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Rogue Waves

Rogue waves, also known as freak waves, are relatively large and spontaneous ocean surface waves which are a threat even to large ships and ocean liners. In oceanography, they are more precisely defined as waves that are more than double the significant wave height (SWH), which is itself defined as the mean of the largest third of waves in a wave record.

Once thought to be only legendary, they are now known to be a natural ocean phenomenon, not rare, but rarely encountered. Anecdotal evidence from mariners' testimonies and damages inflicted on ships suggested they occurred; however, their scientific measurement was only positively confirmed following measurements of a rogue wave at the Draupner oil 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.

In the course of the Project MaxWave, researchers from the GKSS Research Centre, using data collected by ESA satellites, identified a large number of radar signatures that may be evidence for rogue waves. Further research is underway to verify the method that translates the radar echoes into sea surface elevation.

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 in the Gulf Make the News

Hurricane Ivan prompts rogue wave rethink: 100 ft waves could be quite common

By Lucy Sherriff of The Register.co,uk

Hurricane Ivan generated a wave almost 100ft high last September, as the storm passed over the Gulf of Mexico. Scientists reckon it is the tallest wave ever measured in US waters, and that it would have had enough power to literally break a ship in two.

The wave was so large that it is likely to prompt scientists to rethink their models of wave formation. Current theories do not allow for such monsters to be created, the BBC reports.

The wave never reached land, fortunately, but was recorded by the Naval Research Laboratory's ocean-floor-based pressure sensors. The wave measured a whopping 91 feet, or 27.7 meters from its peak to its trough as it passed over the instruments, and would have been around 600 feet long. However, researchers think the instruments could have missed even bigger waves during the storm.

Large waves - known as rogue waves - are occasionally reported, and have been spotted by European Space Agency satellites. However, the mechanism by which they form is poorly understood, and they were dismissed for a long time as exaggerations or fibs told by sailors.

ESA reports several such encounters: On 1 January 1995 the Draupner oil rig in the North Sea was hit by a wave whose height was measured by an onboard laser device at 26 metres, with the highest waves around it reaching 12 meters. Then, in early 2001, two cruising vessels - the Bremen and the Caledonian Star - had their bridge windows smashed by 30-metre rogue waves in the South Atlantic.

But these waves may not be rogue waves at all: lead author David Wang told the BBC: "Our results suggest that waves in excess of 90 ft are not rogue waves but actually are fairly common during hurricanes."
The research is published in the journal Science. ®