for National Geographic News
Imagine a cup of black coffee. Add a spot of cream. Watch. The white glob swirls, tendrils stretch to the rim and get thinner. Black and white meld to milky brown but black patches remain, white tendrils linger. Grab a spoon. Stir. The patches and tendrils disappear. Sip. Enjoy.
The interactions of motion that mix the coffee and cream are difficult to understand, says Dan Rudnick, an oceanographer at the Scripps Institution of Oceanography at the University of California in San Diego. His task is even more difficult. He and his colleagues are on a quest to understand how mixing happens in the oceans.
"Even in the cup of coffee it is extremely complex," he said. "Now imagine it over the global oceans. It's just a very complex process."
Rudnick and his colleagues from six institutions around the world want to understand how ocean mixing works so that they can understand things such as how nutrients rise from the ocean floor to feed plants and animals on the surface and how hot and cold water mix together and move around to drive the global climate.
To investigate the process, Rudnick and his colleagues have spent the last several years probing the waters along the Hawaiian Ridge, a 1,600-mile (2,600-kilometer) long chain of largely submerged volcanic mountains that stretches from the Big Island of Hawaii to Midway Island.
Due to the rough topography of the mountains and valleys, the ridge is thought to be like the spoon that enhances the mixing process in a cup of coffee. The tides broadside the ridge from the northeast, causing water in some places to scatter back as waves, to funnel through valleys in others, and generally swirl around in turbulent pools.
Back in their laboratories, the researchers have analyzed their data from the first field session along the Hawaiian Ridge and concluded that dissipation of tidal energy over rough seafloor features may indeed play a role in the mixing that keeps nutrients in flux and hot and cold waters gurgling together and driving the global climate.
A report on their research findings from the Hawaii Ocean-Mixing Experiment, a U.S. 18 million dollar project sponsored by the National Science Foundation, appeared recently in the journal, Science.
Cascade From Tides to Turbulence
Rudnick and his colleagues started the experiment with the hypothesis that ocean ridges such as the Hawaiian Ridge are places of enhanced ocean mixing driven by tidal energy.
They used a suite of instruments, including satellites in space, computer models, and a slew of gadgets towed behind a research vessel that they cruised in along the Hawaiian Ridge to test their hypothesis.
The team found that the Hawaiian Ridge is indeed a site with vastly increased ocean mixing. They tracked the cascade of tidal energy from the creation of huge internal waves, some as large as 1,000 feet (300 meters), down to small-scale localized mixing of water. All told, they believe they are on the way to understanding the dissipation of most of the tidal energy.
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