Hawaii Ridge Helps "Stir" Ocean Waters, Study Says

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"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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