European waters drive ocean overturning, key for regulating climate

January 31, 2019, Duke University
New research shows the Atlantic meridional overturning circulation, which regulates climate, is primarily driven by waters west of Europe. Credit: Carolina Nobre, WHOI Media

A new international study 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 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, 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.

"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."

The R/V Knorr at the dock in Reykjavik before the start of the first OSNAP cruise, July 2014. The team set out to deploy the very first OSNAP science moorings and deploy the first OSNAP RAFOS floats. Credit: Heather Furey, Woods Hole Oceanographic Institution
"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."

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

"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 the OSNAP (Overturning in the Subpolar North Atlantic Program) , 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. Lozier is lead investigator of the project, which began in 2014.

Overturning in the Subpolar North Atlantic Program (OSNAP) is an international program designed to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. The OSNAP observing system consists of two legs: one extending from southern Labrador to the southwestern tip of Greenland across the mouth of the Labrador Sea (OSNAP West), and the second from the southeastern tip of Greenland to Scotland (OSNAP East). The observing system also includes subsurface floats (OSNAP Floats) in order to trace the pathways of overflow waters in the basin and to assess the connectivity of currents crossing the OSNAP line. Credit: OSNAP
"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.

Explore further: NSF awards grants for deployment of new observing system in the North Atlantic Ocean

More information: M.S. Lozier el al., "A sea change in our view of overturning in the subpolar North Atlantic," Science (2019). science.sciencemag.org/cgi/doi … 1126/science.aau6592

"Taking a close look at ocean circulation," Science (2019). science.sciencemag.org/cgi/doi … 1126/science.aaw3111

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Shootist
2 / 5 (4) Jan 31, 2019
while the Pacific is still being cooled by waters from the little ice age.
dudester
5 / 5 (3) Jan 31, 2019
Shootist's raison d'etre exposed and disintegrated by the blistering light of the march of the scientific method:
'America colonisation cooled Earth's climate'
https://www.bbc.c...47063973
Those bikini clad Vikings swilling home made wine and harvesting wheat on the soil-less mountainsides of Greenland he constantly hallucinates had long fled the skrælings back to the relative warmth of their fjords and snowy forests when the real cause of the "little ice age" arrived-- Spanish and English disease and warfare. These killed so many millions of Native American farmers that their untold acres of land (see Amazonia and the Mississippians) went back to forest in a matter of years not decades. This sucked so much CO2 out of the air it led to centuries of cooling before the boomerang effect of the inflood of European guns, germs, steel, plantations worked by slaves, and small farms worked by settlers cut those forests down again-- and more.
And on it goes.

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