Spatially Broad Observations of Internal Waves in the Upper Ocean at the Hawaiian Ridge

Abstract

Abstract The density and current structure at the Hawaiian Ridge was observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks. Across- and along-ridge changes in internal wave statistics in the upper ocean within 200 km of the ridge are investigated. Internal waves with trough-to-crest amplitude as large as 60 m and horizontal wavelength of about 50 km are observed repeatedly in across-ridge sections of potential density. Within 150 km of the ridge, kinetic and potential energy density exceed open-ocean values with maxima about 10 times Garrett–Munk levels. In the Kauai Channel (KC), the kinetic energy density is largest along an M2 internal tide ray. The ray originates at the northern edge of the ridge peak at a large across-ridge change in topographic slope and terminates at the ocean surface about 30–40 km south of the ridge peak. Kinetic and potential energy density are larger on the south side of the ridge at KC, the side with larger topographic slope. Energy density is also larger on the south side of the ridge at KC in numerical model results and on the side of steeper topographic slope in analytical model results. Along the ridge, the largest observed values of mean-square shear and mean-square slope of isopycnal depth are collocated with the largest energy density in numerical model results. Mean-square shear and mean-square slope increase with decreasing bottom depth and with increasing M2 barotropic tidal forcing.