On the Role of Turbulent Mixing Produced by Vertical Shear Between the Brazil Current and the Intermediate Western Boundary Current

An intensification of the vertical shear is observed below the surface mixed layer at 21${}^{\circ }$S due to the mutually opposing flows of the Brazil Current (BC) and the Intermediate Western Boundary Current (IWBC). 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 BC-IWBC, providing direct quantitative values of the turbulent fluctuations. Peaks of strong dissipation rates of turbulent kinetic energy (TKE) (O(10${}^{-8}$) W~kg${}^{-1}$) were observed close to the base of the surface mixed-layer. On the other hand, prominent peaks of TKE dissipation rates (O(10${}^{-7}$) W~kg${}^{-1}$) of up to two orders of magnitude higher than the background were observed at deeper levels where stratification begins to lose intensity. The combination of the intense vertical shear and weak stratification enables better characterization of mixing processes and the role played by vertical exchanges of biogeochemical properties. Based on the estimated nitrate gradient and the vertical diffusivity, turbulent mixing driven by vertical shear plays an important role in the supply of nitrate to the upper layer.