ON THE PARAMETERIZATION OF EQUATORIAL TURBULENCE - EFFECT OF FINE-SCALE VARIATIONS BELOW THE RANGE OF THE DIURNAL CYCLE

We seek relationships among turbulence and fine-scale and large-scale flow by ensemble averaging the observations taken in the Pacific Equatorial Undercurrent (EUC) at 0 degrees/140 degrees W in April 1987. A combination of fine- and microscale sensors resolves vertical wavenumber spectra of shear and temperature from scales of 300 m to the viscous and thermal diffusive cutoffs. We study the depth range 50-350 m, which encompasses high-shear layers below and above the core of the EUC and the thermostad, but not the diurnal cycle near the surface. Fine-scale shear dominates over large-scale shear (related to the slowly varying EUC) at 50-170 m and below 270 m, where large-scale gradient Froude numbers (Fr) drop below I, while large-scale shear dominates in the weakly stratified thermostad at 170-270 m, where ($) over bar ($) over bar Fr > 1. We analyze shear fluctuations in different vertical wavenumber bands and pragmatically separate nonturbulent and turbulent fluctuations, the latter being associated with vertical overturning and viscous dissipation. In part of the fine-scale range, shear spectra fall off approximately in inverse proportion to vertical wavenumber. The shear variance in this wavenumber band stays close to the squared buoyancy frequency independent of large-scale shear. At a vertical resolution that resolves turbulent overturning, the Kunze et al. (1990) model of shear instability well predicts average dissipation rates below 100 m. Yet instantaneous high-resolution gradient Froude numbers show virtually no correlation with turbulent dissipation rates. At 20-m vertical resolution, mean dissipation rates from below 50 m and total rms shear S-tot are well correlated as ($) over bar epsilon similar to S-tot(-3.5). Fine-scale shear is essential in this relationship. Large-scale gradient Froude numbers and large-scale shear are comparatively poorly correlated with mixing parameters.