Ocean scale dispersion of Amazon river plume by chaotic advection

Dispersion of physical and biogeochemical tracers of the Amazon River plume is an intriguing topic, investigated since the 90s, due to its large impact on the North tropical Atlantic Ocean. Here we quantitatively depict the nonlinear behavior of the Amazon River plume by means of Lagrangian simulations, analyzed through dynamical systems techniques. The interaction between the North Brazil Current, flowing northward along the coast, and its retroflection that diverts part of the coastal waters eastward is mostly responsible for the observed spreading of freshwater at ocean basin scale. By means of the finite scale Lyapunov exponents, we characterize this behavior and provide a quantitative tool that allows us to relate nonlinear dynamics of the Amazon River plume to hydrologic and seasonal variability. Single trajectory analysis identifies three main direct transport patterns, and their related target areas, of Amazon River's nutrient-rich waters; trajectory pair analysis, on the other hand, is consistent with a general scenario of chaotic advection, due to the presence of nonlinear, large-scale dynamical features. In perspective, our findings provide the basis for a quantitative assessment of fate, distribution, and large-scale dispersion of riverine tracers, in relation to specific river discharge and ocean dynamics.