ABSTRACT
Ocean waves move due to gravity: the water at crests is heavier than the air surrounding it and so it falls, overshoots its equilibrium position, and then at a trough feels an upward restoring force. ›e same phenomena occur below the ocean surface. Cold water li¢ed upward into warmer surroundings will feel a downward buoyancy force and downward-displaced warm water will feel an upward buoyancy force. If this motion is periodic in both space and time, it is referred to as an internal gravity wave. Internal gravity waves are manifest in two qualitatively di§erent forms, which we will refer to here as “interfacial waves” and “internal waves.” (›e dynamics of internal waves are broadly discussed in the textbook “Internal Gravity Waves” (Sutherland 2010).)
Interfacial waves exist at the interface between dense and less dense žuid such as the thermocline, which refers to the interface between warm and cold water, or an atmospheric inversion, which refers to the interface between warm and cold air. In the ocean and in laboratory experiments, they can also exist at a halocline, which is the interface between fresh and salty water. An example of such a wave is shown in Figure 28.1a, in which a subsurface disturbance has launched a wave beneath a nearly žat surface. Looking down on the surface, the presence of these waves is evident from žuctuating horizontal žows and, on large scales, changing surface roughness.
