Energy dissipation within an anticyclonic eddy in the South Atlantic.
Supervisor: Maren Walter, Ria Oelerich (Univ. Bremen)
This study examines the dissipation rate of turbulent kinetic energy (ϵ) within an anticyclonic eddy below the mixed layer in the Cape Basin. In the region where two particularly strong counter-directed currents interact, generating eddies and Agulhas rings, heat, salt and high eddy kinetic energy (EKE) are transported from the Indian Ocean to the South Atlantic. In this study, microstructure measurements represent the primary observational data, which are complemented by satellite altimetric data and reanalysis data. In addition to observational data, modelling data are applied to predict barotropic tides. The results demonstrate that ϵ is elevated within the eddy centre, resulting in high diapycnal mixing at rates at O(10−5) m2s−1. The shear-induced Kelvin-Helmholtz instability is identified as a significant contributor to the diapycnal (vertical) mixing at the periphery of the eddy, while the expansion of the thermocline within the anticyclonic eddy entrains other mechanisms that are responsible for the highest energy dissipation and mixing rate within the eddy centre. Local forcing, such as wind and tides, exerts a significant influence on mixing processes. In conditions of moderate wind and spring tide, the tidal cycle exerts a notable influence on the development of low and high stratification. Tidal energy represents a primary source of energy for the ocean, with ramifications for the dissipation of turbulent kinetic energy below the mixed layer. In the absence of eddy centre effects, the diapycnal diffusivity exhibits a range between O(10−7) and O(10−6) m2 s−1 below the mixed layer in the study region across the neap-spring tidal cycle.

Keywords: microstructure measurements, anticyclonic eddy, Agulhas ring, energy dissipation, turbulent mixing, diapycnal diffusivity, below the mixed layer