Search Results

You are looking at 1 - 2 of 2 items for

  • Author or Editor: Terry E. Ewart x
  • Refine by Access: All Content x
Clear All Modify Search
R. Scott McKean and Terry E. Ewart

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 N 2dT/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)∝N 2dT/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)∝N 3N −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 PwN −1(dT/dz)2 predicted by WKB theory.

Full access
Terry E. Ewart and Stephen A. Reynolds

Abstract

Instrumentation for measuring the evolution of volume-scattered acoustic signals in both depth and time is described. Measurements were taken for 12 days during the spring of 1985 with transmitters and receivers suspended beneath arctic pack ice in the Beaufort Sea. These acoustic measurements were made simultaneously with extensive oceanographic measurements taken by other investigators during the Arctic Internal Wave Experiment (AIWEX). A depth cycling vertical array of three receivers and a single fixed horizontal receiver 100 m transverse to the propagation path were deployed 6.43 km from moored transmitters. A vertical depth cycle of 51 m produced a synthetic vertical aperture of 153 m. Pulses spanning three octaves in acoustic frequency (2, 4, 8, 16 kHz) were used. The scattered field was sampled more often at 8 and 16 kHz to assure sufficient sampling (better than Nyquist) of the space/time fluctuations. Ten Gbytes of acoustic pulse receptions were recorded on optical disk for later processing. The measurements provide a unique two-dimensional (space/time) dataset for testing theories of acoustic volume scattering due to internal waves and finestructure.

Full access