No abstract

The Intermediate Western Boundary Current (IWBC) transports Antarctic Intermediate Water across the Vitória–Trindade Ridge (VTR), a seamount chain at ~20°S off Brazil. Recent studies suggest that the IWBC develops a strong cyclonic recirculation in Tubarão Bight, upstream of the VTR, with weak time dependency. We herein use new quasi-synoptic observations, data from the Argo array, and a regional numerical model to describe the structure and variability of the IWBC and to investigate its dynamics. Both shipboard acoustic Doppler current profiler (ADCP) data and trajectories of Argo floats confirm the existence of the IWBC recirculation, which is also captured by our Regional Oceanic Modeling System (ROMS) simulation. An “intermediate-layer” quasigeostrophic (QG) model indicates that the ROMS time-mean flow is a good proxy for the IWBC steady state, as revealed by largely parallel isolines of streamfunction [Formula: see text] and potential vorticity [Formula: see text]; a [Formula: see text] scatter diagram also shows that the IWBC is potentially unstable. Further analysis of the ROMS simulation reveals that remotely generated, westward-propagating nonlinear eddies are the main source of variability in the region. These eddies enter the domain through the Tubarão Bight eastern edge and strongly interact with the IWBC. As they are advected downstream and negotiate the local topography, the eddies grow explosively through horizontal shear production.

An intensification of the vertical shear is observed below the surface mixed layer at 21 • S due to the mutually opposing flows of the Brazil Current and the Intermediate Western Boundary Current. The propensity to develop turbulence and mixing due to vertical shear over intense stabilizing density gradients is an important characteristic of such environments. For the first time, microscale measurements were made in the Brazil Current-Intermediate Western Boundary Current system, providing direct quantitative values of the turbulent fluctuations. Peaks of relative strong dissipation rates of turbulent kinetic energy (O(10 −8 ) W/kg) were observed close to the base of the surface mixed layer. On the other hand, prominent peaks of turbulent kinetic energy dissipation rates of up to 2 orders of magnitude higher than the background were observed at deeper levels, where stratification begins to lose intensity. Analyzing such peaks, caused by intense vertical shear or weak stratification-and sometimes both-, allows a characterization of the local mixing processes and the role played by vertical exchanges of biogeochemical properties. Based on the estimated nitrate gradient and the vertical diffusivity, we show that turbulent mixing driven by vertical shear plays an important role in the supply of nitrate to the upper layer.Plain Language Summary Turbulent mixing across the density surfaces can bring nutrient-rich waters from the subsurface to the upper sunlit layer of the ocean, therefore, modulating the primary productivity in an oligotrophic ocean. Based on measurements of small-scale shear variance, we found that the interaction between the poleward-flowing Brazil Current and the Intermediate Western Boundary Current flowing underneath in the opposite direction enhances the upper-ocean mixing through shear instabilities. The destabilizing influence of the velocity shear overcomes the stabilizing effect of the stratification. The mixing on the interface between these two western boundary currents may provide an important route for local nutrient exchanges.

At latitudes north of which it becomes a robust western boundary current, the newly formed, poleward-flowing Brazil Current (BC) encounters a zonal seamount chain-the Vitória-Trindade Ridge (VTR). Located at 20.5°S, with seamounts reaching up to 30 m below the surface, the VTR poses an obstacle for the typically 150-m-deep BC southward path (