Their effect on the surface of the ocean is negligible, producing a rise of just inches that is virtually imperceptible on a turbulent sea. But internal waves, which are hidden entirely within the ocean, can tower hundreds of feet, with profound effects on the Earth's climate and on ocean ecosystems.
Enlarge / Surprisingly, internal waves can sometimes be seen clearly in satellite imagery (like in the above image of the Luzon Strait). This is because the internal waves create alternating rough and smooth regions of the ocean that align with the crest of the internal wave. Sunlight reflects the smooth sections, appearing as white arcs, while the rough sections stay dark.
Now new research, both in the ocean and in the largest-ever laboratory experiments to investigate internal waves, has solved a longstanding mystery about exactly how the largest known internal waves, in the South China Sea, are produced. The new findings come from a team effort involving MIT and several other institutions, and coordinated by the Office of Naval Research (ONR).
Seen in cross-section, these waves resemble surface waves in shape. The only difference between an underwater wave and the water around it is its density, due to temperature or salinity differences that cause ocean water to become stratified.
Though invisible to the eye, the boundary between colder, saltier water below and warmer, less-salty water above can be detected instrumentally. That boundary layer can resemble the ocean's surface, producing waves that reach towering heights, travel vast distances, and can play a key role in the mixing of ocean waters, helping drive warm surface waters downward and drawing heat from the atmosphere.
Because these internal waves are hard to detect, it is often a challenge to study them directly in the ocean. But now Thomas Peacock, an associate professor of mechanical engineering at MIT, has teamed with researchers from the Ecole Centrale de Lyon, the Ecole Normale Superieure de Lyon, and the University of Grenoble Alpes, all in France, as well as the Woods Hole Oceanographic Institution, to carry out the largest laboratory experiment ever to study such waves. Their results have been published in the journal Geophysical Research Letters.
The team performed laboratory experiments to study the production of internal waves in the Luzon Strait, between Taiwan and the Philippines. "These are the most powerful internal waves discovered thus far in the ocean," Peacock says. "These are skyscraper-scale waves."
This animation shows density layers in the South China Sea being perturbed by the regular back-and-forth tidal flow through the Luzon Strait. This leads to large amplitude internal waves (shown in red underwater, and in white when seen from above), being radiated west to the Chinese continental shelf:
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