Ocean Theory Not Current

August 1, 2009

A fifty-year-old model of the way Atlantic ocean currents flow, which shows a discrete "conveyor belt" of deep, cold water flowing southward from the Labrador Sea and forming a continuous loop with the Gulf Stream, is likely incorrect, according to Duke researchers.

New research led by Duke and the Woods Hole Oceanographic Institution relied on an armada of sophisticated floats to show that much of the cold water, originating in the sea between Newfoundland and Greenland, is diverted eastward by the time it flows as far south as Massachusetts. From there, it disperses to the depths in ways that are difficult to follow.

"Everybody always thought this deep flow operated like a conveyor belt, but what we are saying is that concept doesn't hold anymore," says Susan Lozier, professor of physical oceanography and chair of the Earth and Ocean Sciences Division in the Nicholas School of the Environment.

And since cold Labrador seawater is thought to influence and perhaps moderate human-caused climate change, this finding may affect the work of global-warming forecasters. The research was published in the journal Nature.

Forecasters say effects of global warming are magnified at higher latitudes, making the Labrador Sea an added focus of attention. Surface waters there absorb heat-trapping carbon dioxide from the atmosphere, and a substantial amount of it gets pulled underwater, where it is no longer available to warm the Earth's climate.

Oceanographers long thought all the Labrador seawater moved south along what is called the Deep Western Boundary Current (DWBC), which hugs the eastern North American continental shelf almost to Florida, before veering off to the southeast. But studies in the 1990s using submersible floats showed little evidence of southbound export of Labrador seawater within the DWBC.

Scientists challenged those studies in part because the floats had to return to the surface to report their observations to satellite receivers, meaning the data could have been biased by upper ocean currents. To address the criticisms, the researchers launched seventy-six special floats that could stay submerged at a depth of 700 or 1,500 meters and still communicate their data. Only 8 percent of the floats followed the DWBC, while about 75 percent of them left the underwater pathway and drifted into the open ocean by the time they rounded the southern tail of the Grand Banks of Newfoundland.

Since the float paths could only be tracked for two years, researchers 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, researchers then traced their movements. Both experiments proved very similar.

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

Read about Susan Lozier's previous research in a July-August 2007 Duke Magazine feature story.