Oceanic isopycnal slope spectra. Part II: Turbulence

Isopycnal slope spectra were computed from thermistor data obtained using a microstructure platform towed through turbulence generated by internal tidal motions near the Hawaiian Ridge. The spectra were compared with turbulence dissipation rates epsilon that are estimated using shear probes. The turbulence subrange of isopycnal slope spectra extends to surprisingly large horizontal wavelengths (> 100 m). A four-order-of-magnitude range in turbulence dissipation rates at this site reveals that isopycnal slope spectra alpha epsilon(2/3)k(x)(1/3). The turbulence spectral subrange (k(x) > 0.4 cpm) responds to the dissipation rate as predicted by the Batchelor model spectrum, both in amplitude and towed vertical coherence. Scales between 100 and 1000 m are modeled by a linear combination of internal waves and turbulence while at larger scales internal waves dominate. The broad bandwidth of the turbulence subrange means that a fit of spectral amplitude to the Batchelor model yields reasonable estimates of epsilon, even when applied at scales of tens of meters that in vertical profiles would be obscured by other fine structure.