Perturbation of the Littoral Sound Speed Field by Small-Scale Shelf/Slope Fluid Processes

M.H. Orr and P.C. Mignerey
Acoustics Division

Introduction: We have been studying the smallscale fluid processes that periodically perturb the ocean's sound speed field in the vicinity of the continental shelf break. A high-frequency acoustic backscattering system is used to generate flow visualization images of the processes. The images are used to estimate their impact on the sound speed field variability. A correlation between the occurrence of the small-scale fluid processes and the tidal flow has been observed. Due to the periodicity of the tide, it is felt that we may eventually develop an ability to estimate the variability of both the sound speed field and the acoustic signals that propagate through it. As a result, we may be able to estimate the performance of Navy acoustic antisubmarine warfare (ASW) systems operating in littoral areas.

Fluid Processes: The shelf/slope water column is often composed of layers of water of differing density or sound speed. The layers have range-dependent thickness variability. Navy sonar operators usually treat the layers as time invariant, i.e., that there is no time dependent change in thickness or vertical displacement of the layers. The layers can, however, be temporally perturbed (vertically displaced) as the result of tidal flow over sloping ocean bottoms. A sharp discontinuity in water depth occurs at the shelf/slope break. Tidal flow over the break causes the layers of water to be displaced in the vertical. The displacement generates waves that displace the interfaces between the water layers and propagate away from the shelf break. These waves are called internal waves because they do not noticeably displace the air/sea interface surface as do ocean surface waves. The internal waves are generated on every tide and propagate away from the shelf/slope break with a known speed. Consequently, their distance from the shelf break can be calculated and their influence on the shelf and slope sound speed profile can be estimated.

If there are several water layers, the layers can be vertically displaced by internal waves in a variety of ways. If all the layers are displaced together vertically upward or downward, the displacement is called a mode 1 internal wave. If the top boundary of a layer is displaced upward and the bottom bondary of a layer is displaced downward, the displacement is called a mode 2 internal wave.

Acoustic Flow Visualization: Two-hundred kHz acoustic signals are projected perpendicular to the ocean surface toward the ocean bottom. Acoustic energy is scattered back to the acoustic surface from particles or temperature and salinity fluctuations found in the vicinity of density discontinuities located at the boundary between layers of ocean water. Changes in the depth of the boundaries by fluid processes such as internal waves are extracted from changes in the roundtrip travel time of the scattered acoustic signals. As a result, the fluid process causing the changes can be visualized and studied.

FIGURE 4
The range variability of the depth of an isosound speed layer (blue line) shows both long wavelength and short wavelength features. The ocean bottom is the green line. The direction of propagation of the internal tide, the 20-km depression feature, is shown by arrow 2. The insert shows a section of the acoustic flow visualization data that was digitized to obtain the 27-km realization of the sound speed variability. The vertical displacement of the mixed layer by the internal wave field is clear. The ocean bottom reflection is the upper red reflection. The lower bottom red reflection is an artifact. (From Fig. 24 of Ref. 1.)

Flow Visualization Images: A section of an internal wave packet detected by the acoustic flow visualization system (insert in Fig. 4) shows a downward displacement of the base of the ocean's mixed layer (scattering layer). The data were taken on the New Jersey Shelf.¹ The scattering layer was tracked for more than 27 km. The depth of the base of the mixed layer was digitized and is plotted (Fig. 4). Two features are present: the first is a vertical displacement of the mixed layer (arrow 1). It slowly recovers over a distance of 20 km. This is the internal tide that was propagating shoreward (arrow 2) at ~0.5 m/s. The internal tide is generated on each tidal cycle and is a repeatable ocean process. The short wavelength (100 to 300 m) displacements within the internal tide envelope are caused by internal wave packets. They are dominated by mode 1 internal waves. The repeated generation of the internal tide, when stratified waters are present, suggests that the temporal and spatial variability of the shelf sound speed field may be repeated on each tide.

As mentioned, the interfacial internal waves imaged in Fig. 4 were dominated by the mode 1 component of the internal wave field. This component causes the stratified layers and sound speed field to be vertically displaced in the same direction (upward or downward) together. In a multilayer fluid, interfacial mode 2 internal waves cause the upper and lower boundary of one of the fluid layers to be displaced in opposite directions. This causes the sound speed profile variability to be different than the mode 1- dominant case shown above. Figure 5 shows a mode 2 interfacial internal wave imaged on the New Jersey Shelf during the fall of 2000. The mode 2 internal waves shown are 150 to 200 m in length.

FIGURE 5
A 600-m flow visualization section showing the presence of a mode 2 internal wave on the New Jersey Shelf. The impact of mode 2 interfacial waves on the sound speed variability and acoustic signal variability is being addressed. Water depth is ~75 m.
FIGURE 6
An 1100-m flow visualization record of a series of shear instabilities associated with the first lobe of a Luzon Basin soliton. Water between two layers is being mixed by roughly 40-m amplitude shear instabilities. This will cause short acoustic coherence lengths.

In addition to mode 1 and mode 2 internal wave perturbation of the sound speed field, the internal tide and associated internal waves can also contain shear instabilities that cause water between two dif- ferent fluid layers to mix. The mixing causes sound speed variability that will change the amplitude and phase of an acoustic signal that is propagating through the mixing event. Figure 6 shows a series of shear instabilities detected at the shelf break of the South China Sea. These instabilities were imbedded in the first lobe of an internal wave packet.

First-order calculations indicate that these instabilities could cause a perturbation to the complex properties of an acoustic signal propagating through them. The perturbation or variability is enough to cause an ~10 dB degradation in the performance of a large-aperture acoustic array.

Conclusions: High-frequency acoustic systems are being used to image the fluid processes generated by tidal flow over bathymetry. The fluid processes range in scale from 20-km internal tides to 10 m or less shear instabilities. The larger scale fluid processes appear to repeat on each tide. As a result, it may be possible to estimate the variability of the sound speed profile and naval array performance in littoral areas.

Acknowledgments: The low-noise preamplifier used in the acoustic flow visualization system was designed and integrated into the flow visualization system by Michael McCord. The following individuals helped with the development and installation of equipment and the acquisition of portions of the highfrequency data set: Earl Carey, Steve Wolf, Roger Meredith, James Schowalter, Bruce Pasewark, and John Kemp and his Woods Hole Oceanographic mooring group. We also thank the crews and staff of the UNOLS R/V Endeavor and the Taiwan research vessel Ocean Research III for their generous help.

[Sponsored by ONR]

References
¹ J.R. Apel, M. Baidey, C.-S. Chiu, S. Finette, R. Headrick, J. Kemp, A. Neuhall, M.H. Orr, B. Pasewark, D. Tielbuerger, A. Turgut, K. Von Der Heydt, and S. Wolf, "An Overview of the 1995 SWARM Shallow-Water Internal Wave Acoustic Scattering Experiment," IEEE J. Oceanic Eng. 22, 465-500 (1997).

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