Oceanography

Internal waves

Below the ocean surface, waves form along the density boundaries between water layers of different density just as they do between air and water. These waves are called internal waves.

Internal waves form at the interfaces between layers of different water density. The disturbance needed to begin an internal wave need not be large, small generating forces plus the viscosity of the water results in large-amplitude, slow-moving waves with long wavelengths called internal waves. Internal waves appear to move like shallow water waves, with the thickness of the layers playing a role similar to the water depth in surface shallow-water waves. Internal wave travel more slowly than surface waves of similar amplitude. 

Internal waves may be caused when the surface layer flows over a more static lower layer in the same way the wind produces surface waves. The possible triggers of internal waves include seismic disturbances, long-term wind changes, and tidal forces.

When the surface layer is relatively shallow, a wave moving along an internal boundary may be high enough to have its crest below the surface. This action creates moving bands of smooth water called slicks, each representing a crest position. Slicks are most likely to occur in coastal water, estuaries, or fjords, where the shallow surface layer is diluted by fresh water runoff. These natural slicks reveal the presence of underlying internal waves.

Since internal waves occur below the surface and are usually not visible, instruments that measure the periodic rise and fall of surfaces of constant temperature and salinity record the passing of internal waves. Because they cannot be observed easily and are not common surface waves. The speed of the waves depends on the differences in desntiy between the two fluids. Surface wave can be up to 20m high, whie internal waves can reach a height of 300m or more dependent on the thickness of the upperwater layer. Internal waves are of importance in the context of vertical mixing process, internal waves may transport plankton and fish larvae to other areas.