Atlantic Meridional Overturning Circulation (AMOC), involving the deep-ocean mixing of warm, salty waters with colder, fresher waters in the North Atlantic, is a major influencer of Earth’s climate. As warm tropical currents journey north, pushed by prevailing winds, they cool, become denser, and sink in a process known as overturning.
Historically, most models have shown that the bulk of this overturning occurs in the Labrador Sea, west of Greenland. But a new study indicates that the eastern North Atlantic between Greenland and Scotland may actually be the dominant overturning venue.
The new study is the first report from the initial phase of the Overturning in the Subpolar North Atlantic Program (OSNAP), in which moored instruments and subsurface floats are being used to monitor the rates and patterns of overturning circulation in the North Atlantic.
“As climate is warming and surface waters are warming and sea ice melts and more fresh water is being added to the system, we are trying to figure out how we can expect the AMOC to change in the years and decades ahead,” says Susan Lozier, a physical oceanographer at Duke University and lead author of the new study published in Science. (Editor’s Note: Susan Lozier is AGU’s president-elect.)
OSNAP is not the only program monitoring AMOC. Efforts to begin systematically monitoring variables such as sea surface temperature, salinity, and currents gave rise to the Rapid Climate Change–Meridional Overturning Circulation and Heat-flux Array (RAPID-MOCHA), deployed in 2004. RAPID-MOCHA takes ongoing measurements at a latitude of 26.5°N, across the Atlantic between Florida and Morocco.
Installed in 2014, OSNAP’s array links several existing data collection nodes, including the German Labrador Sea exit array and the recently installed U.S. Ocean Observatories Initiative array in the southwest Irminger Sea, to cover the vast region of ocean that stretches between Newfoundland and Scotland.
“Taking measurements at one latitude [as RAPID-MOCHA does] tells you a lot but not everything,” says Meric Srokosz, a physical oceanographer at the National Oceanography Centre in Southampton, England, who helps run the RAPID program but was not involved in the new study. “OSNAP’s measurements at different latitudes are giving us a more complete picture of what the overall overturning circulation in the Arctic is doing.”
Data Reveal New Patterns
The most unexpected finding of the new OSNAP study is that the overturning cycle is dominated not by conditions in the Labrador Sea, as oceanographic models had long indicated, but by changes in OSNAP East—the waters east of Greenland in the Irminger and Iceland Basins.
Location matters, Lozier says. “As we’re trying to understand the sensitivities of this system to changes, we need to know, where is the water getting warmer? Where is the fresh water coming in from the Arctic? All those factors make a big difference in predicting how the overturning will change over time.”
OSNAP data also revealed new variability patterns for AMOC. For decades, scientists assumed that overturning was consistent over timescales of hundreds to thousands of years or longer, Lozier says.
“But in the 1990s we became concerned that variation could occur on much shorter timescales—on the order of decades to centuries—and that a decline in overturning could perhaps trigger rapid climate change,” she explains.
As it turns out, scientists’ suspicions about short-term variation were indeed correct: RAPID’s data indicate that variability in the rate of overturning is occurring on interannual timescales.
“The AMOC is more variable on much shorter timescales than anybody expected,” says Srokosz. “In 2009 and 2010 there was a big dip when the AMOC dropped by 30% and then rapidly recovered, which was a big surprise. Computer models didn’t indicate that such variability was possible.”
Overall, since monitoring began in 2004, RAPID has recorded an overall decline in the overturning rate of the AMOC.
Monitoring North Atlantic Carbon Sinks
As the overturning of seawater in the North Atlantic changes, so does the ocean’s ability to absorb and store atmospheric carbon, Lozier says.
“Since the Industrial Revolution, the oceans have taken up about a third of the anthropogenic carbon dioxide humans have produced.”
Half of that carbon dioxide is now sequestered in the deep ocean, including the North Atlantic.
“If the overturning slows down, the ocean will take up less anthropogenic carbon, which would leave more anthropogenic carbon in the atmosphere, which could trigger rapid warming,” she says.
To better understand the role of the North Atlantic in carbon storage, the Argo ocean observing program will soon add biogeochemical carbon sensors to their network of data-collecting floats in the North Atlantic. The Argo floats collect data at depths of 1,000 meters, periodically rising to the surface as well as diving to depths of 2,000 meters.
“Adding the carbon monitoring component will be the biggest expansion to the OSNAP project to date,” Lozier says.
The OSNAP team also plans to continue collecting data for the duration of the 5-year initial project phase and to seek funding for continued monitoring.
“As of now, we have a 21-month record, and we can’t yet say anything about whether the AMOC is slowing down, but our observations will provide some important ground truthing for modeling studies that are working to anticipate future changes.”
Srokosz looks forward to when data from RAPID-MOCHA and OSNAP can be overlapped.
“It will be interesting to get 10 years of overlap between the two data sets to help link what’s happening at different latitudes in the North Atlantic,” he says. “The major obstacle will be to keep both programs funded.”
This article is part of a series made possible through the generous collaboration of the writers and editors of Earth magazine, formerly published by the American Geosciences Institute.
5 April 2019: This article has been updated with Susan Lozier’s affiliation with AGU.