Inferences and Observations of Turbulent Dissipation and Mixing in the Upper Ocean at the Hawaiian Ridge

Abstract

The Hawaiian Ridge is one of the most energetic generators of internal tides in the pelagic ocean. The density and current structure of the upper ocean at the Hawaiian Ridge were observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks and up to 200 km from the ridge peak. Survey observations are used to quantify spatial changes in internal-wave-induced turbulent dissipation and mixing. The turbulent dissipation rate of kinetic energy ε and diapycnal eddy diffusivity K? are inferred from an established parameterization using internal wave shear as input. At the Kauai Channel (KC) and French Frigate Shoals/Brooks Banks sites, ε and K? decay away from the ridge with maxima exceeding minima by 5 times. At both sites, average K? is everywhere greater than the canonical open-ocean value of 10?5 m2 s?1. Along the ridge, ε and K? vary by up to 100 times and are largest at sites of largest numerical model internal tide energy density. In the eastern KC, K? > 10?3 m2 s?1 is typical in a patch more than 200 m thick located above the path of an M2 internal tide ray. An upper limit on the dissipation rate from M2 internal tides to turbulence within 50 km of the Hawaiian Ridge is roughly estimated to be in the range of 4?9 GW. At KC, the depth-integrated internal wave energy density and dissipation rate are positively correlated. Potential density inversions occur near the main ridge axis at significant topographic features. Average K? is larger inside inversions.