Rogue waves were once just a spooky bit of marine folklore.
Sailors recounted tales of freakishly large waves that seemingly came out of nowhere, crashing into their ships — or completely submerging others, never to be seen again.
In the last 27 years, scientists have accepted that rogue waves are a real, scientific phenomenon. And when a massive wave hit a Ventura, Calif., beach on Thursday, many were quick to call it a rogue.
But was it? And what are rogue waves, anyway?
Gigantic waves pummelled California’s coast on Thursday, including one in Ventura County that crashed over the sea wall at a beach, causing minor injuries to at least eight people. The waves were caused by storms in the Pacific Ocean, which also led to flooding and evacuation orders, and left beaches covered in debris.
What are rogue waves?
Rogue waves are a relatively rare phenomenon, but they can be dangerous. The largest one ever recorded, measuring 17.6 metres, was detected off the coast of B.C. by researchers at the University of Victoria in November 2020.
Those who study the ocean use a measure called significant wave height — which is the average of the largest 33 per cent of waves observed in the ocean — to determine if a wave is large enough to be considered a rogue.
“A rogue wave is a rogue if an individual wave is at least two times that significant wave height,” said Scott Beatty, the CEO of coastal intelligence firm MarineLabs in Victoria, B.C.
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Rogue waves are a rare and potentially dangerous ocean phenomenon in which waves can suddenly appear that are two to three times higher than the surrounding waves. Johannes Gemmrich, a physical oceanographer at the University of Victoria, published work Scientific Reports analyzing a real whopper recorded off Vancouver Island in 2020 — a wave as high as a four-story building. His research may help scientists understand the physics of these mysterious and frightening waves.
In other words, rogue waves are usually more than twice the size of the waves around them, according to the U.S. National Ocean Service.
The agency’s website also says that the waves are “very unpredictable,” often coming from unexpected directions that are inconsistent with the typical direction of winds and waters.
Rogues might be called “freak” or “killer” waves, but scientists refer to them as “extreme storm waves.”
They’re often described as resembling walls of water, with steep sides and deep troughs.
Was the wave that hit a California beach a rogue?
“Rogue waves are often called rogue waves by accident, because people think just, ‘I saw a big wave, therefore it must be a rogue,'” said Beatty.
However, rogue waves have a technical definition — and Beatty says it’s hard to say whether the wave that crashed into a Ventura, Calif., beach on Thursday met that criteria.
A large individual wave, or set of waves, combined with a high tide might form something that looks like a rogue wave, Beatty explained, “but it is just indeed very large waves that end up running up along the beach and up and over barriers.”
Some of California’s coastal cities have been hit hard this week by massive waves generated by powerful swells from Pacific storms. In Ventura, crews used bulldozers to build protective sand berms along a beach in order to protect nearby homes.
Lisa Phillips, a meteorologist at the National Weather Service in Oxnard, Calif., says that the incident in Ventura was what’s called a “wave run-up.”
“When you have really large surf, some of the waves are bigger than others and the water will wash up much further on shore than some of the other waves. So it can catch people by surprise,” she said.
What causes a rogue wave?
Rogue waves are difficult to predict because scientists don’t fully understand how they’re formed, but they happen more often than we think.
Like an avalanche, there are certain conditions that make a rogue wave more likely to occur, “but we couldn’t say there will be a rogue wave at 11:01 a.m. at this location,” says Beatty, who studied ocean wave energy during a PhD in mechanical engineering at the University of Victoria.
What are those conditions? Extreme waves form because sea swells collide to create larger, taller waves, especially when travelling in the same direction. When the crests (highest points) of two waves meet, they can merge to form a huge rogue wave, according to research published by National Geographic.
There’s another way that rogue waves can emerge, according to the National Ocean Service. When waves that were formed during a storm move against the typical direction of the current, they can interact in a way that leads to more waves passing through a fixed point during a shorter period of time. That can cause waves to join together and form large, long-lasting rogue waves.
Are El Niño or climate change contributing factors?
El Niño is a naturally occurring climate pattern associated with warming of the ocean surface temperatures in the central and eastern tropical Pacific Ocean. That pattern is causing larger waves than usual, according to Beatty.
“We are definitely expecting to see some of this kind of activity for this winter,” said Phillips, the meteorologist. “It could have a correlation to our strong El Niño that we’re in, because strong El Niños mean that low-pressure systems can be more intense.”
What about climate change? It would be challenging to say whether that’s a factor in giant waves, Beatty said. However, observers have noticed that “the wave energy in the ocean is gradually increasing as climate change has more effect,” he added.
“As the ocean is warming, storms are getting more intense, which means winds are more intense. And winds are what create waves, and so that means waves are getting more intense.”