Oceanic Fronts Driven by the Amazon Freshwater Plume and Their Thermohaline Compensation at the Submesoscale

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.