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Volcano Watch — Rogue waves: The Big Kahuna of Earth's oceans

September 16, 2010

Disaster movie fans may remember the 2000 film "The Perfect Storm," in which economic hardship drives a fishing boat to risk a late-season expedition. After weathering two powerful weather fronts, a hurricane, and other requisite disaster movie scenarios, the ship is hit by an enormous rogue wave, capsizes, and sinks. Is this all Hollywood hype or is there fact in the fiction?

Although sailors have been telling stories of encountering monstrous ocean waves for centuries, until relatively recently, rogue waves were considered to be near-mythical. Then, on January 1, 1995, for the first time, a monster wave was successfully identified using a downwards-pointing laser instrument, at the Draupner Oil Rig in the North Sea. What has become known as the "New Year's Wave" or "Draupner Wave" had a maximum wave height of 25 m (84 ft—or about the height of a 10-story building) was 430 m (1,415 ft) across, and inflicted minor damage to the platform. It rose out of a storm-tossed sea of 9 m (30 ft) waves and swept across the deck at 45 miles per hour.

Rogue waves, also known as freak or monster waves, are large and spontaneous ocean surface waves that generally occur far out at sea, are very different in character than the surrounding waves, and frequently, although not exclusively, occur in stormy conditions. In oceanography, they are more precisely defined as waves whose height is more than twice the significant wave height, which is the average height of the tallest third of the waves in a wave record. While waves commonly reach 6 m (20 ft) on the open sea, or even 15 m (50 ft) in extreme weather, 30-m (100-ft) rogue waves have been measured.

Rogue waves occur more often in areas where waves propagate into strong opposing currents such as off the coast of South Africa and in the North Atlantic. They often travel at an angle to the prevailing current and wave direction and, thus, seemingly "come out of nowhere." Most modern merchant vessels are designed to withstand about 15 tons of pressure per square meter (square yard), but rogue waves can exert a pressure of about 100 tons per square meter (square yard).

Rogue waves may be more common than previously thought. Modern techniques such as satellite and radar imagery, and ocean-floor pressure sensors have documented the existence of numerous rogue waves. Among other events, the sinking of the freighter MS Munchen in 1975 may have been caused by a rogue wave.

Rogue waves are distinct from tsunami, which are caused by earthquakes, volcanic eruptions, landslides, and other disturbances above or below water. Tsunami travel at high speed, growing in height as they reach shallower water. Rogue waves occur in deep water or where a number of physical factors, such as strong winds and fast currents, converge. In the original 1972 movie, "The Poseidon Adventure," an ocean liner was hit by a tsunami far out at sea, but tsunami rarely, if ever, affect ships at sea. In the 2006 remake, a rogue wave capsizes the ship.

A number of theories address the cause of these anomalous waves; however, it appears that rogue waves don't behave in accordance with the conventional rules of wave interaction. In the language of mathematics, a wave that doesn't break or curl and is the same size and speed over a long distance is dubbed a soliton. In the ocean, a boat or board riding swells is traveling over solitons. A special type of soliton, named after the mathematician who first theorized its existence, may be used to model rogue waves. A Peregrine soliton occurs when many solitons come together to form one wave that is larger and faster than any individual wave. Recently, scientists have demonstrated the Peregrine soliton in the lab using light pulses traveling through fiber optic cables. It is hoped that this new information can be used to search for and forecast oceanic rogue waves.


Volcano Activity Update

No active surface flows from Kīlauea's east rift zone eruption have been observed over the past week. Incandescence from the recently active lava pond within the Pu‘u ‘Ō‘ō crater faded away early in the week, indicating that activity there had ceased. Lava continues to enter the lava tube system, however, and is carried downslope to Puhi-o-Kalaikini, near Kalapana, where it enters the ocean and creates a robust steam plume.

At Kīlauea's summit, the circulating lava lake deep in the collapse pit within the floor of Halema‘uma‘u Crater has been visible via the Webcam throughout the past week. The lava level has fluctuated slowly in tandem with the frequent deflation/inflation cycles impacting Kīlauea's summit. This slow change has also been interrupted sporadically by abrupt increases in the height of the lava surface. These periods of high lava level were short-lived, lasting up to several hours, and ended with a sudden drop of the lava surface back to its previous level. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

No earthquakes beneath Hawai‘i Island were reported felt during the past week.

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