Temperature Spectra in the Deep Ocean off Hawaii

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Abstract
Estimates of the spatial spectrum of ocean temperature fluctuations at six different depths between 500 and 2500 m are presented. Measurements were made with a depth-stable, self-propelled instrument carrier. Temperature samples were taken at 0.2 m intervals over a track typically 10 km long, corresponding to a spectral bandwidth of 1–2500 cycles per kilometer (cpkm). The observed spectrum falls into three distinct bands; these are tentatively identified with the effects of internal waves (W), fine-scale layering (L), and turbulent mixing (T). Each band has a characteristic wavenumber dependence which is invariant with depth. The intensity in each band scales with depth in characteristic fashion. Depth scaling over the range of interest accords with the empirical formula N2∝dT/dz∝ exp[−(Z−500)/λ], where the stability frequency and mean vertical temperature gradient are equivalent variables, and λ=750 m is the observed local scale height. Then the spectrum in band T is PT=Ak−1, where the intensity decreases with depth as A(z)∝N2∝dT/dz. The L band is limited above and below by wavenumbers 20 and 300 cpkm. The spectrum is PL=Bk−2, with the approximate depth dependence B(z)∝N3∝N−1(dT/dz)2. The W band appears to be fully resolved only at the two shallowest depths, because of limited run length. But the maximum intensity in this band, corrected for fine-structure, is not inconsistent with the depth scaling Pw≈N−1(dT/dz)2 predicted by WKB theory.

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