Internal tides and mixing in the upper ocean at the Hawaiian Ridge

Upper-ocean density and current structure was observed using SeaSoar and Doppler sonar during a survey along the Hawaiian Ridge. These observations are used in Chapter 2 to investigate spatial changes in internal wave statistics within 200 km of the ridge. Internal waves with waveheight up to 60 m and horizontal wavelength 50 km are observed repeatedly. Within 150 km of the ridge, energy density exceeds open-ocean values with maxima about ten times Garrett-Munk levels. At the Kauai Channel (KC), the kinetic energy density is largest along an M2 internal tidal ray originating at the northern edge of the ridge peak. Energy density is larger on the south side of the ridge at KC, the side with larger topographic slope, consistent with numerical and analytical model predictions. Along the ridge, the largest observed values of mean-square shear and isopycnal slope are collocated with largest numerical-model energy density. Mean-square shear and slope decrease with increasing bottom depth and increase with increasing barotropic tidal forcing.; In Chapter 3, the survey observations are used to quantify spatial changes in internal wave dissipation and mixing. The turbulent dissipation rate epsilon and diapycnal eddy diffusivity Krho, inferred from established parameterizations, decay away from the ridge with maxima exceeding minima by 5 times. The e-folding scale of K rho is 80 km for south-side KC and French Frigate Shoals/Brooks Banks (FFS/BB) and 150 km for north-side KC. epsilon and K rho vary along the ridge by up to 100 times and are largest at sites of largest numerical-model energy density. Density inversions are found near the main ridge axis at significant topographic features. K rho is larger inside inversions.; In Chapter 4, microstructure temperature and CTD observations from profiling floats deployed during the survey are used to investigate the temporal and spatial variability of internal wave properties and temperature variance dissipation rate. The observations show an internal wave field dominated by internal tides and confirm across-ridge structure observed in Chap. 2. Internal-tide energy decreases are followed by dissipation increases. The dissipation rate is ten times larger than at a benchmark open-ocean site. Dissipation and mean-square strain and strain rate are positively correlated.