Observations of the Internal Wave to Turbulence Cascade

Abstract Shear spectral energy density (shear spectra) is measured across three decades in vertical wavenumber using a Wirewalker profiler equipped with a microstructure instrument and a pulse-coherent Doppler sonar. We identify the features of the canonical ocean vertical shear spectrum, including an internal wave band, an intermediate saturation band whose spectral level is independent of turbulent dissipation, and a three-dimensional turbulence band. The internal wave band and saturation band of the spectrum scale as and , respectively. These scalings hold despite the deployment location at the head of a La Jolla Canyon, a deep canyon incising the shelf off La Jolla, California, where weakly nonlinear wave–wave interaction is not the primary physical process driving the forward energy cascade. In La Jolla Canyon, high-amplitude tidally driven internal waves generate significant strain, resulting in turbulent events that cover a majority of the water column. During these events, we observe shear spectra with energy above the saturation level, which we interpret as a sign of the wave–turbulence transition. Finestructure parameterizations developed to predict mixing from shear spectra in the open ocean thermocline continue to predict average mixing well. The success of finestructure parameterizations implies a rate of downscale energy transfer consistent with the rate predicted from weakly nonlinear wave–wave interactions and suggests that the theoretical framework of the canonical shear spectrum can be used to make useful predictions in shallow, high-energy environments. Significance Statement Internal waves with vertical scales from tens to hundreds of meters provide much of the energy that drives irreversible turbulent mixing at centimeter scales in the ocean. The pathways energy follows from internal wave scales to mixing scales depend on the internal wave energy and the water depth. Energy pathways in high-energy coastal systems are dominated by highly nonlinear dynamics, while energy at middepths in the open ocean moves between scales via weakly nonlinear interactions. Our observations show that predictions made about the rate of energy transfer from internal wave to mixing scales formulated for the open ocean work surprisingly well in a coastal marine canyon with a high-energy nonlinear internal wave field.