Stéphanie Barrillon (Aix Marseille Univ., Université de Toulon, CNRS, MIO, 13288, Marseille, France, France); Jean-Luc Fuda (Aix Marseille Univ., Université de Toulon, CNRS, MIO, 13288, Marseille, France, France); Robin Rolland (Sorbonne Université, CNRS, IRD, MNHN, LOCEAN-IPSL, 4 place Jussieu, Paris, 75005, France, France); Gérald Grégori (Aix Marseille Univ., Université de Toulon, CNRS, MIO, 13288, Marseille, France, France); Andrea Doglioli (Aix Marseille Univ., Université de Toulon, CNRS, MIO, 13288, Marseille, France, France); Francesco d'Ovidio (Laboratoire d’Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN-IPSL), Sorbonne, France); Ananda Pascual (Mediterranean Institute for Advanced Studies, IMEDEA, (CSIC-UIB), Esporles, Spain, Spain); Anne Petrenko (Aix Marseille Univ., Université de Toulon, CNRS, MIO, 13288, Marseille, France, France)

Ocean fine-scale dynamics such as (sub)mesoscale processes (1-100 km / days to weeks) can significantly impact biogeochemistry and particularly phytoplankton production and distribution. Although difficult to sample due to their short space and time extent, fine-scale processes can induce substantial vertical velocities of a few mm/s.

The SWOT mission, launched in 2022, provides a new tool to better detect fine-scale features from altimetry at an unprecedented resolution, reaching a spatial resolution of 15km (versus >100km for previous missions). The SWOT CalVal phase provided daily data in specific areas. Therefore, the BioSWOT-Med cruise (DOI: 10.17600/18002392) took place in one of these areas in the western Mediterranean Sea, from April to May 2023, and combined modelling, in situ and satellite data to study fine-scale dynamics. Indeed, the North Balearic front between dense saline older Atlantic waters and light fresh newer Atlantic waters and a small but strong anticyclonic eddy were extensively sampled both physically and biologically.

The challenging vertical velocities measurement has been conducted using several in situ instruments and methodologies. The conventional vessel-mounted ADCPs (Acoustic Doppler Current Profilers) provide 3D velocity transects covering the entire cruise period. Two other original methods developed by our group have also been used: the Free-Fall ADCP (FF-ADCP) at stations, and the autonomous Vertical Velocity Profiler (VVP). The FF-ADCP consists of a downward-looking ADCP, connected to the boat by a loose rope, letting the ADCP drop by gravity and make measurements free of the ship's movements. The VVP is inspired from the methodology developed for gliders, using the comparison to a flight model to estimate the direction and intensity of the vertical velocities. In order to focus on physics-driven oceanic vertical velocities, we developed a method to identify and isolate biology-induced vertical velocities. The intensity and variability of the physics-driven vertical velocities have been studied in the frontal zone and in the anticyclonic eddy, dominated by near-inertial internal waves.

In addition to the innovative part of this work using three different methods to measure vertical velocities during this interdisciplinary cruise, it unlocks perspectives for inter-validation of vertical velocities between in situ measurements and sea surface height derived velocities in the context of the SWOT era.