INTERNAL GRAVITY-WAVES IN THE UPPER EASTERN EQUATORIAL PACIFIC - OBSERVATIONS AND NUMERICAL-SOLUTIONS

On the basis of data from a towed thermistor chain collected near 140 degrees W on the equator during April 1987, the zonal wavenumber and v ertical structure of internal gravity waves were observed to vary sign ificantly between wave events. Our hypothesis is that this variability is due to changes in the vertical structure of mean horizontal veloci ty and density. Assuming that the observed waves were the fastest grow ing modes for shear instability, we solve the Taylor-Goldstein equatio n, using different analytical basic states, including a zonal and meri dional flow, to simulate the different conditions during 4 nights of i ntense internal wave activity. We find that while the observed waves a re of finite amplitude, linear shear instability is sufficient to expl ain the wavelength and vertical structure of vertical displacement for most of the waves. The fastest growing, unstable, mode-one solutions have e-folding growth times of less than 10 min. These solutions show wave phase speeds and vertical structures to be highly dependent upon the velocity structure of the uppermost 40 m. Near the base of the mix ed layer at a flow inflection point the kinetic energy of the mean flo w is extracted for wave growth. Wave vertical displacement is maximum near this inflection point. Zonal phase speeds range from -0.8 to -0.1 m/s. The propagation direction of waves with growth rates of 75% of t he maximum growth rate can range from about 45 degrees north to 45 deg rees south of the zonal direction. The vertical wave-induced Reynolds stress divergence could explain a discrepancy in zonal momentum budget s of the upper 90 m of this region. Estimates of this stress divergenc e show that only about 100 days of intense internal wave activity is n eeded per year for these internal waves to explain estimated residuals of the mean zonal momentum budgets of this region at 50- to 100-m dep th.