Surf's up: hurricanes could be a common source of giant waves.© Punchstock

Immense waves capable of sinking the largest ships might not be freaks of nature, but a common result of hurricanes.

That's the implication of new evidence that Hurricane Ivan, which whirled across the Gulf of Mexico last September, probably generated waves greater more that 40 metres high from crest to trough.

David Wang and his colleagues at the Naval Research Laboratory in Mississippi say that the water pressures measured by their array of seafloor sensors, 60 to 90 metres below the ocean off the northeast coast of the gulf, indicate the passage of waves nearly 30 m high.

But they say that the waves near the eye of the hurricane, which unfortunately passed over the sensors while they were not taking measurements, would probably have topped 40 metres.

"These are the largest wave heights ever recorded with instruments in US waters", says Wang's colleague William Teague. "They're larger than we ever thought they would be." Ivan wasn't even a particularly large hurricane, he adds.

Sunken ships

Terrible walls of water are a staple of nautical lore. But oceanographers have only recently come to accept them, as simple statistics suggest that such extreme events should almost never happen.

But there's no fundamental reason why ocean waves can't grow to immense sizes. "Nobody knows what the upper limit might be," says Paul Liu of the National Oceanic and Atmospheric Administration in Ann Arbor, Michigan.

The largest waves to hit shore come from tsunamis - waves generated most often by seafloor movements during major earthquakes. But while at sea these waves are just a few centimetres high, only growing as they reach shallow waters. The Indian Ocean tsunami of December 2004 reached 30 metres above sea level in the hardest hit areas.

There are eyewitness accounts of similarly enormous waves at sea. In 1995, the Queen Elizabeth 2 liner survived an encounter with one about 30 metres high in the North Atlantic, and six years later a similar wave smashed windows on the cruise ship Bremen in the South Atlantic and nearly sank it.

It is now widely suspected that such rogue waves, generated by wind and currents, might explain the mysterious, regular disappearance of large ships at sea. One, the German supertanker München, vanished in 1978.

No freak

Such waves are generally ascribed to unusual circumstances, such as fast-moving storms. But the new findings, reported in Science¹, suggest that waves even larger than these might romp across the oceans whenever a hurricane hits.

Shuyi Chen, a specialist on computer modelling of hurricanes at the University of Miami, agrees, saying that the latest hurricane models predict such waves should occur "very often". The findings are consistent with less accurate measurements made from ships, she adds.

Il Ju Moon, an oceanographer at the University of Rhode Island who studies the interactions of hurricanes and oceans, points out that the study may spell bad news for ships and coastal defences. If hurricane activity increases, as some expect to happen along with climate change, such giant waves could become more frequent. 

Naval Research Laboratory

• References

  1. Wang D. W., et al. Science, 309. 896 (2005). | Article |