ÌìÃÀÓ°ÊÓ´«Ã½ oceanographers made the first detailed measurements at the headwater’s source, a submarine canyon offshore from the strait that separates the U.S. and Canada. Observations show water surging up through the canyon and mixing at surprisingly high rates, according to a published in March in .

“This is the headwaters of Puget Sound,” said co-author , a UW professor of oceanography. “That’s why it’s so salty in Puget Sound, that’s why the water is pretty clean and that’s why there’s high productivity in Puget Sound, because you’re constantly pulling in this deep water.”

It has been known for decades that 20 to 30 times more deep water flows into Puget Sound than from all the rivers combined. Surface tides, while dramatic, play a minor role.

“The tidal currents that slosh the water back and forth, that’s what’s really obvious,” MacCready said. “But there’s also a slow, persistent circulation that is constantly bringing deep water in, mixing it up and sending the surface water out.”

New measurements show this canyon potentially supplies most of the water coming into Puget Sound, the Strait of Juan de Fuca and Canada’s Georgia Strait.

The intense flow and mixing measured inside the canyon could help explain the mysterious productivity of Northwest shores. Coastal winds usually bring nutrients up on the west coast, but the numbers don’t add up for this region.

“Washington is several times more productive – has more phytoplankton – than Oregon or California, and yet the winds here are several times weaker. That’s been kind of a puzzle, for years,” said co-author , an oceanographer with the UW’s Applied Physics Laboratory.

The secret to the Northwest’s outsize productivity could be marine canyons, an idea first suggested by UW oceanographer . The northern section of the west coast has many more canyons than Oregon or California, with 11 along the Washington coast.

The new paper provides the latest evidence for these canyons’ importance. Measurements by another UW oceanographer in the 1970s first showed water flowing through Juan de Fuca Canyon with a direction that depends on the coastal winds. More recently, calculations by Hickey and a colleague in 2008 submarine canyons could play an important role in supplying nutrients to the Northwest coastal waters.

Alford and MacCready measured inside the Juan de Fuca Canyon in April 2013 using an , built at the UW Applied Physics Laboratory with funding from Washington Sea Grant, that takes water measurements near the seafloor. During a day and a half of round-the-clock observations they got lucky with the wind direction and recorded strong flow up through the canyon.

Water flowed as fast as 1.3 feet per second at 500 feet below the surface, and showed mixing up to 1,000 times the normal rate for the deep ocean. The data also showed that the flow is hydraulically-controlled, meaning it flows smoothly over a shallow ridge just off the cape and then forms a turbulent breaking wave on the other side, mixing with the waters far above.

The deep water forced up through the canyon is rich in nutrients that support the growth of marine plants which then feed other marine life. Those waters also are more acidic and lower in oxygen, all of which contribute to water conditions in the Sound.

“The location of this sill would be an outstanding place to fish,” Alford said. “People fish in Juan de Fuca Canyon pretty actively, and that’s probably no coincidence.”

Pinpointing the source of Puget Sound waters will help make better computer models of circulation through the region, and eventually could help forecast ocean acidity, harmful algal blooms and low-oxygen events.

“Canyons might be important not just for coastal productivity, but that mixed water also gets exported into the interior of the ocean,” Alford said. “I look at this as a first step in getting canyons right in coastal models and in global climate models, because I think it could potentially be a very important source of mixing.”

The research was funded by the Office of Naval Research and the National Oceanic and Atmospheric Administration. Ship time aboard the Thomas G. Thompson was provided by the UW.

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For more information, contact Alford at malford@apl.washington.edu or 206-221-3257 and MacCready at pmacc@uw.edu or 206-685-9588.

A ÌìÃÀÓ°ÊÓ´«Ã½ study for the first time recorded such a wave breaking in a key bottleneck for circulation in the world’s largest ocean. The was published online this month in the journal .

The deep ocean is thought of as dark, cold and still. While this is mostly true, huge waves form between layers of water of different density. These skyscraper-tall waves transport heat, energy, carbon and nutrients around the globe. Where and how they break is important for the planet’s climate.

“Climate models are really sensitive not only to how much turbulence there is in the deep ocean, but to where it is,” said lead author , an oceanographer in the UW . He led the expedition to the Samoan Passage, a narrow channel in the South Pacific Ocean that funnels water flowing from Antarctica.

“The primary importance of understanding deep-ocean turbulence is to get the climate models right on long timescales,” Alford said.

Dense water in Antarctica sinks to the deep Pacific, where it eventually surges through a 25-mile gap in the submarine landscape northeast of Samoa.

“Basically the entire South Pacific flow is blocked by this huge submarine ridge,” Alford said. “The amount of water that’s trying to get northward through this gap is just tremendous – 6 million cubic meters of water per second, or about 35 Amazon Rivers.”

In the 1990s, a major expedition measured these currents through the Samoan Passage. The scientists inferred that a lot of mixing must also happen there, but couldn’t measure it.

In the summer of 2012 the UW team embarked on a seven-week cruise to track the 800-foot-high waves that form atop the flow, 3 miles below the ocean’s surface. Their measurements show these giant waves do break, producing mixing 1,000 to 10,000 times that of the surrounding slow-moving water.

“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 , 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 another 40-day cruise in January to collect those instruments and map currents flowing through various gaps in the intricate channel.

Co-authors of the paper are James Girton, Gunnar Voet and John Mickett at the UW Applied Physics Lab; Glenn Carter at the University of Hawaii; and Jody Klymak at the University of Victoria. The research was funded by the National Science Foundation.

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For more information, contact Alford at 206-221-3257 or malford@apl.washington.edu.