Labrador Sea Water Circulation in the Northern North Atlantic Ocean

Direct observation of velocities with drifting floats near 1500m depth in the North Atlantic Ocean north of about 45 N are combined to derive a new basin-wide mean circulation within the layer of Labrador Sea Water. Inverse methods are used to determine a basin-scale circulation constrained by non-divergence and conservation of climatological large-scale potential vorticity, with the aim of investigating large-scale potential vorticity dynamics, improving circulation estimates near the basin boundaries, and determining the relative amount of recirculation in sub-basin gyres with (aliased) direct velocity data. Eulerian averages are created from combined datasets and a stream function of the non-divergent flow is computed from the velocity curl and the imposed boundary conditions. Unlike deep flow regimes at low latitudes, where the mid-depth interior circulation is dominated by zonal currents, the mid-depth subpolar flow is dominated by gyres. As suggested by previous estimates of mid-depth circulation, the Labrador Sea Water layer is rapidly circulating around the periphery of the subpolar basin in northern and western boundary currents. Recirculation within basins and exchange between the Labrador Basin and Irminger Basin is modified by the large-scale PV constraints. Eastward flow across the Mid-Atlantic Ridge is concentrated in the Charlie Gibbs Fracture Zone with return flow above the Reykjanes Ridge. A weaker subtropical return flow around the Azores Plateau appears, but data are insufficient to map adequately this region.