Australasian Science: Australia's authority on science since 1938

Rogue Waves Confirmed

By Stephen Luntz

An Australian–German collaboration has created super rogue waves in a 10 x 1 metre wave tank, swamping a Lego pirate in the process.

The wave created was just 5 cm high, but it provided insight into a phenomenon that, until recently, was widely believed to be a myth.

Rogue waves generally occur in chaotic conditions, and are defined as waves more than 2.2 times the average height of the largest one-third of waves around them. However, for many years reports have circulated of waves that seem to come from nowhere, appearing from calm (or at least regular) seas. Most sailors experiencing these freak events do not survive to report them, but Prof Nail Akhmediev of the Australian National University says that satellite data shows that at any time there may be ten such waves occurring somewhere on the global ocean.

In the 3 weeks prior to the publication of Akhmediev’s work in Physical Review, an Australian was injured off the coast of California and eight of nine people aboard a New Zealand fishing boat died when each was struck by these sorts of waves.

The researchers floated the Lego pirate in a wave tank at the Hamburg University of Technology. Waves were generated using computer-controlled panels, with the pirate bobbing happily on 1 cm swells. However, Akhmediev says that “under certain conditions, small perturbations can grow and be amplified. Waves of certain wavelengths become transformed into other wavelengths and amplification accumulates, transmitting into one big wave.”

The team was able to produce a wave that, starting at 1 cm, reached an amplitude of 5 cm and upended the pirate. “This observation could have far-reaching consequences for our efforts to understand these waves that are, by far, still mysterious,” Akhmediev says.

The researchers dubbed this phenomenon “super rogue waves”. While such a title may seem grandiose for a 5 cm wavelet, Akhmediev says the oceanic equivalents can reach 30 metres in height.

Fortunately the initial conditions required for such monsters are rare, but the more common shipping becomes the more chance there will be of tragic encounters.

“Of course, in real oceans the problem will require more careful analysis,” Akhmediev says, “but we expect the result to have a significant impact on the studies of extreme ocean waves and, more generally, extreme events in nature and society”.

Rogue waves are now being studied in optics, superfluids and Bose–Einstein condensates. Akhmediev says that equivalents have even been considered in the financial markets, leaving apparently secure institutions vulnerable to different sorts of pirates.