Adjustment of the ventilated thermocline

The time dependence of the ventilated thermocline is examined via analytica l and numerical means. The original Henderschott model is modified such tha t the outcrops all occur on the same geopotential surface, rather than at s taggered geopotential surfaces. This model has the advantage that the ocean interior can be ventilated directly by the Sverdrup flow, rather than by w estern boundary processes. The propagation of disturbances governed by line arized forms of the three-layer or four-layer modified Henderschott model, nonlinear solutions of the full modified Henderschott model, and numerical solutions of the planetary geostrophic equations are considered. Low-frequency disturbances are predicted by the linear models to move on ch aracteristics jointly set by advection and wave dynamics. It is shown that perturbations due to wind stress anomalies project strongly onto the first mode and propagate westward similarly to the classical first baroclinic Ros sby mode. They do not experience much interaction with the mean flow (the s o-called non-Doppler effect). On the other hand, perturbations generated by buoyancy anomalies have strong projections onto the second or third modes, and propagate along pathways very close to the mean circulation. Their spe ed is somewhat slower than the current speed, however. These properties app ear in the linearized and simplified nonlinear models and their occurrence in planetary geostrophic results argues the relevance of the Henderschott m odel. Also, these properties are consistent with results from other studies .