Wednesday, February 13, 2002

Scientists find big
undersea waves

By Helen Altonn

Waves 300 to 1,000 feet high are traveling beneath the ocean surface along the Hawaiian Ridge and breaking like surf on beaches, University of Washington researchers have found.

They say energy from "internal" waves along the 1,600-mile-long ridge may affect distant ocean waters.

The scientists were scheduled to discuss "Oceanic Internal Tides" today at the 2002 Ocean Sciences Meeting sponsored by the American Society of Limnology and Oceanography and American Geophysical Union. More than 2,000 scientists worldwide are at the Hawaii Convention Center discussing new technology and research findings on every aspect of the ocean.

Oceanographers Tom Sanford and Eric Kunze, with the University of Washington's Applied Physics Laboratory, are among scientists making presentations today. They are among lead researchers for a $16 million National Science Foundation Hawaii Ocean Mixing Experiment.

They made the first direct measurements of the internal waves at 14 sites along 430 miles of the Hawaiian Ridge, determining the waves' direction and strength or magnitude. They hope to learn what causes 90 percent of the mixing of warm surface waters and cold deep waters. The mixing, in a subsurface layer known as the thermocline, is a key factor in global ocean circulation and forcing nutrients up from the deep as food for tiny surface plants, the scientists said.

In a paper on their research, Sanford said the team found that mixing in the thermocline far from land is weak and can account for only about 10 percent of the mixing that must be occurring.

Sanford, Kunze and other scientists began to theorize about 10 years ago that the energy causing the mixing may be generated in places where surface tides draw deeper waters around and across rough seafloor features.

The Hawaiian Ridge has a lot of "rough topography" with its islands and underwater seamounts, shoals, banks and channels, they noted.

Unlike most continental coasts where surface tides flow along seafloor features, rather than across, tides traveling toward the Hawaiian Ridge from the northeast collide almost directly into the chain, the scientists said.

Mixing rates are 1,000 times more intense where the seafloor is roughest, Sanford said.

Questions still facing the researchers are how far the internal waves persist, how much mixing they may cause in the thermocline, and what is actually happening on the ridge to cause them.

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