Mixed Layer-Thermocline Interaction in a Three-Dimensional Isopycnic Coordinate Model

Abstract

The annual buildup and obliteration of the seasonal thermocline and the associated ventilation of the permanent thermocline in a wind- and thermally driven ocean basin are simulated numerically. The model developed for this purpose is a combination of a single-layer model of the oceanic mixed layer, based on a simple closure of the turbulence kinetic energy equation, and a three-dimensional isopycnic coordinate model of the stratified oceanic interior. The joint model, set in a rectangular ocean basin, is forced by annually varying wind stress and radiative plus turbulent heal fluxes approximating zonnaly averaged conditions over the North Atlantic. Special emphasis is placed on the description of the mixed-layer detrainment process, which requires distributing mixed-layer water of continuously variable density among constant-density interior layers. The truncation errors associated with this process are found to be numerically tolerable. The quasi-Lagrangian character of the model's vertical coordinate permits easy tracking of water masses left behind during the annual retreat of the mixed layer to form the seasonal thermocline. Likewise, the subduction of ventilated water into the permanent thermocline by the horizontal gyre motion is explicitly simulated. While a comparison of simulated mixed-layer characteristics with actual observations is problematic due to the idealized basin configuration, the model appears to be reasonably successful in duplicating the seasonal cycle of the zonally averaged conditions over the North Atlantic.