March 7, 2002
UW project measures ‘internal tide’ in Hawaii
Along the 1,600-mile-long Hawaiian Ridge, the moon’s inexorable pull is creating waves that break in the hidden depths of the ocean just as the surf does on the world-famous beaches of Hawaii, Oahu and Kauai. The energy from these deep waves may be helping stir ocean waters, even those quite distant from the ridge.
The first-ever direct measurements of the energy flux of the “internal” tide along the Hawaiian Ridge were reported last month by UW researchers at the American Geophysical Union and American Society of Limnology’s Ocean Sciences meeting.
With waves — some 300 to 1,000 feet tall — traveling beneath the surface, internal tides at the Hawaiian Ridge and other such spots around the world may help scientists discover what causes 90 percent of the mixing in the world’s ocean, say Tom Sanford and Eric Kunze with the UW’s Applied Physics Laboratory. The two professors of oceanography are among the lead researchers for the National Science Foundation’s $16 million Hawaii Ocean Mixing Experiment, or HOME.
The mixing of warm surface waters and cold deep waters in what’s called the thermocline is an important component in helping drive global ocean circulation and force nutrients up from the deep, where they can be used by tiny plants at the sea surface that are at the base of the ocean’s food web.
Scientists have found that mixing in the thermocline far from land is weak and can account for only about 10 percent of the mixing that must be occurring, Sanford says.
It was less than a decade ago that Sanford, Kunze and other scientists began to hypothesize that the energy causing this mixing may be generated in places where surface tides draw deeper waters around and across rough seafloor features. The Hawaiian Ridge has an abundance of such “rough topography” with its numerous islands and an underwater range of craggy seamounts, shoals, banks and channels.
In contrast to most continental coasts where surface tides flow along — not across — seafloor features, tides traveling toward the Hawaiian Ridge from the northeast collide almost directly into the chain, scattering off some places and passing over and through others. Where the seafloor is roughest, the mixing rates are 1,000 times more intense than places without such topography, Sanford says.
The resulting internal waves, indiscernible at the surface, travel in both directions away from the ridge. The “energy flux” of such internal waves was measured directly for the first time during fieldwork led by UW scientists. Working at 14 sites across 430 miles of ridge, this initial attempt at measuring the flux from top to bottom included stations where the internal waves were expected to be strong and others where they were expected to be weak.
The UW scientists determined the direction of the waves and their strength, or magnitude, in terms of kilowatts per meter of ridge length.
It appears that internal waves generated at the Hawaiian Ridge probably have an average magnitude of about 5 to 10 kilowatts per meter along the entire ridge, Kunze says. That’s a few percent of the considerable energy — 100 kilowatts per meter — coming onto the ridge from the surface tides.
Some places turned out to be hotspots, Kunze says. Water traveling over the submerged Kaena Ridge off western Oahu generated internal waves with magnitudes of more than 40 kilowatts per meter. The most dramatic internal waves, measured at 60 kilowatts per meter, were at the French Frigate Shoals 400 miles northwest of the Hawaiian islands.
Just how far the internal waves persist, how much mixing they may ultimately cause in the thermocline and actually what’s happening on the ridge that causes them in the first place are only a few of the questions now facing researchers, Kunze says.
Kunze, Sanford and their co-authors on the presentation, Craig Lee and Jonathan Nash of the UW’s Applied Physics Laboratory, say measuring the flux from the top to the bottom of the water column is important.
“You miss a significant portion if you don’t measure it all,” Sanford says.
The measurements were made using absolute velocity profilers, developed at the UW 20 years ago, and researchers said modeling work by University of Hawaii’s Mark Merrifield and University of New South Wales’ Peter Holloway was key to deciding where to look for waves that represented both strong and weak fluxes.
Earlier, in 1997, Sanford and Kunze conducted a pioneering study with NSF funding that verified that internal tides were being created where surface tides crossed rough topography — in this case the 10,000-foot-tall Mendocino Escarpment off California.
The Hawaii Ocean Mixing Experiment is led by Scripps Institution of Oceanography’s Rob Pinkel, with principal investigators from the UW, Scripps, Oregon State University, Woods Hole Oceanographic Institution and University of Hawaii.