A Three-Dimensional Simulation of the Formation of Anticyclonic Lenses (Meddies) by the Instability of an Intermediate Depth Boundary Current

Abstract

The evolution of a buoyancy-driven intermediate water current along an idealized continental slope is investigated using a three-dimensional primitive equation ocean model. The outflow current, originating in a marginal sea, flows northward along a linear sloping bottom where it undergoes considerable mixing. For a parameter range resembling the Mediterranean outflow, the flow becomes hydrodynamically unstable. The intruding, relatively dense outflow water interacts with the overlaying slope water by inducing vortex compression and stretching. The phase relation between the upper-layer and lower-layer perturbations indicates that baroclinic instability is the driving mechanism in the initial destabilization. As the disturbances grow to large amplitude, dipoles are formed and move away from the boundary current in a westward direction. While the amplifying anticyclones wrap around the intruding low potential vorticity water, the cyclones usually entrain ambient water and decrease in strength. Isolated anticyclones finally separate from the boundary current and develop a southward translation component. In some cases dipolar structures remain intact and propagate to the west. In the slope water strong cyclones and somewhat weaker anticyclones are formed periodically and travel in the direction of the bottom flow. The upper-layer flow shows surface expressions of the middepth lenses that develop as meddies. These penetrate into the ambient slope water and gain a barotropic velocity component. The adjustment process is discussed in terms of potential vorticity conservation. The instability mechanism discussed here does not require any topographic irregularities. Sensitivity experiments, however, suggest that canyons or capes introduce an additional source of vorticity and may lead to the observed localization of spawning events.