The causes of freak waves, which have been known to reach heights of 18 metres and are often blamed for the mysterious loss of ships at sea, have long been a matter of conjecture. But scientists at San Francisco State University and the Naval Postgraduate School in Monterey, California, have used a computer simulation to show that a combination of ocean currents and undersea topography are most likely to blame.

In a paper published in the Journal of Physical Oceanography, Tim Janssen and Thomas HC Herbers showed that when waves are focused by variations in depth and/or currents, a rapid increase in their energy can cause them to significantly increase in height. The conditions most conducive to the production of freak waves typically occur in coastal areas, and the simulations should enable the identification of extreme wave ‘hotspots’.

‘We found that if the focusing [of the waves] is sufficiently strong and abrupt, wave interactions create conditions favourable to extreme waves,’ said Janssen. ‘It appears that wherever waves undergo a rapid transformation, freak waves can be much more likely than we would otherwise expect.’

October 2009