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Getting the drift of ocean circulation.

Sopping up greenhouse gases and heat from the atmosphere, the world's oceans function as a climatic brake, slowing down the pace of global warming. Just how well this brake works depends on how quickly the oceans stir themselves, a process that carries away materials dissolved at the surface and stores them in deeper water. Results of an experiment in the North Atlantic give oceanographers their first direct measurement of how quickly ocean water mixes in the vertical direction.

To gauge the rate of stirring, an international team pumped 139 kilograms of an inert "tracer" called sulfur hexafluoride into the ocean last May and then tracked its dispersal. The oceanographers released the liquid at a depth of 310 meters, 1,200 kilometers west of the Canary Islands.

The experiment's results confirm suspicions that vertical mixing occurs exceptionally slowly, report James R. Ledwell of the Woods Hole (Mass.) Oceanographic Institution and Andrew J. Watson and Clifford S. Law of the Plymouth (England) Marine Laboratory. They discuss their work in the Aug. 19 NATURE.

Immediately after a first ship injected the tracer, a second ship measured the tracer's concentration in the water and found that the compound formed horizontal streaks with a vertical width of 7 meters. When they returned to the region six months later, the oceanographers located some of the plumes, which had spread very little vertically, measuring only about 18 meters from top to bottom. A year after the initial release, the height of the plumes had reached roughly 30 meters, says Watson.

The tracer spread so slowly in the vertical dimension because water density increases with depth, inhibiting mixing of different layers. Instead of crossing these density surfaces, water moves far more readily along the plane of a single layer. Such density surfaces are inclined rather than horizontal because of the forces induced by Earth's spin. Water can therefore rise and sink by moving along one of the sloping density layers.

The findings of the tracer experiment show how water moves in the upper kilometer of the ocean, which not only affects greenhouse gases but also controls the movement of nutrients. Researchers are now considering conducting a similar experiment for the deep ocean, at depths of 3,000 to 4,000 meters.

Chris Garrett, a physical oceanographer at the University of Victoria in British Columbia, says the results of the recent experiment will attract attention because they provide solid proof that water does not flow readily across density layers. Less direct tests in the last few years have suggested similar conclusions, but they did not resolve the issue. "What the [tracer] experiment does very nicely is it allows us to uncross our fingers," says Garrett.

The tracer release operation is only one part of a seven-year international effort called the World Ocean Circulation Experiment, which started in 1990.
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Title Annotation:rate of vertical mixing studied
Author:Monastersky, Richard
Publication:Science News
Article Type:Brief Article
Date:Aug 21, 1993
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