Two Years in the Life of a Mediterranean Salt Lens

Abstract

A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s?1, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to be the most important mode of mixing in terms of salt and heat loss. Radial profiles of azimuthal velocity revealed a core in almost solid body rotation, with a period of 5?6 days corresponding to 0.35 times the local Coriolis parameter. During the October 1984 survey, the azimuthal speed had a maximum of 0.3 m s?1 at a radius of 24 km. Both the radius and magnitude of the velocity maximum decreased with time. The anticyclonic circulation attained a maximum at the radius of the salinity front. As the lens was eroded from the sides, the radius of maximum velocity and the maximum velocity both decreased, but the rotation rate of the core remained fairly steady.