Oceanographers have put a stethoscope on the coursing circulatory system of the Atlantic Ocean, and they have found a skittish pulse that’s surprisingly strong in the waters east of Greenland—data that should improve climate models.
Floats have helped map the ocean currents in the Atlantic Meridional Overturning Circulation. WHOI
The powerful currents in the Atlantic, formally known as the Atlantic meridional overturning circulation (AMOC), are a major engine in Earth’s climate. The AMOC’s shallower limbs—which include the Gulf Stream—transport warm water from the tropics northward, warming Western Europe. In the north, the waters cool and sink, forming deeper limbs that transport the cold water back south—and sequester anthropogenic carbon in the process. This overturning is why the AMOC is sometimes called the Atlantic conveyor belt.
Last week, at the American Geophysical Union’s (AGU’s) Ocean Sciences meeting here, scientists presented the first data from an array of instruments moored in the subpolar North Atlantic. The observations reveal unexpected eddies and strong variability in the AMOC currents. They also show that the currents east of Greenland contribute the most to the total AMOC flow. Climate models, on the other hand, have emphasized the currents west of Greenland in the Labrador Sea. “We’re showing the shortcomings of climate models,” says Susan Lozier, a physical oceanographer at Duke University in Durham, North Carolina, who leads the $35-million, seven-nation project known as the Overturning in the Subpolar North Atlantic Program (OSNAP).
Four years ago, researchers began placing OSNAP’s 53 moorings, studded with sensors to measure temperature, salinity and current flow, near 55°N in the waters between Labrador and Greenland and between Greenland and Scotland. The moorings, galvanized steel wires as thick as a pinkie finger, are anchored to the ocean floor and tugged vertically by submerged floats. Since 2004, researchers have gathered data from another array, at 26°N, stretching from Florida to Africa. But OSNAP is the first to monitor the circulation in the waters of the subpolar North Atlantic, where a critical aspect of the overturning occurs. It’s here in the frigid Nordic Seas that water masses become cold and dense, sinking in streams that snake along the basin bottom, eventually turning southward and reaching the subtropics in about a decade.
Some climate models suggest that the formation of large amounts of this so-called “deep water” should lead to a stronger AMOC, but the first 21 months of data from OSNAP aren’t conclusive. Both of the recorded winters were unusually cold and created similarly large amounts of deep water, but the strength of the AMOC whipsawed wildly between 8 and 25 sverdrups, a unit of flow roughly equivalent to the total flow of all the world’s rivers. However, this variability was on such short timescales—months—that it might not be linked to the deep water at all, Lozier says. “We need more winters.”