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 fi laments, 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 SYMP LEX 1998 survey are analyzed to describe the physical dynamics of small mes oscale features along the Atlantic-Ionian stream (western Ionian Sea). The data were optimally interpolated over a regular grid and used to compute th e 3D ageostrophic circulation by solution of the Q-vector formulation of th e omega equation. The relative importance of stratification, relative vorti city, and twisting terms in the Rossby-Ertel potential vorticity is thus ex amined along selected isopycnals, together with the associated vertical mot ions and vortex stretching. A surface cyclonic eddy similar to 15 km of rad ius was observed near a meander of the Atlantic-Ionian stream. This small c yclone is characterized by high potential vorticity values similar to that of the meander, by a relative vorticity of 0.5 f, and by vertical velocitie s of the order of 10-14 m d(-1). A subduction process of similar to 15 m d( -1) was found in correspondence of the meander trough, giving rise to a dee p cold anticyclone, while peak upward velocities reached the same order of magnitude at the trough.