Mixing Structure of High-Frequency Internal Waves in the Upper EasternEquatorial Pacific

Abstract

A thermistor chain towed from 140° to 110°W along the equator revealed the presence of high-frequency internal waves in the upper 125 m having zonal wavelengths of 150?250 m. Turbulence dissipation rates, ε, observed from a free-falling profiler were high when wave packets were present. Unfortunately, the frequency of the vertical profiles of ε taken did not resolve the internal wave cycle, so a dynamical link between the waves and the mixing could not be directly observed with vertical profiler data. It is presumed that either wave-induced shear instability or advective instability destabilized the waves and led to increased ε. The thermistor data, which were sampled at 20 Hz or approximately 12.5 cm horizontally, resolved part of the inertial subrange of turbulence and are used to determine the structure of turbulence within an internal wave cycle. A temperature gradient variance method for estimating ε relies on a fully resolved Batchelor spectrum that, for this experiment, would have required resolution of scales less than 2?10 mm. Nevertheless, the authors use the observed, underestimated temperature gradient variance in this study as a surrogate for ε. That is, an activity index AT was used as an indicator of the turbulence dissipation rate. Observations of AT as a function of internal wave phase and depth reveals a consistent structure of turbulent mixing within the wave cycle. This structure, having relatively higher AT associated with wave crests near 20-m depth and wave troughs near 60-m depth, is consistent with purely wave-induced shear instability based on its criterion and is not consistent with purely advective instability. The index, AT, as a function of Uz/N (Uz is the mean vertical shear of zonal velocity and N is the mean buoyancy frequency) and wave slope (defined as the product of the wavenumber and the wave displacement amplitude) demonstrates agreement between mixing and a neutral stability curve for the combined effects of advective and shear instabilities. However, the background shear and stratification are such that the vast majority of observed waves are associated with purely shear instability.