Buoyancy- and wind-driven circulation in an extended model of potential vorticity homogenization

An analytical model is proposed on the middepth water circulation between the surface wind-driven circulation and the deep buoyancy-driven circulation. The model consists of three and a half layers, with the second and third layers being the middepth layers. The model includes thermohaline processes by allowing diapycnal flows at interfaces between layers, in addition to the potential vorticity homogenization proposed by Rhines and Young. The velocities of diapycnal flow are calculated from the density stratification, which can be given solely by a wind-driven model as a good approximation. The divergence of diapycnal velocity causes a circulation in addition to the wind-driven circulation. Although, in pure wind-driven theories, motion was absent outside the region of homogenized potential vorticity, the present model gives significant currents there. In particular, a prominent eastward flow appears in the third layer along the southern rim of the homogeneous potential vorticity region in the second laver. The flow pattern in the present model is consistent with that estimated diagnostically from the climatological density distribution in the North Pacific. The diagnosed flow field is further supported by the oxygen distribution there.