Attributing Recent Variability in the Subpolar AMOC to Surface Buoyancy Forcing

Abstract Variability in the Atlantic Meridional Overturning Circulation (AMOC) on interannual to multidecadal timescales can primarily be linked to the strength of deep-water formation in the Subpolar North Atlantic, where surface buoyancy forcing transforms light surface waters to the dense waters of the southward-flowing lower-limb of the AMOC. In a water mass transformation (WMT) 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 Subpolar North Atlantic (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. To further partition dense waters contributing to the AMOC, the SFOC is longitudinally partitioned into an East component, comprising the Irminger/Iceland basins, and a West component, comprising the Labrador Sea. Interannual and multidecadal changes in the dominant location of deep-water formation in the Subpolar North Atlantic are thus elucidated. The analysis provides transport estimates complementary to those obtained with observations from the RAPID array since 2004, and OSNAP array since 2014, revealing that recent (post-2014) domination of overturning in the Eastern Subpolar Gyre may be transient.