Jan. 22, 2008 -- A 900-mile-long string of scientific instruments across a stretch of the open ocean has revealed the first evidence of giant internal waves partially "breaking" inside the oceans.
Tide-generated internal waves up to 300 feet tall are thought to mix shallow and deep waters when they break -- and so play a role in climate-critical ocean currents. Despite their size, they have been very hard to find in the act of fully breaking.
"We know where the waves are generated but we really don't know where they break," said Matthew Alford of the University of Washington, Seattle Applied Physics Laboratory and School of Oceanography.
Like the surface waves that travel at the interface of water and air -- which have very different densities -- internal waves propagate deep down in the oceans where denser, colder and saltier deep waters meet warmer, fresher and less dense upper waters.
To find out where they break, Alford and his team ventured to French Frigate Shoals northwest of the Hawaiian Islands, where the twice-a-day tidal sloshing of water across the undersea ridge churns out internal waves that roll northwards.
They installed a series of moorings in the three-mile-deep water, each equipped with a robot that chugged up and down the cable every few hours to collect data on water temperature, salinity as well as speed and direction of the water flow.
The moorings were placed along what ocean models had predicted was a fairly straight and likely path for internal waves created by tidal flow squeezing over the ridge at French Frigate Shoals. The team also used a radar-gun-like method to detect and observe the waves along the line of the moorings from aboard the research vessel.
"This is the first effort to look in the ocean for the theoretical breakdown in the internal wave," said internal wave researcher John Toole of Woods Hole Oceanographic Institution, who was not a member of the expedition.
Once the data were in, they revealed the internal waves moving along north. But instead of fully breaking, the data showed the waves only "sloshed over" a bit, said Alford, conserving most of their energy.
"The rest is just rocketing off to parts unknown," said Alford. Perhaps they break closer to the Aleutians or even on the Oregon coast, he said. Their findings were published in the current issue of the journal Geophysical Research Letters.
Discovering where the tidally-induced internal waves break is especially important for accurately tracking how oceans move heat and energy around the planet. It's thought that internal waves play a big role in allowing cold, deep waters to well up to the surface in lower latitudes.
Those upwellings are part of a global heat conveyor belt known as the thermo-haline circulation which carries warmer saltier waters like the Gulf Stream poleward to cool and sink. Then the waters move along the bottom of the oceans to lower latitudes where the waters then rise -- if there are internal waves in the right places to mix things up and help push the cold waters upward.
"The real key is not only how much turbulence, but where the turbulence is," said Alford. His team's work is the first step in that deep-sea search.
Related Links:
Larry O'Hanlon's blog: Earth Matters
University of Washington, Seattle, School of Oceanography
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