A simple model of the adiabatic pole-to-pole overturning circulation

The adiabatic pole-to-pole cell of the meridional overturning circulation (MOC) is studied using a simple, analytically-tractable zonally averaged model. Several elements are found to be necessary for the existence of a quasi-adiabatic pole-to-pole cell: (1) a thermally-indirect, wind-driven overturning circulation in the zonally-reentrant channel portion of the domain, analogous to the Deacon cell in the Antarctic Circumpolar Current (ACC), (2) a set of outcropping isopycnals shared between the channel region and the semi-enclosed region in the Northern Hemisphere, (3) a mixed-layer which transmits the surface heat fluxes into the adiabatic interior. The two-dimensional model parametrizes the streamfunction associated with adiabatic eddy buoyancy fluxes in the ACC region as downgradient diffusion of buoyancy thickness. The streamfunction associated with the buoyancy transport due to eddies, gyres, and boundary currents in the semi-enclosed region of the Northern Hemisphere is also paramtrized as downgradient diffusion of buoyancy thickness, with a diffusivity much larger than that used in the ACC region. Because gyres and boundary currents are much more effective than mesoscale eddies at restratifying the fluid, their "diffusivity" is larger. The simple model shows that the disparity in restratification efficiency in the periodic and enclosed region is necessary to obtain a substantial pole-to-pole residual overturning circulation. The simple model also illustrates how the geometry of the isopycnals is shaped by the interhemispheric residual overturning, leading to three major thermostads, which we identify with the major water masses of the Atlantic, i.e. North Atlantic Deep Water, Antarctic Intermediate Water and Antarctic Bottom Water.