Lagrangian overturning pathways in the eastern Subpolar North Atlantic

The strength of the Atlantic Meridional Overturning Circulation (AMOC) at subpolar latitudes is dominated by water mass transformation in the eastern Subpolar North Atlantic (SPNA). However, the distribution of this overturning across the individual circulation pathways of both the Subpolar Gyre (SPG) and the Nordic seas overflows remains poorly understood. Here, we introduce a novel Lagrangian measure of the density-space overturning to quantify the principal pathways of the time-mean overturning circulation within an eddy-permitting ocean model hindcast. By tracing the trajectories of water parcels initialised from the northward inflows across the OSNAP East section, we show that water mass transformation along the pathways of the eastern SPG accounts for 55% of the mean strength of the eastern subpolar AMOC. Water parcels following the dominant SPG pathway, sourced from the Sub-Arctic Front, form upper North Atlantic Deep Water by circulating horizontally across sloping isopycnals in less than 2 years. A slower SPG route, entrained by overflow waters south of the Iceland-Faroes Ridge, is a crucial conduit for subtropical-origin water masses to penetrate the deep ocean on subdecadal timescales. On reproducing our findings using time-averaged velocity and hydrographic fields, we further show that the Nordic seas overflow pathways integrate multiple decades of water mass transformation before returning across the Greenland-Scotland Ridge. We propose that the strong disparity between the overturning timescales of the SPG (interannual) and the Nordic seas overflows (multi-decadal) has important implications for the propagation of density anomalies within the eastern SPNA and hence the sources of AMOC variability