14 May 2009
Back to the drawing board for North Atlantic circulation
by Kate Melville
The conveyor belt paradigm that is used to describe the North Atlantic ocean's circulation has it that 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. But this is a vast oversimplification, say oceanographers.
Woods Hole Oceanographic Institution (WHOI) and Duke University scientists have now teased out a new piece of the puzzle, publishing their findings in Nature. Using field observations and computer models, their new work 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.
"This new path is not constrained by the continental shelf. It's more diffuse," said study author Amy 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," adds Susan Lozier, the study's co-author. "We thought we could just measure them in the Deep Western Boundary Current, but we really can't."
Studies 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. 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 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 downloaded to 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). About 75 percent of them "escaped" that coast-hugging deep underwater pathway and instead drifted into the open ocean before they get around 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 note.
Since the RAFOS float paths could only be tracked for two years, the team 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," Lozier said.
"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," the report concludes.
Source: Woods Hole Oceanographic Institution