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NSF PR 03-74 - July 17, 2003
Hawaii Ocean Mixing Experiment Scientists Close in on Puzzle of Ocean Energy
Wide range of instruments, equipment document giant 1,000-foot undersea waves
Scientists are closing the gap in deciphering one of the most puzzling aspects of the world's oceans, ocean mixing. These complex motions of seawater that span large-scale phenomena down to tiny, centimeter-sized turbulence serve a key role in redistributing heat throughout the oceans. But a mystery remains in accounting for how these mixing processes unfold.
In a research paper published in the July 18 issue of Science, scientists representing six institutions, including Scripps Institution of Oceanography at the University of California, San Diego, describe ocean mixing in unprecedented detail, capturing intriguing phenomena such as undersea waves that spanned nearly 1,000 feet. Using an array of technologies and instruments, scientists in the Hawaii Ocean-Mixing Experiment (HOME), an $18 million National Science Foundation-sponsored project, focused on pinpointing, dissecting and analyzing ocean mixing.
The paper in Science is the first effort by HOME investigators to collectively document their findings.
The researchers chose to study the Hawaiian Ridge, a 1,600-mile largely submerged
volcanic mountain chain that stretches from the Big Island of Hawaii to Midway Island, due to its rough topography, including large underwater mountains and valleys. Such areas are sometimes referred to as the "stirring rods" of the oceans. Prior to the HOME project, areas like the Hawaiian Ridge were thought to be a major energy pathway for ocean mixing turbulence.
"Understanding where, when and how turbulent mixing takes place in the ocean is one of the most outstanding problems in oceanography," said Eric Itsweire, director of NSF's physical oceanography program, which funded the research. "Mixing processes occurring at centimeter scales, in large part, control the degree to which the ocean is stratified. Spatial differences in stratification, in turn, drive its large-scale circulation and affect climate on decadal to centennial time scales. Places like the Hawaiian ridge are thought to be hot spots for ocean mixing. HOME was designed to understand how energy from tides is dissipated in the abyssal ocean through complex interactions with the rough bottom topography associated with mid-ocean ridges."
Traveling across the Pacific, oceanic tides crash upon the Hawaiian Ridge and dissipate. To help explain how such areas help mix warm, low latitude waters and cool polar waters, HOME investigators undertook a comprehensive survey to track ocean energy and turbulence.
"One of the triumphs of the HOME experiment was being able to measure the cascade from thousands of meters down to centimeter scales," said Dan Rudnick, an oceanographer at Scripps and lead author of the Science paper. "I don‚t think this effort is rivaled in terms of measuring detailed dissipation over a topographic feature."
Using the research vessel Roger Revelle, the towed instrument SeaSoar, which took a variety of measurements of upper ocean properties, a new Doppler sonar developed by Scripps scientist Robert Pinkel, and a variety of other instruments and equipment, the scientists found that the Hawaiian Ridge is indeed a site with vastly increased ocean mixing. They documented undersea internal wave energy that was enhanced 10 times at the Hawaiian Ridge as compared with normal open ocean areas.
With the details of the cascading processes described in the Science paper, the
co-authors helped further close the gap of how energy is dissipated in ocean mixing. But the paper notes that the energy puzzle is not completely solved with these results. Even more energy for ocean mixing must be found elsewhere.
"Our conclusion is interesting because we found that there was certainly a lot of energy loss occurring at the Hawaiian Ridge, but much of it propagates away and doesn't get dissipated at the ridge. So we're approaching closure of this phenomenon," said Rudnick. "But until we have a firmer understanding of this process, until we get a better handle on mixing, climate models will be of limited use."
Planning for the HOME project began in 1996. The final field phases of the project were concluded last month. Scientific analysis of the data set is planned through 2005.
Scripps Media Contacts: Mario Aguilera or Cindy Clark, (858) 534-3624, firstname.lastname@example.org
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