platforms and other equipment cannot get out of the way.

"So when there were reports of an 80-foot (24-meter) wave striking an oil platform, they called it a rogue wave. We think it wasn't a rogue wave," Teague said.

Ivan sank seven oil platforms and set five adrift, Teague said, and his team's findings might help explain it.

It was pure luck that they did. The instruments the researchers used were out there to measure currents.

They were recorded by sensors about 75 miles south of Gulfport, Mississippi on six moorings resting on the ocean floor, Teague, Mitchell and colleagues report in Friday's issue of the journal Science.

"It was very fortuitous that our instruments were there to measure the wave in the first place and even more fortuitous that they survived the storm," Teague said.

The six sensors turned on only once every eight hours, and sample water flow and pressure for 9 minutes.

"We would see 50 or more waves," Teague said. "It turned out that during one of these periods of recording, the biggest crest-peak wave that we measured was 91 feet."

There were five taller than 65 feet, he added.

The previous record height recorded by the U.S. National Oceanic and Atmospheric Agency was 52 feet, also recorded during Ivan.

What would a 90-foot wave look like? Mitchell said the film "The Perfect Storm" might provide an idea. In "The Perfect Storm" a 100-foot (30 meter) wave sinks a fishing vessel, drowning its crew. The images are computer-generated.

"The waves are about 600 feet long and there are ships out there that are that long," Mitchell said. "If the wave is under the ship, with a crest in front and a crest in back, there is nothing supporting the middle," he added.

"The chances of a ship surviving something like that are not good."