The ocean’s overturning circulation transports heat and carbon around the globe and stores it in the deep ocean, playing a crucial role in regulating Earth’s climate. A crucial component in this system is the Atlantic Meridional Overturning Circulation (AMOC), which moves warm surface waters poleward, where they cool, become saltier and denser, and sink to form deeper water masses that are then exported back toward the equator over hundreds to thousands of years.
Scientists have long believed that deepwater formation in the Labrador Sea, between Canada and Greenland, is one of the strongest controls on the AMOC. Now the first long-term continuous observational data from the Irminger Sea, just east of the Labrador Sea, are elucidating how convective water masses form and are entrained into boundary currents. The data reveal that intermediate-depth waters formed in the Irminger Sea may play a larger role in the AMOC than previously thought.
Using data gathered between August 2014 and July 2016 from moorings deployed southeast of Greenland as part of the Overturning in the Subpolar North Atlantic Program and the Ocean Observatories Initiative, Le Bras et al. report two intermediate water masses formed by convection in the Irminger Sea. The upper Irminger Sea Intermediate Water (ISIW) forms at the edge of the sea near the western boundary current, whereas deep ISIW forms in the sea’s interior. The researchers found that the upper ISIW is entrained into the sea’s western boundary current, the East Greenland–Irminger Current, and exported within about 3 months after it forms, whereas export of deep ISIW can lag its formation by more than a year.
Profilers on the moorings, along with instruments tethered at specific depths, measured conductivity (salinity), temperature, depth, and current velocity. From these data, the researchers reconstructed the layering and stratification of the water column, as well as the formation and movement of water masses, through the winters of 2014–2105 and 2015–2016, a period of intense convection.
The team found that between 2014 and 2016, the boundary current transported about twice as much water from the upper Irminger water mass as from the deeper one. The researchers conclude that shallow intermediate waters could be a larger contributor to the overturning circulation than waters that form by dramatic deep convection, implying that cooling near boundary currents may be more important for climate than previously thought.
Climate models have suggested that as temperatures warm and increased melting sends more fresh water into polar oceans, overturning circulation and deepwater formation could slow. (Geophysical Research Letters, https://doi.org/10.1029/2019GL085989, 2020)
—Sara E. Pratt, Science Writer