UC San Diego

Wind-driven motions in the upper ocean represent one of the largest inputs of mechanical energy the ocean receives, but where that energy goes is harder to say. Some stays in the mixed layer, some radiates downward as near-inertial internal waves, and some is traded with the eddies these motions pass through. The exchanges that decide between these outcomes occur at scales smaller than most models resolve and are rarely linear, making the fate of wind-driven energy an inherently multiscale problem. This dissertation follows wind-driven energy from mixed-layer near-inertial oscillations, to the internal waves they radiate, to the turbulence that dissipates them, using observations from the North Atlantic and the Indian Ocean. Chapter 1 analyzes data from ten Lagrangian surface drifters deployed in the Lofoten Basin Eddy (LBE), a semi-permanent Norwegian Sea anticyclone, combined with sea-surface height altimetry and reanalysis winds. A Lagrangian slab model including eddy pressure-gradient terms reproduces the observed inertial response, while a wind-forced-only model does not, showing that not all near-inertial energy near the eddy core is wind-driven. A mechanism is proposed whereby correlations between near-inertial velocities and eddy pressure gradients, modulated by a near-inertial oscillation of the eddy center, transfer energy into the inertial band. Chapter 2 resolves the sub-eddy scale in the LBE with a denser 28-drifter array deployed the following year and using three different analysis methods: drifter-triplet kinematics, a gridded stream-function decomposition, and Gaussian Process Regression reconstruction. The eddy is not a smoothly rotating axisymmetric vortex but is permeated by intense, short-lived vorticity patches with cyclonic filaments encircling the anticyclonic core. Near-inertial motions oscillate below the local inertial frequency throughout the interior, tracking the coherent, time-integrated eddy rather than instantaneous local vorticity. Chapter 3 follows the energy to dissipation using microstructure and velocity profiles collected in the Bay of Bengal before and after severe cyclonic storm Remal. Ray tracing indicates most pre-storm near-inertial packets are not wind-generated, and the storm reorganized turbulence into fewer, thinner shear-driven layers while reducing mixing efficiency, reshaping the geometry of mixing more than its magnitude.