On the parameterization of equatorial turbulence: Effect of fine‐scale variations below the range of the diurnal cycle

Abstract

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°/140°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 Proude numbers ( ) drop below 1, while large‐scale shear dominates in the weakly stratified thermostad at 170–270 m, where > 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 Stot are well correlated as . Fine‐scale shear is essential in this relationship. Large‐scale gradient Froude numbers and large‐scale shear are comparatively poorly correlated with mixing parameters.