Simple Mechanistic Models of Mid-Depth Meridional Overturning

Two idealized, three-dimensional, analytical models of mid-depth meridional overturning in a basin with a southern-hemisphere circumpolar connection are described. In the first, the overturning circulation can be understood as a ‘pump and valve’ system, in which the wind forcing at the latitudes of the circumpolar connection is the pump, and surface thermodynamic exchange at high northern latitudes is the valve. When the valve is on, the overturning circulation extends to the extreme northern latitudes of the basin, and the mid-depth thermocline is cold. When the valve is off, the overturning circulation is short-circuited and confined near the circumpolar connection, and the mid-depth thermocline is warm. The meridional overturning cell in this first model is not driven by turbulent mixing, and the subsurface circulation is adiabatic. In contrast, the pump that primarily drives the overturning cell in the second model is turbulent mixing, at low and mid-latitudes, in the ocean interior. In both models, however, the depth of the mid-latitude deep layer is controlled by the sill depth of the circumpolar gap. The thermocline structures in these two models are nearly indistinguishable. These models suggest that northern hemisphere wind and surface buoyancy forcing may influence the strength and structure of the circumpolar current in the southern hemisphere.