Modeling and analysis of internal-tide generation and beamlike onshore propagation in the vicinity of shelfbreak canyons

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.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.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-1060430.2014-09-0

Piecewise coherent mode processing of acoustic data recorded on two horizontally separated vertical line arrays

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): EL492-EL498, doi:10.1121/1.4722193.Motivated by measurements made in the 2004 Long-Range Ocean Acoustic Propagation Experiment (LOAPEX), the problem of mode processing transient acoustic signals collected on two nearby vertical line arrays is considered. The first three moments (centroid, variance, and skewness) of broadband distributions of acoustic energy with fixed mode number (referred to as modal group arrivals) are estimated. It is shown that despite the absence of signal coherence between the two arrays and poor high mode number energy resolution, the centroid and variance of these distributions can be estimated with tolerable errors using piecewise coherent mode processing as described in this paper.This work was supported by the Office of Naval Research, Code 322, Grant Nos. N00014-06-1-0245, N00014-08-1-0195, and N00014-11-1-0194

Mode filters and energy conservation

Author Posting. © Acoustical Society of America, 2010. 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 127 (2010): EL185-EL191, doi:10.1121/1.3327240.The discrete form of the mode filtering problem is considered. The relevant equations constitute a linear inverse problem. Solutions to problems of this type are subject to a well-known trade-off between resolution and precision. But unlike the typical linear inverse problem, the correctly formulated mode filtering problem is subject to an energy conservation constraint. This letter focuses on the importance of satisfying, approximately at least, the energy conservation constraint when mode filtering is performed.This work was supported by the Office of Naval Research, Code 321, Grant Nos. N000140610245 and N000140810195

Three-dimensional numerical modeling of sound propagation and scattering in the deep ocean with elastic bottoms [poster]

Poster presented AGU Ocean Sciences Meeting, Honolulu, Hawaii USA, February 23-28, 2014A challenging problem in bottom-interacting ocean acoustics and marine seismology is to accurately describe environmental variability in a computationally feasible model. Wave field predictions are often difficult in environments with strong range dependence, with rapid bathymetric variations, with multiple scattering regions, with interface waves at fluid/solid boundaries, and/or with shear waves in the bottom. In this presentation, we are using an existing three-dimensional spectral-finite-element code (SPECFEM3D, distributed and supported by the NSF funded program, Computational Infrastructure for Geodynamics), originally developed for simulations of seismic wave propagation at the local or regional scale, to bottom interaction problems in underwater acoustics. Recent developments of the SPECFEM3D model include full compressional attenuation in elastic media and improved transparent boundary conditions. Numerical results from SPECFEM3D are compared with wave fields simulated using acoustic propagation model based on the parabolic equation (PE) method, for a 10 Hz broadband acoustic pulse propagating in the deep ocean. The importance of out-of-plane scattering and bottom shear properties on resulting wave fields are investigated.Work supported by the National Science Foundation in the U. S. and L'Agence Nationale de la Recherche and Le Centre National de la Recherche Scientifique in Franc

Eulerian and Lagrangian correspondence of high-frequency radar and surface drifter data : effects of radar resolution and flow components

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 Atmospheric and Oceanic Technology 31 (2014): 945–966, doi:10.1175/JTECH-D-13-00146.1.This study investigated the correspondence between the near-surface drifters from a mass drifter deployment near Martha’s Vineyard, Massachusetts, and the surface current observations from a network of three high-resolution, high-frequency radars to understand the effects of the radar temporal and spatial resolution on the resulting Eulerian current velocities and Lagrangian trajectories and their predictability. The radar-based surface currents were found to be unbiased in direction but biased in magnitude with respect to drifter velocities. The radar systematically underestimated velocities by approximately 2 cm s−1 due to the smoothing effects of spatial and temporal averaging. The radar accuracy, quantified by the domain-averaged rms difference between instantaneous radar and drifter velocities, was found to be about 3.8 cm s−1. A Lagrangian comparison between the real and simulated drifters resulted in the separation distances of roughly 1 km over the course of 10 h, or an equivalent separation speed of approximately 2.8 cm s−1. The effects of the temporal and spatial radar resolution were examined by degrading the radar fields to coarser resolutions, revealing the existence of critical scales (1.5–2 km and 3 h) beyond which the ability of the radar to reproduce drifter trajectories decreased more rapidly. Finally, the importance of the different flow components present during the experiment—mean, tidal, locally wind-driven currents, and the residual velocities—was analyzed, finding that, during the study period, a combination of tidal, locally wind-driven, and mean currents were insufficient to reliably reproduce, with minimal degradation, the trajectories of real drifters. Instead, a minimum combination of the tidal and residual currents was required.I.R. was supported by the WHOI Coastal Ocean Institute Project 27040148 and by the WHOI Access to the Sea Program 27500036. I.R. and A.K. acknowledge support fromthe NSF project 83264600. A.K. acknowledges support from the Massachusetts Clean Energy Center (MassCEC) via the New England Marine Renewable Energy Center (MREC).2014-10-0

Three-dimensional bottom diffraction in the North Pacific

Author Posting. © Acoustical Society of America, 2019. 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 146(3), (2019): 1913-1922, doi:10.1121/1.5125427.A significant aspect of bottom-interaction in deep water acoustic propagation, from point sources to point receivers, is the diffraction (or scattering) of energy from discrete seafloor locations along repeatable, deterministic paths in three-dimensions. These bottom-diffracted surface-reflected (BDSR) paths were first identified on the North Pacific acoustic laboratory experiment in 2004 (NPAL04) for a diffractor located on the side of a small seamount. On the adjacent deep seafloor, ambient noise and propagation in the ocean sound channel were sufficiently quiet that the BDSRs were the dominant arrival. The ocean bottom seismometer augmentation in the North Pacific (OBSANP) experiment in June–July 2013 studied BDSRs at the NPAL04 site in more detail. BDSRs are most readily identified by the arrival time of pulses as a function of range to the receiver for a line of transmissions. The diffraction points for BDSRs occur on the relatively featureless deep seafloor as well as on the sides of small seamounts. Although the NPAL04 and OBSANP experiments had very different geometries the same diffractor location is consistent with observed arrivals in both experiments within the resolution of the analysis. On OBSANP the same location excites BDSRs for 77.5, 155, and 310 Hz transmissions.We greatly appreciate the support from Captain Curl, the officers, and crew of the R/V Melville (MV1308). The OBS data used in this research was acquired on instruments from the ocean bottom seismograph instrument pool (OBSIP) at Scripps Institution of Oceanography. Ernie Aaron (SIO) was responsible for shipboard OBS operations. The multi-beam data was processed using the MB-System (Caress and Chayes, 1996). Figure 1 was prepared using the generic mapping tool (Wessel and Smith, 1998). Feedback and reviews from an anonymous reviewer and the editorial staff of JASA are also greatly appreciated. The OBSANP experiment was funded by the ONR Ocean Acoustics Program (Code 322 OA) under Grant Nos. N00014-10-1-0987 and N00014-10-1-0510. Analysis was carried out under ONR Grant Nos. N00014-14-1-0324, N00014-16-1-2337, and N00014-17-C-7043.2020-03-3

Modeling deep ocean shipping noise in varying acidity conditions

Author Posting. © Acoustical Society of America, 2010. 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 128 (2010): EL130–EL136, doi:10.1121/1.3402284.Possible future changes of ambient shipping noise at 0.1–1 kHz in the North Pacific caused by changing seawater chemistry conditions are analyzed with a simplified propagation model. Probable decreases of pH would cause meaningful reduction of the sound absorption coefficient in near-surface ocean water for these frequencies. The results show that a few decibels of increase may occur in 100 years in some very quiet areas very far from noise sources, with small effects closer to noise sources. The use of ray physics allows sound energy attenuated via volume absorption and by the seafloor to be compared.This work was supported by the Ocean Acoustics Program at the U.S. Office of Naval Research, Code 321, including an ONR Postdoctoral Fellowship award to the first author

Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) - cruise report

The Ocean Bottom Seismometer Augmentation in the North Pacific Experiment (OBSANP, June-July, 2013, R/V Melville) addresses the coherence and depth dependence of deep-water ambient noise and signals. During the 2004 NPAL Experiment in the North Pacific Ocean, in addition to predicted ocean acoustic arrivals and deep shadow zone arrivals, we observed "deep seafloor arrivals" (DSFA) that were dominant on the seafloor Ocean Bottom Seismometer (OBS) (at about 5000m depth) but were absent or very weak on the Distributed Vertical Line Array (DVLA) (above 4250m depth). At least a subset of these arrivals correspond to bottomdiffracted surface-reflected (BDSR) paths from an out-of-plane seamount. BDSR arrivals are present throughout the water column, but at depths above the conjugate depth are obscured by ambient noise and PE predicted arrivals. On the 2004 NPAL/LOAPEX experiment BDSR paths yielded the largest amplitude seafloor arrivals for ranges from 500 to 3200km. The OBSANP experiment tests the hypothesis that BDSR paths contribute to the arrival structure on the deep seafloor even at short ranges (from near zero to 4-1/2CZ). The OBSANP cruise had three major research goals: a) identification and analysis of DSFA and BDSR arrivals occurring at short (1/2CZ) ranges in the 50 to 400Hz band, b) analysis of deep sea ambient noise in the band 0.03 to 80Hz, and c) analysis of the frequency dependence of BR and SRBR paths. On OBSANP we deployed a 32 element VLA from 12 to 1000m above the seafloor, eight short-period OBSs and four long-period OBSs and carried out a 15day transmission program using a J15-3 acoustic source.Funding was provided by the Office of Naval Research under contract #'s N00014-10-1-0987 and N00014-10-1-051

Analysis of Deep Seafloor Arrivals observed on NPAL04

This report gives an overview of the analysis that was done on Deep Seafloor Arrivals since they were initially presented in Stephen et al (2009). All of the NPAL04/LOAPEX (North Pacific Acoustic Laboratory, 2004/ Long Range Ocean Acoustic Propagation Experiment) data on three ocean bottom seismometers (OBSs) at ~5,000m depth and the deepest element of the deep vertical line array (DVLA) at 4250m depth has been analyzed. A distinctive pattern of late arrivals was observed on the three OBSs for transmissions from T500 to T2300. The delays of these arrivals with respect to the parabolic equation predicted (PEP) path were the same for all ranges from 500 to 2300km, indicating that the delay was introduced near the receivers. At 500km range the same arrival was observed throughout the water column on the DVLA. We show that arrivals in this pattern converted from a PEP path to a bottom-diffracted surface reflected (BDSR) path at an off-geodesic seamount.Funding was provided by the Office of Naval Research under Contract No. N00014-10-1-0510