Kinematics and Dynamics of a Mediterranean Salt Lens

Abstract

Two surveys of the absolute velocity field of an eddy of Mediterranean Water (meddy) in the Eastern North Atlantic were conducted one year apart in 1984 and 1985. Two velocity regimes were revealed. Within the radius of maximum velocity, the meddy rotated anticyclonically as a solid body with a depth-dependent rotation period near 6 days at its mid-depth (1000 m). One year later the radius of the core had decreased by one third. The rotation rate of the lens also decreased, except at its mid-depth where there was a small but perceptible increase. There was a sharp (5 km or less) transition between the core and the outer region where the velocity decayed exponentially with radius. A strong potential vorticity front, due to the abrupt change in sign of horizontal shear, kept the core isolated from the outer region. Potential vorticity was nearly constant within the upper confines of the core over the study period, whereas, there was a notable increase in potential vorticity in the lower portion of the core due to erosion from underneath. Although there were significant azimuthal velocities beyond the transition, the potential vorticity was nearly that of the background field. The horizontal uniformity of the potential vorticity field suggests free exchange along isopycnal surfaces.