$16m Study Traces Changes in Ocean Circulation

Thursday, October 17, 2013

Oceanographers from Duke University, Woods Hole Oceanographic Institution and the University of Miami have received $16 million in grants from the National Science Foundation for the deployment of a new observing system in the subpolar region of the North Atlantic. The observing system will measure the ocean’s overturning circulation, a key component of the global climate system.

The five-year initiative is part of the $32 million, U.S.-led Overturning in the Subpolar North Atlantic Program (OSNAP). International collaborators include scientists from Canada, the United Kingdom, Germany, France and the Netherlands.

The goal of the program is to simultaneously measure the surface ocean currents that carry heat northward toward the Arctic Ocean and the deep ocean currents that carry cooler waters southward toward the equator. Together, these currents form the overturning circulation that plays a role in redistributing heat from the equator to the poles. Recent modeling studies have shown that changes in this circulation would have a critical impact on temperatures and precipitation in North America, Europe and Africa.

“In addition to measuring the variability of the ocean overturning, OSNAP is strongly focused on understanding what factors create those changes,” said Susan Lozier, the international project lead and a physical oceanographer at Duke’s Nicholas School of the Environment.

“For decades, oceanographers have understood the overturning circulation to be highly susceptible to changes in the temperature and salinity of surface waters in the subpolar North Atlantic. With increasing ocean temperatures, and increased ice melt that impacts the salinity of the surface waters, it is timely to establish just how climate changes might affect the strength of the overturning circulation,” Lozier explained.

Likewise, the OSNAP array affords the opportunity to study how overturning changes impact the environment. OSNAP measurements will facilitate the study of how changes in the northward flow of warm water affects the reduction of Arctic sea ice and the shrinking of the Greenland Ice Sheet.

Duke, Woods Hole and Miami oceanographers, along with their international partners, will deploy moored instruments and sub-surface floats across the subpolar North Atlantic during the summer of 2014. The measurement period will last until 2018.

The array of instruments will stretch along two lines, from Labrador to southern Greenland and from Greenland east to Scotland. The instruments will provide the scientists with continuous measurements of surface-to-bottom water temperature, salinity and velocities in areas of the subpolar ocean that historically have been under-sampled. Trajectories of the subsurface floats will provide the first look at deep-water pathways in the North Atlantic.

The OSNAP measurement system complements a joint U.K. and U.S. program that has been measuring the overturning circulation in the subtropical North Atlantic since 2004. Differences and similarities in these measures will provide oceanographers insight into the working of the ocean’s overturning.

Overturning measures are also critical for an understanding of the ocean’s continued ability to act as one of Earth’s most important carbon sinks.

Surface waters absorb heat-trapping carbon dioxide from Earth’s atmosphere. When cold, dense south-flowing waters from subpolar regions sink, they carry the surface water -- and much of the CO2 it contains -- into the ocean’s depths, where it is no longer available to heat Earth’s climate.

“Because the storage of carbon at depth is linked to the overturning circulation, our OSNAP measures take on added importance,” Lozier said. “A critical question for climate scientists today is: How much carbon will continue to be stored in the ocean?”

The OSNAP program was designed at an international workshop Lozier led at Duke in April 2010.

Principal U.S. investigators of the new program are Amy Bower, Fiamma Straneo and Robert Pickart, scientists in physical oceanography at Woods Hole Oceanographic; William Johns, professor of meteorology and physical oceanography at the University of Miami; and Lozier.

OSNAP will be one of the first projects to make use of the new, NSF-funded Ocean Observatories Initiative’s (OOI) array of moored sensors that will be installed in the Irminger Sea, off the southern tip of Greenland, in 2014. The Irminger Sea is one of four planned global observing sites of the OOI program, a networked infrastructure of sensor systems measuring physical, chemical, geological and biological variables in high-latitude and coastal ocean locations as well as at the seafloor.

The OSNAP project is funded by two NSF grants, OCE-1259102 and OCE-1259103.

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