Waves Above and Below the Water

Waves Above and Below the Water

Two kinds of waves are visible in the image above, yet neither is the kind you are probably familiar with.

At 11:05 a.m. local time (03:05 Universal Time) on February 10, 2016, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this natural-color image of wave patterns off the coast of Western Australia. Well offshore to the north and west, atmospheric waves are made visible by parallel bands of white clouds. Closer to the coast, the bright area of water is sunglint—the reflection of sunlight directly back toward the satellite imager. That sunglint makes it possible to see the faint ripples of internal waves; that is, large waves that propagate below the water surface, within the depths of the sea.

Waves form in the atmosphere for a variety of reasons. Sometimes the movement of an air mass over a bumpy feature—a mountain ridge, a volcano, or an island amidst a flat sea—will force air to rise or sink, creating ripples in the sky like those propagating across the surface of a pond. Other times, the collision of different air masses will cause a rippling effect.

It is unclear what caused the atmospheric waves in the image above. Off the west coast of Africa, we often see waves form when the dry air from the Sahara moves out over the much moister air over the tropical Atlantic Ocean. The dry air tends to push the moist air higher in the atmosphere, causing water vapor to form droplets and amass into clouds. The moist air rises, then gravity pulls it back down; the warm air rises again, then falls again. A series of cloud ripples mark the edges of the wave front as it propagates and dissipates.

It is also possible—though perhaps less likely because of the distance—that the wave patterns in the image above have their origin inland. Western Australia is mostly desert and relatively flat, so it is possible that an atmospheric wave pattern formed when an air mass rode up over the Hamersley Range (just outside the scene) and out toward the sea.

Internal waves are quirky phenomena that were scarcely known to science until the satellite era. They can be hundreds of meters tall and tens to hundreds of kilometers long. Enhanced by sunglint in the image above, these long wave forms moving across the sea surface are a visible manifestation of slow waves moving tens to hundreds of meters beneath the sea surface.

Internal waves form because the ocean is layered. Deep water is cold, dense, and salty, while shallower water is relatively warmer, lighter, and fresher. The differences in density and salinity cause layers of the ocean to behave like different fluids. When tides, currents, and other large-scale effects of Earth’s rotation and gravity drag water masses over some seafloor formations, it creates wave actions within the sea that are similar to those happening in the atmosphere.

If you were on a boat, you would not necessarily see or feel internal waves because they are not expressed at the surface in different wave heights. Instead, they show up as smoother and rougher water surfaces that are visible from airplanes and satellites. As internal waves move through the deep ocean, the lighter water above flows up and down the crests and troughs. Surface water bunches up over the troughs and stretches over the crests, creating alternating lines of calm water at the crests and rough water at the troughs. 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.

“There are definitely ocean internal waves in this image,” said physical oceanographer Nicole Jones of The University of Western Australia. “We have measured them off the coast of Ningaloo with instruments in the water. The different directions of the wave fronts are most likely due to the different seafloor slope directions in this region.” She notes that internal waves play an important role in global ocean circulation and mixing, which is critical to understanding the ocean’s role in climate and in the movement of nutrients and carbon from the depths to the surface and back. Jones and colleagues also study internal waves for their potential impact on drill rigs and other offshore structures.

NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response. Caption by Mike Carlowicz and Holli Riebeek.

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