• A satellite image of internal waves (with a wavelength of about 2 kilometers), which are generated by flow from the the Atlantic ocean to the Mediterranean Sea, at the east of Gibraltar and Ceuta.

    European Space Agency

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  • Photo of a Morning Glory cloud formation taken from a plane near Burketown, Australia.

    Mick Petroff/wikimedia

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  • Thomas Peacock, an associate professor of mechanical engineering, on a research trip earlier this year to study internal waves in the ocean.

    Courtesy of Thomas Peacock

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Hidden waves pack a big punch

Thomas Peacock sheds light on an odd but powerful phenomenon of sea and sky

The Air Canada plane from Victoria to Toronto was cruising last year in clear skies; neither the pilots' eyes nor the air traffic controllers' radar screens saw any bad weather. Then, abruptly, the plane plummeted 2,000 feet in 15 seconds, and then another 2,000, before the pilot was able to regain control and level off.

The terrified passengers and crew — 10 of whom suffered injuries that would require hospital treatment — had experienced a phenomenon called an internal wave, something that is relatively unknown to the general public, and which is beginning to yield its secrets to scientists under new observational and modeling techniques.

Internal waves, which are prevalent in the oceans as well as in the atmosphere, are hidden from obvious view most of the time. But these colossal phenomena — a single wave can span 1,000 kilometers or more — can have profound effects on Earth's climate as well as on drilling rigs, undersea cables and even vehicles such as submarines and, as those Air Canada passengers and crew can attest, to aircraft in flight.

Thomas Peacock, an associate professor of mechanical engineering at MIT, has been studying these waves for more than five years, and has uncovered important new details of how they form and propagate. His two latest papers on the subject are being published over the coming months in the Journal of Fluid Mechanics. One of these provides new insight into the forms of internal waves generated in the oceans, and the other helps explain the mystery of a narrow, focused beam of internal waves that recurs regularly twice a day - tied to the tides - in a channel in the Hawaiian Islands, but vanishes near the ocean surface.

Understanding these waves is important for models of climate change, because breaking internal waves in the ocean are believed to be a significant part of the mixing process by which warmer surface ocean water can be carried to the depths and colder water to the surface. This potentially makes them one of several important ocean mechanisms that impact the Earth's climate.

Ron Prinn, director of MIT's Center for the Science and Policy of Global Climate Change, says the mixing rate in the oceans — the rate at which warm surface waters get mixed with colder deep water and remove heat from the atmosphere — is one of the biggest remaining uncertainties in climate modeling, so understanding the mechanisms better could make a big difference in the accuracy of climate projections.

"It's one of the outstanding problems" in climate modeling, says Raffaele Ferrari, professor of physical oceanography in the Department of Earth, Atmospheric and Planetary Sciences. He notes that although researchers have made great progress in understanding how internal waves are produced, when it comes to figuring out how they break — that is, how their energy is dissipated — "we can probably account for 20 percent, but we can't account for the other 80 percent. It's a missing link." 

Since such waves can disrupt both moving airplanes and underwater vehicles, and stationary equipment such as undersea drilling rigs and communications cables, this research could also have important practical consequences - at some point perhaps yielding improved ways of predicting the times and locations where they may occur. Because of that, Peacock's work has drawn funding from the National Science Foundation, the MIT France Program and the Office of Naval Research.

The existence of internal waves has been known for more than a century, but exactly how they form and dissipate, and their effects on both natural and technological systems, are still being explored and are yielding new insights. Peacock and his students in the Experimental and Nonlinear Dynamics Lab (ENDLab) have made significant strides by coupling mathematical modeling of the behavior of these waves with laboratory experiments in wave tanks he has designed, and participation in field research at sea, in an effort to better understand not only how the waves form, but also how they then lose their energy. Bruce Sutherland, professor of physics at the University of Alberta, Canada, says that this combined approach is unique among climate scientists. "Such a holistic view has already benefited our understanding of climate," he says, "through his studies of wave generation by tidal flow over ridges, and by the examination of the life-cycle of these waves emanating from sills and seamounts."

The internal waves themselves are made up of moving regions of air or water that are more dense or less dense than their surroundings because of differences in temperature and, in the water, differences in salinity. In principle, they are similar to the familiar waves on the ocean's surface, but because they occur within the water their visible manifestations are subtle, or sometimes nonexistent.

In the ocean, these waves form when tidal currents pass over an obstacle such as a submerged ocean ridge. "Cold, heavy water from the bottom gets pushed up over the ridge, and sets up a disturbance," Peacock explains. They can also be generated by powerful storms, such as hurricanes, displacing the ocean surface. In the atmosphere, internal waves can be produced by thunderstorms and when air passes over a mountain range, in which case they are sometimes called "mountain waves." It was just such a mountain wave, in the lee of the Rockies, that caused last year's Air Canada plunge.

Besides his efforts to understand these waves, Peacock has been working to increase public understanding of these little-known yet widespread effects. To illustrate their power and immense scale, he plans to travel to Australia this fall to film a segment for a Discovery Channel program he is co-producing about internal waves; two segments in the South China Sea and off the West Coast of Australia have already been completed. He and the film crew hope to be able to catch an atmospheric internal wave that produces something called a Morning Glory cloud, in a location where they typically form at this time of year, and "surf" that wave in a glider. Because the long, narrow cylindrical cloud formation can span hundreds of miles, the popular Lonely Planet travel guide has described it as the most exciting natural phenomenon to observe in the sky next to a total eclipse.

"It's a challenge, to try to be there when it happens," he says. "But it will be a once-in-a-lifetime experience to surf the Morning Glory."

Topics: Environment and energy, Aeronautical and astronautical engineering, Environment, Earth and atmospheric sciences, Space, astronomy and planetary science, Oceanography and ocean engineering


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