Internal gravity waves are waves that occur in density stratified fluids in the presence of a gravitational field. They arise as gravitational restoring forces act on vertically displaced fluid.
Internal gravity waves are all around us, but difficult to see. They are ubiquitous in stratified lakes, in the ocean, and in the atmosphere. They are easily seen in a glass jar containing cooking oil and water. Internal waves are the waves you see on the interface. Better yet, try a layer of fresh water over salty water dyed with food colouring.
This animation shows how internal waves can be generated in a lake. Daytime winds drag warm surface water down wind forming a tilted epilimnion, or thermocline (the thin layer across which the water temperature changes). When the winds die down, the thermocline is released. The rocking basin scale standing wave, or seiche, evolves into a complex wave field, including trains of nonlinear solitary waves.
When a single solitary wave shoals along a sloping bottom it breaks, dissipating energy, mixing fluid and resuspending bottom sediments. This has important implications for biological and chemical processes.
Internal Waves in the ocean and lakes lie beneath the surface. Their associated currents can modify the surface wave field, creating slicks on the surface which are visible from land or air. Here is a photo of internal wave slicks taken from the space shuttle.
Large amplitude internal waves in the ocean are usually generated by tidal flow over topographic features such as a bank edge.
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