Thermohaline gradients and frontal regimes in the northwestern Tropical Atlantic

Abstract. At the edge of the Amazon River plume, stirring by the North Brazil Current (NBC) and its eddies creates sharp surface thermohaline gradients on scales of O(1–100) km. This study provides a comprehensive picture of these gradients and fronts associated with the region's distinctive dynamics. Saildrone observations show that the plume amplifies density gradient variability at all scales from 1–100 km, with frontal sharpness up to 75x stronger inside the plume than outside, with differences reaching 100x at scales below 3 km. Partial temperature-salinity compensation and reinforcement act on density gradients, with net frontogenesis observed in both regions. To expand in-situ observations, we use a 1-km resolution CROCO simulation to assess the spatial distribution of surface fronts and their spatio-temporal variability. We characterize three distinct frontal regimes: broken-up fronts parallel to shore occupy the plume core over the continental shelf, thin elongated fronts associated with NBC-plume interactions connect nearshore and offshore regions, and pools of anisotropic fronts driven by the mixed-layer cycle are present offshore. Salinity dominates density gradients throughout the year north of 15 °N, whereas near-shore fronts exhibit seasonal shifts in temperature-salinity dominance linked to the Amazon discharge seasonality and NBC strength. Within the plume, freshwater filaments stirred by NBC rings systematically generate compensated fronts on their inner edge and reinforced fronts on their outer rim, a pattern with implications for energy cascades and tracer export.