Rogues, called extreme storm waves by scientists, are those waves which are greater than twice the size of surrounding waves, are very unpredictable, and often come unexpectedly from directions other than prevailing wind and waves. Most reports of extreme storm waves say they look like walls of water and they are often steep-sided with unusually deep troughs.
Exactly how and when rogue waves form is unknown, but there are several known causes:
Constructive interference. The waves often form because swells, while traveling across the ocean, do so at different speeds and directions. As these swells pass through one another, their crests, troughs, and lengths sometimes coincide and reinforce each other which can form unusually large, towering waves that quickly disappear. If the swells are traveling in the same direction, these mountainous waves may last for several minutes before subsiding.
Focusing of wave energy. When waves formed by a storm develop in a water current against the normal wave direction, it results in a shortening of the wave frequency. This can cause the waves to join together, forming very big waves. The currents where these are sometimes seen are the Gulf Stream and Agulhas current. Extreme waves developed in this fashion tend to be longer lived.
Off the coast of Georgia, early on Saturday, April 16, 2005, a giant, seven-story wave appeared out of nowhere and crashed into the bow of a cruise ship, sent deck chairs flying, smashed windows, raced as high as the 10th deck, flooded 62 cabins, injured 4 passengers and sowed widespread fear and panic. On passenger claimed the "ship was like a cork in a bathtub.”
Over the decades oceanographers were skeptical and doubted their existence and tended to lump them together with sightings of mermaids and sea monsters. But, as is sometimes the case, the "experts" were wrong. Scientists are now finding that rogue waves are far more common and destructive than once imagined.
In size and reach these waves are quite different from earthquake-induced tsunamis, which form low, almost invisible mounds at sea before gaining height while crashing ashore. Rogue waves seldom, if ever, hit land because they cannot get into shallow water.
By one definition, rogue waves rise to heights of at least 82 feet, about the height of an eight-story building. Scientists have calculated their theoretical maximum at 198 feet, but so far they have documented nothing that big. Large rogues seem to average around 100 feet.
Most waves, big and small alike, form when the wind blows across open water and the wind’s force, duration and sweep determine the size of the swells. Waves of about 6 feet are common, though ones up to 30 or even 50 feet are considered unexceptional; frightening if you have ever seen one. As waves gain energy from the wind, they become steeper and the crests can break into whitecaps. The trough preceding a rogue wave can be quite deep, called a “hole in the sea.” For a ship, it is a roller coaster plunge that can be disastrous.
In 1933 in the North Pacific, the Navy oiler Ramapo encountered a huge wave with an estimated its height at 112 feet. In 1966, the Italian cruise ship Michelangelo was steaming toward New York when a giant wave tore a hole in its superstructure, smashed heavy glass 80 feet above the waterline, and killed a crewman and two passengers. In 1978, the München, a German barge carrier, sank in the Atlantic. Surviving bits of twisted wreckage suggested the cause was a rogue wave. Even with these eyewitness accounts many oceanographers were skeptical; people tend to embellish, they said. Another factor causing doubt was the fact that bobbing ships don't make good reference points for trying to determine the size of a wave.
Scientific models predicted that giant waves were statistical improbabilities that would happen only once every 10,000 years or so. Then on New Year’s Day in 1995, when an oil platform in the North Sea, which had a laser designed to measure wave height was hit with a rogue wave that was 84 feet high. In February 2000, a British oceanographic research vessel caught in a gale west of Scotland measured waves up to 95 feet, making them largest waves ever recorded by scientific instruments. These two incidents convinced scientists that rogue waves were real.
It was found that rogue waves regularly form in regions where there are powerful currents: the Agulhas off South Africa, the Kuroshio off Japan, and the Gulf Stream off the eastern United States; the Gulf Stream also flows through the Bermuda Triangle, famous for the disappearance of large numbers of ships.
One way that rogue waves apparently form is when the strong currents meet winds and waves moving in the opposite direction with the result that due to a concentration of forces, rogue waves form. For example, when a small, fast wave catches up with a large, slow wave, the energy of both can combine to create a single, high-intensity mutant wave.
There is one area in particular where big oil tankers coming from the Middle East ride the Agulhas current around South Africa. There, the westward-flowing current meets prevailing easterly winds, at times disastrously. Three or four tankers a year there get badly damaged in the area. In September 2004 when Hurricane Ivan swept through the Gulf of Mexico it passed directly over six wave-tide gauges that the Naval Research Laboratory had placed about 50 miles east of the Mississippi Delta and the waves measured more than 90 feet from trough to crest. Even the best physical protections may fail under assault by tons of roiling water, so the best precaution is to learn how to avoid rogue waves in the first place, but there prediction is still not possible.
In addition to wind-current interactions, is the amplification that occurs when different storms come together. Sometimes the waves cancel each other out, but sometimes they get much larger. Another cause is choppy seas where several waves moving in different directions merge. In that case scientists claim a giant wave would normally last for no more than a few seconds or minutes. However, some are suspected of lasting for hours and traveling long distances.
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Watch a rogue wave hit a US Navy ship on Youtube.
US Naval officer Dan Butterfield relates what happened in 1961 when his submarine encountered a rogue wave.