Wednesday
December 26, 2001
Web Posted: 11:00 am

After reading a newspaper article about a rogue wave damaging a fishing vessel just outside of Sitka Sound, I thought it would be a good topic for this column. Rogue waves, sometimes referred to as "freak" waves, are generally defined as unexpectedly high waves which may come from a direction other than the prevailing sea state.

A single rogue wave can wreak havoc on even the sturdiest vessels, and our maritime history is littered with the lore and legend of these sea monsters. In 1942, the Queen Mary was struck by a mountainous wave that rolled her over. Fortunately, the ship righted herself and continued on to England. In 1965, the U.S.S. Pittsburgh lost 90 feet of her bow to a rogue wave in the North Pacific. In 1966, while crossing from Lisbon to New York, the S.S. Michelangelo was stuck by an 80 foot wave that tore 30 feet of bulwark off, smashing it into the bridge and first class rooms. Every year, major ocean vessels suffer structural damage while traveling south along the standard route from the Middle East to the United States or Europe.

Because the rogue wave phenomenon is so fleeting, it is difficult to document for study and analysis. But there are several possible causes for its development which are well accepted among oceanographers and meteorologists. The most common explanation for its random occurrence is constructive interference, or the coincidence of several different wave trains meeting at the same time. In this way, the crests of the waves may be superimposed so that an extremely amplified wave results. Waves generated this way are typically short-lived since the wave trains separate as they continue to move on.

Rogue waves may also result from the focusing of wave energy by ocean currents, such as the Gulf Stream or the Kuroshio Current. When storm forced waves or a steady ocean swell runs against a strong ocean current, unusually high waves may develop. Waves generated this way tend to be longer lived and may be very steep as the wave frequency is shortened by the current interaction.

Mariners who have experienced rogue waves often say the trough is far more sinister than the crest. Where the troughs of several wave trains coincide, the effect has been described as an "enormous hole" developing in the sea. When a ship's bow falls unexpectedly into a rogue trough, its momentum and the force of gravity can drive it downward so steeply it cannot rise over the next crest.

Rogue waves are predictable only in the sense that they are a normal part of the wave spectrum. Sea state is commonly reported in terms of significant wave height, defined as the average of the one third-highest waves. The probability of encountering a wave which is at least twice the significant height is about 1 in 1000. Certainly uncommon, but far from impossible.

Note: Aimee Devaris is a Public & Marine Program Manager with the National Weather Service in Anchorage, AK.

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