Wednesday, 18 February 2009

Rogue Waves

So that last story, not to mention all this Volvo Ocean Race writing, got me thinking about, and researching, this whole rogue wave thing.

When Miles Smeeton published his book, it joined a building folklore about rogue waves. No less a ship than the Queen Elizabeth had her bridge windows taken out by a wave in a North Atlantic storm in 1943 – they were 28 metres above the waterline. In 1995, her successor, the QE2 took what was estimated to be a 29 meter wave over the bow. Then there was the Esso Languedoc, caught in a storm off South Africa in 1980.

Philippe Lijour, the first mate aboard the oil tanker, was fortunate enough to have a camera handy (fifth picture down) when the breaking crest of a wave roared past, just short of the top of the ship’s cranes some 25 metres above the waterline. At the time, Lijour reckoned the average wave height to be somewhere between five to ten metres from trough to crest.

But despite the stories and even despite Lijour’s photo, oceanographers and meteorologists refused to believe these freak waves existed in any number – their theories simply didn’t predict them. Conventional, linear mathematics states that waves should vary in a pattern around the average, called the significant wave height, defined as the mean of the largest third of the waves recorded. According to this analysis, in a storm sea of 12 metres, a 15 metre wave will pop up about once every 25 years.

A rogue wave - one defined as twice that of the significant wave height - is theoretically possible, but you’ll have to wait about ten thousand years to see one. It seemed to the seafaring community that they were appearing a lot more often than that, but the scientists were about as interested in the anecdotal evidence as they were in reports of the Loch Ness Monster. And that’s how things stood, when on New Year’s Day 1995, the equivalent of the dead bloated body of Nessie floated up on the shore of the Loch.

The winds howling down the North Sea had peaked at hurricane force that afternoon, and the captain of the ferry Colour Viking later described going eyeball to eyeball with wave crests from his bridge, 30 metres above sea level. It would have gone down as just another account, if what’s become known as the New Year Wave hadn’t roared under the Draupner oil platform just after three o’clock that afternoon.

It was measured by a laser wave sensor at a maximum height of 25.6 metres – twice the size of the average wave at the time. If that wasn’t enough, five years later, a British oceanographic research vessel in the Rockall shipping area, to the west of Scotland, used an onboard recorder to measure a wave 29.1 metres from the crest to trough. Suddenly, the accounts of walls of water approaching at twice the height of the waves around them were no longer quite so unbelievable – even to the scientists.

The implication for the safety of ships and oil platforms was not good – their design had always been based on the assumptions of linear mathematics. But the maths was flawed and the theory shot to hell, so the science community went back to its other mainstay – observation. The European Union started up a project called MaxWave, which used images from satellite radar to ‘measure’ wave height across broad swathes of ocean.

I’ve used quotes, because there’s a fair bit of theory between the radar images and the computed wave heights. Nevertheless, from three weeks of images taken from a period and place when two cruise ships had almost been sunk by rogues, the project measured ten waves bigger than 25 metres – and that kicked the linear maths model into touch once and for all. A subsequent project called WaveAtlas used radar images to measure rogue waves across the globe, and provided maps of size and frequency. The latest plan is to provide the radar images in real-time, allowing dangerous swells to be tracked and forecast.

But we still don’t have an explanation of why these waves occur. Counter currents and the sea bed provide known mechanisms for throwing up bigger waves, but that explanation only works in areas like Cape Horn, where an easterly gale can meet the westerly Antarctic circumpolar current across an area of shoaling water. It doesn’t tell us why some waves roar under North Sea shipping platforms at twice the height of all those around them.

The most likely explanations use the same equations as the ghostly world of quantum mechanics – so-called non-linear mathematics. It seems that somehow, the energy from several separate waves is being focused into just one or two of these monsters. But until these theories are better refined and established, our best chance of predicting these waves is the up-coming radar tracking. In the meantime, if you’re in the wrong place at the wrong time, you’ll need all the luck and all the determination of the Smeeton’s, Guzzwell and the Tzu Hang to come back and tell the world about it.

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