Ocean small-scale fronts in the Northwestern Tropical Atlantic: Assessment from the EUREC4A-OA/ATOMIC field experiment

Upper-ocean fronts are dynamically active features of the global ocean that have significant implications for air-sea interactions, vertical mass and heat transfers, stratification and phytoplankton production and export. They have a large range of spatial scales from submesoscale (1 – 10 km) to mesoscale (10 – 100s km) characterized by temporal variability from days to months. The small dimensions and short duration of these structures have limited our capacity of observing, modelling and understanding fully these processes and their impact.

The EUREC4A-OA/ATOMIC field experiment, that took place during January-February 2020 in the Northwest Tropical Atlantic, has tried to address this challenge. In particular, five Saildrones, which are uncrewed platforms instrumented to measure the air-sea interface, have been deployed. This region showed to be a well-suited laboratory to investigate horizontal density surface gradients over a wide range of scales. Strongly affected by the outflow of the Amazon River, the generation of fine-scale horizontal thermohaline gradients is favored by the stirring of this freshwater input by large anticyclonic eddies (a.k.a. North Brazil Current Rings). The distribution of these frontal structures highlights the presence of very intense gradients, including at the smaller spatial scales. The coherence of temperature and salinity fronts was estimated by a wavelet transform analysis. It reveals that large-scale density fronts are primarily controlled by horizontal variations in salinity but with increasing temperature-salinity coherence at the small scales range of the spectrum (O (0.1 km)) for strong gradients whereas they are poorly correlated for weaker fronts.

Our study shows that processes such as the mixed layer depth, the diurnal cycle, and air-sea exchanges are strongly affected by these small-scale frontal regimes. The parallel and quasi synchronous tracks of a 4-Saildrone formation provide a detailed picture of the upper ocean vorticity, divergence, and strain from their ADCP current measurements. Overall the methodology that has been developed could be extended on other datasets in order to assess the phenomenology of fine-scale structures in other dynamical regions.