How to predict killer waves in the ocean?

Predicting when killer waves, dangerous waves as tall as skyscrapers will appear, is still a headache for researchers.

In 1826, Captain Jules Dumont d'Urville, a French scientist and naval officer, encountered a strong storm while passing through the Indian Ocean. He observed the wall of water rising 30 meters above the Astrolabe. This was one of several waves over 24 meters high that Dumont d'Urville had seen during the storm. One of his crew members is missing. However, after Dumont d'Urville returned to the mainland, his story of three witnesses seemed so strange that it was considered paranoia by everyone.

Scientists at that time thought that the waves could only be about 9 meters high, so some reports of giant waves in the ocean in the 19th century were considered maritime mysteries. It was not until much later that researchers realized that the records were very rare because many sailors who had encountered killer waves or poison waves did not survive to tell about them.

Killer waves in the Southern Ocean taken from the French icebreaker Astrolabe.

Today, a killer wave is defined as a wave that is more than twice the height of the surrounding waves. These are large waves that can appear suddenly and from any direction. With its steep edge and deep trench below, the killer wave resembles a wall of water jutting straight out of the sea. They can occur during storms in rough seas but are also recorded in calm waters so they are difficult to predict.

Scientists have recognized killer waves as a real phenomenon since the mid-1990s, but ensuring safety for seafarers against these waves is still a big challenge. Although relatively rare, killer waves can cause serious damage to life and property when crashing into ships on the open sea. In the middle of the vast ocean, the interaction of many forces forming a killer wave is difficult to define. Recently, mathematicians combined real-time data collected from buoys with statistical modeling to better understand what causes killer waves to form. Their research could help predict killer waves before they hit.

The theory of the formation of a killer wave

There are two main mathematical theories that explain the wave motion that leads to the generation of killer waves. According to the linear addition hypothesis, waves move across the ocean at different speeds. When they collide with each other, they become stronger and create toxic waves. The second theory, called nonlinear focusing, states that waves move in clusters and energize each other, forming killer waves. The reason researchers can't be sure is because killer waves are so rare. Even today, quality tracking data is scant.

"In general, killer waves in the ocean are measured from buoys in a particular location without knowing what happened before or after," said Amin Chabchoub, a wave physicist at the University of Sydney at the University of Sydney. Australia, said. A 2019 study conducted by Chabchoub evaluated several observations and patterns of killer waves. The research team concludes that the mechanism that generates killer waves can vary depending on many factors at sea, called sea surface state.

To support the limited observation of killer waves, the scientists relied on wave pools. "The laboratory reconstruction almost exactly simulates what happens on the ocean's surface," says Chabchoub. Experiments of this kind can even incorporate both currents and winds, although the control environment has some limitations of its own.

When water is trapped in a narrow trench like a wave pool, large waves are much easier to form and observe. However, such an experiment is somewhat impractical because waves cannot spread in all directions like in the ocean, said Francesco Fedele, an engineer at the Georgia Institute of Technology.

Development of predictive technology

The US National Oceanic and Atmospheric Administration is developing a system that can predict the potentially hazardous area in the ocean every hour, using the WAVEWATCH III program. The latest version, published in 2019, uses a probabilistic formula that Fedele developed in 2012 to predict extreme conditions in the sea at a specific time and place. This is a useful tool to help navigators avoid dangerous waters, but not enough to protect them against sudden killer waves.

Johannes Gemmrich, a researcher at Victoria University in Canada, who analyzed the 2020 killer wave near Vancouver Island, says killer waves appear most often when waves travel at different speeds and sometimes crash into each other. But he thinks wave asymmetry (waves with higher peaks and lower troughs) also plays a role.

Scientists are developing technology to predict waves in real time, but the new method needs to be tested in real-world conditions, a big challenge given the rarity of killer waves. In many cases, the computation needs to be accelerated to match the wave speed. Killer waves can form in just 10-15 seconds in rough seas, so it is difficult to make quick and accurate predictions in such a short time.

To make predictions, scientists will need radar systems that continuously measure waves near the ship, so they can run the data through a mathematical model that simulates the sea surface at that time. The model, which calculates sea surface conditions once every 5 minutes, provides a relatively accurate prediction of how ocean waves will develop over the next few minutes. But such a system has not yet become a reality. "The technology is already there. The problem now is how can you predict fast enough?", emphasized Fedele.