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).
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.
