Wednesday, September 1, 2010
September 1, 2010 : Rogue Waves
Rogue waves (also known as freak waves, monster waves, killer waves, extreme waves, and abnormal waves) are relatively large and spontaneous ocean surface waves that occur far out in sea, and are a threat even to large ships and ocean liners. In oceanography, they are more precisely defined as waves whose height is more than twice the significant wave height (SWH), which is itself defined as the mean of the largest third of waves in a wave record. Therefore rogue waves are not necessarily the biggest waves found at sea; they are, rather, surprisingly large waves for a given sea state. "Rogue waves are not tsunamis, which are set in motion by earthquakes [and] travel at high speed, building up as they approach the shore. 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."
Once thought by scientists to exist only in legends, rogue waves are now known to be a natural ocean phenomenon. Eyewitness accounts from mariners and damages inflicted on ships have long suggested they occurred; however, their scientific measurement was only positively confirmed following measurements of the "Draupner wave," a rogue wave at the Draupner platform, in the North Sea on January 1, 1995. During this event, minor damage was inflicted on the platform, confirming that the reading was valid. Satellite images have also confirmed their existence.
Freak waves have been cited in the media as a likely source of the sudden, inexplicable disappearance of many ocean-going vessels. One of the very few cases in which evidence exists that may indicate a freak wave incident is the 1978 loss of the freighter MS München, detailed below. In February 2000, a British oceanographic research vessel sailing in the Rockall Trough west of Scotland encountered the largest waves ever recorded by scientific instruments in the open ocean, with a SWH of 18.5 meters (61 ft) and individual waves up to 29.1 meters (95 ft). "In 2004 scientists using three weeks of radar images from European Space Agency satellites found ten rogue waves, each 25 metres or higher."
A rogue wave is not the same as a tsunami. Tsunamis are mass displacement-generated waves which propagate at high speed and are more or less unnoticeable in deep water; they only become dangerous as they approach the shoreline and do not present a threat to shipping (the only ships lost in the 2004 Asian tsunami were in port). A rogue wave, on the other hand, is a spatially and temporally localized event that most frequently occurs far out at sea.
Rogue waves may sometimes be referred to as "hundred-year waves," due to the supposed likelihood of their occurrence. They should not be confused, however, with the hundred-year wave, which is a statistical prediction of the highest wave likely to occur in a hundred-year period in a particular body of water. These predictions are typically based on wave models which do not take rogue waves into account.