“For God’s sake, hold on! It’s got us!” When explorer Ernest Shackleton uttered these words in Antarctica in 1916, his ship Endurance had already been crushed by ice and sunk. Desperately rowing to the island of South Georgia with a small crew, Shackleton spotted another disaster heading their way: an enormous wave.
“During twenty-six years’ experience of the ocean in all its moods I had not encountered a wave so gigantic. It was a mighty upheaval of the ocean, a thing quite apart from the big white-capped seas that had been our tireless enemies for many days,” he later wrote, “but somehow the boat lived through it.”
Advertisement
Although freak waves like Shackleton’s “mighty upheaval” are peppered through mariners’ tales, on dry land, accounts were met with raised eyebrows. However, when a gargantuan wall of water slammed into the Draupner oil platform in the North Sea on 1 January 1995, science finally caught up with folklore. Dubbed the New Year’s wave, it was the first official recording of a rogue wave. The 25-metre giant rose from a surrounding sea churned by waves averaging 12 metres.
Since then, our understanding of the complex forces that drive water to abruptly rise to create rogue waves far taller than those around them has become clearer, propelled by more reliable measurements, advances in wave modelling and ramped-up computational power.
Destructive power
But to protect ships and lives at sea, we need to predict when these rogues will occur. Given the complex patterns of waves across the vast reaches of the seas, making accurate forecasts is no simple task. Still, the need for such predictions may be getting more urgent; as climate change intensifies weather systems, we may see even more of these ocean monsters.
Waves are swells of energy, created mainly by wind. They grow bigger over distance when egged on by strong winds, and very occasionally the conditions cause one to rise far higher and much more steeply than its neighbours. While there is no set definition of a rogue wave, it is generally accepted that they have a of the tallest third of the surrounding waves.
In essence, a rogue wave is a very high concentration of energy, says Alessandro Toffoli at the University of Melbourne, Australia. These monsters can appear as walls of water reaching close to 30 metres in height, with great destructive power (see “Four freaks“). They pose a serious threat to even the largest vessels, and are estimated to have between 1969 and 1994 with the loss of more than 500 lives. Even when less extreme, they can still be deadly. In South Africa, many anglers have died after being washed from the rocks by freak waves on calm days in False Bay, a location that has consequently been dubbed “death coast”.
These giants can come in groups, too. On 30 November 2018, a were spotted by radar in the North Sea. Dubbed the Justine Three Sisters, this was the first formal detection of a rogue triplet.
Rogues appear unexpectedly, so are very challenging to study at sea. Ironically, wave researchers are unlikely to ever see one. Instead, they rely on data from remote monitoring and laboratory simulations to understand and visualise their behaviour.
So what causes them? Originally, they were thought to arise through a straightforward mechanism, where waves with different speeds and directions interact with each other and, under the right conditions, merge. But this so-called linear approach doesn’t account for all rogues, and it also predicts that they should be extremely rare – yet we now know that freak waves aren’t so freakishly uncommon.
When the MaxWave project carried out the first census of rogue waves using European Space Agency satellites, for instance, it within a three-week period in 2001. And a .
According to an alternative approach to modelling the ocean surface, instead of being created by simple merging, these giants can be understood in terms of the physics of the movement of wave energy. This uses equations such as the Schrödinger equation, which can help predict future behaviour of chaotic systems such as stock markets or weather patterns. This “non-linear” method predicts that rogue waves aren’t so rare after all, as confirmed by observations in the real world.
“We believe there are many more of these waves than we expect,” says , who studies rogue waves at DNV, a consultancy in Oslo, Norway. Toffoli came to a similar conclusion while “playing around” with the conditions under which wind-generated waves would turn rogue in a 2017 study. He and colleagues used a ring-shaped tank at the University of Turin, Italy, to run their tests. The customary laboratory practice is to generate waves with paddles in a straight tank, but their novel circular tank allowed fan-generated waves to flow freely and, in principle, indefinitely.
Rogues in the wild
The researchers measured the surface elevation as the wind blew over the water, increasing the wind speed until the waves were saturated with energy. At that point, just before the wave broke, the probability of extreme waves “went sky high”, says Toffoli.
But it is all very well understanding waves in the controlled environment of a lab. It is a completely different matter out at sea. “The big question is: can I find the results that I’ve seen in the model, and in the lab, in the real ocean?” says Toffoli. In 2017, after 15 years of studying centimetre-high freak waves in labs, Toffoli set out to do just that. He was on board South Africa’s Antarctic research vessel, the SA Agulhas II, as part of a University of Cape Town-led expedition. They were on a mission to characterise the waves in the Atlantic and Indian ocean sectors of the Southern Ocean, especially at the margin with the sea ice. One of Toffoli’s aims was to establish the probability of extreme waves in this area.
“These monster waves can appear as walls of water up to 30 metres in height”
Unexpectedly, the conditions turned. “We were so lucky to find ourselves at the edge of the sea ice in the middle of a polar hurricane,” says Toffoli. “It was a once-in-a-lifetime experience.” With the data still to be confirmed, Toffoli is careful not to claim that he saw an actual rogue, although he describes the findings as “very exciting”. When that research is published later this year, it should provide insight into how closely his experimental wave patterns map onto waves seen in the wild.
Meanwhile, also of great interest to researchers are the shipping lanes off the east coast of South Africa, where dozens of ships have been damaged or sunk as a consequence of rogue waves. Here, the Agulhas current thunders southwards at speeds up to 8 kilometres an hour, eventually meeting up with massive ocean swells from the Southern Ocean running in the opposite direction. In 1991, a large oil tanker called the ULCC Mimosa was hit by a wave that its captain described as the biggest he had ever seen. The ship limped to port with a hole of more than 20 square metres in its side.
Researchers are hopeful their models can make these channels safer, but something in the data wasn’t adding up. “We were very much aware that the Agulhas current influences the wave climate along the east coast,” says Christo Rautenbach, former marine scientist at the South African Weather Service (SAWS), now at the National Institute for Water and Atmospheric Research in Hamilton, New Zealand. Yet the actual wave recordings from the region didn’t correlate with the researchers’ models of the movement of energy in the waves, he says.
Then in 2020, new computer simulations revealed how other factors – the strength of the current and its direction relative to the direction of the waves – affect wave height. “When the waves oppose the direction of the current, the current will slow the wave down,” says Michael Barnes at SAWS. “This results in wave steepening.” It also focuses the energy into a rhythmic succession of waves known as a wave train. Based on , the SAWS has which includes the effect that the Agulhas current has on wave steepening. Although it is still a far cry from predicting individual rogues, Rautenbach says it can statistically paint a better picture of where and when to be on the lookout for extreme waves, and these forecasts are now broadcast through maritime alerts.
Death coast
New research has also clarified the mechanics of South Africa’s “death coast”. This , refracting the incoming open ocean swell and focusing the wave energy towards the shore, creating unexpectedly large waves under certain conditions. Researchers are hopeful that this information could ultimately prove useful for creating coastal warning systems.
Rogue-wave forecasting has moved forward in the North Atlantic too, through Extreme Wave Warning Criteria for Marine Structures or ExWaMar, a Norwegian project that aimed to develop warning criteria based on weather forecasts. ExWaMar further highlights the challenge of predicting these rare events.
“We were lucky to find ourselves in the middle of a polar hurricane”
First, the ExWaMar researchers used wave and weather data to see if they could simulate irregular waves. They used a non-linear approach called higher order spectral modelling, which, unlike models based on the Schrödinger equation, accounts for the direction of waves. This approach was able to successfully recreate that famous Justine Three Sisters triplet, proving that accurate forecasting is in fact possible.
There was, however, a snag. This process is so computationally “intense”, says Bitner-Gregersen, who led the project, that it is impractical for meteorological offices to use. Instead, the ExWaMar researchers turned to less computationally demanding alternatives, including using machine learning, to predict indicators of rogue waves. They had some promising results, but it is still not enough to accurately forecast individual rogues.
Bitner-Gregersen thinks the solution may be to zoom out a bit. “The sea surface is random. It oscillates,” she says, and so it doesn’t make sense to develop warning criteria for a single point. Instead, the ExWaMar criteria for the risk of rogue waves should be applicable to an area of 2.5 square kilometres. This strategy is currently being tested, with the aim of including these predictions in the Norwegian Meteorological Institute’s open access weather maps, which look ahead to the next six days.
If this proves accurate, the strategy could be rolled out internationally, she says. This would complement the predictions of organisations such as the European Centre for Medium-Range Weather forecasts, which provide estimates of the tallest waves to expect in an area as well as the indicators of freak waves.
Making better predictions of rogue waves could already help to make the seas safer for ships in potentially dangerous waters, but many believe this need will become even more pronounced in future. Extreme waves may become , both due to an increase in storm activity and the fact that melting polar ice will give the wind a larger sea surface to blow over.
It is also possible that the changing climate won’t cause more rogue waves, but instead fewer, bigger ones, as has been observed in a off the western US coast.
“In the future, extreme waves may be more likely due to climate change”
Predicting these trends is even more tricky than forecasting rogues. Another Norwegian project, called , ran from 2013 to 2016 to investigate changes to the North Atlantic wave climate with an eye on safe ship design. Large variations in climate change projections, future ice coverage and winds made it tough to draw many robust conclusions.
However, it did highlight some places in the North Atlantic that could face more rogue waves. In one area off the coast of northern Norway, for example, they could result from melting sea ice combined with potential increases in wind duration and speed. The results show that conditions ripe for rogues will be more common, says Bitner-Gregersen. Changes to the extent of sea ice and more swell due to climate change in the Arctic may also increase the occurrence of rogue waves there.
There is a long way to go to be able to accurately predict rogues in the wild. In the meantime, watch out. The maths up to five times higher than those around them are theoretically possible. Not only that, but they have been . What would Shackleton have said if he had encountered one of those?
Since this article was first published, we have clarified the methods used by ExWaMar researchers and the information on rogue waves in European Centre for Medium-Range Weather forecasts
Was The great wave a rogue?
The Great Wave off Kanagawa is one of the most famous images in Japanese art. Katsushika Hokusai’s image, printed in 1831-33, captures the moment three cargo boats battle a huge wave, with Mount Fuji in the background. “For a long time, this painting was used as an illustration of a tsunami, but a tsunami doesn’t look like this at all,” says Frédéric Dias at University College Dublin, Ireland.
In 2013, Dias co-authored a which concluded that a process called linear focusing can predict characteristics similar to those of the Great Wave. The authors also pointed out the remarkable similarities of the wood print to a rogue wave photographed in the sub-Antarctic by Véronique Sarano, founder of the French organisation Longitude.
Sarano happened to witness the wave while on board the French icebreaker L’Astrolabe on the way to Antarctica in 1991. It was a sunny day and sea conditions were “not very rough”, she says. Suddenly, a much bigger wave, around 7 metres from peak-to-trough, emerged, and Sarano, alone on deck, quickly took a shot of it.
It isn’t the first time the Great Wave has been called a rogue. A pointed out that the wave, estimated at 10 metres, is so much larger than the average waves in Tokyo Bay that it must be a freak. A tsunami at sea, in comparison, is just “an unnoticeable small amplitude swell with a very long wavelength”, the authors wrote.
To ram this point home, in 2019 researchers created a wave strikingly similar to the Great Wave in their laboratory, using two smaller wave groups travelling at a crossing angle.
