Abstract

Lagrangian trajectories from the SOFAR float Mediterranean outflow experiment are used to estimate the low frequency variability and mixing in the vicinity of the Mediterranean salt tongue. Two dominant patterns of Lagrangian variability are observed, (1) nearly zonal low frequency motions and (2) wave-like oscillations with northwest to southeast orientation. The zonal motions are found near the core of the salt tongue in the Canary Basin while the oscillations are generally found to the south and east. It is suggested that the zonal motions are the result of baroclinic instability of the large-scale flow. They have zonally enhanced low frequency variability (periods greater than 200 days) and nearly isotropic mesoscale variability (periods 50 to 200 days). The wave motions are believed to be the signature of radiating baroclinic Rossby waves generated to the south at the Cape Verde Frontal Zone. They are strongly peaked at the mesoscale band and have an essentially isotropic low frequency component. Integral time scales for the zonal motions are relatively long (23 and 13 days for the zonal and meridional directions) while for the wave motions they are short (7.7 and 5.0 days). The resulting eddy diffusivities are found to be non-isotropic and non-homogeneous with (Kxx, Kyy) = (21 and 8.4 × 106 cm2 s−1) in the core of the salt tongue (mainly zonal motions) and (Kxx, Kyy) = (4.3 and 3.5 × 106 cm2 s−1) to the south of the core (mainly wave-like motions). A simple scale analysis indicates that these time dependent motions play the dominant role in the spread of the Mediterranean salt tongue in both the zonal and meridional directions.

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