The time dependence of the ventilated thermocline is examined via analytical and numerical means. The original Henderschott model is modified such that the outcrops all occur on the same geopotential surface, rather than at staggered geopotential surfaces. This model has the advantage that the ocean interior can be ventilated directly by the Sverdrup flow, rather than by western boundary processes. The propagation of disturbances governed by linearized 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 characteristics 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 Rossby mode. They do not experience much interaction with the mean flow (the so-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 speed is somewhat slower than the current speed, however. These properties appear in the linearized and simplified nonlinear models and their occurrence in planetary geostrophic results argues the relevance of the Henderschott model. Also, these properties are consistent with results from other studies.
Journal of Physical Oceanography – American Meteorological Society
Published: May 11, 1999