The so-called thermohaline conveyor belt—a strong northward flow of warm water near the surface of the Atlantic balanced by a southward flow of cold water near the bottom—carries heat from warm equatorial regions to the frigid north, keeping Europe relatively balmy. The fancy term—made famous from a Hollywood disaster movie and Al Gore's slides—is really just the Gulf Stream and other related currents. But because this circulation involves water flowing in different directions across the entire surface of the Atlantic and every layer below, it has been difficult to measure. Now, strung between the Bahamas and the coast of Morocco, a necklace of moored wires—stretching from the seafloor to the surface, at points as much as three miles above—has for the first time revealed the Atlantic's complex movements.
Oceanographers Stuart Cunningham and Torsten Kanzow of the U.K.'s National Oceanography Center, Southampton, led an international effort that both sank and monitored the moorings as well as analyzed the data. The scientists captured the fickle flow of Atlantic currents by comparing what the moorings' current meters revealed with measurements of the streams' effects on an unused telephone wire stretching between Florida and the Bahamas as well as satellite measurements of surface winds.
"With our moorings we measure current velocities, temperatures, salinities and pressures every 15 minutes, from which meaningful daily values of the [meridional overturning circulation] strength can be derived," Kanzow says. This crucial current "has a mean strength of 19 sverdrups [more than 5 billion gallons per second] but varied substantially over the course of one year showing values between four and 35 [sverdrups]."
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That large fluctuation calls into question earlier studies that showed this meridional overturning circulation (MOC) might be slowing based on snapshot current measurements from research vessels. Over time, the flow of water above and below balances out, but the strongest variability comes from the Atlantic's uppermost and deepest layers. It is not a surprise that weather events or salinity changes might cause fluctuations in near-surface currents but "why we also find intensified current fluctuations at very large depths is an open question that still needs to be investigated," Kanzow notes.
The future of the currents, whether slowing, stopping or reversing (as was observed during several months measurements), could have a profound effect on regional weather patterns—from colder winters in Europe to a much warmer Caribbean (and hence warmer sea surface temperatures to feed hurricanes).
"Are you seeing changes in the rate of transfer? Or storage of heat in the Caribbean?" Cunningham asks. "We can't say we have defined the seasonal cycle yet."
That, he notes, will require many more years of monitoring. Data has been collected for 2005 and 2006 but it has yet to be analyzed. But the measurements collected and analyzed from March 2004 to March 2005 provides a real-world baseline against which various models of Earth's climate can be tested, Cunningham says. "Do [climate models] have the right sort of nature of the overturning and its variability?" he asks. "There are questions to answer now before you believe their predictions."
Among those predictions is a gradual slowing of this Atlantic overturning itself, difficult to detect at this early stage of monitoring. And the Atlantic is not the only ocean on the globe with such mercurial currents: A similar effort to understand the dynamics of the southern ocean around Antarctica would help explain the complexity of ocean and climate interactions.
But the Atlantic circulation is an important one, responsible for as much as a quarter of the Earth's heat transfer. Therefore, this study "is a giant leap forward," says physicist John Church of Australia's Commonwealth Scientific and Industrial Research Organization. "To think that we can measure continuously the integrated north Atlantic overturning at 26 [degrees north latitude] is remarkable."