The Role of Finite Mixed-Layer Thickness in the Structure of the Ventilated Thermocline

A model of the ventilated thermocline consisting of three adiabatic layers surmounted by a mixed layer of finite thickness is presented. The mixed-layer depth density increase continuously northward, and these attributes of the mixed layer are specified. The effect of the mixed layer on the thermocline circulation is explicitly calculated. The mixed-layer thickness and its variation play a significant role in shifting the trajectories of the streamlines westward. The shadow zones enlarge more rapidly south of the outcrop lines. The finite mixed-layer depth and its increase northward produce shadow zones in each of the adiabatic layers of the thermocline. The augmentation of the ventilation rate, i.e., the rate at which fluid enters the thermocline from the mixed layer in excess of the Ekman pumping is directly proportional to the northward gradient of the mixed-layer potential vorticity. The enhancement is greatest in regions where the circulation is surface-intensified i.e., above the eastern shadow zones.