"Oceanographers used to talk about the so-called 'dark mixing' problem, where they knew that there should be a certain amount of turbulence in the deep ocean, and yet every time they made a measurement they observed a tenth of that," Alford said. "We found there's loads and loads of turbulence in the Samoan Passage, and detailed measurements show it's due to breaking waves."
It turns out layers of water flowing over two consecutive ridges form a lee wave, like those in air that passes over mountains. These waves become unstable and turbulent, and break. Thus the deepest water, the densest in the world, mixes with upper layers and disappears.
This mixing helps explain why dense, cold water doesn't permanently pool at the bottom of the ocean and instead rises as part of a global conveyor-belt circulation pattern.
The Samoan Passage is important because it mixes so much water, but similar processes happen in other places, Alford said. Better knowledge of deep-ocean mixing could help simulate global currents and place instruments to track any changes.
On a lighter note: Could an intrepid surfer ride these killer deep-sea waves?
"It would be really boring," admitted Alford, who is a surfer. "The waves can take an hour to break, and I think most surfers are not going to wait that long for one wave."
In fact, even making the measurements was painstaking work. Instruments took 1.5 hours to lower to the seafloor, and the ship traveled at only a half knot, slower than a person walking, during the 30-hour casts. New technology let the scientists measure turbulence directly and make measurements from instruments lowered more than 3 miles off the side of the ship.
The researchers left instruments recording long-term measurements. The team will do a
|Contact: Hannah Hickey|
University of Washington