Extracting the Buoyancy-Driven Atlantic Meridional Overturning Circulation

Variations in the Atlantic Meridional Overturning Circulation (AMOC) driven by buoyancy forcing are typically characterized as having a low frequency timescale, interhemispheric structure, cross-equatorial heat transport, and linkages to the strength of North Hemisphere gyre circulations and the Gulf Stream. This study first tests whether these attributes ascribed to the AMOC are reproduced in a coupled model that is mechanically decoupled and, hence, is only buoyancy coupled. Overall, the mechanically decoupled model reproduces these attributes, with the exception that in the subpolar gyre, buoyancy forcing drives AMOC variations on interannual to multidecadal timescales, yet only the multidecadal penetrate into the subtropical gyre. A stronger AMOC is associated with a strengthening of the North Hemisphere gyre circulations, the Gulf Stream, and northward heat transport throughout the basin. We then determine whether the characteristics in the mechanically decoupled model can be recovered by low pass filtering the AMOC in a fully coupled version of the same model, a common approach used to isolate the buoyancy-driven AMOC. A major conclusion is that low pass filtering the AMOC in the fully coupled model reproduces the buoyancy-driven AMOC pattern and most of the associated attributes, but not the temporal variability. The AMOC-Gulf Stream connection is also not recoverable. The analyses reveal caveats that must be considered when choosing indexes and filtering techniques to estimate the buoyancy-driven AMOC in coupled models. Results also provide insight on the latitudinal dependence of timescales and drivers of ocean circulation variability in coupled models, with potential implications for the measurement and detection of the buoyancy-driven AMOC in the real world.