Quantifying the Contribution of Surface Buoyancy Forcing to Recent Subpolar AMOC Variability

In the Subpolar North Atlantic (SPNA), interannual to multidecadal variability in the Atlantic Meridional Overturning Circulation (AMOC) is primarily attributed to surface buoyancy forcing. Here, warm surface waters arriving via the Gulf Stream and North Atlantic Current undergo an intense loss of heat and freshwater to the atmosphere and are thus transformed to cold and dense waters which subsequently sink and are returned southward at depth. Quantifying the contribution of surface buoyancy forcing to AMOC variability is essential for modelling how the AMOC will respond to predicted warming and freshening at high latitudes. In a water mass transformation framework, fields of surface density and surface density flux from the GODAS ocean reanalysis are used to construct the surface-forced overturning circulation (SFOC) streamfunction for the SPNA (48-65°N) in an operational assimilation over 1980-2020. Computed and plotted in latitude-density space, the SFOC reconstruction compares favourably with the corresponding AMOC, computed from GODAS currents. We thus conclude that subpolar AMOC variability is largely explained by changing air-sea heat and freshwater fluxes controlling water mass transformation across the region. We further highlight the changing relative influences of water mass transformation in the eastern and western subpolar gyre, by partitioning SFOC longitudinally into an East component (5-43 °W) comprising the Irminger and Iceland basins, and a West component (43-60 °W) comprising the Labrador Sea. Our analysis demonstrates that interannual to multidecadal SFOC variability is dominated by changing water mass transformation in the western subpolar gyre. This challenges a shifting consensus that highlights the eastern subpolar gyre as dominant in driving the AMOC across subpolar latitudes.