Spinning down. When the North Atlantic’s subpolar gyre began losing less heat (black) in the mid-1990s, it slowed (red).

Labrador Sea Slows Its Swirl

Dick writes about Earth and planetary science for Science magazine.

The giant vortex of ocean current tucked into the far northwestern North Atlantic appears to have slowed. It's no climate catastrophe--notwithstanding next month's climate disaster movie The Day After Tomorrow, which depicts chilling consequences of a breakdown of the North Atlantic currents--but the effects could be very real.

The vortex, or gyre, in question is a cog in the conveyor belt of currents, including the mighty Gulf Stream, carrying warm surface waters from the South Atlantic into the far northern North Atlantic. A slowdown could have dramatic effects, including a cooling of northern Europe, fewer Atlantic hurricanes, and more drought in the Sahel of Africa.

It's too early to say whether these events will come to pass, but the gyre does seem to be losing steam, according to a report published online by Science 15 April. Oceanographers Sirpa Häkkinen of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and Peter Rhines of the University of Washington, Seattle, analyzed 10 years worth of satellite measurements of sea surface height over the far northern North Atlantic. These measurements revealed that the low-lying interior of the gyre, nestled between Labrador and Iceland just south of Greenland, rose by about 4 to 9 centimeters. Because a lower interior translates to a faster swirl, the gyre should have slowed by more than 1 centimeter per second per decade, or about one-quarter of its flow. That's what seems to have happened: A set of current meters moored in the western edge of the gyre recorded a slowing as the satellite photos showed a shallowing. The researchers don't know if the weakening gyre is just a random fluctuation or part of a long-term trend.

All the same, the evidence that the gyre is slowing “is quite convincing,” says oceanographer Jochem Marotzke of the Max Planck Institute for Meteorology in Hamburg, Germany. However, there's no agreement that slowing of the conveyor belt would make much difference to climate. Even if the gyre continues to slow, most researchers agree it's unlikely New York City will flood and freeze over in summertime, as it does in The Day After Tomorrow. Good thing they don't work in Hollywood.

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