Rogue wave

Rogue waves (also known as freak waves, monster waves, killer waves, extreme waves, and abnormal waves) are relatively large and spontaneous ocean surface waves that occur far out in sea, and are a threat even to large ships and ocean liners.^[1] In oceanography, they are more precisely defined as waves whose height is more than twice the significant wave height (SWH), which is itself defined as the mean of the largest third of waves in a wave record. Therefore rogue waves are not necessarily the biggest waves found at sea; they are, rather, surprisingly large waves for a given sea state. Rogue waves seem not to have a single distinct cause, but occur where physical factors such as high winds and strong currents cause waves to merge to create a single exceptionally large wave.^[1]

Contents 1 Background 2 History 3 Occurrence 4 Causes 5 Applications 6 Reported encounters 6.1 Nineteenth century 6.2 Twentieth century 6.3 Twenty-first century 6.4 Loss estimates 7 See also 8 References 9 External links 9.1 MaxWave report and WaveAtlas 9.2 Other

Background

Once lacking hard evidence for their existence, rogue waves are now known to be a natural ocean phenomenon. Eyewitness accounts from mariners and damages inflicted on ships have long suggested they occurred; however, their scientific measurement was only positively confirmed following measurements of the "Draupner wave", a rogue wave at the Draupner 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. Satellite images have also confirmed their existence.^[2]

Freak waves have been cited in the media as a likely source of the sudden, inexplicable disappearance of many ocean-going vessels. One of the very few cases in which evidence exists that may indicate a freak wave incident is the 1978 loss of the freighter MS München. In February 2000, a British oceanographic research vessel sailing in the Rockall Trough west of Scotland encountered the largest waves ever recorded by scientific instruments in the open ocean, with a SWH of 18.5 meters (61 ft) and individual waves up to 29.1 meters (95 ft).^[3] "In 2004 scientists using three weeks of radar images from European Space Agency satellites found ten rogue waves, each 25 meters or higher."^[1]

A rogue wave is distinct from a tsunami.^[1] Tsunamis are caused by mass displacement, such as sudden movement of the ocean floor. They propagate at high speed over a wide area and are more or less unnoticeable in deep water, only becoming dangerous as they approach the shoreline and the ocean floor becomes shallower. They do not present a threat to shipping at sea (the only ships lost in the 2004 Asian tsunami were in port). A rogue wave, on the other hand, is a highly localized phenomenon both in space and duration, most frequently occurring far out at sea.^[1]

Rogue waves may sometimes be referred to as "hundred-year waves," due to the supposed likelihood of their occurrence.^[4] They should not be confused, however, with the hundred-year wave, which is a statistical prediction of the highest wave likely to occur in a hundred-year period in a particular body of water. These predictions are typically based on wave models which do not take rogue waves into account.

History

It is common for mid-ocean storm waves to reach 7 meters (23 ft) in height, and in extreme conditions such waves can reach heights of 15 meters (49 ft). However, for centuries maritime folklore told of the existence of much larger waves — up to 30 meters (98 ft) in height (approximately the height of a 10-story building) — that could appear without warning in mid-ocean, against the prevailing current and wave direction, and often in perfectly clear weather. Such waves were said to consist of an almost vertical wall of water preceded by a trough so deep that it was referred to as a "hole in the sea"; a ship encountering a wave of such magnitude would be unlikely to survive the tremendous pressures exerted by the weight of the breaking water, and would almost certainly be sunk in a matter of minutes.

Research has confirmed that waves of up to 35 meters (115 ft) in height are much more common than mathematical probability theory would predict using a Rayleigh distribution of wave heights. In fact, they seem to occur in all of the world's oceans many times every year. This has caused a re-examination of the reasons for their existence, as well as reconsideration of the implications for ocean-going ship design.

Rogue waves may also occur in lakes. A phenomenon known as the "Three Sisters" is said to occur in Lake Superior when a series of three large waves forms. The second wave hits the ship's deck before the first wave clears. The third incoming wave adds to the two accumulated backwashes and suddenly overloads the ship deck with tons of water. The phenomenon was implicated in the sinking of the SS Edmund Fitzgerald on Lake Superior in November 1975.^[5]

Occurrence

In the course of Project MaxWave, researchers from the GKSS Research Centre, using data collected by ESA satellites, identified a large number of radar signatures that have been portrayed as evidence for rogue waves. Further research is under way to develop better methods of translating the radar echoes into sea surface elevation, but at present this technique is not proven.^[6][7]

Causes

Because the phenomenon of rogue waves is still a matter of active research, it is premature to state clearly what the most common causes are or whether they vary from place to place. The areas of highest predictable risk appear to be where a strong current runs counter to the primary direction of travel of the waves; the area near Cape Agulhas off the southern tip of Africa is one such area; the warm Agulhas current runs to the southwest, while the dominant winds are westerlies. However, since this thesis does not explain the existence of all waves that have been detected, several different mechanisms are likely, with localised variation. Suggested mechanisms for freak waves include the following:

• Diffractive focusing — According to this hypothesis, coast shape or seabed shape directs several small waves to meet in phase. Their crest heights combine to create a freak wave.^[8]
• Focusing by currents — Waves from one current are driven into an opposing current. This results in shortening of wavelength, causing shoaling (i.e., increase in wave height), and oncoming wave trains to compress together into a rogue wave.^[8] This happens off the South African coast, where the Agulhas current is countered by westerlies.
• Nonlinear effects — It seems possible to have a rogue wave occur by natural, nonlinear processes from a random background of smaller waves.^[9] In such a case, it is hypothesised, an unusual, unstable wave type may form which 'sucks' energy from other waves, growing to a near-vertical monster itself, before becoming too unstable and collapsing shortly after. One simple model for this is a wave equation known as the nonlinear Schrödinger equation (NLS), in which a normal and perfectly accountable (by the standard linear model) wave begins to 'soak' energy from the waves immediately fore and aft, reducing them to minor ripples compared to other waves. The NLS can be used in deep water conditions. In shallow water, waves are described by the Korteweg–de Vries equation or the Boussinesq equation. These equations also have non-linear contributions and show solitary-wave solutions. A rogue wave consistent with the nonlinear Schrödinger equation was produced in a laboratory water tank in 2011.^[10]
• Normal part of the wave spectrum — Rogue waves are not freaks at all but are part of normal wave generation process, albeit a rare extremity.^[8]
• Wind waves — While it is unlikely that wind alone can generate a rogue wave, its effect combined with other mechanisms may provide a fuller explanation of freak wave phenomena. As wind blows over the ocean, energy is transferred to the sea surface. When strong winds from a storm happen to blow in the opposing direction of the ocean current the forces might be strong enough to randomly generate rogue waves. Theories of instability mechanisms for the generation and growth of wind waves—although not on the causes of rogue waves—are provided by Phillips^[11] and Miles.^[12]

The spatio-temporal focusing seen in the NLS equation can also occur when the nonlinearity is removed. In this case, focusing is primarily due to different waves coming into phase, rather than any energy transfer processes. Further analysis of rogue waves using a fully nonlinear model by R.H. Gibbs (2005) brings this mode into question, as it is shown that a typical wavegroup focuses in such a way as to produce a significant wall of water, at the cost of a reduced height.

A rogue wave, and the deep trough commonly seen before and after it, may last only for some minutes before either breaking, or reducing in size again. Apart from one single rogue wave, the rogue wave may be part of a wave packet consisting of a few rogue waves. Such rogue wave groups have been observed in nature.^[13]

There are three categories of freak waves:

• "Walls of water" travelling up to 10 km (6.2 mi) through the ocean
• "Three Sisters", groups of three waves^[14]
• Single, giant storm waves, building up to fourfold the storm's waves height and collapsing after some seconds^[15]

A research group at the Umeå University, Sweden in August 2006 showed that normal stochastic wind driven waves can suddenly give rise to monster waves. The nonlinear evolution of the instabilities was investigated by means of direct simulations of the time-dependent system of nonlinear equations.^[16]

Applications

The possibility of the artificial stimulation of rogue wave phenomena has attracted research funding from DARPA, an agency of the United States Department of Defense. Bahram Jalali and other researchers at UCLA studied microstructured optical fibers near the threshold of soliton supercontinuum generation and observed rogue wave phenomena. After modeling the effect, the researchers announced that they had successfully characterized the proper initial conditions for generating rogue waves in any medium.^[17] Additional works carried out in optics have pointed out the role played by a nonlinear structure called Peregrine soliton that may explain those waves that appear and disappear without leaving a trace.^[18][19]

Reported encounters

It should be noted that many of these encounters are only reported in the media, and are not examples of open ocean rogue waves. Often, in popular culture, an endangering huge wave is loosely denoted as a rogue wave, while it has not been (and most often cannot be) established that the reported event is a rogue wave in the scientific sense — i.e. of a very different nature in characteristics as the surrounding waves in that sea state and with very low probability of occurrence (according to a Gaussian process description as valid for linear wave theory).

This section lists a limited selection of notable incidents.

Nineteenth century

• The Eagle Island lighthouse (1861) – water broke the glass of the structure's east tower and flooded it, implying a wave that surmounted the 40 m (130 ft) cliff and overwhelmed the 26 m (85 ft) tower.^[20]
• Flannan Isles (1900) – three lighthouse keepers vanished after a storm that resulted in wave-damaged equipment being found 34 meters (112 ft) above sea level.^[21][22]

Twentieth century

• SS Waratah - In 1909, it left Durban, South Africa with 211 passengers and crew but did not reach Cape Town, South Africa.^[1]
• Voyage of the James Caird - In 1916 Sir Ernest Shackleton encountered a wave he termed "gigantic" while piloting a lifeboat/whaler from Elephant Island to South Georgia Island.^[23]
• USS Ramapo (1933) – triangulated at 112 feet (34 m).^[24]
• RMS Queen Mary (1942) – broadsided by a 92-foot (28 m) wave and listed briefly about 52 degrees before slowly righting.
• SS Michelangelo (1966) – hole torn in superstructure, heavy glass smashed 80 feet (24 m) above the waterline, and 3 deaths.^[24]
• SS Edmund Fitzgerald (1975) – lost on Lake Superior. A Coast Guard report blamed water entry to the hatches, which gradually filled the hold, or alternatively errors in navigation or charting causing damage from running onto shoals. However, another nearby ship, the SS Arthur M. Anderson, was hit at a similar time by two rogue waves and possibly a third, and this appeared to coincide with the sinking around ten minutes later.^[5]
• MS München (1978) – lost at sea leaving only "a few bits of wreckage" and signs of sudden damage including extreme forces 66 feet (20 m) above the water line. Although more than one wave was probably involved, this remains the most likely sinking due to a freak wave.^[9]
• Esso Languedoc A 25–30m wave washed across the deck from the stern of the French supertanker, and was photographed by the first mate, Philippe Lijour, in 1980.^[25]
• Fastnet Lighthouse Struck by 48 m (157 ft) wave in 1985 ^[26]
• Draupner wave (North Sea, 1995) – First rogue wave confirmed with scientific evidence, it had a maximum height of 25.6 meters (84 ft).^[27]
• RMS Queen Elizabeth 2 – North Atlantic, September 1995, 29 meters (95 ft), during Hurricane Luis: The Master said it "came out of the darkness" and "looked like the White Cliffs of Dover." ^[28] Newspaper reports at the time described the cruise liner as attempting to "surf" the near-vertical wave in order not to be sunk.

Twenty-first century

• MS Bremen and Caledonian Star (South Atlantic, 2001) encountered 30-meter (98 ft) freak waves. Bridge windows on both ships were smashed, and all power and instrumentation lost.^[28]
• U.S. Naval Research Laboratory ocean-floor pressure sensors detected a freak wave caused by Hurricane Ivan in the Gulf of Mexico, 2004. The wave was around 27.7 meters (91 ft) high from peak to trough, and around 200 meters (660 ft) long.^[29]
• Norwegian Dawn, (Georgia,[U.S.] 2005) On April 16, 2005, after sailing into rough weather off the coast of Georgia, Norwegian Dawn encountered a series of three 70-foot (21.34 m) rogue waves. The third wave damaged several windows on the 9th and 10th decks and several decks were flooded. Damage, however, was not extensive and the ship was quickly repaired.^[30] Four passengers were slightly injured in this incident.^[31]
• Aleutian Ballad, (Bering Sea, 2005) footage of what is identified as a rogue wave appears in an episode of Deadliest Catch. The wave cripples the vessel, causing the boat to tip for a short period onto its side. This is one of the few video recordings of what might be a rogue wave.^[32]
• It has been suggested that these types of waves may be responsible for the loss of several low-flying aircraft, namely U.S. Coast Guard helicopters on Search and Rescue missions.^[33]
• MS Louis Majesty, (Gulf of Marseille, Mediterranean Sea, March 2010). An unexpected pack of three waves of 26 feet (7.9 m) hit the ship while on a cruise between Carthagena and Marseille. Two passengers were killed in a lounge by flying glass from a shattered window. Damage to the ship was done by the second and third waves.^[34][35] While this wave was much lower than freak waves appearing in open oceans, published evidence indicates that its behaviour was similar to that of freak waves.

Loss estimates

Freak waves have been cited in the media as a likely source of the sudden, inexplicable disappearance of many ocean-going vessels. However, although this is a credible explanation for unexplained losses, there is to date little clear evidence supporting this hypothesis nor any cases where the cause has been confirmed, and the claim is contradicted by information held by Lloyd's Register. A press release by the European Space Agency in 2004 made the claim that "200 large ships of 600ft long or more in the past two decades sunk without trace".^[27] However, at the time the claim was made, there had only been 142 ships of that size lost at sea in the time frame, all with clear, known causes.^[36] The main culprits were the Iranian and Iraqi air forces in the 1980s during the Iran-Iraq war. One of the very few cases in which clear evidence exists that may indicate a freak wave incident is the 1978 loss of the freighter MS München.

See also

Antarctic Circumpolar Current (ACC) or West Wind Drift Agulhas Current Bermuda Triangle Clapotis

Extreme value theory Gulf Stream Kuroshio Current Megatsunami Ocean current

Ocean surface wave Sneaker wave Soliton White squall Resonance

References

External links

MaxWave report and WaveAtlas

• Freak waves spotted from space, BBC News Online
• Ship-sinking monster waves revealed by ESA satellites
• MaxWave project
• Rogue Wave Workshop (2005)
• Rogue Waves 2004

Other

• BBC News Report on Wave Research, 21 August 2004
• The BBC's Horizon "Freak waves" first aired in November 2002
• 'Giant Waves on the Open Sea', lecture by Professor Paul H Taylor at Gresham College, 13 May 2008 (available for video, audio or text download)
• TV program description
• New Scientist article 06/2001
• Freak Wave Research in Japan
• Rogue Giants at Sea, New York Times, July 11, 2006
• Kristian B. Dysthe; Harald E. Krogstad; Hervé Socquet-Juglard; Karsten Trulsen. "Freak waves, rogue waves, extreme waves and ocean wave climate". http://www.math.uio.no/~karstent/waves/index_en.html. Retrieved 2008-11-01.  Illustrations of the ways rogue waves can form — with descriptions for layman, photos and animations.

• Rogue Waves-Monsters of the deep, The Economist, September 17, 2009, p. 94

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