An international team of scientists has developed a relatively¹ simple mathematical explanation for the rogue² ocean waves that can develop seemingly out of nowhere to sink ships and overwhelm oil platforms with walls of water as much as 25 meters high. The waves stem from a combination of constructive³ interference - a known phenomenon of colliding waves - and nonlinear effects specific to the complex dynamics⁴ of ocean waves. An improved understanding of how rogue waves originate could lead to improved techniques for identifying ocean areas likely to spawn⁵ them, allowing shipping⁶ companies to avoid dangerous seas.

 

Based on an analysis of three rogue waves observed at different oil platforms in the North Sea over the course of a decade, the research was scheduled to be reported June 21 in the journal Scientific Reports. The work was done by researchers at the Georgia Institute of Technology, University College Dublin, and the Institut FEMTO-ST CNRS-Université de Franche-Comté.

 

"We saw similar wave behaviors at all three oil platforms," said Francesco Fedele, a professor in the Georgia Tech School of Civil and Environmental Engineering. "We found that the main mechanism⁷ responsible for generating these waves is the constructive interference of elementary waves enhanced by second-order bound nonlinearities."

 

Rogue waves have been observed in oceans around the world. They typically last only 20 seconds or so before disappearing, and are different from tsunami⁸ waves that can travel great distances after being created by underwater earthquakes or landslides⁹.

 

Earlier research had suggested a phenomenon known as "modulational instability" to explain the rogue waves. That theory had been demonstrated in laboratories, but didn't adequately explain the complex three-dimensional waves that were being measured in the open ocean without boundaries to constrain¹⁰ them, Fedele said.

 

Though ocean waves have a predominant direction, in the open ocean, waveforms from other directions can arrive. In rare conditions, those waves arrive in an organized way or almost in phase, leading to an unusual case of constructive interference that can double the height of the resulting wave.

 

But this doubled height still cannot explain the size of the rogue waves observed in the North Sea - and elsewhere. That difference can be accounted for by the nonlinear nature of the waves, which are not sinusoidal - but instead have rounded troughs, along with sharp peaks that result from 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 crests¹¹ and the troughs," said Fedele, who also has an appointment in Georgia Tech's School of Electrical and Computer Engineering. "These nonlinear effects can produce an enhancement of 15 to 20 percent in wave height, which adds onto the effects of constructive interference."

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