UC San Diego

Turbulent mixing in the ocean surface boundary layer (OSBL) mediates the transfer of energy, momentum, and gases between the atmosphere and the ocean. While the dynamics that drive these exchanges happen on small lateral and vertical scales relative to the size of the ocean, they are crucial in setting the behavior of Earth’s climate on global scales. Historically, small-scale processes near the ocean’s surface have been difficult to observe. Using novel measurements of the OSBL enabled by advances in instrumentation and multi-platform observational techniques, this thesis investigates specific OSBL turbulent dynamics. The onset and growth of Langmuir circulations (LCs) is observed from simultaneous airborne infrared imagery of sea surface temperature and subsurface in-situ measurements. For weak, fetch-limited wind wave forcing with stabilizing buoyancy forcing, LCs appear non-uniformly in space. During a period of LC growth and diurnal warm layer (DWL) deepening, subsurface temperature structures show temperature intrusions into the base of the DWL of the same scale as bubble entrainment depth during the deepening period. A large-eddy simulation run with observed initial conditions and forcing reproduces the onset and rate of DWL deepening, but exhibits coherent temperature structures with a larger aspect ratio than in observations, with large sensitivity to the numerical representation of surface radiative heating. At the base of the mixed layer, a drifting thermistor chain is used to observe temperature fluctuations consistent with turbulence in a stratified shear layer resulting from a mixture of Kelvin-Helmholtz and Holmboe instabilities. The size and frequency of these structures depends on the surface forcing regime, defined by the balance of wind-shear, wave-shear, and convective turbulent kinetic energy production. Thorpe scale estimates of dissipation and entrainment are consistent with the observed rate of mixed layer deepening, while the outer vertical scale of the turbulent region is correlated with the wind forcing magnitude. Below the mixed layer, observations of velocity from an array of drifting profiling instruments are used to relate spatial gradients in near-inertial wave energy flux to array-scale lateral vorticity gradients. These unique observational studies can aid in improving future numerical simulations and parametrizations used in global climate simulations.