High-frequency internal waves in the upper eastern equatorial Pacific
From towed thermistor chain data collected near 140-132$\sp\circ$W on the equator during April 1987 a diurnally cycling high-frequency internal wave field was observed. Observations from 10-s-averaged data include zonal wavelengths of 150-250 m and vertical structures of wave displacement. From high-passed 20-Hz data the turbulence dissipation rate, $\varepsilon,$ is estimated within the internal wave cycle. To understand wave dynamics both the linear, diagnostic Taylor-Goldstein equation and a nonlinear, prognostic numerical model are solved for conditions representative of the upper eastern equatorial Pacific during the period of interest. Unstable Taylor-Goldstein solutions scaled by observed wave displacement amplitudes agreed relatively well with observations. This indicates that a linear instability of the background stratified flow is a possible mechanism for internal wave generation. The wave-induced Reynolds stress divergence from scaled solutions shows that only 100 days of intense wave activity is needed per year for these waves to explain estimated residuals of the mean zonal momentum budget of this region at 50- to 100-m depth. Observed $\varepsilon$ shows a pattern of turbulent mixing within the wave cycle consistent with wave-induced shear instability but inconsistent with advective instability. The 2-dimensional, nonhydrostatic numerical model was forced with a diurnally cycling surface heat flux and a steady surface wind stress. Waves generated in the model are due to the combined effects of a shear instability of the background flow and a type of boundary layer forcing known as the obstacle effect. The waves grow rapidly after initial convection and subsequent development of boundary layer eddies. ^
Andrew P Mack,
"High-frequency internal waves in the upper eastern equatorial Pacific"
Dissertations and Master's Theses (Campus Access).