Abstract

Data from the Tourbillon Experiment in the eastern North Atlantic indicate clearly the stirring of waters with contrasting thermohaline properties by a mesoscale eddy, and the ensuing mixture which occurred. The observed features are discussed in relation to a mixing scenario which considers the salinity distribution in the eastern N. Atlantic associated with the Mediterranean Water (MW) outflow through the Straits of Gibraltar to provide a large-scale context. A mesoscale eddy near the boundary of this water mass advected and deformed a blob of MW, sharpening thermohaline fronts so that double diffusive frontal intrusions developed. Double diffusion processes are invoked as the basic mixing mechanism between the contrasting waters, and following the model of Joyce the lateral mesoscale diffusivity across these fronts is estimated to be 4 m2 s−1. Estimates are made of the lateral fluxes to sub-eddy scales (<20 km) by a number of essentially independent approaches, viz: (a) evaluating the changes in the temperature, salinity and potential vorticity of a particular patch of water, the successive positions of which are deduced from daily optimal streamfunction charts constructed from direct current measurements; (b) evaluating the rate of increase of salinity of the "inner shell" of the eddy which is attributed to mixing with the more saline "outer shell," (c) considering the warm salty blob of MW which was drawn into the eddy circulation as a dye patch and determining its rate of spreading from the increase of its radially symmetrical variance. All of these approaches indicate downgradient mixing of temperature, salinity and potential vorticity anomalies with effective lateral diffusivity of the order of 102 m2 s−1. This is considered to be a shear-augmented diffusivity. Using a salinity flux deduced from the eddy heat fluxes computed from the 8-month moored current meter data together with the large-scale salinity gradient implies large-scale diffusivities of the order 5 × 102 m2 s−1; these summarize the averaged effect of many eddy events and can be used to parameterize lateral mesoscale eddy fluxes. It is shown that salt fluxes of the magnitude estimated are of the order required to balance the input of salt through the Straits of Gibraltar and maintain the large-scale salinity distribution in the eastern North Atlantic.

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