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  • 1
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    Modeling weak fluctuations of undersea telemetry signals (2007)
    IEEE
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © IEEE, 1991. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 16 (1991): 3-11, doi:10.1109/48.64880.
    Description: Numerical calculation of acoustic field perturbation expressions can be used to predict fluctuations after propagation through ocean sound-speed structures, but before the onset of multipath. The general form of the expressions for signal spectra or correlation functions allow numerical evaluation for an unlimited quantity of vector wave-number spectral models of refractive index. In order to help define the bounds of applicability of the theory, log-intensity fluctuation variances have been calculated for three major situations: ocean internal waves, ocean turbulence, and continuous strong large-scale turbulence. Propagation through ocean thermocline internal waves, realistically weak thermocline turbulence, and unrealistically strong turbulence show that scintillations of intensity can be predicted and understood to first order up to ranges of tens of kilometers, given the proper transmission geometry. Internal wave effects dominate over any effects from expected microstructure. Nonhorizontal transmission yields small fluctuations, but eventually refractive effects of the sound channel will contribute some additional spatial variability and multipath, complicating the use of the theory. Multipath due to the sound channel can exist at ranges where the random small-scale structures would contribute only small perturbations (no multipath from small structures)
    Description: This work was supported by the Office of Naval Research, Ocean Acoustics Program.
    Keywords: Wave propagation ; Forward scattering ; Internal waves ; Microstructure
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Temporal and cross-range coherence of sound traveling through shallow-water nonlinear internal wave packets (2008)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2006. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 119 (2006): 3717-3725, doi:10.1121/1.2200699.
    Description: Expressions governing coherence scales of sound passing through a moving packet of nonlinear internal waves in a continental shelf environment are presented. The expressions describe the temporal coherence scale at a point, and the horizontal coherence scale in a plane transverse to the acoustic path, respectively. Factors in the expressions are the wave packet propagation speed, wave packet propagation direction, the fractional distance from the packet to the source, and the spatial scale S of packet displacement required to cause acoustic field decorrelation. The scale S is determined by the details of coupled mode propagation within the packet and the waveguide. Here, S is evaluated as a function of frequency for one environment, providing numerical values for the coherence scales of this environment. Coherence scales derived from numerical simulation of coupled mode acoustic propagation through moving wave packets substantiate the expressions.
    Description: This work was funded by grants from the Ocean Acoustics Program of the U.S. Office of Naval Research.
    Keywords: Underwater sound ; Acoustic wave propagation ; Acoustic field ; Acoustic waveguides ; Acoustic wave scattering
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Horizontal Lloyd mirror patterns from straight and curved nonlinear internal waves (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 131 (2012): 1689-1700, doi:10.1121/1.3666004.
    Description: Experimental observations and theoretical studies show that nonlinear internal waves occur widely in shallow water and cause acoustic propagation effects including ducting and mode coupling. Horizontal ducting results when acoustic modes travel between internal wave fronts that form waveguide boundaries. For small grazing angles between a mode trajectory and a front, an interference pattern may arise that is a horizontal Lloyd mirror pattern. An analytic description for this feature is provided along with comparisons between results from the formulated model predicting a horizontal Lloyd mirror pattern and an adiabatic mode parabolic equation. Different waveguide models are considered, including boxcar and jump sound speed profiles where change in sound speed is assumed 12 m/s. Modifications to the model are made to include multiple and moving fronts. The focus of this analysis is on different front locations relative to the source as well as on the number of fronts and their curvatures and speeds. Curvature influences mode incidence angles and thereby changes the interference patterns. For sources oriented so that the front appears concave, the areas with interference patterns shrink as curvature increases, while convexly oriented fronts cause patterns to expand.
    Description: The authors thank the Office of Naval Research for funding this work. Additionally, the first author is supported through an ONR Ocean Acoustics Traineeship.
    Keywords: Acoustic waveguides ; Nonlinear acoustics ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea (2006)
    IEEE
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © IEEE, 2004. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 29 (2004): 1105-1130, doi:10.1109/JOE.2004.836998.
    Description: A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.
    Description: This work was supported in part by grants from the U.S. Office of Naval Research, Physical Oceanography and Ocean Acoustics Programs, and by the National Science Council of Taiwan.
    Keywords: Baroclinic tides ; Internal waves ; Nonlinear waves
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
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    The unpredictable nature of internal tides on continental shelves (2012)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 1981–2000, doi:10.1175/JPO-D-12-028.1.
    Description: Packets of nonlinear internal waves (NLIWs) in a small area of the Mid-Atlantic Bight were 10 times more energetic during a local neap tide than during the preceding spring tide. This counterintuitive result cannot be explained if the waves are generated near the shelf break by the local barotropic tide since changes in shelfbreak stratification explain only a small fraction of the variability in barotropic to baroclinic conversion. Instead, this study suggests that the occurrence of strong NLIWs was caused by the shoaling of distantly generated internal tides with amplitudes that are uncorrelated with the local spring-neap cycle. An extensive set of moored observations show that NLIWs are correlated with the internal tide but uncorrelated with barotropic tide. Using harmonic analysis of a 40-day record, this study associates steady-phase motions at the shelf break with waves generated by the local barotropic tide and variable-phase motions with the shoaling of distantly generated internal tides. The dual sources of internal tide energy (local or remote) mean that shelf internal tides and NLIWs will be predictable with a local model only if the locally generated internal tides are significantly stronger than shoaling internal tides. Since the depth-integrated internal tide energy in the open ocean can greatly exceed that on the shelf, it is likely that shoaling internal tides control the energetics on shelves that are directly exposed to the open ocean.
    Description: This research was supported by ONR Grants N00014-05-1-0271, N00014-08-1-0991, N00014-04- 1-0146, and N00014-11-1-0194.
    Description: 2013-05-01
    Keywords: Internal waves ; Nonlinear dynamics ; Tides
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
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    A higher-order split-step Fourier parabolic-equation sound propagation solution scheme (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 132 (2012): EL61-EL67, doi:10.1121/1.4730328.
    Description: A three-dimensional Cartesian parabolic-equation model with a higher-order approximation to the square-root Helmholtz operator is presented for simulating underwater sound propagation in ocean waveguides. The higher-order approximation includes cross terms with the free-space square-root Helmholtz operator and the medium phase speed anomaly. It can be implemented with a split-step Fourier algorithm to solve for sound pressure in the model. Two idealized ocean waveguide examples are presented to demonstrate the performance of this numerical technique.
    Description: This work was sponsored by the Office of Naval Research under Grants No. N00014-10- 1-0040 and No.N00014-11-1-0701.
    Keywords: Acoustic waveguides ; Helmholtz equations ; Parabolic equations ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 7
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    Comparison of deep-ocean finescale shear at two sites along the Mid-Atlantic Ridge (2006)
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © The Author, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 53 (2006): 207-225, doi:10.1016/j.dsr2.2005.08.021.
    Description: Four drifting floats were used to measure the magnitude of the vertical derivative of horizontal velocity in waters above the rough bathymetry of the Mid Atlantic Ridge. This derivative is typically the dominant component of the velocity gradient (the shear). Two floats were at the site of the Brazil Basin Tracer Release Experiment (BBTRE) in the South Atlantic, and two were near the site of the Guiana Abyssal Gyre Experiment (GAGE) in the North Atlantic. Floats operated for one year except for one BBTRE float which operated for 100 days. Shear was measured over a vertical span of 9.5 m using drag elements that caused the floats to rotate slowly in response to shear. For each float, the first, second and fourth moments of shear were elevated above levels associated with the Garrett-Munk model internal-wave spectrum. Three of the four floats were tracked as they moved over mountainous terrain, allowing shear intensity to be measured as a function of height above the bottom. A deep BBTRE float showed enhancement of rms shear near the bottom. Floats at both areas provided measurements at 2000 m above the bottom, with differing results: The GAGE site had a lower fourth moment of shear (diapycnal diffusivity proxy) than the BBTRE site. However, application of normalization factors accounting for differences between the sites in bottom roughness, latitude-dependent internal-wave dynamics, and tidal current speeds brings the results into agreement.
    Description: This work was funded by the National Science Foundation under grants OCE9416014 and OCE9906685.
    Keywords: Internal waves ; Internal wave shear ; Diapycnal mixing
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 8
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    Internal tidal modal ray refraction and energy ducting in baroclinic Gulf Stream currents (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 1969-1993, doi:10.1175/JPO-D-18-0031.1.
    Description: Upstream mean semidiurnal internal tidal energy flux has been found in the Gulf Stream in hydrodynamical model simulations of the Atlantic Ocean. A major source of the energy in the simulations is the south edge of Georges Bank, where strong and resonant Gulf of Maine tidal currents are found. An explanation of the flux pattern within the Gulf Stream is that internal wave modal rays can be strongly redirected by baroclinic currents and even trapped (ducted) by current jets that feature strong velocities above the thermocline that are directed counter to the modal wavenumber vector (i.e., when the waves travel upstream). This ducting behavior is analyzed and explained here with ray-based wave propagation studies for internal wave modes with anisotropic wavenumbers, as occur in mesoscale background flow fields. Two primary analysis tools are introduced and then used to analyze the strong refraction and ducting: the generalized Jones equation governing modal properties and ray equations that are suitable for studying waves with anisotropic wavenumbers.
    Description: The Woods Hole research was supported by National Science Foundation Grant OCE-1060430 and by the Office of Naval Research Grants N00014-11-1-0701 and N00014-17-1-2624. The USM research was supported by ONR Grant N00014-15-1-2288 and National Science Foundation Grant OCE-1537449.
    Description: 2019-02-28
    Keywords: Internal waves ; Wave properties ; Tides ; Differential equations ; Numerical analysis/modeling
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Modeling and analysis of internal-tide generation and beamlike onshore propagation in the vicinity of shelfbreak canyons (2014)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 834-849, doi:10.1175/JPO-D-13-0179.1.
    Description: A hydrostatic numerical model with alongshore-uniform barotropic M2 tidal boundary forcing and idealized shelfbreak canyon bathymetries is used to study internal-tide generation and onshore propagation. A control simulation with Mid-Atlantic Bight representative bathymetry is supported by other simulations that serve to identify specific processes. The canyons and adjacent slopes are transcritical in steepness with respect to M2 internal wave characteristics. Although the various canyons are symmetrical in structure, barotropic-to-baroclinic energy conversion rates Cυ are typically asymmetrical within them. The resulting onshore-propagating internal waves are the strongest along beams in the horizontal plane, with the stronger beam in the control simulation lying on the side with higher Cυ. Analysis of the simulation results suggests that the cross-canyon asymmetrical Cυ distributions are caused by multiple-scattering effects on one canyon side slope, because the phase variation in the spatially distributed internal-tide sources, governed by variations in the orientation of the bathymetry gradient vector, allows resonant internal-tide generation. A less complex, semianalytical, modal internal wave propagation model with sources placed along the critical-slope locus (where the M2 internal wave characteristic is tangent to the seabed) and variable source phasing is used to diagnose the physics of the horizontal beams of onshore internal wave radiation. Model analysis explains how the cross-canyon phase and amplitude variations in the locally generated internal tides affect parameters of the internal-tide beams. Under the assumption that strong internal tides on continental shelves evolve to include nonlinear wave trains, the asymmetrical internal-tide generation and beam radiation effects may lead to nonlinear internal waves and enhanced mixing occurring preferentially on one side of shelfbreak canyons, in the absence of other influencing factors.
    Description: All three authors were supported by Office of Naval Research (ONR) Grant N00014-11-1-0701. WGZ was additionally supported by the National Science Foundation (NSF) Grant OCE-1154575, and TFD was additionally supported by NSF Grant OCE-1060430.
    Description: 2014-09-01
    Keywords: Circulation/ Dynamics ; Baroclinic flows ; Internal waves ; Ocean circulation ; Topographic effects ; Waves, oceanic ; Models and modeling ; Numerical analysis/modeling
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 10
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    Acoustic mode coupling by nonlinear internal wave packets in a shelfbreak front area (2005)
    IEEE
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © IEEE, 2004. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 29 (2004): 118-125, doi:10.1109/JOE.2003.822975.
    Description: A computational case study of coupled-mode 400-Hz acoustic propagation over the distance 27 km on the continental shelf is presented. The mode coupling reported here is caused by lateral gradients of sound-speed within packets of nonlinear internal waves, often referred to as solitary wave packets. In a waveguide having unequal attenuation of modes, directional exchange of energy between low- and high-loss modes, via mode coupling, can become time dependent by the movement of waves and can cause temporally variable loss of acoustic energy into the bottom. Here, that bottom interaction effect is shown to be sensitive to stratification conditions, which determine waveguide properties and, in turn, determine modal attenuation coefficients. In particular, time-dependent energy loss due to the presence of moving internal wave packets is compared for waveguides with and without a frontal feature similar to that found at the shelfbreak south of New England. The mean and variability of acoustic energy level 27 km distant from a source are shown to be altered in a first order way by the presence of the frontal feature. The effects of the front are also shown to be functions of source depth.
    Description: This work was supported by the Office of Naval Research Grants N00014-99-1-2074 and N00014-01-1-0772.
    Keywords: Continental shelf ; Internal waves ; Mode coupling ; Shallow water ; Shelfbreak front ; Solitary waves ; Sound propagation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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