A simple model of the large-scale circulation of Mediterranean Water and Labrador Sea Water

Abstract

A steady, 2 1 2 layer planetary geostrophic model is used to study the circulation and mixing of Mediterranean Water (MW) and Labrador Sea Water (LSW). The model includes parameterizions of salt fingering, mesoscale variability, and meddies. A close relationship between the vertical density ratio (and expected strength of salt fingering) and the potential vorticity anomaly of the upper salt tongue is identified and used to parameterize salt fingering as a spatially nonhomogeneous diapycnal mass flux. A balance between baroclinic Rossby wave propagation and salt fingering produces an asymmetric tongue of potential vorticity extending into the interior from the eastern boundary, consistent with the observed Mediterranean salt tongue. The resulting internal pressure gradients give rise to a large-scale anticyclonic circulation in the upper layer of the salt tongue with northward flow at mid-latitudes, consistent with geostrophic flow estimates based on hydrographic data. Salt fingering that attains maximum strength at mid-latitudes forces a large-scale circulation in the lower MW/LSW depth range that advects water from the western boundary into the eastern basin near 50°N, where it then flows to the south under the upper MW, and finally returns to the western boundary. This pattern is generally consistent with a variety of estimates for the circulation of LSW, and suggests that the eastward flow of LSW off the western boundary may be, at least in part, forced by salt fingering in the interior. The addition of mixing due to mesoscale eddies modifies the details of the potential vorticity distribution, but does not change the qualitative behavior. Representing meddies as a distributed source of mass to the upper layer results in a further westward extension of the salt tongue signature and enhances the strength of the deep recirculation.