Rogue Waves

Lots of people who are afraid to fly think nothing of taking long ocean cruises. They might think again in the wake of the European Space Agency’s report this week that rogue waves are far more common than anyone ever expected.

Rogue waves are monster, ship-smashing walls of water that rise to seemingly impossible heights, three to five times as high as the waves around them, and often come from unexpected directions.

In February of 1995, the Queen Elizabeth II met a 29-metre-high wave during a hurricane in the North Atlantic. Captain Ronald Warwick described it as "a great wall of water...it looked as if we were going into the white cliffs of Dover."

Between February and March 2001 two tourist cruisers, the Bremen and the Caledonian Star, encountered 30-metre rogue waves in the South Atlantic. In both cases the waves smashed the bridge windows; in the case of the Bremen, it killed the ship’s electronics, leaving it drifting without navigation or propulsion, parallel to the waves, for two hours, vulnerable to capsizing should another big wave come along.

In the last two decades more than 200 supertankers and container ships more than 200 metres long have been sunk by severe weather; rogue waves are believed to have been the cause of many of those losses.

One cubic metre of water weighs one tonne. A storm wave 12 metres high hits a ship with a force of six tonnes per square metre. Ships are designed to take hits of 15 tonnes per square metre without damage, but a rogue wave, engineers estimate, could hammer a ship with 100 tonnes per square metre.

Scientists and engineers have traditionally been skeptical about the reports of these monsters because the wave models they’ve been using for decades almost never produce a rogue wave. Statistically, according to those models, a rogue wave should arise only once every 10,000 years.

But that skepticism--and those models--have been demolished by recent data. On January 1, 1995, the Draupner oil rig in the North Sea was hit by a rogue wave that a laser device measured as 26 metres high that swept across the deck at 72 kilometres per hour. In all, North Sea oil rig radar records revealed 466 rogue waves in just 12 years.

This greater-than expected frequency of rogue waves has major implications for shipping, because ships and offshore platforms are built to withstand maximum wave heights of only 15 metres. As a result, the European Union launched a project called MaxWave in December of 2000 to investigate rogue waves. One goal was a global rogue wave census, using data from two European Space Agency satellites, ERS-1, launched in 1991, and ERS-2, launched in 1995. Every 200 kilometres while over the ocean these satellites take 10-kilometre by five-kilometre radar "imagettes." Normally, computer analysis of these imagettes produces averaged-out breakdowns of wave energy and direction, called ocean-wave spectra, for use by weather forecasters. The MaxWave researchers decided to look at 30,000 of the raw imagettes, taken over three weeks at around the same time that the Bremen and Caledonian Star were hit by rogue waves.

The results were announced last week. During the three-week study period, more than 10 giant waves more than 25 metres tall turned up worldwide--far more than anyone expected. That gives added urgency to the next phase of research, WaveAtlas, which study two years’ worth of imagettes to create a worldwide atlas of rogue waves and analyze them for patterns, with the ultimate goal of forecasting them.

One pattern has already been found: rogue waves often occur where ordinary waves encounter ocean currents and eddies. That wouldn’t surprise rogue-wave researcher Alfred Osborne, a native of Texas who now teaches at the University of Torino, Italy. He thinks he has a better mathematical model of rogue waves, one that may help with predicting when and where they form. The key is that they aren’t like regular waves, that arise from the action of the wind on the water and result in a regular pattern of crests and troughs. In fact, they perhaps shouldn’t be thought of as waves at all, but more like one-of-a-kind events. They arise when the energy inherent in a collection of random, unstable waves--particularly waves moving in various directions at various speeds mixing together to create a choppy sea--somehow gets focused in a single location. A rogue wave rises up, races across the sea, and then dissipates again. If radio waves behaved the same way, the volume would suddenly blare three to five times as loud as usual for a few seconds, then fade back to normal.

Osborne has been able to use his mathematical model to create rogue waves at will in a wave tank, something no one else has been able to do, so it appears he’s on to something. If so, his better mathematical model of rogue waves should help with the effort to predict when and where rogue waves will occur.

So just to recap: if you’re on a winter cruise to the Bahamas and you see what looks like the White Cliffs of Dover dead ahead--you might want to go below.