Turbulence and Internal Waves at the Equator. Part I: Statistics from Towed Thermistors and a Microstructure Profiler

Abstract

High correlations between turbulent dissipation rates and high-wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the Equatorial Undercurrent. An extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high-wavenumber internal waves. In the low Richardson number (Ri) thermocline below the mixed layer but above the core of the Equatorial Undercurrent, and when winds were moderate and steadily westward, it was found that: ? the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band (corresponding to 150?250-m zonal wavelength) of internal waves; ? both turbulence and internal waves varied diurnally?hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a factor of 100 at night; and ? correlations between turbulent dissipation rate and several measures of internal wave activity (wave isotherm displacement, wave slope, and wave potential energy) were high. Little or no high wavenumber internal wave activity was observed when winds were light or eastward: Superposing plane waves with the observed characteristics on the observed background field suggests that they are inherently unstable to both adjective and shear instability above the core of the Equatorial Undercurrent. These waves are due either to locally generated internal gravity waves or to Kelvin-Helmholz-type instabilities generated in the shear flow; from our measurements these two phenomena could not be distinguished.