FaSt-SWOT field campaigns combining high-resolution observations and modelling for SWOT validation in the Western Mediterranean Sea

The FaSt-SWOT sea trial experiments, conducted in the Balearic Sea (Western Mediterranean Sea) between 25-28 April and 7-10 May 2023, aimed at collecting multi-platform in-situ observations of meso- and submesoscale ocean structures in the area covered by the SWOT satellite during its initial fast-sampling phase. The general objectives of the FaSt-SWOT project are twofold: 1) participate with these data to the satellite cal/val activities, and 2) improve the characterization and understanding of the fine-scale dynamics by combining in-situ multi-platform and satellite data with high-resolution numerical models and machine-learning-based computational techniques. The experiments consisted in 2 phases both using multi-scale ship-based instruments (CTD, Moving Vessel Profiler, thermosalinograph, ADCP and GoPros), autonomous platforms (surface drifters and gliders), and satellite observations (SST, ocean color and altimetry). In addition, 2km-resolution data-assimilative modelling simulations were produced to provide a complementary view of the fine-scale ocean variability. Finally, machine-learning-based optimization algorithms were also tested to define adaptive sampling strategies during the experiment.  The sampling first focused on a 20km-diameter anticyclonic eddy detected under the swath of the satellite thanks to satellite imagery and drifter trajectories. Several cross-sections of the Moving Vessel Profiler and underwater gliders provided insights into the vertical structure of temperature and salinity fields and the associated signals in chlorophyll and dissolved oxygen. Two gliders were programmed to perform back-and-forth sections during a 3-week time with a 1-day delay between them, allowing to evaluate the temporal variability of the ocean fields at the period of repetitivity of the satellite. The second phase started 9 days after the end of the first one. A 48-hour dense radiator-like pattern was performed by R/V SOCIB, allowing to characterize the evolution of the small eddy observed during the first leg.  A total of 45 surface drifters were deployed during the two phases to evaluate in-situ surface currents and their associated convergence and divergence in the vicinity of the eddy. While conventional altimetry was not able to properly represent the sea level signature of the observed eddy, initial SWOT measurements indicate an improved detection capability by the new satellite. In addition, high-resolution numerical simulations reproduce a small anticyclonic eddy with similar characteristics as that of the observed eddy. These simulations are used to provide a more general understanding of the situation, indicate the origin of the eddy in the frontal area between recent and modified Atlantic waters, and provide insights into the vertical extension of the small mesoscale structure.   We provide here an overview of the whole FaSt-SWOT dataset, including both observing and modelling components. A more detailed analysis of the measurements is provided in a companion presentation.