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Study finds surprising new pathway for North Atlantic circulation

by Woods Hole Oceanographic Institution on 27 May 2009
For this study, Dr. Amy Bower, shown here aside a RAFOS float on the deck of the research vessel Oceanus, and her colleagues deployed 76 RAFOS floats over three years. The floats are programmed to drift at a specific depth for two years, recording location information and temperature and pressure measurements once a day. After two years, the floats returned to the surface and transmitted all their data through the ARGOS satellite-based data retrieval system and downloaded to scientists in the lab. (Woods Hole Oceanographic Institution) Woods Hole Oceanographic Institution (WHOI) © http://www.whoi.edu/

Oceanographers have long known that the 20-year-old paradigm for describing the global ocean circulation – called the Great Ocean Conveyor – was an oversimplification. It’s a useful depiction, but it’s like describing Beethoven’s Fifth Symphony as a catchy tune.

The ocean conveyor paradigm says the Gulf Stream-warmed ocean releases heat to the atmosphere in the northern North Atlantic, leaving ocean water colder and denser as it moves north. The cold waters sink and flow southward along the 'deep western boundary current' that hugs the continental slope from Canada to the equator. To replace the down-flowing water, warm surface waters from the tropics are pulled northward along the conveyor’s upper limb.

But while the conveyor belt paradigm establishes the melody, the subtleties and intricacies of the symphony of global ocean circulation largely remain a puzzle.

Now, research led by oceanographers at Woods Hole Oceanographic Institution (WHOI) and Duke University have teased out a new piece of that puzzle, expanding our understanding of this circulation model. Using field observations and computer models, the study shows that much of the southward flow of cold water from the Labrador Sea moves not along the deep western boundary current, but along a previously unknown path in the interior of the North Atlantic.

The study by co-principal authors Amy Bower, a senior scientist in the WHOI Department of Physical Oceanography, and Susan Lozier, a professor of physical oceanography at Duke University’s Nicholas School of the Environment, will be published in the May 14 issue of the research journal Nature.

'This new path is not constrained by the continental shelf. It’s more diffuse,' said Bower. 'It’s a swath in the wide-open, turbulent interior of the North Atlantic and much more difficult to access and study.'

And since this cold southward-flowing water is thought to influence and perhaps moderate human-caused climate change, this finding may impact the work of global warming forecasters.

'This finding means it is going to be more difficult to measure climate signals in the deep ocean,' Lozier said. 'We thought we could just measure them in the Deep Western Boundary Current, but we really can’t.'

Lozier and Bower first conceived of this program eight years ago. Studies led by Lozier and other researchers had previously suggested cold northern waters might follow such 'interior pathways' rather than the conveyor belt in route to subtropical regions of the North Atlantic.
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But testing the idea meant developing an elaborate WHOI-led field program involving the launching of 76 special Range and Fixing of Sound (RAFOS) floats into the current south of the Labrador Sea between 2003 to 2006. The ambitious program would have been prohibitively expensive had it not been for a collaboration with Eugene Colbourne of the Northwest Atlantic Fisheries Center in St. Johns, Newfoundland. Colbourne regularly conducts hydrographic surveys around the Grand Banks, and agreed to deploy the team’s RAFOS floats in groups of six every three months for three years.

Bower worked with a team at WHOI to build the floats and develop the plan for their deployment.

The RAFOS floats were configured to submerge at 700 or 1,500 meters depth – within the layer of the ocean where one constituent of the cold southward-flowing water, called Labrador Sea Water, travels. They drifted with the currents for two years, recording location information as well as temperature and pressure measurements once a day. After two years, the floats returned to the surface and transmitted all their data through the ARGOS satellite-based data retrieval system and were downloaded by scientists in the lab.

To communicate with the floats and to track their position, the team deployed anchored low-amplitude sound beacons in the general area of the experiment, which were set to 'ping' automatically every day. The RAFOS floats’s onboard hydrophones detect the sound from the beacons, enabling scientists to determine the distance from the float to the beacon, based on the time delay between when the ping went off and when it was detected.

But only 8 percent of the RAFOS floats followed the conveyor belt of the Deep Western Boundary Current (DWBC), according to the Nature report. About 75 percent of them 'escaped' that coast-hugging deep underwater pathway and instead drifted into the open ocean before rounding the Grand Banks. Eight percent 'is a remarkably low number in light of the expectation that the DWBC is the dominant pathway for Labrador Sea Water,' the researchers wrote.

Since the RAFOS float paths could only be tracked for two years, Lozier, her graduate student Stefan Gary, and German oceanographer Claus Boning also used a modeling program to simulate the launch and dispersal of more than 7,000 virtual 'e-floats' from the same starting point.

Subjecting those e-floats to the same underwater dynamics as the real ones, the researchers then traced where they moved. 'The spread of the model and the RAFOS float trajectories after two years is very similar,' they reported.

'The new float observations and simulated float trajectories provide evidence that the southward interior pathway is more important for the transport of Labrador Sea Water through the subtropics than the DWBC, contrary to previous thinking,' their report concluded.

Next, Bower and Lozier hope to extend their research to study the southward flow of cold water originating even farther north in the Greenland Sea.

This research was supported by the National Science Foundation.

The Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the oceans’ role in the changing global environment - http://www.whoi.edu

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