Rogue waves are dangerous, fairly unpredictable beasts.
Rogue ocean waves are not something you ever want to come across. The are dangerous, can be massive, as much as 25 meters (82 feet) high, and often develop seemingly out of nowhere and with little warning. Although they are short-lived — about 20 seconds — they have been known to sweep people away, sink ships, and overwhelm oil rig platforms.
An international team of scientists from the Georgia Institute of Technology (GIT), University College Dublin (UCD), and the Institut FEMTO-ST CNRS-Université de Franche-Comté, has determined that these massive waves are birthed through a combination of constructive interference — a phenomenon where colliding waves line up crest (wave top) to crest or trough (wave bottom) to trough, combining the strength of the two waves — and nonlinear effects, such as wave shape and height.
The discovery came after the analysis of three rogue waves observed at three different oil platforms in the North Sea in 1995, 2007, and 2015.
"We saw similar wave behaviors at all three oil platforms," said Francesco Fedele, a professor in the GIT School of Civil and Environmental Engineering and first author of the paper, in a GIT news release.
Although ocean waves generally travel in a particular direction, in the open ocean other waveforms can arrive from different directions. In rare circumstances, these waves can arrive almost in phase with the others, leading to constructive interference that can double the height of the resulting wave.
However, this doubled height is not enough to explain the size of rogue waves observed in the oceans. The researchers hypothesized that the resulting size must be due to the nonlinear nature of waves — with rounded troughs, and sharp peaked crests due to the water being pushed upward against the pull of gravity.
"You have to account for the nonlinearity of the ocean, which is manifested in the lack of symmetry between the crests and the troughs," explained Fedele. "These nonlinear effects can produce an enhancement of 15 to 20 percent in wave height, which adds onto the effects of constructive interference."
To test their idea, the researchers modeled how waves could combine to produce the rogue waves measured at the three different oil platforms. Amazingly, their model predictions matched the waves measured to a tee. The results are published in the journal Scientific Reports.
"These are fascinating results," said John Dudley, a professor at FEMTO-ST and co-author of the paper. "Many of us have spent years studying the effects of nonlinearity in wave amplification, but it is essential as a scientist to keep an open mind. It is not for us to tell Nature how to work — we must follow where it leads us, even if it means changing our ideas."
Understanding how rogue waves originate could help identify which ocean regions are likely to produce them, allowing shipping companies to avoid dangerous areas, and also produce better warning systems.
Editor's note (June 23): The title has been updated to reflect the fact that the mystery of rogue waves has not been completely solved.