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Research > Science Highlights > Doing the Wave

Doing the Wave

Geologist Associate Scientist Tim Duda demonstrates with a wave tank what it is difficult to see in the ocean: how differences in fluid properties can lead to waves beneath the waves. (Photo by Tom Kelindinst)
Surfers and satellites make unlikely oceanographic assistants, but both are helping investigators from the Applied Ocean Physics and Engineering Department study the basic physics of ocean waves on scales varying from centimeters to hundreds of kilometers.

For more than a decade, Associate Scientist Britt Raubenheimer and Senior Scientist Steve Elgar have been working together to decipher the patterns and processes of the shore environment. Most of their work takes place in the breaking waves of the surf and swash zones, from “where the water barely covers your feet to where it just covers your head,” as Elgar says.

Along the U.S. coastline, from Truro, Massachusetts to Duck, North Carolina, they have fought the pounding surf to set up current meters, pressure gauges, and other sensors that measure the movement of currents, waves, and sand. Their work could help coastal policymakers and managers understand how the movement of water affects the evolution of coastlines, the safety of beachgoers, and the dispersal of runoff and pollutants.

In the fall of 2003, their work took them to Scripps Canyon near La Jolla, California, for a study of how deep submarine canyons can produce incredible waves, rip currents, and placid lulls all within just a few miles of Pacific shoreline. Working with fellow principal investigator Bob Guza of the Scripps Institution of Oceanography, Elgar and Raubenheimer led a team of 25 scientists, divers, and engineers—plus a few surfers and lifeguards who kept kelp and people off their instruments—in the Nearshore Canyon Experiment (NCEX).

Offshore canyons are thought to focus and channel the energy of ocean waves as they pile up along the continental margins. But until NCEX, very little real-life data had been collected to support the theories and models (most previous work had been done on simpler, smoother shelves).

Britt Raubenheimer (second from left), Steve Elgar (fourth from left), and their research team struggle to deploy current- and sediment-observing equipment near La Jolla, California. (Photo by Susan Green, SIO)

“As waves pass over the canyons,” Raubenheimer said, “the steep topography can act like a magnifying glass and concentrate ocean wave energy in hot spots where waves are large. Alongshore variation of waves and currents can result in rip currents.”

Working in arduous conditions—“We’ve never worked where the circulation is so crazy,” Elgar said—the team collected “a spectacular data set.” They also showed, in real time, the societal benefits of their research. When 130,000 gallons of sewage spilled from a local water system into the ocean, the NCEX team provided information on how the contaminants were dispersing along the coast.

In the deeper waters of the East China and South China seas, Associate Scientist Tim Duda examines waves up to 100 kilometers long, but so subtle that they are rarely visible at the surface. Duda studies a phenomenon called internal waves, which form along the intersection of waters of different density or temperature in the interior of the ocean (such as saltier water flowing beneath fresher water). These waves can pulse through a sea in cyclic frequencies known as internal tides.

At the extreme, internal waves can grow 150 meters tall, yet the average boater wouldn’t notice the passage of an internal wave unless trained to detect the telltale bands or slicks that sometimes form on the surface. Most often, these huge phantoms (and the enormous yet diffuse energies they carry) are detected only by satellites or innovative underwater instruments.

A few years ago, Duda and more than a dozen WHOI colleagues deployed temperature-sensing moorings to examine internal waves and tides as part of the Asian Seas International Acoustics Experiment. Duda and scientists from China and the United States found the internal waves and tides of the South China Sea to be much more potent than expected. The complex seafloor of the region seems unusually efficient at generating internal waves, so the team is now analyzing its new data to figure out where those waves are generated and where their energy goes.

For Duda, understanding the motions and patterns of internal waves could help improve our understanding of how water masses of different densities mix and drive ocean circulation. Energy is radiated throughout the oceans by internal waves, which randomly break down and cause mixing. (By comparison, when atmospheric internal waves mix, you usually spill your coffee as your airplane bounces in the turbulence.)

“If we can demonstrate in a few spots that we can model the tides and waves correctly,” Duda said, “then we may improve the fidelity of ocean and climate models by properly accounting for mixing.” Understanding internal waves has important implications for understanding acoustic communication in the ocean, sonar performance, and the biological productivity of certain regions.

óMike Carlowicz (mcarlowicz@whoi.edu)

Related Web Sites
Nearshore Canyon Experiment (NCEX)
Asian Sea International Acoustics Experiment (ASIAEx)
Drifting Deep Ocean Shearmeter
Internal Wave Online Atlas