Scientists at Johns Hopkins and the University of California, Irvine have finally been able to field-test theories about how wind transfers energy to ocean waves, a topic of debate since the 19th century that had previously proved impossible to settle experimentally.
The new results may help lead the way to the resolution of a long-standing problem in scientists' understanding of how energy and momentum are exchanged between the atmosphere and the oceans.
Scientists have shown that interactions between wind and the surface of the ocean creates waves of varying sizes, from the waves that crash ashore at a beach to the waves that imperil ships at sea during storms. They also have demonstrated that waves can grow by extracting energy from the wind. However, when they tried to bring together models for atmospheric dynamics and ocean circulation, the ocean models suggested that the atmosphere should contribute much more energy to the ocean than the atmospheric models seem to suggest the atmosphere could possibly provide.
The new findings, reported in the March 6 issue of Nature, are focused on kinetic energy and momentum rather than thermal energy but could potentially help scientists improve climate modeling research by further refining their picture of energy flow between the atmosphere and the ocean. They also will help efforts to improve prediction of weather and wave activity.
"Until now, we've had lots of theories [on wind-to-wave energy transfer] but no experimental confirmation because it's been so hard to make the theory and the experiment talk to each other," says Tihomir Hristov, an associate research scientist in the Department of Earth and Planetary Sciences at Johns Hopkins and lead author on the new paper. "The patterns we see in the new data are very consistent with the theory originally proposed by John Miles in 1957."
Hristov notes that speculation about wind-wave energy transfer dates all the way back to the work of Lord Kelvin, a 19th-century British physicist, mathematician and inventor. Attempts to study the transfer in laboratory simulations of the ocean have met with only limited success, Hristov says.
"There's a parameter that's involved in the calculations for studying this transfer, the Reynolds number, that is different over the ocean than it is in the laboratory simulations," he explains.
Study of the energy transfer over the actual ocean also presented scientists with a problem: a completely uncontrollable mix of chaotic variables like wave height and speed, as well as wind turbulence.
Hristov and his colleagues at the University of California, Irvine, Scott Miller and Carl Friehe, have begun to tame this wild and wooly mix of variables by taking measurements from a device known as the Floating Instrument Platform, or FLIP. Owned by the U.S. Navy and operated by the Marine Physical Laboratory of the Scripps Institution of Oceanography, FLIP is a 355-foot-long platform that scientists can haul out to sea and turn vertical to create a stable ocean perch for taking measurements. (The platform's design gives it too much inertia for it to bob in the waves, Hristov explains.)
To avoid airflow distortions created by the platform itself, researchers monitored wind speed with instruments on a mast extending away from the platform. They simultaneously measured wave height underneath the mast. In 1998, they announced that these observations had enabled them to develop a method to separate random, turbulent air motion from airflow involved in transferring energy to waves.
The follow-up study that was published in Nature found a pattern of wind-wave interaction that matched "very consistently" with Miles' model over a period of five days, Hristov says.
Hristov remembers that Miles, a professor at the Scripps Institution and the University of California, San Diego, came to Johns Hopkins in 1998 to speak at a meeting in honor of Owen Philips, a professor of Earth and planetary sciences at Johns Hopkins who produced a competing wind-wave interaction model in 1957. At one of the sessions, Miles spoke of his wind-wave interaction theory.
"A central concept in his theory is the so-called 'critical layer,'" Hristov says. "Somebody from the audience asked him what he thought about the critical layer. And since at that time no experiments had suggested otherwise, he said, 'The critical layer is just a convenient mathematical notion.'
"Our data are now showing that the theory has been right all along," Hristov says, smiling. "The critical layer is observable, and it is essential in the energy transfer."
Hristov is currently preparing for a follow-up study that will be based on observations taken off the coast of Massachusetts.
This research was funded by the Office of Naval Research's Marine Meteorology section.