Characterization, distribution, and evolution of surface fronts in the Amazon Plume region

The Amazon River runoff reigns absolute as the most prominent river discharge to the ocean, with about 0.2 Sv of freshwater entering the Northwest Atlantic. The Amazon River outflow together with the North Brazil Current (NBC), dominates the low sea surface salinity spread into the open ocean. At the edge of the river plume, stirring by the NBC and its eddies generates sharp gradients at scales from Ο(0.1-100) km. These (equatorial to tropical) submesoscale fronts are important, for example, in modulating air-sea interactions and the energy cascade. In the EUREC4A-OA project, we use state-of-the-art Saildrone observations and numerical simulation to assess surface gradients in the northwestern tropical Atlantic. Our objective is to provide a comprehensive picture of surface gradients and associated fronts in the Amazon Plume region. From observations, we find that the plume influences density gradients from scales l < 30 km; sharp gradients skyrocket within the plume at l < 10 km, a scale that has recently been shown to mark a shift from an inverse to a forward energy cascade. Using a Δx ≅1 km CROCO simulation, we assess the spatial distribution of surface fronts and their spatio-temporal variability. Salinity dominates surface gradients even outside the plume due to an almost permanent barrier layer formed by mixing of low salinity water from previous seasons. Near the shelf, the Amazon runoff controls the formation and evolution of fronts. As we move poleward, the NBC dictates the distribution of the surface fronts. The influence of the NBC gradually decreases until the distribution of fronts closely follows the mixed layer dynamics.