Topographic coupling of the atlantic overturning and gyre circulations

The vorticity dynamics associated with the mean and time-varying gyre and overturning circulations of the Atlantic Ocean are examined in a realistic ocean model hindcast simulation of the late twentieth century. Abyssal flow interaction with sloping bottom bathymetry gives rise to the bottom pressure torque (BPT) term of the vertically integrated vorticity equation. The dominance of this term in the closure of the barotropic gyre circulation noted in previous studies is corroborated here for both non-eddy-resolving and eddy-resolving versions of the Parallel Ocean Program (POP) model. This study shows that BPT is also a dominant term in the vorticity balance of the Atlantic meridional overturning circulation (AMOC) and therefore represents a key dynamical link between the overturning and gyre streamfunctions. The interannual variability of the Atlantic circulation over the last several decades, viewed in terms of time-varying integral vorticity balances, demonstrates the fundamental role played by BPT in coupling the large-scale barotropic and baroclinic flows. Forcing perturbation experiments show how flow-bathymetry interactions mediate buoyancy-driven changes in the gyre circulation and momentum-driven changes in the AMOC. Examples of topographic coupling of the overturning and gyre circulations that this analysis elucidates include the covariation of the high-latitude AMOC and subpolar gyre flows on decadal time scales, buoyancy-forced variance of the Gulf Stream, and large wind-driven variations in AMOC at subtropical latitudes.