A Slowing Cog in the North Atlantic Ocean's Climate Machine

Oceanographers, who have begun to watch the slow churnings of the ocean much the way meteorologists observe the daily weather in the atmosphere, believe they have seen a new shift in ocean “climate.” The giant vortex of an ocean current, or gyre, tucked into the northwestern North Atlantic appears to have slowed.

The weakening of this subpolar gyre in the 1990s may have been just a random fluctuation in one part of the complex of ocean currents that carries warm waters into the high North Atlantic. If so, this single cog in the Atlantic “conveyor belt” of north-south currents could soon recover.

Or the subpolar gyre might continue to slow through this century as the whole conveyor belt brakes under the arresting hand of global warming. That would be no climate catastrophe—notwithstanding next month's climate disaster movie The Day After Tomorrow, which depicts chilling consequences of a breakdown of the conveyor. But the effects could be real enough, including a cooling of northern Europe, fewer Atlantic hurricanes, and more drought in the Sahel of Africa. The record of subpolar gyre behavior is decades shy of revealing what may be in store, but “there may well be a consequence” for the conveyor, says oceanographer Jochem Marotzke of the Max Planck Institute for Meteorology in Hamburg, Germany.

This latest hint of global change comes from what might be described as an ocean weather satellite. To take a snapshot of atmospheric weather, meteorologists measure atmospheric pressure so they can map out the centers of high and low pressure around which the winds blow. In the ocean, the highs and lows around which currents circle to form gyres are palpably manifest in the height of the sea surface at their centers. In a 15 April online report in Science (www.sciencemag.org/cgi/content/abstract/1094917), oceanographers Sirpa Häkkinen of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and Peter Rhines of the University of Washington, Seattle, give satellite measurements of sea surface height over the far northern North Atlantic. The measurements were made by the U.S.-French TOPEX/Poseidon radar-altimeter satellite.

During the decade of observations between 1992 and 2002, the interior of the subpolar gyre, which is most intense between Labrador and Iceland just south of Greenland, rose by about 4 to 9 centimeters depending on location, Häkkinen and Rhines report. With a shallower and thus weaker low, the gyre should have slowed by more than 1 centimeter per second per decade, or about one-fifth of its flow. That's what seems to have happened: A set of current meters that was moored in the western edge of the gyre for 2 years in the mid-1990s recorded a slowing as the satellite altimetry showed a shallowing. “I think that is quite convincing,” says Marotzke.

If the subpolar gyre continues to slow, oceanographers wonder what effect it could have on the conveyor belt, which they variously term the thermohaline circulation or the meridional overturning circulation (MOC). The gyre and the Labrador Sea that it encompasses make up “the Grand Central Station of global circulation,” says Rhines. “There's so much happening there.” On the broadest scale, the MOC carries warm surface waters from the South Atlantic into the far northern North Atlantic. The most obvious warm-water route is the Gulf Stream, whose northernmost branch abuts the southern edge of the subpolar gyre. The MOC also sinks cold, saltier water that moves southward in currents at mid depths and along the ocean bottom. Some of that sinking occurs in the Labrador Sea.

The forces driving the subpolar gyre are varied. The wind has immediate as well as delayed effects; anything affecting the buoyancy of surface water—heating or cooling, and evaporation or the addition of fresh water—can influence the sinking of surface waters; and the effects of these forces elsewhere in the Atlantic can propagate into the gyre. In the case of the 1990s slowing, Häkkinen and Rhines deduce that surface waters within the gyre lost less heat to the atmosphere in the 1990s, warming and expanding waters there and thus raising the sea surface and slowing the gyre.

Oceanographers can't say whether the subpolar gyre's heating-induced slowing will continue. “We don't claim to show there is an irreversible global warming effect” on the gyre, says Rhines. Researchers have only two snippets of record before the launch of TOPEX/Poseidon in 1992, so they have no way to tell whether they're seeing a long-term, greenhouse-induced slowing of the gyre or just random natural variations.

Even if the subpolar gyre were to continue to slow, there's no agreement that it would make much difference to the MOC or climate around the Atlantic. On page 400, ocean modelers Andrew Weaver of the University of Victoria, British Columbia, and Claude Hillaire-Marcel of the University of Quebec in Montreal argue from published modeling and paleoclimate records that global warming might in fact shut down the sinking of surface waters in the gyre's Labrador Sea, as happened in 1995. But that needn't slow the MOC as a whole, they say, and would have a minor climate effect downwind in Europe. That scenario is less dramatic than an inundated New York City freezing up one summer's night, as Hollywood has it in The Day After Tomorrow, but likely closer to the truth.