The Ventilated Thermocline

TLDR

The oceanic thermocline dynamics are governed by Sverdrupian balances. The study presents a simple theoretical model of the oceanic thermocline and its associated current field. The model represents the thermocline as many inviscid, homogeneous layers of differing density, with density interfaces outcropping at the surface to allow Ekman‑pumping ventilation to excite deep motion even beyond the outcrop latitude. Analytical solutions for the model show non‑self‑similar behavior, with critical potential‑vorticity trajectories delineating ventilated versus unventilated lower‑thermocline regions, and the model’s predictions compare favorably with Atlantic observations.

Abstract

A simple theoretical model for the oceanic thermocline and the associated field of current is presented. The model consists of a finite but arbitarily large number of inviscid, homogeneous fluid layers each with a different density. The dynamical balances everywhere are Sverdrupian. IN regions where the Ekman pumping is negative (downward) the surface density is specified, i.e., the position of the outcrop of density interfaces is specified. This outcropping of density layers allows deep motion to be excited by the ventilation provided by Ekman pumping even in latitudes far south of the outcrop where the layer is shielded from direct influence of the wind. Analytical solutions are presented in the case where the density-outcrop lines are coincident with latitude circles. The solutions are not self-similar and important sub-domains of the solution are defined by critical potential vorticity trajectories which separate the ventilated from the unventilated regions in the lower thermocline. These critical trajectories also separate regions of strong variations in potential vorticity from regions of fairly weak variation in potential vorticity although the small variations in potential vorticity in the latter are crucial to the dynamics. Comparison is made between the predictions of the model and data from the Atlantic with encouraging results.