Physical and dynamical characteristics of a 300 m-deep anticyclonic eddy in the Ligurian Sea (Northwest Mediterranean Sea): Evidence from a multi-platform sampling strategy

We describe the physical and dynamical characteristics of a mesoscale anticyclonic eddy observed in August 2013 over the shelf-break region of the northeastern sector of the Ligurian Sea, between the northeastern edge of the Northern Current (NC) and the coast. Results derive from a dense dataset of temperature, salinity and current measurements obtained from a multi-platform sampling strategy as well as from a diagnostic simulation with the Regional Ocean Modeling System (ROMS) at a horizontal resolution of 1.8km. Model results are obtained from a strong nudging to observations and, as they are physically balanced, they allow a three-dimensional diagnosis of the dynamics and physical characteristics of the eddy. The eddy is centered around 9.5°E, 43.94°N, about 20km from the coast, and has a radius of 16km. It is characterized by low-density waters and penetrates the thermocline down to at least 300m reflecting the main features of the NC. Horizontal velocities near the surface are around 0.4ms−1, while at 150m are still significantly high and close to 0.2ms−1. Vertical velocities were estimated from model results; absolute values are below 4mday−1 until depths shallower than 150m and increase with depth to 15mday−1. The eddy's presence inverts the northwestwards flow traditionally portrayed in the region determining a southeastwards coastal circulation that replenishes coastal waters with those originating from the NC. We discuss several possible eddy formation mechanisms and suggest that its formation depends on the directionality of the NC when it enters the Ligurian Sea, as a result of the adjustment of the shear and orbital components of the current's relative vorticity.