While the competitors of the 9th Vendée Globe are familiar with the screaming fiftieths, a painful event comes back to mind: in January 1997, the Canadian sailor Gerry Roufs disappeared during the third edition of the event, not far from the Nemo point, a lost place in the Pacific, the most remote place on earth. “The waves are not waves, they are the Alps!” he will say in one of his last messages.
Seen from the sky, by satellite, the waves exceeded – on average – 16 to 18 meters in the vicinity. Given the local weather situation conducive to liquid overflows, Gerry Roufs may have been the victim of a 35-meter-high monster. A rogue wave, the name given to those terrifying events that suddenly occur and reach twice the height of the surrounding waves. Anglophones qualify these waves as “rogue” (rascals, thugs, unscrupulous) or “freak” (abnormal).
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In the 19th century, the explorer Dumont d’Urville, who crossed for a long time south of the fortieth parallel, was the laughing stock of everyone when he spoke about his encounters with these scoundrels. During the century which followed, similar testimonies multiplied – with sometimes impressive images of damage to large ships – without science paying an interest to the height of the phenomenon, for lack of data to dissect: one story remains obviously suspicious as long as an instrument has not corroborated the saying.
The equivalent of an eight-story building
It will therefore be necessary to wait for the dawn of a new year, on January 1, 1995, for the science of rogue waves to finally emerge from limbo: on that day, a 25.6-meter wave broke on the Norwegian gas platform Draupner , in the North Sea. The equivalent of an eight-story building, which arose in a sea of 12 meters high… Little damage, but recordings of a radar which bring the proof which the scientists waited, finally.
Two years ago, in December 2018, a group from the British University of Oxford presented results and an astonishing video: in a barely stirred pool, a wave suddenly arises before surging. It bears a disturbing resemblance to the famous Great Wave overlooking Mount Fuji in the print by 18th-century Japanese artist Hokusai.
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Oxford scientists present their wave as a faithful replica of Draupner’s wave. “It is certainly very visual, but the conditions in the basin are unfortunately far from the reality of those which prevail at sea”, warns Frédéric Dias of University College Dublin in Ireland, who devotes his research to the scoundrels. This observation is one of the conclusions of a European research project (2011-2016) in which the scientist participated.
An event every … ten thousand years
“If we have data on this wave of 1995, we are far from having enough information to reconstruct what happened that day. What is missing are repeated observations, and for good reason: the probability of a rogue wave forming in the oceans is estimated at around 1 in 100,000, whatever its size, ”explains the researcher. In other words, the probability that a rogue wave occurs in a given place is one event every… ten thousand years! “In Europe, there are on average ten big storms each winter. Each probably produces a few 20-meter waves. This is little!”
In 2010, work had given hope that we could better understand the formation of rogue waves, by reproducing it with… light waves in an optical fiber. “This had raised a lot of hopes because we understand optical phenomena well, remembers Frédéric Dias. But it didn’t go as planned. ” To tell the truth, it is rather the opposite which occurred: it is the knowledge in hydrodynamics which, thanks to this analogy between waves and light, made progress the knowledge of optical phenomena!
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So today what have we understood from these waves? Much effort has been made to try to show that it is instabilities specific to each wave that can lead to its transformation into a villain. But, in 2016, a study has shown that this is not the dominant cause of what is observed in the ocean. In contrast, the interaction between waves – what is called nonlinear interference – is probably the dominant cause of rogue formation, apart from well-known situations related to local conditions.
Underwater acoustic sensors
For example the meeting between a swell and an opposing current, like the detestable sea which forms south of the Cape of Good Hope, when the Aiguilles current – which descends to the south to the east of the Africa – hits the mighty west-east swell of the Southern Ocean. Or when the bottom of the sea rises suddenly. Near the coast, in places popular with surfers, but sometimes offshore, such as on the Rochebonne plateau, off La Rochelle, in the Bay of Biscay, a place hated by Vendée Globe sailors.
To go further, researchers need more observations. One of the keys could be stereo-video, recording – in relief – waves. “This allows us to obtain a description of their evolution in space and time, but it generates so much data that we cannot do it continuously,” says Frédéric Dias. Campaigns are therefore regularly organized, but intermittently, on an offshore platform or a ship. The scientist and his colleagues are also using underwater acoustic sensors that measure water height and wave speeds in western Ireland, an area battered by winter storms.
There remains the satellite, underemployed in this area. “It gives the significant height of the waves but does not measure their period well – their spacing in time – and cannot determine the maximum height”, points out the researcher, however. Because scientific satellites are in fairly low orbit and move fast. There is therefore little chance that they are in the right place at the right time! When will we see a geostationary satellite capable of monitoring “rogue waves”?
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