What causes rogue waves?

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Maritime history has long told of infeasibly tall waves that strike suddenly during calm seas and topple boats. And yet to date little is understood about what causes these mystery waves. An ESA project confirmed the existence of these mammoth swells when it recorded ten waves all over 25 metres (80 feet) during a three-week period in 2001.

A rogue wave is defined as being around three times the average height of the other waves around it. So they needn’t actually be massive – just surprisingly large compared with the general sea state. Their very nature makes it difficult to predict or pinpoint their exact cause as factors such as water depth, currents and many other variables will all affect the propagation and development of a single wave.

Energy can be exchanged between multiple waves to generate abnormally large ones. For example, when a small, fast wave catches up with a large, slow wave, the energy of both can combine to create a single, high-intensity mutant wave.

There are also specific regions of Earth more prone to rogues. The interaction of surface waves and the Agulhas Current near South Africa’s east coast, for example, is thought to breed giant waves that propagate from east to west. Environmental engineers at the University of Wisconsin-Madison discovered that when fast waves from one direction interacted with the strong currents moving in the opposite direction, a wave could rise up and ‘climb’ the current’s wall.

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