Internal Waves

An internal wave is a wave that develops below the surface of a fluid, along with changes in density. With increased depth, this change may be gradual, or it may occur abruptly at the interface. Similar to the transmission of energy by wind along the surface of the ocean, the density interface beneath the ocean surface transmits energy to produce internal waves. The greater the difference in density between the two fluids, the faster the wave will move.

There are a variety of causes for internal waves, including the tidal pull of water, wind stress, and energy put into the water by moving vessels. Year-round internal waves caused by tidal forces carry between 30 and 50 percent of their energy away from their source. Seasonal (stormy winter months) internal waves caused by wind and storms carry at least 15 to 20 percent of the energy input from their source.

Internal waves reach greater highs (above 100 m, or 328 ft.) from a smaller energy input than do the waves resulting from large energy input at the ocean’s surface. This is because they move along interfaces with less density difference than between the ocean surface and the atmosphere. 

Eventually, internal waves run out of energy and break, similar to surface waves. When they break in the deep ocean, they create turbulence. Where they create this turbulence, heat can be transferred from the upper ocean and stored in the deep ocean, with the exception of the Arctic Ocean, which is warmer in deeper water than on the surface. In the Arctic, turbulence transfers heat from the deep ocean to the surface.

Beneath the surface of the ocean exists a unique weather and climate resulting from the fluctuating currents driving wind in the atmosphere and from deep waves with similar patterns to those on the surface of the ocean—but unable to be seen from the surface—created by ocean movement

caused by the tides. Internal tide is an internal wave created from the back-and-forth flows of water over geographic features at lower depths. These internal waves radiate away in the form of tide current, carrying energy away from the source.

Internal waves are of special interest to climate modeling researchers because heat transfer is one of the roles of the ocean in regulating and changing climate. For example, the amount of heat transferred from the deep ocean in the Arctic will affect the amount of floating sea ice above. In the same way that heat from the sun at the equator is transferred to the atmosphere, the upwelling of water enhances the global ocean circulation; otherwise, the depths of the ocean would be much colder and retain nutrients deeper down and away from supporting life or the ability for continuance of the carbon dioxide cycle absorption and release.

Internal waves mix and redistribute heat, salt, and nutrients in the oceans; mixing is accomplished more easily in water having uniform density. Most of this mixing occurs where internal waves break, overturning the density stratification of the ocean and creating patches of turbulence. Scientists have observed that the internal wave rates of dissipation and the redistribution of heat and nutrients is 10 percent less near the equator than at midlatitudes. This ocean dynamic would have to be accounted for in climate models.

Researchers are studying internal waves, as determining where internal wave energy might be high and where it might be low will help researchers distinguish between fluctuations in the data record originating from ocean currents and fluctuations resulting from internal waves/tides. Thus, oceanographic instruments can be deployed in oceanographically interesting locations where scientists can quantify the vertical redistribution of heat or assess the potential contribution to climate change and variability.