General Discussion

Wind waves, swell, and internal waves make up the dominant action in the upper layers of the ocean. Internal waves in coastal waters have been known since the late 1800s. Internal waves in the open ocean and nonlinear solitons have been recognized only in the past two decades, as a result of human observations and remote sensing from space platforms (Apel et al., 1975; Fedorov, 1976).

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Wind waves,
North Pacific
northeast Pacific
Tropical Pacific
Internal wave
packets, Hainan

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Internal waves,
equatorial Indian
internal waves,
Nicobar Islands
Internal waves,
Red Sea
Internal waves,

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Alboran Sea
(100 mm)
(250 mm)

      Just as there are waves on the sea's surface generated by the wind, so too are there waves below the surface. Internal waves are most obvious at density interfaces within the ocean, such as the base of the upper mixed layer and the permanent thermocline. At that surface, the up-and-down wavy motion of an isotherm can be measured in situ by bathythermographs or, as has been done in a few cases, thermistor chains. That interface has a far smaller density gradient than that between the sea's surface and the overlying air. Internal waves are far more subdued in frequency, therefore, than surface gravity waves (LaFond, 1962). On the other hand, a result of the small density difference is that internal waves reach far greater amplitudes than those at the sea surface, up to200 meters.
      Internal waves occur in a great variety of wavelengths. The most obvious are those with wavelengths from two to six kilometers, as measured from crest to crest. Where the waves are the result of daily tidal forcing, they occur in packets of four to eight waves, the lead wave being most dominant. Where the density interface (usually the thermocline) is shallow enough to permit the internal wave crests to interact with the sea surface, the waves can be seen, and photographed, in the resulting textural change of the surface ocean (Basovitch, 1979).
      Fundamentally, solitons are nonlinear, localized, traveling waves that maintain their shape and identity through a balance between the nonlinear and dispersive wave effects. Oceanic solitons (or VBrand waves, after Vance Brand, the astronaut who first discovered them) manifest themselves as large internal waves with a small surface expression.
      Typically, solitons come in groups of six, the flrst being the most intense. In the Andaman Sea, the location of the flrst known oceanic solitons, crest lengths of 160 kilometers have been observed, with wavelengths of some 10 kilometers (Osborne and Burch, 1980). The subsurface vertical amplitude of the Andaman Sea waves, as measured from current meters and thermistors on oil drilling rigs, reached 120 meters. The whole packet of waves there moved forward as a group with speeds up to eight kilometers per hour.
      The tides are also forcing oceanic functions for solitons, for example, those entering the Alboran Sea through the Strait of Gibraltar. Southern Hemisphere storm surges seem to be the cause of solitons in the Andaman and Sulu seas of Indonesia, although the tides clearly influence the large nonlinear internal waves in the Celebes and East China seas.
      There are no measurements or observations of solitons in the open oceans of the world. There has been speculation that the loss of the USS Thresher came from a soliton carrying the submarine rapidly deeper than its crush depth. There is no evidence for that theory, however.

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