Small Mesoscale Features at a Meandering Upper-Ocean Front in the Western Ionian Sea (Mediterranean Sea): Vertical Motion and Potential Vorticity Analysis

Transient mesoscale and submesoscale processes, such as small eddies and filaments, could play a fundamental role in the marine ecosystem, especially in oligotrophic seas like the Mediterranean. However, very little is known about the biological and physical dynamics characterizing such structures. In this work, the 8-km horizontal resolution data collected during the SYMPLEX 1998 survey are analyzed to describe the physical dynamics of small mesoscale features along the Atlantic–Ionian stream (western Ionian Sea). The data were optimally interpolated over a regular grid and used to compute the 3D ageostrophic circulation by solution of the Q-vector formulation of the omega equation. The relative importance of stratification, relative vorticity, and twisting terms in the Rossby–Ertel potential vorticity is thus examined along selected isopycnals, together with the associated vertical motions and vortex stretching. A surface cyclonic eddy ∼15 km of radius was observed near a meander of the Atlantic–Ionian stream. This small cyclone is characterized by high potential vorticity values similar to that of the meander, by a relative vorticity of 0.5f, and by vertical velocities of the order of 10–14 m d−1. A subduction process of ∼15 m d−1 was found in correspondence of the meander trough, giving rise to a deep cold anticyclone, while peak upward velocities reached the same order of magnitude at the trough.