By Vladimir Brezina
As kayakers, we are intimately familiar with waves on the surface of the water. But waves produced by the same basic physical mechanism—gravity waves—can form anywhere where a perturbation sets off oscillations in a density-stratified fluid. The surface of the water—an interface between two fluids of different densities, water and air—is just the most familiar location. But essentially similar waves, albeit now internal rather than surface waves, can form deep below in the water, and high above in the air.
In the water
Different water layers may have different densities because of differences in temperature or salinity. At the boundaries between the layers—themoclines and haloclines, respectively—internal waves can form and propagate along the boundary. If there is no sharp boundary and the water density varies continuously, internal waves can still form that, in that case, can propagate in three dimensions.
These internal waves have many of the properties of the familiar surface waves: they can, for instance, break just like waves on a beach. But because of the smaller density differences involved, the internal waves can have much larger amplitudes than surface waves (in deep enough water, amplitudes of a hundred meters or more) and much longer wavelengths. Here are two satellite images of internal ocean waves with wavelengths of kilometers:
Typically, the internal waves are not felt as waves—as changes in water level—at the surface at all. So how are they visualized in these images? According to NASA’s Earth Observatory,
[a]s internal waves move through the lower layer of the ocean, the lighter water above flows down the crests and sinks into the troughs. This motion bunches surface water over the troughs and stretches it over the crests, creating alternating lines of calm water at the crests and rough water at the troughs.
It is the pattern of calm and rough water that makes the internal wave visible in satellite images. Calm, smooth waters reflect more light directly back to the satellite, resulting in a bright, pale stripe along the length of the internal wave. The rough waters in the trough scatter light in all directions, forming a dark line.
Fram [Nansen's ship] appeared to be held back, as if by some mysterious force, and she did not always answer the helm. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose.
This happens when the progress of the ship is impeded by subsurface waves—while the surface may be completely calm—that in some cases may even be generated by the movement of the ship itself, as in this simulation (description here):
In the air
Similarly in the air. An initial perturbation such as uplift of an air stream over a mountain range can lead to downstream oscillations where the air repeatedly rises (and cools) and falls (and warms). If the moisture content is right, periodic standing wave clouds form at the crest of each oscillation. Here are a few spectacular examples:
As these examples show, these atmospheric internal waves exhibit classic wave properties such as refraction and interference. And when the waves collide or break, clear-air turbulence may strike those of us flying by…
Everything still seems well-behaved, though, in the following aerial views:
In sum, as NASA’s Earth Observatory remarks, “If air were visible, it would be a thing of mesmerizing beauty and motion. Air courses in streams and eddies; it rises and falls and flows.” The same goes for the ocean waters. Those motions are there whether we see them or not. And sometimes, indeed, the circumstances are just right to reveal to us, suddenly, this beauty and this motion.