UC San Diego

Direct Numerical Simulation (DNS) of simplified model problems is used to investigate the role of turbulence and internal waves in the Equatorial Undercurrents (EUC) system. Prior observational studies of the Pacific EUC have established a strong correlation between deep-cycle turbulence in the thermocline and internal waves. The object of the thesis is to understand and quantify the dynamical processes underlying turbulence and internal waves in the EUC. The investigation has three phases. In the first phase, an idealized problem of a weakly- stratified shear layer located above a thermocline is simulated to investigate internal waves excited by unstable shear. The evolution of the shear layer consists of coherent Kelvin-Helmholtz (KH) rollers and small-scale turbulence. Internal waves excited by the KH rollers are narrow-band and of stronger amplitude that of the broadband wave field generated by turbulence. Internal waves are shown to carry significant amount of momentum and energy away from the shear layer. In the second phase, the EUC is represented by a weakly stratified shear layer on top of a stable stratified jet. The objective is to investigate the interaction between the jet and the waves excited by the adjacent shear layer. Two simulations are performed: one with the jet located far from the shear layer (far jet) and the other with the shear layer on top of the jet (near jet). In the far jet, waves excited by the KH rollers are reflected and trapped in the region between the shear layer and jet and lead to little dissipation. In the near jet, more representative of the EUC configuration, waves with wavelength larger than that of the KH rollers are found in and below the jet. Pockets of hot fluid associated with horseshoe vortices that originate from the shear layer penetrate into the jet region, initiate turbulence and disrupt the internal wave field. In the third final phase of the thesis, a stratified jet situated below a well-mixed surface layer driven by a constant wind stress and a surface buoyancy flux is considered. Turbulence is generated in the surface layer and deepens into the jet upper-flank. Waves generated by the turbulent surface layer propagate downward across the jet. The momentum flux and energy flux carried by the waves are significantly weaker than the waves generated by the unstable shear in the problem studied during the first phase. Intermittent patches of intense dissipation inside the jet upper-flank are the result of ejections of fluid parcels