Mixing and submesoscale dynamics in the western South Atlantic Ocean

• Main Author: Lazaneo, Cauê Zirnberger
• Other Authors: Silveira, Ilson Carlos Almeida da
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Biblioteca Digital de Teses e Dissertações da USP 2021
• Subjects: Cascata de energia direta; Interação escoamento-topografia; Turbulência e mistura oceânica; Vórtice coerente de submesoescala; Flow-topography interaction; Forward energy cascade; Submesoscale coherent vortex; Turbulence and ocean mixing; South Atlantic Ocean
• Online Access: https://www.teses.usp.br/teses/disponiveis/21/21135/tde-03052022-111245/; https://doi.org/10.11606/T.21.2021.tde-03052022-111245

This dissertation addresses two oceanic processes that result in the dissipation of turbulent kinetic energy in the western South Atlantic Ocean. We first approach the vertical turbulent flux and mixing at pycnoclinic level related to the flow reversal between the Brazil Current (BC) and the Intermediate Western Boundary Current (IWBC). We secondly address the effect of the topography of the Vitória-Trindade Ridge (VTR) on the generation of subsurface submesoscale vortices, which results in the dissipation of turbulent kinetic energy as well as mixing. In the first study, we investigate the role of the vertical shear generated by the flow reversal between the poleward-flowing Brazil Current (BC) and the equatorward-flowing Intermediate Western Boundary Current (IWBC), which occurs just below the mixed layer at the latitude of 21.6º S. From unprecedented measurements of microstructure in the region, we observe that the vertical shear at the interface between the two currents locally destabilizes the water column, and may overcome the stabilizing effect of stratification. Thus, mixing processes occur, resulting in vertical exchanges of various properties at the base of the mixed layer. In particular, we compute the vertical nitrate flux, and observe that turbulence may supply the upper layer with nutrients. In the second study, we seek intrapycnoclinic, submesoscale coherent vortices (SCVs) in the VTR region from synoptic observations. We captured two anticyclonic SCVs embedded in a meander of the South Equatorial Current (SEC) from high-resolution measurements of temperature, salinity, and velocity. The SCVs were found at the lee of the Columbia Seamount (20.5º S, 32.3º W). As these structures are adjacent and interacting, we interpret their observed structure as a submesoscale version of the Fujiwhara effect in the ocean. Both eddies present low potential vorticity, and distinct signatures of temperature and salinity relatively to surrounding waters. The more homogeneous water characteristics are a result of ...