To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

This study of the first continuous multiyear observations of the East Reykjanes Ridge Current (ERRC) reveals a highly variable, mostly barotropic southwestward flow with a mean transport of 10-13 Sv. The ERRC effectively acts as a western boundary current in the Iceland Basin on the eastern flank of the Reykjanes Ridge. As part of the Overturning in the Subpolar North Atlantic Program (OSNAP), continuous measurements of the ERRC have been maintained for the first time using acoustic Doppler current profilers, current meters, and dynamic height moorings at six mooring sites near 58°N since 2014. Together with satellite altimetry and Argo profile and drift data, the mean transport, synoptic variability, water mass properties, and upstream and downstream pathways of the ERRC are examined. Results show that the ERRC forms in the northeastern Iceland Basin at the convergence of surface waters from the North Atlantic Current and deeper Icelandic Slope Water formed along the Iceland-Faroe Ridge. The ERRC becomes denser as it cools and freshens along the northern and western topography of the Basin before retroflecting over the Reykjanes Ridge near 59°N into the Irminger Current. Analysis of the flow-weighted density changes along the ERRC's path reveals that it is responsible for about one third of the net potential density change of waters circulating around the rim of the subpolar gyre. Plain Language Summary The East Reykjanes Ridge Current (ERRC) is an important but poorly studied flow in the Iceland Basin of the northern North Atlantic Ocean. This study investigates the ERRC using underwater floats, satellites, and 4 years of continuous mooring data to determine the current's pathways, water properties, and the strength of its transport. From its formation to the south of Iceland near 62°N, 18°W, the ERRC travels westward along the Icelandic slope before turning to the southwest as it follows the eastern flank of a shallow portion of the Mid-Atlantic Ridge, known as the Reykjanes Ridge. As the Reykjanes Ridge deepens, the ERRC eventually reaches its termination as it crosses westward over the ridge into the next basin. Results of this study show that the ERRC is approximately 900 km in length and has an average transport of 10-13 Sv (1 Sv = 1,000,000 m 3 /s). Significant water densification occurs along the ERRC's path, which makes it an important component in the broader picture of the Earth's climate system.

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<p>Since 2014, the Overturning in the Subpolar North Atlantic Program (OSNAP) has maintained the first continuous Eulerian array across the North Atlantic Subpolar Gyre to monitor changes in the Atlantic Meridional Overturning Circulation (AMOC).  The deep limb of the AMOC – the Deep Western Boundary Current (DWBC) – forms in the North Atlantic subpolar gyre from the combination of cold, dense waters of Norwegian Sea origin with the ambient waters within the gyre.  Norwegian Sea Water enters the gyre by crossing southward over the Greenland Scotland Ridge as Denmark Strait Overflow Water to the west of Iceland and Iceland Scotland Overflow Water to the east.  As these waters descend into the Irminger and Iceland Basins (respectively), they entrain the surrounding waters, which are primarily comprised of Labrador Sea Water and Subpolar Mode Water, to increase their transport.  These waters mostly flow cyclonically along the bathymetry of the gyre before merging along the eastern flank of Greenland.  At the eastern tip of Greenland, near Cape Farewell, OSNAP maintains moorings consisting of current meters, acoustic doppler current profilers and temperature-salinity recorders to capture the transport of the DWBC.  This presentation will give new estimates of the DWBC from 6 years of OSNAP observations and shed new light into the current’s variability and long-term trend.  Previous observations at this location found 9-13 Sv of transport, including 10.8 Sv from the first two years of OSNAP data. </p>