Observation-Based Estimates of Eulerian-Mean Boundary Downwelling in the Western Subpolar North Atlantic

A significant fraction of the Eulerian downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) has been proposed to occur around the subpolar North Atlantic's continental slopes. While this downwelling ultimately takes place in a thin boundary layer where relative vorticity can be dissipated via friction, it is maintained by a large-scale geostrophic balance and an along-shore densification of the boundary current. We here use modern hydrography data (Argo and shipboard hydrography mainly) to map the long-term mean density field along the continental slope via an optimal interpolation method specifically adapted to the length scales of the boundary current. The overall downstream densification of the boundary region implies a Eulerian-mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. While seasonal variations appear to be relatively limited, a clear regional pattern emerges with Eulerian-mean downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross-basin estimates confirm that overturning transport across the marginal seas of the subpolar North Atlantic is mainly explained by vertical volume fluxes along the continental slopes, and suggest the usefulness of hydrographic data alone to estimate the regional pattern of the sinking branch of the AMOC.