Tuesday, August 19, 2003

Undersea Hawaii ridge
may play role in climate

Scientists are studying how it
helps distribute heat from the tropics

University of Hawaii scientists are participating in an $18 million, seven-year research project to try to better understand the ocean and its effect on climate.

The Hawaii Ocean-Mixing Experiment focuses on the Hawaiian Ridge, a 1,600-mile underwater volcanic mountain chain that stretches from the Big Island to Midway.

"The big picture is one of the structure of the ocean -- what maintains the thermal (temperature) and salinity structure of the deep ocean," said UH oceanographer Douglas Luther. This is important because global ocean circulation, driven primarily by winds, involves movement of warm water up from the tropics to high latitudes, where it is cooled and sinks to the bottom, he said.

Ocean mixing has a lot to do with how heat is distributed in the ocean, said Mark Merrifield, another UH oceanographer working on the project. "It is a big issue for climate. How far is it mixed down in the ocean?"

The ocean and atmospheric systems are linked with changes in sea surface temperatures affecting dispersal of nutrients for sea life and causing such events as El Nino and La Nina, which trigger storms or dry weather.

Estimates of how the heat is redistributed are needed for models of the atmosphere to predict climatic changes, Merrifield said.

Luther said the HOME project is testing a theory that sea water mixing does not occur over the entire ocean, but at boundaries, in places like islands, midocean ridges or seamounts.

"Wherever some kind of topography is sticking up through ocean layers, it acts as a catalyst for ocean processes which mix the ocean," he said.

Mixing or stirring of sea water by tides causes processes ranging from large-scale internal waves to tiny turbulent motions, the scientists said.

Undersea internal wave energy at the Hawaiian Ridge was 10 times greater than in normal open ocean areas, they found.

Luther said currents from the surface to the ocean bottom are not strong in the open ocean, but when they crash into the Hawaiian Ridge, they concentrate into the channels, speed up and go over the ridge.

"If we get very strong currents, they produce instabilities in the water column which will generate mixing," he said.

The scientists are trying to close gaps in their knowledge of how much energy goes into tides, boundary layers and mixing, and how much escapes, Luther said.

About 25 investigators at six institutions are participating in the experiment, including UH oceanographers Eric Firing and Pierre Flament, as well as Luther and Merrifield.

The team used the Scripps Institution of Oceanography's research vessel Roger Revelle, a towed instrument called SeaSoar for upper ocean measurements, and a new Doppler sonar developed by Scripps professor Robert Pinkel.

Reporting their findings in the July 18 issue of the journal Science, Scripps oceanographer and lead author Dan Rudnick said one of the "triumphs" was being able to measure a cascade of ocean energy and turbulence from thousands of feet down to a few inches.

After a survey in 2000, the HOME investigators moored instruments up and down the Hawaiian Ridge to find the best location to study the energy conversion process, Luther said.

Kauai Channel was identified as a strong generator of mixing, and Luther, Merrifield and Murray Levine, of Oregon State University, were principal investigators for two other experiments there.

They moored instruments to the ocean bottom last summer to measure temperature, salinity and currents, and picked them up this summer, Luther said. The researchers are doing intensive data analysis and will meet the end of this month in Oregon for "show and tell," he said.

The other institutions include the Scripps Institution of Oceanography, Oregon State University, the Pacific Islands Fisheries Science Center, University of Washington and University of New South Wales, Australia.

They are about halfway through the project, funded by the National Science Foundation. Luther estimated the Hawaii scientists got $2.3 million.


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