Subpolar North Atlantic Overturning and Gyre-Scale Circulation in the Summers of 2014 and 2016

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate system through its transport of heat and freshwater. The subpolar North Atlantic (SPNA) is a region where the AMOC is actively developed and shaped though mixing and water mass transformation and where large amounts of heat are released to the atmosphere. Two hydrographic transbasin sections in the summers of 2014 and 2016 provide highly spatially resolved views of the SPNA velocity and property fields on a line from Canada to Greenland to Scotland. Estimates of the AMOC, isopycnal (gyre-scale) transport, and heat and freshwater transport are derived from the observations. The overturning circulation, the maximum in northward transport integrated from the surface to seafloor and computed in density space, has a high range, with 20.6 +/- 4.7 Sv in June-July 2014 and 10.6 +/- 4.3 Sv in May-August 2016. In contrast, the isopycnal (gyre-scale) circulation was lowest in summer 2014: 41.3 +/- 8.2 Sv compared to 58.6 +/- 7.4 Sv in 2016. The heat transport (0.39 +/- 0.08 PW in summer 2014, positive is northward) was highest for the section with the highest AMOC, and the freshwater transport was largest in summer 2016 when the isopycnal circulation was high (-0.25 +/- 0.08 Sv). Up to 65% of the heat and freshwater transport was carried by the isopycnal circulation, with isopycnal property transport highest in the western Labrador Sea and the eastern basins (Iceland Basin to Scotland).