These isolated waves, towering at 20 meters (or more), suddenly appear in the open . They exist for a brief moment, typically less than a minute, before vanishing. Due to their height and power, they pose a significant threat to vessels.
To gain a deeper understanding of what killer waves are and what causes them, an international team of scientists led by Francesco Fedele from the Georgia Institute of Technology (USA) conducted a unique study.
Over 18 years, researchers analyzed high-frequency laser measurement data from the Ecofisk oil platform in the central North Sea. Ultimately, the team concluded that killer waves are not random phenomena. They form according to the natural laws of the sea, and they are not mysterious; rather, they are quite straightforward.
How Was the Research Conducted?
The scientists examined approximately 27,500 half-hour recordings of sea state collected between 2003 and 2020. These data, recorded every half hour, captured how high the sea surface rose compared to the average sea level. This included information about major storms.
Under normal conditions, waves arise due to wind blowing across the sea surface. This is similar to how we blow on a cup of coffee, creating small ripples on the surface. Meanwhile, in the ocean, given enough time and space, these ripples can transform into large waves.
The team focused on understanding the reasons why waves suddenly become anomalous and rise significantly above neighboring waves. One theory is based on modulation instability. When waves are confined in a narrow channel, modulation instability theory describes their wavelike motion well. However, it falters when it comes to the real ocean. In open environments like the North Sea, waves can freely propagate in various directions.
To grasp the difference, imagine a crowd of spectators leaving a stadium after a football match. If they must exit through a long, narrow corridor with high walls, people are forced to move in one direction. Those at the back try to push forward, and some even climb over others. This catastrophic bottleneck resembles a killer wave caused by limited space, as explained by Science Alert.
Researchers can generate killer waves in a confined channel under laboratory conditions, where they obey modulation instability. However, without channel confinement, killer waves typically do not conform to this theory and do not form in the same way in open water.
What Is Constructive Interference?
Using statistical methods, the scientists analyzed sea state data to identify patterns underlying these rare events. The results indicated that extreme waves are more likely to form not due to modulation instability but as a result of a process known as constructive interference.
Constructive interference occurs when two or more waves align to form one large wave. This effect is amplified by the natural asymmetry of sea waves: their crests are typically sharper and steeper than the flat troughs.
Killer waves form when numerous small waves line up in a row, and their steeper crests begin to overlap, transforming into one massive wave that towers above the surrounding waters for a brief moment.
These killer waves rise and fall in less than a minute, following a so-called quasi-deterministic pattern in space and time. As the wave gains height and energy, it cannot remain above a certain point of no return. The peak of the wave spills over the edge and breaks into foam, releasing excess energy.
What Does Quasi-Determinism of Waves Have to Do With It?
Killer waves are not only found in the ocean. The theory of oceanographer Paolo Boccotti, known as quasi-determinism of waves, explains how killer waves form both in the ocean and in other wave systems.
The team applied Boccotti’s theory to identify patterns in the wave recordings from the North Sea. The giant waves captured in these recordings carried a sort of signature or imprint of a group of waves, helping to explain the emergence of rogue waves.
The researchers reported that on November 24, 2023, a powerful storm occurred in the North Sea. A camera on the Ecofisk platform captured a killer wave measuring 17 meters high. The scientists utilized the theory of quasi-determinism and an artificial intelligence model to investigate the origins of this extreme wave, which formed from numerous smaller waves repeatedly overlapping.
Understanding how killer waves form can help engineers and designers build safer vessels and offshore platforms, as well as better predict risks.
The study’s findings were published in the journal Nature Scientific Reports.
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