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

Abstract

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.

Submesoscale Coherent Vortices in the South Atlantic Ocean: A Pathway for Energy Dissipation

“…Low PV values are prone to instabilities (D'Asaro et al, 2011;Thomas et al, 2008Thomas et al, , 2013. Here, we evaluate the two-dimensional PV (e.g., Lazaneo et al, 2020;Napolitano et al, 2021;Ramachandran et al, 2018;Thomas et al, 2016) from the objectively mapped field. If one assumes that the features are indeed SCVs and that the sampling of the SCV currents and density occurs through their centers, then an approximation of the Ertel PV is possible.…”

“…For this study, we chose segments of the water column corresponding to 1 s to convert temporal derivatives to spatial derivatives, assuming a frozen field hypothesis (Taylor, 1938). Lazaneo et al (2020) showed that there is no significant ɛ variation from the different lengths of segments. Since the VTR presents very abrupt changes of depth, using a bin size of 1 s to estimate ɛ provides a reasonable number of values, mainly over shallow regions.…”

“…Lazaneo et al. (2020) showed that there is no significant ɛ variation from the different lengths of segments. Since the VTR presents very abrupt changes of depth, using a bin size of 1 s to estimate ɛ provides a reasonable number of values, mainly over shallow regions.…”

Revisiting the Atlantic South Equatorial Current

“…Figure 10b shows the Ilhéus Anticyclone being formed by the bifurcation of the 14°S band; the Royal‐Charlotte Anticyclone being formed by the BC and the 18°S SSEC band; and the Abrolhos Anticyclone being formed by the BC and the 21°S SSEC band. The latter band simply adds volume to the BC, as this boundary current reattaches itself to the continental slope south of the Vitória‐Trindade Ridge crossing (e.g., Lazaneo et al., 2020; Napolitano et al., 2021).…”

Submesoscale Ageostrophic Motions Within and Below the Mixed Layer of the Northwestern Pacific Ocean

“…Recently, the physics of submesoscale turbulence has attracted great attention as it is not well understood due to the difficulty and challenge in observing submesoscale dynamics (McWilliams, 2016). Recent literature has reported that submesoscale processes play a crucial role in the forward energy transfer from the geostrophic scale toward dissipation (e.g., Cao et al., 2021; Capet et al., 2008a; D’Asaro et al., 2011; Jing et al., 2021; Kaneko et al., 2013; Lazaneo et al., 2020; McWilliams, 2016; Thomas et al., 2008; Wang et al., 2018; Zhang et al., 2016) and highlighted their importance to the vertical transport of heat, salt, and nutrients, which ultimately is important for biological dynamics (e.g., Lévy et al., 2012, 2001; Mahadevan & Tandon, 2006; Rosso et al., 2014; Siegelman et al., 2020; Su et al., 2018).…”