Subducting filaments in frontal zones in the Western Mediterranean Sea: Physical, turbulent and biological evidences

Oceanic frontal areas are well known as sites prone to the generation of submesoscale instabilities that can lead to surface waters subduction along isopycnal surfaces well below the mixed layer. Those processes can play an important role in the vertical exchange of physical properties and biogeochemical tracers.
In the framework of the CALYPSO DRI research initiative (Coherent Lagrangian Pathways from the Surface Ocean to Interior), turbulent dissipation rates characterizing subducting filaments originated form frontal areas were studied with a free falling microstructure profiler. Microstructure profiles, along ancillary data, were collected on several transects along and across frontal areas and mesoscale eddies in the Western Mediterranean Sea during two cruises: one in the Alboran Sea (March/April 2019) and one in the Balearic Sea (February/March 2022).
The presence of subducting filaments moving along isopycnal surfaces was identified at depths between 100 and 250 m by the combined analysis of physical (i.e. temperature and salinity), chemical (i.e. dissolved oxygen) and biological properties (i.e. high chlorophyll concentration well below the mixed layer and the deep chlorophyll maximum). The majority of the subducting filaments were characterized by turbulent kinetic energy dissipation rates (TKE, values of 10^-7 W·m^-2) much higher than rates generally observed at such depths. The TKE values were found in conjunction with an increase in Brunt Vaisala frequency and low Thorpe scale values. The same conclusion can be drawn from Turner angle values.