Abstract
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).
Highlights
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global 51 climate system through its transport of heat and freshwater
We describe the character of the subpolar North Atlantic (SPNA) AMOC which is complicated by the presence of the cold and fresh shallow boundary currents (LC, EGC, East Greenland Coastal Current (EGCC), West Greenland Current (WGC) and West Greenland Coastal Current (WGCC))
We present two definitions of AMOCσ which we discuss later; the first is the maximum value of the overturning streamfunction (Tamoc accumulated from low to high density, AMOCσ-max adapted from Mercier et al, 2015), the second is the sum of all the northward transport in the upper layer of Tamoc (AMOCσ-n adapted from Li et al., (2017))
Summary
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global 51 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. It is a region where large amounts of heat transported northwards by the ocean are released to the atmosphere thereby modifying the climate of northwest Europe. The SPNA has complex topography with a series of basins (Fig. 1) in which the large scale circulation is characterised by cyclonic boundary currents and interior recirculation. Part of the NAC flows into the Norwegian Sea, and some recirculates within the boundary currents of the subpolar gyre (e.g. Hansen and Osterhus, 2000)
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