Climate Change/Environmental
9/27/2009 10:43:40 PM EST
Rogue waves are not a myth, but do exist, and happen much more often than previously believed
Partner, Ropers Majeski Kohn & Bentley
Sailors have long reported the presence of “rogue waves” (huge swells often 75’ or more in height) at sea. Most such reports have been written off as tall-tales and fabrications. Now, the evidence is developing that they are in fact real.
Then, in 1995, an oil rig in the North Sea recorded a 25.6-meter wave. In 2000, a British oceanographic vessel recorded a 29-meter wave off the coast of Scotland. In 2004, scientists using three weeks of radar images from European Space Agency satellites found ten rogue waves, each 25 meters or more high.
A typical ocean wave forms when wind produces a ripple across the surface of the sea. [See, for example, http://en.wikipedia.org/wiki/Ocean_surface_wave.] If the wind is strong, the ripples grow larger. A hurricane can amplify a wave to a few storeys. But trying to create giant rogue waves in a laboratory tank is very difficult, making them hard to study. Now researchers at Harvard University and Tulane University have started using microwaves rather than water waves to create a laboratory model.
Rogue waves are not tsunamis, which are set in motion by earthquakes. [See, for example, http://en.wikipedia.org/wiki/Tsunamis.] These travel at high speed, building up as they approach the shore (because the shore area is becoming shallower). Rogue waves seem to occur in deep water or where a number of physical factors such as strong winds and fast currents converge. This may have a focusing effect, which can cause a number of waves to join together. Such conditions exist along Africa’s “wild coast” (home to many reports in past centuries of rogue waves), where strong winds blowing from the northwest interact with the swift and narrow Agulhas current flowing down the coast to produce enormous waves. The researchers note that there may be other mechanisms at work, including an interference effect that causes different ocean swells, traveling at different speeds, to add up to produce a rogue, and a non-linear effect in which a small change in something like wind direction or speed causes a disproportionately large wave.
To study the phenomenon the researchers created a platform measuring 26cm by 36cm on which they randomly placed around 60 small brass cones to mimic random eddies in ocean currents. When microwaves were beamed at the platform, the researchers found that hot spots (the microwave equivalent of rogue waves) appeared far more often than conventional wave theory would predict; they were between ten and 100 times more likely.
The researchers note that the results tend to support anecdotal evidence from seamen that rogue waves are not as rare as once thought.
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