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Barry R. Ruddick

Abstract

A three-layer model is used to study the effects of pure strain flow and simple shearing flow on isolated, anticyclonic, baroclinic vortices such as Mediterranean salt lenses. Exact steady solutions are found representing elliptical vortices with uniform interior vorticity. These solutions become increasingly elliptical with increasing strain or shear, with the major axis always 45° clockwise from the principal (outflow) axis of the strain field. This is shown to be necessary so that the mean flow not exchange energy with the lens. At some Critical value of strain or shear, these solutions cease to exist. The results suggest that for a lens of a given Rossby number, there is a maximum large-scale strain beyond which the lens must undergo drastic changes in order to survive.

The geostrophic adjustment of an infinitely long strip aligned with a simple shearing flow is also investigated. It is found that the shear modifies the distance of outward adjustment, but not the profile of the adjusted region. The strong flow and vorticity near the edge, and the assumed infinite length, allow the strip to persist in environmental slim as strong as f, the Coriolis parameter.

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Trevor J. McDougall and Barry R. Ruddick

Abstract

It is shown that two methods used for eliminating the effects Of internal-wave straining on fine-scale salinity-temperature-depth profiles also eliminate finestructure in which Rp = α∂z T/β∂z S is locally constant. It is expected that finestructure indicative of double-diffusive convection will have a near-constant value of Rp and hence be suppressed (or grossly distorted) by the methods. In those few cases where vertical internal-wave advection is the only process affecting the measurements, we suggest a modification to the method of Johnson et al. (1978) which overcomes this disadvantage. When this modification cannot be applied, it is suggested that the modification developed by Joyce et al. (1978) he adopted.

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Barry R. Ruddick and Terrence M. Joyce

Abstract

A total of four moorings from POLYMODE array I and II were analyzed in an investigation of the interaction of wavefields and mean flow. In particular, evidence for internal wave-mean flow interaction was sought by searching for time correlations between the vertically acting Reynolds stress of the wavefield (estimated using the temperature and velocity records), and the mean shear. No significant stress-shear correlations were found at the less energetic moorings (≲10 cm s−1), indicating that the magnitude of the eddy viscosity was under 200 cm2 s−1, with the sign of the energy transfer uncertain. This is considerably below the O(4500 cm2 s−1) predicted by Müller (1976). An extensive error analysis indicates that the large wave stress predicted by the theory should have been observable clearly under the conditions of measurement. At moorings typified by a higher mean velocity (≈25 cm s−1), statistically significant stress-shear correlations were found, and the wavefield energy level was observed to modulate with the strength of the mean shear. The observations were consistent with generation of short (∼1 km horizontal wavelength) internal waves by the mean shear near the thermocline, resulting in an effective eddy viscosity of ∼100 cm2 s−1.

Theoretical computations indicate that the wavefield “basic state” may not be independent of the mean flow as assumed by Müller (1976) but can actually be modified by large-scale vertical shear and still remain in equilibrium. In that case, the wavefield does not exchange momentum with a large-scale vertical shear flow and, excepting critical-layer effects, a small vertical eddy viscosity is to be expected. Using the Garrett-Munk (1975) model internal wave spectrum, estimates were made of the maximum momentum flux (stress) expected to be lost to critical-layer absorption. This stress was found to increase almost linearly with the velocity difference across the shear zone, corresponding to a vertical eddy viscosity of −100 cm2 s−1. Stresses indicative of this effect were not observed in the data.

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William F. Simmons and Barry R. Ruddick

Abstract

Four designs of single electrode conductivity probes, three of them new, were tested for response characteristics in conditions simulating laboratory internal wave flows. Two of the new designs were shown to be significantly superior in sensing vertical motion without deleterious time lags. Amplitude and phase lag, and inherent nonlinearity of response are evaluated quantitatively for all designs.

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Berta Biescas, Barry Ruddick, Jean Kormann, Valentí Sallarès, Mladen R. Nedimović, and Sandro Carniel

Abstract

Marine multichannel seismic (MCS) data, used to obtain structural reflection images of the earth’s subsurface, can also be used in physical oceanography exploration. This method provides vertical and lateral resolutions of O(10–100) m, covering the existing observational gap in oceanic exploration. All MCS data used so far in physical oceanography studies have been acquired using conventional seismic instrumentation originally designed for geological exploration. This work presents the proof of concept of an alternative MCS system that is better adapted to physical oceanography and has two goals: 1) to have an environmentally low-impact acoustic source to minimize any potential disturbance to marine life and 2) to be light and portable, thus being installed on midsize oceanographic vessels. The synthetic experiments simulate the main variables of the source, shooting, and streamer involved in the MCS technique. The proposed system utilizes a 5-s-long exponential chirp source of 208 dB relative to 1 μPa at 1 m with a frequency content of 20–100 Hz and a relatively short 500-m-long streamer with 100 channels. This study exemplifies through numerical simulations that the 5-s-long chirp source can reduce the peak of the pressure signal by 26 dB with respect to equivalent air gun–based sources by spreading the energy in time, greatly reducing the impact to marine life. Additionally, the proposed system could be transported and installed in midsize oceanographic vessels, opening new horizons in acoustic oceanography research.

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