Seafaring folk have told of huge ‘rogue’ or ‘freak’ waves for hundreds of years. But it wasn’t until New Year’s Day 1995 when a 26m high surge was recorded at a gas platform in the North Sea, that scientists started taking their reports more seriously.
The extreme waves have been blamed for numerous incidents at sea. In 2018, for example, eight crew members had to be rescued when massive waves hit and sank a fishing boat off the coast of Hawaii.
More recently, a rogue wave slammed into a cruise ship in the Southern Ocean, shattering windows and injuring several passengers, one fatally. However, it remains unclear how common rogue waves actually are.
One study based on media reports captured just 210 rogue waves worldwide between 2011-2018, but the real number is thought to be much higher.
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It’s worth noting that rogue waves differ from tsunamis. The latter are large waves (or series of waves) that crash onto a coastline, usually following an earthquake or volcanic eruption under the water. Whilst tsunami waves can be tall, they are also long in profile.
Rogue waves, by contrast, have much steeper slopes and they don’t just break on shorelines – they can break in the open ocean. Technically, they’re defined as being at least twice the height of the surrounding waves, though, as University of South Florida researcher Laura Azevedo says, this definition can be misleading.
“It’s a bit of a problem because you can have a rogue wave that is one metre high and that won’t do anything to anybody,” she says.
She favours a threshold of four metres, which she says is the height at which a wave starts to become dangerous.
Exactly what triggers rogue waves is up for debate, but it’s known they’re not related to movement on the seabed. It’s thought that there are several different factors at play.
A team at the University of Oxford are attempting to understand their root causes by combining computer modelling with lab-based experiments in large tanks that use paddles to generate waves.
The team has explored the influence of sudden depth changes, simulated using a step set up in their tank.
“This is a very important problem, for example, around continental shelves, where the waves propagate from the deep ocean to coastal areas,” says team member and engineer Dr Tianning Tang. It also affects the banks created to house offshore wind turbines.
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However, the team’s results show that although depth changes do generate bigger waves, the effect is suppressed when the waves are spreading rather than all heading in the same direction. According to Tang, this means sharp drop-offs are less concerning in the ocean than in the lab, because real waves do tend to spread.
Interestingly, these results chime with what Azevedo found when studying ocean waves at Tampa Bay, Florida. Thanks to a monitoring buoy, Azevedo was able to get four years’ worth of high-quality data on wave heights at the entrance to the bay, identifying 7,593 waves meeting the technical definition of rogue waves (including 372 over four metres).
She says she “couldn’t find one specific reason” why they occurred, but they were more common when waves were travelling in one direction as opposed to spreading out.
“It’s understandable,” she says. “When you have all the energy of the sea creating that one wave, that’s a big wave.”
Her work also shows that Tampa Bay, at least, is relatively sheltered from the east, with most rogue waves coming from the west and more often when conditions are stormy or windy.
However, what causes rogue waves is not thought to be universal, so these results may only be relevant for bays comparable to Tampa’s.
All this means that it's difficult to predict when rogue waves will strike. But as understanding grows, scientists could use modelling techniques to make more accurate long-term predictions. This kind of prediction might, for example, tell the owner of an offshore wind turbine how many waves over four metres they could expect during the next 20 years in the location of their turbine.
But, as Tang notes, it’s much harder to make short-term predictions that tell crew members whether a rogue wave is going to hit their vessel in the next, say, 20 minutes.
That said, in 2016, Massachusetts Institute of Technology researchers did claim to have developed a prediction tool capable of spotting waves that could become a problem within the next 2-3 minutes.
Improved monitoring could boost efforts to understand and predict rogue waves. Right now, Azevedo says, most monitoring buoys don’t transmit maximum wave heights – they send ‘significant wave height’, which is an average of the tallest 33 per cent of waves over a given period.
“So you’re missing the big waves,” she says, adding that this would be simple to change. Better data could help those involved in designing and building for the open ocean to account for extreme waves – which, by some estimates, are already increasing due to climate change.
Azevedo suggests one solution could be for ship designers to make their ships resilient enough to withstand double the significant wave height, though they’d no doubt need some convincing due to the increased costs.
About our experts
Laura Azevedo is a graduate student based at The University of South Florida's College of Marine Science. Her work focuses on meteorological, oceanographic and weather data. Her work has been published in the journals Limnology and Oceanography and MDPI.
Dr Tianning Tang is a postdoctoral researcher based at The University of Oxford, where he specialises in machine learning. His work has been published in journals including the Journal of Fluid Mechanics, Applied Ocean Research and Ocean Engineering.
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