Hawaii Ridge Helps "Stir" Ocean Waters, Study Says

John Roach
for National Geographic News
August 18, 2003
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.

"A big part, say 80 to 90 percent, of the energy appears to be radiated away [as internal waves] and about 10 to 20 percent is lost to turbulence especially over rough topography between the Hawaiian Islands," said Eric Kunze, an oceanographer and team member at the University of Washington in Seattle.

One of the questions raised by the observations is where and when does the energy that propagates away from the ridge ultimately dissipate, said Rudnick.

"It looks like a great deal of energy just propagates away," he said. "But the dissipation locally is significant; it is about ten times as big as you find the open ocean."

Chris Garrett, an ocean scientist at the University of Victoria in British Columbia, Canada, who is an expert on ocean mixing but not a member of Rudnick's team, said he was somewhat surprised at the amount of energy lost locally.

"The preliminary results indicate more energy is lost locally than one might have expected, that is more local mixing than simple [computer] models and concepts might have suggested," he said.

Computer Models

Giving a computer model the parameters to accurately predict how ocean mixing occurs, and thus how nutrients cycle in the ocean and hot and cold masses of water combine and move to drive global climate systems, is one of the ultimate goals of Rudnick and colleagues. Rudnick says there is still much work to be done.

For example, in the ocean, water is heated on the surface near the Equator and cooled at the surface near the Poles. Since cold water is denser than warm water, it sinks to the bottom. If it weren't for ocean mixing the ocean would be divided into two layers with warm water on top and cold water on the bottom, said Rudnick.

Because of ocean mixing, however, there is not a sharp boundary between the warm and cold waters. Oceanographers like Rudnick and his colleagues are trying to figure out where that mixing is taking place. The one thing they know now is that it is not all happening in Hawaii.

"When we look at local mixing at the Hawaiian Ridge, while significant and big, it is not big enough to account for the large-scale structure [of the oceans]," he said. "There is a lot of mechanical energy out there, but local mixing is not the whole story."

Further studies will look at other parts of the oceans that may account for more of the mixing, as well as other sources of energy, such as the wind. Ultimately, the researchers hope to create a computer model that accounts for all the ocean mixing.

"If we can improve the mixing parameters, that could lead to better climate models and better prediction of long-term climate," said Kunze, who added that he has little confidence in the current long-term climate models that are based on a limited understanding of ocean mixing.

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