A three-dimensional model study of the Mediterranean outflow

Abstract

Evaporation over the Mediterranean basin produces a salty water mass that overflows the relatively narrow and shallow Strait of Gibraltar. The outflow is investigated with the three-dimensional Princeton Ocean Model. The model makes use of a bottom-following, sigma-coordinate system and an imbedded turbulence closure scheme to simulate the bottom boundary layer. Starting as a channel flow confined by the sidewalls of the trench of the westernmost part of the Strait, the bottom boundary current descends the steep continental slope of the eastern Gulf of Cadiz. The flow is controlled by a balance of the pressure gradient and the Coriolis acceleration, entrainment and bottom friction. Downslope Ekman fluxes are largest at the shelf break near the entrance of the Strait where we observe the most pronounced entrainment of North Atlantic Central Water. Profiles of modeled horizontal velocity, as well as scalar and turbulent quantities, show a remarkable resemblance to observed features. The model reproduces the “nose” shape of the velocity profile that is typical for a density current; below the velocity maximum, there is a well-mixed layer and a stratified shear layer exists above the nose. The evolution of water mass properties along the path of the outflow agrees favorably with observations and the sensitivity of the Mediterranean outflow (MO) to varying source water properties is investigated. The combined effect of initial stratification, differential entrainment, and routing by the topography leads to the evolution of the well-known two-core structure of the bottom layer. Whereas the upper part of the outflow continues to follow the continental slope northward in the form of a bottom boundary current after it passes Cape St. Vincent, the lower core separates from the slope and there is a lateral spreading of the saline and warm outflow water in the depth range of the lower salinity maximum (centered at about 1200 m). The outflow becomes hydrodynamically unstable and lenses of saline water shed from the core, carrying their water mass characteristic into the Atlantic. The model results confirm that the area of Cape St. Vincent is a prominent formation area for Mediterranean water (MW) eddies (Meddies).