"Monster" Waves Suprisingly Common, Satellites Show
for National Geographic News
|August 10, 2004|
In the recently released movie Riding Giants, big-wave surfers search the globe for the ultimate ride at legendary surf spots such as California's Mavericks. But at unnamed spots in the ocean, there are huge waves that can't be so easily caught.
Known as rogue, or freak, waves, these giants seem to spring up from the ocean floor. They are so large that they can overwhelm and sink even the sturdiest ships.
Now researchers have started using satellites to try to identify and track these unruly waves. And the results have been surprising. While rogue waves were once thought to be the stuff of sailing legend, satellite photos found ten huge waves welling up one three-week period.
"We thought we'd have difficulties finding so many large waves," said Wolfgang Rosenthal, a research scientist at Germany's GKSS research center. "But roughly two ships each week are affected."
Rosenthal and colleagues at GKSS have spent several years looking at 30,000 satellite images of the ocean surface taken by two European Space Agency satellites over a three-week period in 2001.
The satellite images were processed at the Cologne-based German Aerospace Center. This research was part of a European Union program called MaxWave, started in 2000 to document the occurrence of rogue waves.
With this and other studies, researchers want to develop a wave map covering more than a decade of observations of the world's oceans. The map could be used to forecast the likelihood of the giant waves' appearance.
While forecasting rogue waves can be very complex, the goal of the global satellite measurement project is to improve forecasting by looking at the differences between wave models and actual observations, said Wolfgang Lengert, ERS mission manager for the European Space Agency,
During the last two decades, more than 200 supertankersships over 200 meters (656 feet) longhave sunk beneath the waves. Rogue waves are thought to be the cause for many of these disasters, perhaps by flooding the main hold of these giant container ships.
In a single week in winter 2001, two tourist ships were hit by rogue waves in the South Atlantic. The Bremen and the Caledonian Star had their bridge windows smashed by the errant wall of water reaching 30 meters (98 feet) high.
"For tourist ships, they like to have these large windows so that people can look out," said Susanne Lehner, a mathematician at the University of Miami in Florida who studies wave dynamics.
For the Bremen, water pouring in the smashed windows damaged the ship's instruments and power, setting it adrift for two hours while the crew worked to restart the engines.
"The same phenomenon could have sunk many less lucky vessels: two large ships sink every week on average," said Rosenthal in an ESA press release. "But the cause is never studied to the same detail as an air crash. It simply gets put down to 'bad weather.'"
In 1978, when the 43,000-ton cargo ship Mûnchen went down with all hands during an Atlantic crossing, all that remained was a battered lifeboat. Now researchers suspect the sinking likely happened after the ship was hit with a rogue wave.
Offshore oil rigs also get hit by rogue waves. Radar reports from the North Sea's Gorm oil field show 466 rogue-wave encounters in the last 12 years.
Researchers have some clues to what causes these monster waves. Rogue waves seem to be clumped near a strong currentsuch as the Gulf Stream off the North American east coast and the Agulhas Current off South Africa. The current, Lehner said, can act like a lens, focusing wave energy so that small waves combine into larger ones.
But sometimes rogue waves appear out of the deep blue sea, in parts of the ocean without major currentsand scientists aren't quite sure why.
The giant waves sometimes appear in areas of extended storms or converging weather fronts. Waves coming in from different systems could build up into big waves, Lehner said. Looking at more satellite data could help researchers learn even more about how and where the waves form.
These days oil rigs and ships are built to withstand waves of 15 meters (49 feet). Learning more about rogue waves, the researchers said, could help ship designers bolster the seaworthiness of their crafts.
Following the Waves
To track rogue waves, researchers looked at data from the European Space Agency's archives of images shot from the satellites. The satellites' radar makes images of ten-by-five-kilometer (six-by-three-mile) patches of the sea surface every 200 kilometers (120 miles).
With the consecutive images, scientists get a bird's-eye view of the ocean's dynamics. "You can really 'fly' around the globe," Lehner said. "People have never looked at the sea surface in this way before."
Lehner and other researchers then broke down the images into elements of wave energy and wave direction, called ocean-wave spectra, which can be used by weather stations for forecasting.
In the process the researchers spotted more than ten rogue waves during the three-week period. Each wave swelled to 25 meters or more (82 feet) in height. By comparison, big wave surferswho chase monster waves around the globe haven't documented surfing waves over 70 feet (21 meters) high. One of the biggest waves ever surfed was Mike Parsons's 66-foot (20-meter) wave off California's remote Cortes Bank.
Now Lehner and Rosenthal are starting a new project, called WaveAtlas, which will pore through two years of satellite images to create a larger-scale understanding of where and when rogue waves happen throughout the seasons and the years.
Rosenthal, an experienced sailor, may also head out to sea to take measurements at the surface. But he hopes to avoid any errant giants. "I'm glad I've never seen a rogue wave," he said.
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