The Role of the Mixed Layer in Setting the Potential Vorticity of the Main Thermocline

A steady ventilation model is used to assess the effect of the mixed layer on the structure of the main thermocline; the potential vorticity is found in a subtropical gyre after imposing the thickness and density of the mixed layer, the Ekman pumping, and the hydrography on the eastern boundary. The modeled potential vorticity becomes comparable in value to observations in the North Atlantic if the mixed layer deepens poleward as is observed in winter. The isopycnal gradients in potential vorticity are reduced on the denser ventilated surfaces if the mixed-layer outcrops deviate from latitude circles and, more realistically, sweep southward along the eastern boundary; the age of the subducted fluid is also in reasonable agreement with observations of the tritium-helium age by Jenkins. This study suggests that ventilation may form much of the extensive region of nearly uniform potential vorticity observed on the σθ = 26.75 surface in the North Atlantic, with lateral mixing by eddies being required only in the unventilated pool on the western side of the gyre.