High-Frequency Internal Waves at 0°, 140°W and Their Possible Relationship to Deep-Cycle Turbulence

Abstract

An analysis of a moored time series at 0°, 140°W from November to December 1991 found a nighttime enhancement of isotherm displacement variance and of zonal velocity variance below the surface mixed layer at frequencies higher than 1 cph. The nighttime enhancement was generally not seen below the core of the equatorial undercurrent. At 45-m depth, the potential energy and the horizontal kinetic energy of high-frequency waves were strongly correlated and similar in magnitude. The shear production of turbulence kinetic energy calculated from the mooring measurements is strongly correlated with the turbulence kinetic energy dissipation rate observed from the nearby R/Vs Wecoma and Moana Wave during the Tropical Instability Wave Experiment. This suggests a dynamical link between the observed high-frequency internal waves and deep-cycle turbulence. The relationship between internal waves and turbulence in the thermocline was further explored in a case study of one energetic wave packet. This wave packet propagated westward and downward with a horizontal wavelength of no less than 200 m. The potential energy was similar to the horizontal kinetic energy of the wave packet, with the dominant variance occurring in a frequency band close to the local buoyancy frequency. The estimated vertical flux of the horizontal momentum of waves during the event was 0.3 Pa, three times the surface wind stress. About 2 hours after the wave packet passed the mooring site, an anomalous turbulence dissipation rate with a magnitude similar to that of the estimated shear production of the wave packet was observed from the R/V Wecoma. The observed time tag was likely the result of the spatial separation of the observing platforms.