How internal waves in the ocean can be spotted on the surface.
Under certain conditions, internal waves in the ocean can be spotted at the ocean’s surface due to changes in surface roughness or to the movement of floating foam or debris. They can be spotted if half their wavelength is longer than the distance between the interface on which the internal wave is traveling and the water surface, so that the orbital movement caused by the internal waves reaches the water surface. In the tank, they can also be seen – for example by adding small floating particles to the water surface.
In the movie below, you can see the interface between water layers of different densities and the water surface with particles on it. The particles make it easy to spot how the water surface is being stretched and squeezed as internal waves travel through underneath.
For more information on internal waves, check out these posts [which are scheduled to go online over the next couple of days]: