Investigation of the Spatial and Temporal Structure of Internal Waves

Abstract

A collection of field experiments and model simulations are used to investigate the spatial and temporal aspects of internal waves, with a focus on near-inertial waves and internal tides. First, the near-inertial wave field in the North Pacific is investigated using an array of 5 profiling moorings and shipboard ADCP transects. The ship transects reveal the horizontal structure of near-inertial shear, which is coherent over large distances ($\sim 80$km) and exhibits a change in character crossing $28.9^o$N. Second, the spatial structure of internal tidal beams is studied with repeated shipboard ADCP transects across Kaena Ridge, Hawaii. Harmonic fits are used to isolate the $M_2$ component of velocity, revealing a beam structure that compares well with theoretical ray paths and numerical simulations. Third, internal tides are investigated with moorings deployed in Luzon Strait, a major internal tide generation site with complex bathymetry and mesoscale variability. The coherence of internal tides over 4 spring-neap tidal cycles is quantified, and a model is used to diagnose the mechanisms responsible. Intrusions of the Kuroshio current shift large-scale patterns of internal tide pressure, leading to lower coherence and possibly affecting the wave field radiated from Luzon Strait. Finally, the turbulent mixing caused by tidal flow over topography in southern Luzon Strait is investigated with a profiling mooring and model simulations. Depth-integrated turbulent dissipation varies by an order of magnitude between spring and neap tides. Model simulations show that turbulence is due to the formation and breaking of lee waves. Dissipation varies for similar barotropic forcing over the 50 day record, suggesting that mesoscale variability is important in modulating mixing.