Upper ocean fronts are dynamically active features of the global ocean playing a key role in the air-sea exchanges of properties and their transport in the ocean interior. With scales ranging from the submesoscale (0.1–10 km) to the mesoscale (10–100s km) and a temporal variability from hours to months, collecting in situ observations of these structures is challenging and this has limited our understanding of their associated processes and impacts. During the EUREC4A-OA/ATOMIC field experiment, which took place in the northwest tropical Atlantic in January–February 2020, a large number of uncrewed platforms, including five Saildrones, were deployed to provide a detailed picture of the upper-ocean fine-scale variability. This region is strongly influenced by the outflow of the Amazon River, even in winter, which is the minimum outflow season. Here, the generation of fine-scale horizontal thermohaline gradients is driven by the stirring of this freshwater river input by large anticyclonic eddies, the so-called North Brazil Current Rings. Vertical shear estimates using the Saildrones ADCP show that partial temperature compensation occurs along restratifying submesoscale salinity-dominated fronts. The distribution of surface along-track gradients, as sampled by different horizontal length-scales, reveals the prevalence of submesoscale fronts. This is supported by a flattening of the spectral slopes of surface density at the submesoscale. This study emphasizes the need to resolve the upper ocean at high spatial resolution to understand its impact on the broader circulation and to properly represent air-sea interactions.

Key Points

Vertical shear sections from the Saildrone ADCP data reveal that temperature compensation develops along restratifying submesoscale fronts

Temperature and salinity spectra show an increased temperature contribution to salinity-dominated fronts at the submesoscale

A flattening of the density spectra from ~−3 to ~−2 at the submesoscale (1–10 km) is observed in the Boulevard des Tourbillons

Plain Language Summary

Oceanic eddies and filaments that range between 10 and 100 km in size can be identified in the study region of the northwestern tropical Atlantic using ocean color as viewed from space (a proxy for chlorophyll-a). The ocean color maps show that these eddies and filaments are associated with the detachment from the shelf of a freshwater Amazon plume and its interaction with the larger oceanic motions

(100 km). Field observations from different measurements acquired from research vessels and five uncrewed surface vehicles (USVs) reveal the prevalence and scale of upper ocean fronts, whose magnitude results from the combined effect of temperature and salinity. The Amazon freshwater plume is key to the formation of strong salinity-driven density fronts. However, when looking at (1 km) scales, we detect horizontal temperature variations along slumping fronts that partially counteract the effect of salinity. This leads to a damping of the lateral density fronts. This study contributes to the development of a detailed picture of the ocean fine scales, which is necessary to improve our understanding of air-sea interactions over frontal regions.