Waters West of Europe Drive Ocean Overturning, Key for Regulating Climate

A new international study that includes researchers from Scripps Institution of Oceanography at UC San Diego finds that the Atlantic meridional overturning circulation (MOC), a deep-ocean process that plays a key role in regulating Earth’s climate, is primarily driven by cooling waters west of Europe.

In a departure from the prevailing scientific view, the study released Thursday shows that most of the overturning and variability is occurring not in the Labrador Sea off Canada, as past modeling studies have suggested, but in regions between Greenland and Scotland. There,  warm, salty, shallow waters carried northward from the tropics by currents and wind, sink and convert into colder, fresher, deep waters moving southward through the Irminger and Iceland basins.

Overturning variability in this eastern section of the ocean was seven times greater than in the Labrador Sea, and it accounted for 88 percent of the total variance documented across the entire North Atlantic over the 21-month study period.

These findings, unexpected as they may be, can help scientists better predict what changes might occur to the MOC and what the climate impacts of those changes will be, said Susan Lozier, the Ronie-Rochele Garcia-Johnson Professor of Earth and Ocean Sciences at Duke University’s Nicholas School of the Environment.

“To aid predictions of climate in the years and decades ahead, we need to know where this deep overturning is currently taking place and what is causing it to vary,” said Lozier, who led the international observational study that produced the new data.

Among contributors to the international study known as OSNAP (Overturning in the Subpolar North Atlantic Program) is Scripps oceanographer Fiamma Straneo, who is one of five principal investigators from the United States. Straneo’s team has deployed moorings off the southeastern coast of Greenland to measure the transport of mass, heat, and freshwater there.

“OSNAP offers us for the first time the possibility to unravel how changes in the warm waters flowing towards the Arctic can impact ice melt and how this meltwater, in turn, may affect the warm water flow,” Straneo said. “This feedback is often invoked in explaining climate variability from decadal to longer timescales but we have never had the measurements to verify it. It will take a longer record to address it but at least we have started.”

Scientists from 16 research institutions from seven countries collaborated on the new study. They published their peer-reviewed findings Feb. 1 in the journal Science.

“Overturning carries vast amounts of anthropogenic carbon deep into the ocean, helping to slow global warming,” said co-author Penny Holliday of the National Oceanography Center in Southampton, U.K. “The largest reservoir of this anthropogenic carbon is in the North Atlantic.”

“Overturning also transports tropical heat northward,” Holliday said, “meaning any changes to it could have an impact on glaciers and Arctic sea ice. Understanding what is happening, and what may happen in the years to come, is vital.”

“I cannot say enough about the importance of this international collaboration to the success of this project,” Lozier said. “Measuring the circulation in the subpolar North Atlantic is incredibly challenging so we definitely needed an ‘all hands on deck’ approach.”

This paper is the first from the $32 million, five-year initial phase of OSNAP in which scientists have deployed moored instruments and sub-surface floats across the North Atlantic to measure the ocean’s overturning circulation and shed light on the factors that cause it to vary.

"As scientists, it is exciting to learn that there are more pieces to the overturning puzzle than we first thought," said co-author Johannes Karstensen of the GEOMAR Helmholtz Centre for Ocean Research Kiel, in Germany.

"Though the overturning in the Labrador Sea is smaller than we expected, we have learned that this basin plays a large role in transporting freshwater from the Arctic,”  Karstensen said. “Continued measurements in that basin will be increasingly important,” as the Arctic changes unexpectedly.

The new paper contains data collected over a 21-month period from August 2014 to April 2016.

Primary funding came from the U.S. National Science Foundation’s Physical Oceanography Program and the United Kingdom’s Natural Environment Research Council. Additional funding came from the European Union 7^th Framework Programme and Horizon 2020.

Co-authors came from Duke; the National Oceanography Centre in Southampton, U.K.; Woods Hole Oceanographic Institution; the Scottish Association for Marine Sciences; the Royal Netherlands Institute for Sea Research and Utrecht University; Memorial University in St. John’s, Canada; GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany; and the Bedford Institute of Oceanography in Dartmouth, Canada.

They were also scientists from the University of Miami Rosenstiel School of Marine and Atmospheric Science; the University of Oxford; the Ocean University of China and Qingdao National Laboratory for Marine Science and Technology; the National Oceanography Center in Liverpool, U.K.; the Laboratoire d’Oceanographie Physique et Spatiale in Plouzané, France; and the University of Liverpool.