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July 24, 2003

Internal waves generate great energy, researchers learn

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When internal waves up to 300 feet first form they cause a mighty churning of ocean waters — something invisible to and unfelt by anyone at the surface.

Now in a novel use of mooring data, some of it three decades old, a UW researcher has calculated just how much punch these waves appear to carry as they travel, or propagate, thousands of miles from where they originate.

It’s energy that appears to be crucial to the conveyor-belt-like circulation wherein millions of cubic meters of icy-cold water sink each second at high latitudes and are driven to upwell at lower latitudes. Without such upwelling, global ocean circulation would stall, causing the entire ocean to fill with cold water. Further, nutrients that have drifted down to the ocean depths would remain in the deep instead of being carried back to surface waters for use by plankton, the tiny plants and animals on which all other marine life depend and which greatly affect how much carbon dioxide is absorbed and released by the oceans.

Internal waves are just like waves one sees breaking on beaches except they occur completely below the surface. For the most energetic such waves, which get started either during stormy weather or when tides pull water across especially rough places on the seafloor, up to 50 percent of their original energy can be carried far away as the waves undulate through the ocean depths, according to Matthew Alford, an oceanographer with the UW’s Applied Physics Laboratory.

That’s important because climate modelers need to know all the key factors that affect ocean mixing that occurs at mid-latitudes. Previous measurements and calculations by Alford, and by National Aeronautics and Space Administration and Oregon State University scientists, show that internal waves have the 2 trillion watts that would account for such ocean mixing.

Alford provides the first glimpse of how much of that energy actually makes it very far from where the waves originate.

“The fluxes . . . are large enough to transport, across ocean basins, globally significant amounts of energy available for mixing,” Alford wrote in the journal Nature last month. “Global mapping of their fluxes is an important step in determining how and where internal-wave mixing is accomplished.”

Alford, an Office of Naval Research Young Investigator, combed historical data from more than 1,000 moorings — some used for projects as far back as 1973. Sixty had instruments at the right depths and collected data for long enough, at least a winter, for Alford’s purposes. Moorings provide fair coverage of ocean waters in the North Atlantic and Pacific but very poor coverage in low latitudes in the Southern Hemisphere, he says, something that could be considerably improved by deploying several well-placed moorings in the future.

He found that the kind of internal waves caused by tidal forces pulling water across underwater ridges, shoals, channels and other rough places on the seafloor, for example around the Hawaiian Islands, carry 30 percent to 50 percent of their energy directly away from their sources, a process that goes on year-round.

A second type of internal waves, one caused by sudden wind events and storms, appear to carry at least 15 percent to 20 percent of the energy input from where they originate, mainly during the stormy months of winter, and that energy is usually directed toward mid-latitudes areas. Alford says the focus in recent years has been on the importance of internal waves generated by internal tides, whereas his work suggests that internal waves generated by winds are just as significant.

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Hawaiian waves reach height of 1,000 feet

The cascade of energy — from tides to internal waves to turbulence — that results in ocean mixing is so strong around the Hawaiian Islands that the seafloor ridge there is one of the places considered to be a stirring rod of the ocean.

Traveling across the Pacific, oceanic tides crash upon the Hawaiian Ridge, a 1,600-mile largely submerged volcanic mountain chain that stretches from the Big Island of Hawaii to Midway Island. Large internal waves, some as tall as 1,000 feet, occur and the resulting turbulence is 10 times that of what’s typical in open ocean areas of the region.

The exchange of energy from tides to turbulence has been hypothesized to serve as a major energy pathway for ocean mixing, wrote researchers in the July 18 issue of Science, including the Applied Physics Laboratory’s Glenn Carter, Michael Gregg, Eric Kunze, Craig Lee and Thomas Sanford.

The energy “extracted” from the surface tide at the Hawaii Ridge – about 20 gigiwatts according to the just-published results – makes the ridge one of the most prominent areas for dissipating deep-ocean surface tidal energy in the world. As such, it’s been the subject of an $18 million National Science Foundation-sponsored project, the Hawaii Ocean-Mixing Experiment, that has involved some 20 UW oceanographers and engineers as well as researchers from five other institutions.