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December 2, 2005

CLIMATE CHANGE

Oceanic Heat Conveyor Slows

Atlantic current that transports heat from tropics to higher latitudes weakens

Bette Hileman

© NATURE 2005
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HEAT MACHINE The Atlantic meridional overturning circulation, known as the Gulf Stream, consists of basically two currents. One is a subtropical gyre of warm surface water, a giant horizontal swirl that circulates clockwise. But part of this water heads northeast and forms two branches, one heading toward Scandinavia and crossing the Greenland-Scotland Ridge (GSR), and the other toward northern Canada. As these waters cool, they sink in the regions marked with stars, and flow south at great depths down the western edge of the Atlantic Basin.

Measurements indicate that the ocean current that gives Northern Europe a relatively warm climate has slowed 30% since 1957 (Nature 2005, 438, 655). This observation is consistent with global climate models, which predict that as greenhouse gases increase in the atmosphere, the Atlantic oceanic circulation that moves warm water from the tropics and subtropics to polar regions will slow and, in extreme cases, shut down altogether.

Harry L. Bryden and colleagues from the National Oceanography Center in Southhampton, England, measured temperature and salinity at various depths along a transect at 25o N between Florida and Africa. From those measurements taken aboard a ship at 50-km intervals, Bryden was able to calculate ocean flow at various depths. By comparing the new measurements with similar data taken on the same transect four times over the past five decades, he found that the Atlantic meridional overturning circulation decreased 30%.

In the North Atlantic, most warm surface waters circulate clockwise in a giant horizontal swirl in the subtropics. Some of this water, however, branches off and flows north at the surface, passing near the U.K. and Scandinavia, keeping most of the Nordic Seas free of ice. As the water approaches Greenland and cools, it sinks because it is saltier than the underlying water as a result of evaporation in the tropics. Bryden’s measurements show that the horizontal swirl has strengthened, while the northern flow has weakened, says Detlef Quadfasel, an oceanographer at the University of Hamburg, in Germany.

In a Nature commentary, Quadfasel explains that increased precipitation in the northern oceans and land masses, as well as accelerated melting of the Greenland ice sheet, has increased the amount of freshwater going into the polar oceans. This dilutes the salty water moving north on the surface from the subtropics and makes less of it sink when it reaches cold regions.

But, he cautions, measurements of ocean flow are highly uncertain, so the magnitude of the decline may be much less than 30%. Even though Bryden’s results are statistically significant, it will take one or two more decades to establish whether the circulation that transports heat from the tropics to higher latitudes has actually slowed, Quadfasel says.

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