March 6, 2010

Rogue Waves Wreak Havoc on the High Seas

Known as rogue or freak waves, these towering walls of water are simply called monsters of the sea by some people.

No matter what they are called, these oversized swells can appear suddenly on the open water and can be large enough to overwhelm even the biggest of ships, according to experts.

These rogue waves are believed to be what caused havoc on the cruise ship Louis Majesty this past Wednesday off the Mediterranean coast of Spain. Though not confirmed, three 26-foot waves reportedly smashed against the vessel, killing two people and seriously injuring a third.

Rogue waves, which can form in different ways, are almost always generated from storm-related winds, either near or far. "The winds transfer energy into the waves," said Peter Challenor, an oceanographer at the National Oceanography Centre in Southampton in Britain. "Then you get interactions among the waves, with the large ones taking energy from the smaller ones, getting bigger and bigger in the process," he said in a telephone interview.

The biggest rogue wave ever recorded on the open water was a 112-foot giant spotted in the Pacific by a U.S. Navy tanker in the 1920s. An 84-foot wall of water -- known to oceanographers as the "new year wave" -- slammed into an oil rig off the Norwegian coast in the North Sea on January 1, 1995.

Between 1985 and 2005, more than 200 tankers and cargo ships longer than 650 feet have been sunk during severe weather, many of which are now thought to have been struck by freak killer waves, according to University of Hawaii oceanographer Peter Muller.

Before there were good scientific tools to see and measure these waves, many believed sailors were too drunk to know what really had happened. "Today"¦ we know they are real," said Christian Kharif, a scientist in Marseille and co-author of "Rogue Waves in the Ocean".

The rogue waves are generated through amplification of smaller waves moving in the same direction. "As wind increases in intensity, it is first going to create small waves, and then bigger ones, which travel faster. Eventually the big ones will catch up, and the energy is concentrated as the waves pile up," Kharif said by phone.

Another scenario that may be the cause of rogue waves also fits the conditions leading up to the incident on Wednesday.

"There is a mechanism where crisscrossing swells meet, creating a sudden upsurge," Kharif explained. Two prevailing wave patterns affected the region where the Louis Majesty was hit. One wave was driven by northeasterly winds, while a second was created by a distant weather disturbance, and they converged on one another at a right angle, said an official from the French national weather bureau.

Although the scenario is plausible, that same official cited data that could throw the rogue wave theory out the window as well.

According to data collected by a Spanish weather buoy just before the accident, the waves were measuring about 16 feet on average, Jean-Michel Lefevre said by phone. "Under those conditions we would expect 26-foot waves every 15 minutes."

The size of the waves reported seem to fall short for the culprit to have been a rogue wave, which is defined as a wave at least two times as high as the so-called significant wave height, the average of the largest third of waves over a certain period.

There is some debate over the frequency in which rogue waves occur. Some studies suggest that an estimated 1 in 3,000 swells become rogues, while others suggest that the occurrence is even rarer. But they all do agree that they most often occur in sets of three. The phenomenon -- known as the "Ëœthree sisters' -- is a much feared topic in the world of sailors.

In contrast, the tsunami waves generated by earthquakes, while devastating to coastal areas, are "barely perceptible" in open waters, he said.