Parameter study of a continuously stratified model of the ideal-fluid thermocline

The parameter sensitivity of a continuously stratified model of the ideal-f luid thermocline in the subtropical gyre interior is studied. A one-dimensi onal advection-diffusion model is used to set up a background stratificatio n that can provide both the potential vorticity function for the unventilat ed thermocline and the mixed layer depth used in the ideal-fluid thermoclin e model. The wind-driven circulation is treated as a perturbation to this b ackground stratification. Although the perturbation solution excludes mixin g/diffusion, the dynamic effect of diapycnal mixing is included in the unpe rturbed solution; therefore, the ideal-fluid solution should correspond to a nonzero diffusion solution for the wind-driven and thermohaline circulati on in the ocean. It is shown that the model can reproduce the thermocline structure, which c orresponds to either finite or infinitely weak mixing. Under the extreme we ak diffusion limit, the model produces a thermocline that looks like a step function in the stratification, which separates the wind-driven gyre above it and the stagnant abyssal water underneath it. It is shown that the subduction rate and production of mode water with low- potential vorticity are closely related to the stratification (or the poten tial vorticity) of the unventilated thermocline, the geometry of the mixed layer, the Ekman pumping rate, and the orientation of the intergyre boundar y. Changes in the structure of the thermocline in response to different upp er boundary conditions are explored. It is found that cooling and southward migration of the jet stream induce the production of low potential vortici ty mode water, while changes in the vertical density profile have an appear ance like the second baroclinic mode.