Chiu Ching-Sang

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Acoustic intensity fluctuations induced by South China Sea internal tides and solitons

(IEEE, 2004-10) Chiu, Ching-Sang ; Ramp, Steven R. ; Miller, Christopher W. ; Lynch, James F. ; Duda, Timothy F. ; Tang, Tswen Yung

Between late April and May 23, 2001, a suite of acoustic and oceanographic sensors was deployed by a team of U.S., Taiwan, and Singapore scientists in the northeastern South China Sea to study the effects of ocean variability on low-frequency sound propagation in a shelfbreak environment. The primary acoustic receiver was an L-shaped hydrophone array moored on the continental shelf that monitored a variety of signals transmitted along and across the shelfbreak by moored sources. This paper discusses and contrasts the fluctuations in the 400-Hz signals transmitted across the shelfbreak and measured by the vertical segment of the listening array on two different days, one with the passage of several huge solitons that depressed the shallow isotherms to near the sea bottom and one with a much less energetic internal wavefield. In addition to exhibiting large and rapid temporal changes, the acoustic data show a much more vertically diffused sound intensity field as the huge solitons occupied and passed through the transmission path. Using a space-time continuous empirical sound-speed model based on the moored temperature records, the observed acoustic intensity fluctuations are explained using coupled-mode physics.
Enhanced acoustic mode coupling resulting from an internal solitary wave approaching the shelfbreak in the South China Sea

(Acoustical Society of America, 2013-03) Chiu, Linus Y. S. ; Reeder, D. Benjamin ; Chang, Yuan-Ying ; Chen, Chi-Fang ; Chiu, Ching-Sang ; Lynch, James F.

Internal waves and bathymetric variation create time- and space-dependent alterations in the ocean acoustic waveguide, and cause subsequent coupling of acoustic energy between propagating normal modes. In this paper, the criterion for adiabatic invariance is extended to the case of an internal solitary wave (ISW) encountering a sloping bathymetry (i.e., continental shelfbreak). Predictions based on the extended criterion for adiabatic invariance are compared to experimental observations from the Asian Seas International Acoustics Experiment. Using a mode 1 starter field, results demonstrate time-dependent coupling of mode 1 energy to higher adjacent modes, followed by abrupt coupling of mode 5–7 energy to nonadjacent modes 8–20, produces enhanced mode coupling and higher received levels downrange of the oceanographic and bathymetric features. Numerical simulations demonstrate that increasing ISW amplitude and seafloor slope enhance the coupling of energy to adjacent and nonadjacent modes. This enhanced coupling is the direct result of the simultaneous influence of the ISW and its proximity to the shelfbreak, and, compared to the individual effect of the ISW or shelfbreak, has the capacity to scatter 2–4 times the amount of acoustic energy from below the thermocline into the upper water column beyond the shelfbreak in realistic environments.
A report on the 3-D acoustic working group meeting at Long Beach, MS July 7-8, 1988

(Woods Hole Oceanographic Institution, 1989-06) Lynch, James F. ; Chiu, Ching-Sang

At the request of ONR Code 11250A, the Woods Hole Oceanographic Institution (Dr. James F. Lynch) convened a workshop to bring together a group of acoustic and ocean modelers to review and discuss 1. the state of development and the need for three-dimensional numerical acoustic research propagation and scattering models; 2. the interfacing of acoustic models with available oceanographic data and ocean model outputs. The workshop was hosted by the Institute for Naval Oceanography (Dr. Ching-Sang Chiu) at Long Beach, MS on July 7-8, 1988. This report summarizes the research presentations and the recommendations made by the group. The workshop was an initial attempt to promote the interaction between the ocean and acoustic modeling communities. This interaction between the communities is essential to the development of truly interactive basic research acoustic and ocean models. We anticipate more workshops of such nature to be held in the future. The findings and recommendations generated by these workshops are expected to have a strong impact on the direction of future three-dimensional modeling research in both acoustics and oceanography .
Estimate of the bottom compressional wave speed profile in the northeastern South China Sea using "Sources of Opportunity"

(IEEE, 2004-10) Lin, Ying-Tsong ; Lynch, James F. ; Chotiros, Nicholas P. ; Chen, Chi-Fang ; Newhall, Arthur E. ; Turgut, Altan ; Schock, Steven G. ; Chiu, Ching-Sang ; Bartek, Louis R. ; Liu, Char-Shine

The inversion of a broad-band "source of opportunity" signal for bottom geoacoustic parameters in the northeastern South China Sea (SCS) is presented, which supplements the towed source and chirp sonar bottom inversions that were performed as part of the Asian Seas International Acoustics Experiment (ASIAEX). This source of opportunity was most likely a "dynamite fishing" signal, which has sufficient low-frequency content (5-500 Hz) to make it complimentary to the somewhat higher frequency J-15-3 towed source (50-260 Hz) signals and the much higher frequency (1-10 kHz) chirp signals. This low frequency content will penetrate deeper into the bottom, thus extending the other inverse results. Localization of the source is discussed, using both a horizontal array for azimuthal steering and the "water wave" part of the pulse arrival for distance estimation. A linear broad-band inverse is performed, and three new variants of the broad-band inverse, based on: 1) the Airy phase; 2) the cutoff frequency; and 3) a range-dependent medium are presented. A multilayer model of the bottom compressional wave speed is obtained, and error estimates for this model are shown, both for the range-independent approximation to the waveguide and for the range-dependent waveguide. Directions for future research are discussed.
Shelf-edge frontal structure in the central East China Sea and its impact on low-frequency acoustic propagation

(IEEE, 2004-10) Ramp, Steven R. ; Chiu, Ching-Sang ; Bahr, Frederick L. ; Qi, Yiquan ; Dahl, Peter H. ; Miller, James H. ; Lynch, James F. ; Zhang, Renhe ; Zhou, Ji-Xun

Two field programs, both parts of the Asian Seas International Acoustics Experiment (ASIAEX), were carried out in the central East China Sea (28 to 30 N, 126 30 to 128 E) during April 2000 and June 2001. The goal of these programs was to study the interactions between the shelf edge environment and acoustic propagation at a wide range of frequencies and spatial scales. The low-frequency across-slope propagation was studied using a synthesis of data collected during both years including conductivity- temperature-depth (CTD) and mooring data from 2000, and XBT, thermistor chain, and wide-band source data from 2001. The water column variability during both years was dominated by the Kuroshio Current flowing from southwest to northeast over the continental slope. The barotropic tide was a mixed diurnal/semidiurnal tide with moderate amplitude compared to other parts of the Yellow and East China Sea. A large amplitude semidiurnal internal tide was also a prominent feature of the data during both years. Bursts of high-frequency internal waves were often observed, but these took the form of internal solitons only once, when a rapid off-shelf excursion of the Kuroshio coincided with the ebbing tide. Two case studies in the acoustic transmission loss (TL) over the continental shelf and slope were performed. First, anchor station data obtained during 2000 were used to study how a Kuroshio warm filament on the shelf induced variance in the transmission loss (TL) along the seafloor in the NW quadrant of the study region. The corresponding modeled single-frequency TL structure explained the significant fine-scale variability in time primarily by the changes in the multipath/multimode interference pattern. The interference was quite sensitive to small changes in the phase differences between individual paths/modes induced by the evolution of the warm filament. Second, the across-slope sound speed sections from 2001 were used to explain the observed phenomenon of abrupt signal attenuation as the transmission range lengthened seaward across the continental shelf and slope. This abrupt signal degradation was caused by the Kuroshio frontal gradients that produced an increasingly downward-refracting sound-speed field seaward from the shelf break. This abrupt signal dropout was explained using normal mode theory and was predictable and source depth dependent. For a source located above the turning depth of the highest-order shelf-trapped mode, none of the propagating modes on the shelf were excited, causing total signal extinction on the shelf.
Overview of results from the Asian Seas International Acoustics Experiment in the East China Sea

(IEEE, 2004-10) Dahl, Peter H. ; Zhang, Renhe ; Miller, James H. ; Bartek, Louis R. ; Peng, Zhauhui ; Ramp, Steven R. ; Zhou, Ji-Xun ; Chiu, Ching-Sang ; Lynch, James F. ; Simmen, Jeffrey A. ; Spindel, Robert C.

The Asian Seas International Acoustics Experiment (ASIAEX) included two major field programs, one in the South China Sea and the other in the East China Sea (ECS). This paper presents an overview of research results from ASIAEX ECS conducted between May 28 and June 9, 2001. The primary emphasis of the field program was shallow-water acoustic propagation, focused on boundary interaction and geoacoustic inversion. The study area's central point was located at 29/spl deg/ 40.67'N, 126/spl deg/ 49.39'E, which is situated 500 km east of the Chinese coastline off Shanghai. The acoustic and supporting environmental measurements are summarized, along with research results to date, and references to papers addressing specific issues in more detail are given.
Array data acquisition with wireless LAN telemetry as applied to shallow water tomography in the Barents Sea

(Woods Hole Oceanographic Institution, 1992-12) von der Heydt, Keith ; Kemp, John N. ; Lynch, James F. ; Miller, J. ; Chiu, Ching-Sang

This report describes the application of a new technique of digital radio telemetry, based on a recently available wireless Local Area Network Ethernet adapter, to the need for realtime transmission of data from a vertical line array (VLA) of hydrophones to a nearby ship. The report is technical in nature and discusses the design and performance of the system as used during the Barents Sea Polar Front Experiment in August 1992. A key feature of the use of LAN technology in a "telemetry" application is the availability of Transmission Control Protocol (TCP) software for Ethernet hardware that greatly eases the task of achieving error free digital data over a radio link prone to dropouts.
South China Sea internal tide/internal waves-impact on the temporal variability of horizontal array gain at 276 Hz

(IEEE, 2004-10) Orr, Marshall H. ; Pasewark, Bruce H. ; Wolf, Stephen N. ; Lynch, James F. ; Schroeder, Theodore ; Chiu, Ching-Sang

The temporal variability of the spatial coherence of an acoustic signal received on a bottomed horizontal array has been calculated for 276-Hz narrow-band signals. A conventional plane wave beamformer was applied to the received signals. The temporal variability of the array's omnipower, beam power, and array gain are related to variability in the sound-speed field. The spectral characteristics of array omnipower are nonstationary and changed as the spectral characteristics of the temperature field varied. The array omnipower and beam-power variability tracked each other in time and varied by as much as 15 dB over time intervals as short as 7 min. Array gain varied up to 5 dB and usually tracked the omnipower variability. A contiguous 24-h section of data is discussed in detail. This data section is from a time period during which the high-frequency fluid dynamic perturbation of the sound-speed field was of smaller amplitude than other sections of the 16-d data set. Consequently, this section of data sets an upper bound for the realizable array gain. The temporal variability of array gain and spatial coherence at times appears to be correlated with environmental perturbation of the sound-speed field, but are also correlated with changes in the signal-to-noise ratio. The data was acquired during the Office of Naval Research's South China Sea Asian Seas International Acoustics Experiment. The 465-m 32-channel horizontal array was placed on the bottom in 120 m of water at the South China Sea shelf break. The acoustic source was moored in 114 m of water /spl sim/19 km from the receiving array.
Introduction to the special issue on three-dimensional underwater acoustics

(Acoustical Society of America, 2019-09-30) Lin, Ying-Tsong ; Porter, Michael B. ; Sturm, Frédéric ; Isakson, Marcia J. ; Chiu, Ching-Sang

This special issue focuses on compelling three-dimensional (3D) volumetric and boundary effects on underwater sound propagation and scattering in complex and time-varying (thus four-dimensional) underwater environments. It consists of 24 papers covering analytical, numerical, and experimental studies and presents a collection of up-to-date research on this active and relevant topic.
Internal solitons in the northeastern south China Sea. Part I: sources and deep water propagation

(IEEE, 2004-10) Ramp, Steven R. ; Tang, Tswen Yung ; Duda, Timothy F. ; Lynch, James F. ; Liu, Antony K. ; Chiu, Ching-Sang ; Bahr, Frederick L. ; Kim, Hyoung-Rok ; Yang, Yiing-Jang

A moored array of current, temperature, conductivity, and pressure sensors was deployed across the Chinese continental shelf and slope in support of the Asian Seas International Acoustics Experiment. The goal of the observations was to quantify the water column variability in order to understand the along- and across-shore low-frequency acoustic propagation in shallow water. The moorings were deployed from April 21–May 19, 2001 and sampled at 1–5 min intervals to capture the full range of temporal variability without aliasing the internal wave field. The dominant oceanographic signal by far was in fact the highly nonlinear internal waves (or solitons) which were generated near the Batan Islands in the Luzon Strait and propagated 485 km across deep water to the observation region. Dubbed trans-basin waves, to distinguish them from other, smaller nonlinear waves generated locally near the shelf break, these waves had amplitudes ranging from 29 to greater than 140 m and were among the largest such waves ever observed in the world’s oceans. The waves arrived at the most offshore mooring in two clusters lasting 7–8 days each separated by five days when no waves were observed.Within each cluster, two types of waves arrived which have been named type-a and type-b. The type-a waves had greater amplitude than the type-b waves and arrived with remarkable regularity at the same time each day, 24 h apart. The type-b waves were weaker than the type-a waves, arrived an hour later each day, and generally consisted of a single soliton growing out of the center of the wave packet. Comparison with modeled barotropic tides from the generation region revealed that: 1) The two clusters were generated around the time of the spring tides in the Luzon strait; and 2) The type-a waves were generated on the strong side of the diurnal inequality while the type-b waves were generated on the weaker beat. The position of the Kuroshio intrusion into the Luzon Strait may modulate the strength of the waves being produced. As the waves shoaled, the huge lead solitons first split into two solitons then merged together into a broad region of thermocline depression at depths less than 120 m. Elevation waves sprang up behind them as they continued to propagate onshore. The elevation waves also grew out of regions where the locally-generated internal tide forced the main thermocline down near the bottom. The “critical point” where the upper and lower layers were equal was a good indicator of when the depression or elevation waves would form, however this was not a static point, but rather varied in both space and time according to the presence or absence of the internal tides and the incoming trans-basin waves themselves.
Barotropic tide in the northeast South China Sea

(IEEE, 2004-10) Beardsley, Robert C. ; Duda, Timothy F. ; Lynch, James F. ; Irish, James D. ; Ramp, Steven R. ; Chiu, Ching-Sang ; Tang, Tswen Yung ; Yang, Yiing-Jang ; Fang, Guohong

A moored array deployed across the shelf break in the northeast South China Sea during April-May 2001 collected sufficient current and pressure data to allow estimation of the barotropic tidal currents and energy fluxes at five sites ranging in depth from 350 to 71 m. The tidal currents in this area were mixed, with the diurnal O1 and K1 currents dominant over the upper slope and the semidiurnal M2 current dominant over the shelf. The semidiurnal S2 current also increased onshelf (northward), but was always weaker than O1 and K1. The tidal currents were elliptical at all sites, with clockwise turning with time. The O1 and K1 transports decreased monotonically northward by a factor of 2 onto the shelf, with energy fluxes directed roughly westward over the slope and eastward over the shelf. The M2 and S2 current ellipses turned clockwise and increased in amplitude northward onto the shelf. The M2 and S2 transport ellipses also exhibited clockwise veering but little change in amplitude, suggesting roughly nondivergent flow in the direction of major axis orientation. The M2 energy flux was generally aligned with the transport major axis with little phase lag between high water and maximum transport. These barotropic energy fluxes are compared with the locally generated diurnal internal tide and high-frequency internal solitary-type waves generated by the M2 flow through the Luzon Strait.
Fluctuation of 400-Hz sound intensity in the 2001 ASIAEX South China Sea experiment

(IEEE, 2004-10) Duda, Timothy F. ; Lynch, James F. ; Newhall, Arthur E. ; Wu, Lixin ; Chiu, Ching-Sang

We present analyses of fluctuations seen in acoustic signals transmitted by two 400-Hz sources moored as part of the ASIAEX 2001 South China Sea (SCS) experiment. One source was near the bottom in 350-m deep water 31.3 km offshore from the receiving array, and the other was near the bottom in 135-m deep water 20.6 km alongshore from the array. Time series of signal intensity measured at individual phones of a 16-element vertical line array are analyzed, as well as time series of intensity averaged over the array. Signals were recorded from 2 May to 17 May 2001. Fluctuations were observed at periods ranging from subtidal (days) to the shortest periods resolved with our signaling (10 s). Short-period fluctuations of depth- and time-averaged intensity have scintillation indexes (computed within 3-h long windows) which peak at values near 0.5 during an interval of numerous high-amplitude internal gravity waves, and which are lower during intervals with fewer internal waves. The decorrelation times of the averaged intensity (energy level) are also closely related to internal wave properties. Scintillation indexes computed for unaveraged pulses arriving at individual phones often exceed unity.
Research highlights from the Asian Seas International Acoustics Experiment in the South China Sea

(IEEE, 2004-10) Lynch, James F. ; Ramp, Steven R. ; Chiu, Ching-Sang ; Tang, Tswen Yung ; Yang, Yiing-Jang ; Simmen, Jeffrey A.

The Asian Seas International Acoustics Experiment (ASIAEX) included two major field programs, one in the South China Sea (SCS) and the other in the East China Sea (ECS). This paper summarizes results from the work conducted during April and May 2000 and 2001 over the continental shelf and slope in the northeastern South China Sea, just east of Dongsha Island (Pratis Reef). The primary emphasis of the field program was on water-column variability and its impact on acoustic propagation loss. The reader is steered to the appropriate paper within this Special Issue when more information on a specific topic is desired.
Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea

(IEEE, 2004-10) Duda, Timothy F. ; Lynch, James F. ; Irish, James D. ; Beardsley, Robert C. ; Ramp, Steven R. ; Chiu, Ching-Sang ; Tang, Tswen Yung ; Yang, Yiing-Jang

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.
A large-amplitude meander of the shelfbreak front during summer south of New England : observations from the Shelfbreak PRIMER experiment

(American Geophysical Union, 2004-03-04) Gawarkiewicz, Glen G. ; Brink, Kenneth H. ; Bahr, Frank B. ; Beardsley, Robert C. ; Caruso, Michael J. ; Lynch, James F. ; Chiu, Ching-Sang

In order to examine spatial and temporal variability of the shelfbreak front during peak stratification, repeated surveys using a towed undulating vehicle (SeaSoar) are used to describe the evolution of shelfbreak frontal structure during 26 July to 1 August 1996 south of New England. Spatial correlation (e-folding) scales for the upper 60 m of the water column were generally between 8 and 15 km for temperature, salinity, and velocity. Temporal correlation scales were about 1 day. The frontal variability was dominated by the passage of a westward propagating meander that had a wavelength of 40 km, a propagation speed of 0.11 m s−1, and an amplitude of 15 km (30 km from crest to trough). Along-front geostrophic velocities (referenced to a shipboard acoustic Doppler current profilers) were as large as 0.45 m s−1, although subject to significant along-front variations. The relative vorticity within the jet was large, with a maximum 0.6 of the local value of the Coriolis parameter. Seaward of the front, a small detached eddy consisting of shelf water was present with a diameter of approximately 15 km. Ageostrophic contributions to the velocity field are estimated to be as large as 0.3 m s−1 in regions of sharp curvature within the meander. These observations strongly suggest that during at least some time periods, shelfbreak exchange is nonlinear (large Rossby number) and dominated by features on a horizontal scale of order 10 km.
Estimation of planetary wave parameters from the data of the 1981 ocean acoustic tomography experiment

(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1985-08) Chiu, Ching-Sang

Using the maximum-likelihood estimation method and minimization techniques, quasi-geostrophic wave solutions were fitted to the observations of the 1981 Ocean Acoustic Tomography Experiment. The experiment occupied a 300 km square area centered at 26°N, 70°W, and had a duration of ~80 days. The data set consisted of acoustic travel-time records, temperature records and CTD profiles, obtained from the acoustic tomographic array, moored temperature sensors and recorders, and ship surveys, respectively. While the latter two were conventional spot measurements, the former corresponds to integral measurements of the temperature (or sound-speed) field. The optimal fit to the data corresponded to 3 waves in the first baroclinic mode, evolving under the presence of a westward mean flow with vertical shear. The flow was estimated to be weak (~2 cm/s), but it changed the wave periods significantly by producing large Doppler shifts. The waves were dynamically stable to the mean flow, had weak nonlinear interactions with each other and did not form a resonant traid; thus they constituted a fully linear solution. Evidence for the existence of the waves was strongly supported by the high correlation (~0.9) between the data and the fit, the large amount of signal energy resolved (~80 percent), the excellent quality of the wave-parameter estimate (only about 10 percent in error), and the general agreement between the observations and quasi-geostrophic linear dynamics.

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