Observation of Mesoscale Instabilities of the Northern Current in the North Western Mediterranean Sea : a Combined Study Using Gliders, Surface Drifters, Moving Vessel Profiler and Vessel Data in the Ligurian Sea

Abstract:

The Northern Current (NC) is a branch of the general North-Western Mediterranean cyclonic circulation extending from the Ligurian to the Catalan Sea (Millot, Dynamics of Atmospheres and Oceans, vol. 15, 1991). In winter and early spring, instabilities of this slope current are intense and can generate eddies, meanders and filaments. The study of mesoscale structures is crucial in the coastal area because of their physical and biogeochemical impact on ecosystems. They play a role in water, chemicals and nutrients transport, vertical mixing and possible trapping of biological materials.

Results from a combined observational effort put forth in March 2012 during the IMEDIA cruise dedicated to eddy tracking are presented. This work aims at providing experimental evidence of the effects that mesoscale exerts on the NC dynamics via an innovative and complementary data set. A Slocum Glider equipped with a CTD was deployed along the French coast from Nice to Toulon for one month and performed cross-current sections down to 600m depth. Drifters were deployed before the cruise in the Corsica Channel and close to Italy. Concurrent observations were obtained along the vessel track by a thermosalinograph and a fluorometer (subsurface measurements), and using a CTD and a Vessel-Mounted ADCP during the 11-day oceanographic cruise on board of the Research Vessel Tethys II. Additional water profiling was performed using a Moving Vessel Profiler (MVP) equipped with a CTD and capable of profiling down to 400m depth at a speed of 4 knots.

The combined use of data from the MVP and the ship-based ADCP measurements reveals the presence of an instability of the NC north of the Corsica Island. The main branch of the current is moved westward and forms a meander. It is characterized by a density decrease down to 350m of the water column, associated with a salinity decrease of 0.4-0.6 psu (see Figure). Its location is confirmed by high-resolution satellite images as the associated water masses are cooler and more oligotrophic than the adjacent ones.

Additional measurements from the glider deployed off the French coast show that inside the NC vein, the water column has a multi-layer structure with the presence of filaments of differing salinity and temperature.