Mechanism for low‐frequency variability and salt flux in the Mediterranean salt tongue

The role of baroclinic instability in the generation of low‐frequency variability in the interior of ocean subtropical gyres and its possible importance in the zonal flux of salt and heat within the Mediterranean salt tongue are discussed. The observed zonal enhancement of the low‐frequency variability is interpreted as the signature of baroclinic instability of the large‐scale wind‐driven flow. The zonal orientation results from the relatively weak vertical shear of the large‐scale upper ocean flow and the stabilizing influence of the planetary vorticity gradient. There is a reversal in the vertical shear of velocity at middepths in the eastern basin of the North Atlantic that results in a local middepth maximum in the zonal eddy flux of density. Linear theory predicts a vertical structure of this density flux that is nearly coincident with the observed vertical distribution of the warm, salty water of the Mediterranean salt tongue. A primitive equation model is used to investigate the nonlinear, large‐amplitude regime. The zonal eddy density (or salt) flux averaged over several wavelengths and several cycles of wave growth and breaking indicates that this mechanism may be an important component in the overall salt balance in this region.