Thursday, July 13, 2006

From myth to real monsters ... the rogue waves


The storm was nothing special. Its waves rocked the Norwegian Dawn just enough so that bartenders on the cruise ship turned to the usual palliative -- free drinks.

Then, off the coast of Georgia, early on Saturday, April 16, 2005, a giant, seven-story wave appeared out of nowhere. It crashed into the bow, sent deck chairs flying, smashed windows, raced as high as the 10th deck, flooded 62 cabins and injured four passengers.

"The ship was like a cork in a bathtub," recalled Celestine Mcelhatton, a passenger who, along with 2,000 others, eventually made it back to Pier 88 on the Hudson River in Manhattan. Some vowed never to sail again.

Enormous waves that sweep the ocean are traditionally called rogue waves, implying that they have a freakish rarity. Over the decades, skeptical oceanographers have doubted their existence and tended to lump them with sightings of mermaids and sea monsters.

But scientists are now finding that these giants of the sea are far more common and destructive than once imagined, prompting a rush of new studies and research projects. The goals are to understand why they form, explore the possibility of forecasts and learn how to better protect ships, oil platforms and people.

The stakes are high. In the past two decades, freak waves are suspected of sinking dozens of big ships and taking hundreds of lives.

The upshot is that the scientists feel a sense of urgency about the work and growing awe at their subjects.

"I never met, and hope I never will meet, such a monster," said Wolfgang Rosenthal, a German scientist who helped the European Space Agency pioneer the study of rogue waves by radar satellite.

Drawing on recent tallies and making tentative extrapolations, Rosenthal estimated that at any given moment 10 of the giants are churning through the world's oceans.

In size and reach these waves are quite different from earthquake-induced tsunamis, which form low, almost invisible mounds at sea before gaining height while crashing ashore.

"We know these big waves cannot get into shallow water," said David Wang of the Naval Research Laboratory, the science arm of the Navy and Marine Corps. "That's a physical limitation."

By one definition, the titans of the sea rise to heights of at least 25 meters, or 82 feet, about the size of an eight-story building. Scientists have calculated their theoretical maximum at 198 feet -- higher than the Statue of Liberty. So far, however, they have documented nothing that big. Large rogues seem to average around 100 feet.

Most waves, big and small alike, form when the wind blows across open water. The wind's force, duration and sweep determine the size of the swells. Waves of about 6 feet are common, though ones up to 30 or even 50 feet are considered unexceptional.

The trough preceding a rogue wave can be quite deep, what nautical lore calls a "hole in the sea." For anyone on a ship, it is a roller coaster plunge that can be disastrous.

Over the centuries, many accounts have told of monster waves that battered and sank ships. In 1933 in the North Pacific, the Navy oiler Ramapo encountered a huge wave. The crew, calm enough to triangulate from the ship's superstructure, estimated its height at 112 feet.

Despite such accounts, many oceanographers were skeptical. The human imagination tended to embellish, they said.

Then, in February 2000, a British oceanographic research vessel fighting its way through a gale west of Scotland measured titans of up to 95 feet, "the largest waves ever recorded by scientific instruments," seven researchers wrote in the journal Geophysical Research Letters.

It quickly became apparent that the big waves formed with some regularity in regions swept by powerful currents: the Agulhas off South Africa, the Kuroshio off Japan, and the Gulf Stream off the eastern United States, where the Norwegian Dawn got into trouble off Georgia.

Already, the scientists said, naval architects and shipbuilders are discussing precautions. Some of the easiest are seen as increasing the strength of windows and hatch covers. But the best precaution will be learning how to avoid the monsters.

Increasingly, scientists are focusing on better understanding how the big waves form and whether that knowledge can lead to accurate forecasts -- a feat that, if achieved, may save hundreds of lives and many billions of dollars in lost commerce.

Oceanographers are focusing on the interplay of exceptionally strong winds and currents, especially in the Agulhas off South Africa. Bengt Fornberg, a mathematician at the University of Colorado, said that several years ago South African authorities began issuing predictions. "That's the only place the theory has succeeded," he said.

Rosenthal said that in the future the continued proliferation of radar satellites should help, bringing about a safer relationship between people and the sea.

"There will be warnings, maybe in 10 years," he said. "It should be possible."

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