The Influence of Air–Sea Interaction on the Ventilated Thermocline

Air–Sea interaction influences the ventilated thermocline by forcing the mixed layer to deepen and cool poleward. When there is flow from the mixed layer into the interior, the mixed-layer depth and density fields help to set the potential vorticity of the subducted fluid. The importance of this process is assessed by incorporating a depth-varying mixed layer in a ventilation model which is forced by Ekman pumping and implied surface heating. The formulation of the ventilation problem is simplified by only allowing density surfaces to outcrop along latitude circles, and by assuming that there is no zonal inflow along the eastern boundary. The surface heating enables a cross-isopycnal flow within the mixed layer. The volume of ventilated fluid within the subtropical gyre is increased by including the depth-varying mixed layer, and this fluid partly originates from the western boundary, as well as from the Ekman layer. The depth-varying mixed layer increases the depth at which isopycnals are subducted and changes the value of the potential vorticity injected into the main thermocline. However, the mixed layer only alters the detail of the general streamline pattern, with an increase in the subducted potential vorticity leading to the surface flow strengthening and the deeper flow weakening