Throughout history, sailors have recounted tales of massive “rogue” or “abnormal” waves. However, it wasn’t until a 26-meter high storm surge hit on New Year’s Day in 1995 that scientists began to take these reports seriously. The surge was recorded on a gas platform in the North Sea.
These extreme waves are often implicated in sea accidents. For instance, in 2018, a fishing boat off the coast of Hawaii was struck by large waves, causing it to sink and leading to the rescue of eight crew members.
More recently, a fierce wave collided with a cruise ship in the Southern Ocean, resulting in broken windows and injuries to multiple passengers, one of whom tragically passed away. The frequency of rogue waves, however, remains uncertain.
Based on a recent media report, there were 210 abnormal waves documented worldwide between 2011 and 2018, though this number is believed to be higher in reality.
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It’s important to note that rogue waves differ from tsunamis, which are typically large waves caused by underwater earthquakes or volcanic eruptions that crash onto coastlines. Tsunami waves may be tall but are also broad.
In comparison, rogue waves have steep slopes and can break not only near shores but also in open waters. According to researchers at the University of South Florida, they are technically defined as waves at least twice the height of surrounding waves. Laura Azevedo suggests that this definition may be misleading.
“It can be problematic. Sometimes, we encounter rogue waves as high as one meter without any adverse effects,” she explains.
Azevedo advocates for a standardized height of 4 meters, at which point waves become dangerous.
The exact cause of rogue waves is a topic of debate, but it is known that they are unrelated to movements on the ocean floor. Various factors are believed to play a role.
An Oxford University team, led by engineers such as Dr. Tan Tianning, is utilizing computer modeling and lab-based experiments in large aquariums with paddles to generate waves in an effort to understand the underlying causes.
The team conducted simulations involving steps placed within an aquarium to explore the impact of abrupt changes in depth.
“This is crucial, especially around continental shelves where waves transition from deep ocean to coastal regions,” Tan explains. These simulations are also relevant for embankments that support offshore wind turbines.
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However, the team’s findings suggest that despite depth variations, larger waves are created, but the effect diminishes as waves disperse. When they spread out, waves in the ocean tend to calm, reducing concerns about sudden drops in the ocean observed in labs.
Coincidentally, these results align with Azevedo’s findings from studying ocean waves in Tampa Bay, Florida. By leveraging monitoring buoys, Azevedo acquired four years’ worth of high-quality wave height data at the bay entrance, uncovering 7,593 waves that met the technical definition of rogue waves (over 4 meters) (including 372).
Azevedo notes that she “couldn’t pinpoint a specific cause” for this phenomenon but observed that it often occurred when waves travel in a single direction without dispersion.
“This is understandable,” she remarks. “When all the ocean’s energy is concentrated into a single wave, it becomes a significant wave.”
Her research also indicates that Tampa Bay is relatively shielded, especially from eastern directions. Most rogue waves approach from the west during storms and high winds.
Nevertheless, as the causes of rogue waves likely differ across regions, these findings may apply mainly to bays similar to Tampa.
Therefore, predicting the occurrence of rogue waves remains challenging. However, with increased understanding, scientists can use modeling techniques to offer more accurate long-term forecasts. Such forecasts could inform offshore wind turbine owners, for instance, about the number of waves exceeding 4 meters expected over 20 years at specific sites.
On the other hand, Tan points out that short-term predictions, like anticipating rough seas within the next 20 minutes, are much harder to make.
Notably, in 2016, MIT researchers developed a prediction tool capable of spotting impending rogue waves in mere minutes.
Enhanced monitoring could bolster efforts to comprehend and predict rogue waves. Azevedo mentions that most monitoring buoys presently transmit significant wave heights instead of peak wave heights, which are averaged from the highest waves over a specific period.
“This means that some significant waves go unnoticed,” she explains, emphasizing the need for a change. Better data could aid stakeholders in open ocean design and construction by considering extreme waves, which are increasing due to climate change.
Azevedo proposes that ship designers aim for durability against twice the significant wave height, despite the cost, to prepare for rogue waves. Convincing stakeholders to raise standards may be challenging.
About our experts
Laura Azevedo is a graduate student in the College of Marine Sciences at the University of South Florida. Her research focuses on meteorology, oceanography, and weather data, with her work published in journals like Limnology and Oceanography and MDPI.
Dr. Tan Tianning is a postdoctoral researcher from the University of Oxford, specializing in machine learning. His published works include articles in journals such as Fluid Mechanics Journal, Applied Ocean Research, and Marine Engineering.
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Source: www.sciencefocus.com
