9.10: Internal Waves

Wind waves are oscillations that occur at the ocean surface where there is a sharp density discontinuity between water and the considerably less dense overlying atmosphere. Waves can be created at any density interface with a sharp density gradient where a fluid of low density overlies a fluid of higher density. Such sharp vertical density gradients, called pycnoclines, occur in the water column in many areas of the oceans (Chap. 8).

If a pycnocline is displaced up or down, the displacement will create internal waves that travel on the pycnocline. Relatively little is known about internal waves, but their principal causes appear to be tidal motions, the effects of topography on currents moving in ocean water mass layers, and shear stress created between water layers as they flow across one another. Occasionally, where the pycnocline is shallow, internal waves can be generated by propellers or bow waves of ships.

The wave height of internal waves can be great (up to 100 m) in comparison with that of surface waves because a given amount of energy causes waves of larger amplitude where the density difference across the wave interface is smaller. The density difference across the pycnocline is much smaller than that across the sea surface. Internal waves travel approximately one-eighth as fast as surface waves with similar period, and they typically have periods of 5 to 8 min and wavelengths of 0.6 to 1 km. They have much less energy than surface waves, but they can be very important to sea life, submarines, offshore oil platforms, and vertical mixing of ocean waters.

Internal waves behave just like surface waves as they enter shallow water and interact with the seafloor. They slow, their wavelength is reduced, and eventually their wave height increases until they break. Because of their long wavelength, internal waves generally break on the outer part of the continental shelf. In some locations, including the continental shelf offshore of New York and New Jersey, breaking internal waves may mix nutrient-rich, cold water from below the pycnocline into warmer, nutrient-poor surface waters. This mechanism may supply nitrogen, phosphorus, and other nutrients to phytoplankton and thereby support larger populations of fishes and other animals than would otherwise be present (Chaps. 12, 13). Internal waves may also move sediments up and down canyons.

Submarines must be wary of internal waves because such waves can substantially change the submarine’s depth in an uncontrollable and unpredictable way. Internal waves are speculated to have caused the sinking and loss of the nuclear-powered submarine USS Thresher, with its crew of 129, in 1963. Some types of drilling and oil production platforms can also be affected by internal waves. These are designed to withstand surface waves and are constructed on a tower resting on the seabed or on pontoons that float well below the depth of significant surface wave motion. In 1980, a series of internal waves rotated a production platform to face almost 90° from its original direction, although the platform did not topple.

Internal waves are generated not only on the main pycnocline but also at the interfaces between water mass layers and by interactions of the tide wave (Chapter 10) with seafloor topography. Thus, internal wave action occurs throughout the depths of the oceans and is a major contributor to vertical mixing that forms part of the MOC by returning deep-ocean water progressively to the surface layers.