Internal waves in coastal waters have been known since the late 1800s. 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. That interface has a far smaller density gradient than that between the sea's surface and the overlying air. 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. Fundamentally, solitons are nonlinear, localized, traveling waves that maintain their shape and identity through a balance between the nonlinear and dispersive wave effects. Typically, solitons come in groups of six, the flrst being the most intense. The subsurface vertical amplitude of the Andaman Sea waves, as measured from current meters and thermistors on oil drilling rigs, reached 120 meters.