Characterization of physical properties of  a coastal upwelling filamentwith evidence of enhanced submesoscale activityand  transition from balanced to unbalanced motions in the South Atlantic

We combine high-resolution in-situ data (ADCP/Scanfish and surface drifters) and remote sensing to investigate the physical characteristics of a major filament observed in the South Atlantic in comparison to its nearby and distant environment explored by three cruises. Observations of the  Rossby number Ro from the various platforms provide the spatial distribution of Ro for different resolutions. Remote sensing focuses on geostrophic motions of the region related to the mesoscale eddies that drive the filament formation and thereby reveals Ro<0.1.  Ship based measurements in the surface mixed layer reveal 0.5<Ro<1, indicating the presence of unbalanced, ageostrophic motions. Timeseries of Ro from triplets of surface drifters trapped within the filament confirm these relatively large Ro and show a high variability along the filament. A scale dependent analysis of Ro, which relies on the second order velocity structure function, was applied  to  the latter drifter-group and to another drifter-group released in the upwelling zone. The two releases explored  the area nearly distinctly and simultaneously, and reveal that at small scales (< 15 ~km) Ro values are twice as large in the filament in comparison to its environment with Ro >1  for scales smaller than  500 m. This suggests that filaments are hotspots of ageostrophic dynamics, pointing to the presence of a forward energy cascade.  The different dynamics indicated by our Ro-analysis are confirmed by horizontal kinetic energy wavenumber spectra, which exhibit a power law  k-α with α= 5/3 for wavelengths 2π/k smaller than a transition scale of ~15 km, supporting significant submesoscale energy at scales smaller than the first baroclinic Rossby Radius (Ro1 ~ 30 km). The detected transition scale  is smaller than those found in regions with less mesoscale eddy energy, consistent with previous studies.