SPRING 2019

A Sea Change In Oceanography

A Nicholas School-led study has found 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.

These findings 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, Ronie- Rochele Garcia-Johnson Professor of Earth and Ocean Sciences, who led the study.

“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,” she said.

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, deep waters moving southward through the Irminger and Iceland basins.

Overturning variability in this eastern section of the ocean was seven times greater than that of 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.

Overturning carries vast amounts of anthropogenic carbon deep into the ocean, helping to slow global warming. It also transports tropical heat northward, 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. The largest reservoir of this anthropogenic carbon is in the North Atlantic.

Scientists from 16 research institutions collaborated on the study. They published their findings in Science (DOI: 10.1126/ science.aau6592).

This paper is the first from the $32 million OSNAP (Overturning in the Subpolar North Atlantic Program) research project, in which scientists have deployed moored instruments and sub-surface floats across the North Atlantic to measure overturning circulation and shed light on the factors that cause it to vary. Lozier is lead investigator of the project, which began in 2014.