Abstract
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 21.6° 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 (20.5° S, 32.3° 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 mixing in their interior. Through microstructure measurements taken in one of the SCVs, we observe turbulent kinetic energy dissipation rates similar in magnitude to those measured in the mixed layer. To the best of our knowledge, these are the first microstructure measurements taken in the interior of an intrapycnoclinic SCV. We cannot determine the region of formation of such SCVs from one hydrographic transect. However, we suggest that they could be generated through flow-topography interaction since the SEC meander was observed interacting with the Trindade Island two months earlier. In the third study, we investigate whether the VTR is prone to generate submesoscale vortices. We then simulate, with a regional ocean numerical model, the SEC interacting with the ridge. We observe that this type of vortex is often formed in the region at different topographic features and different (intra- and subpycnoclinic) depths along its 900km extension. We simulate SCV generation with both polarities. From the interaction between the SEC and the topographic features of the VTR, vortical filaments of Rossby number O(1) are formed downstream of the topography, creating potential vorticity anomalies. These filaments eventually evolve, roll up and form SCVs. We suggest that VTR can be considered a hotspot for the generation of submesoscale vortices. Finally, we highlight the importance of the western South Atlantic Ocean regarding the development of small scale processes and energy dissipation. This region is key to the comprehension of the ocean energy budget.