Four oceanographic moorings were deployed across the South China Sea
continental slope near 21.85∘ N, 117.71∘ E, from 30 May
to 18 July 2014 for the purpose of observing high-frequency ...nonlinear
internal waves (NLIWs) as they shoaled across a rough, gently sloping
bottom. Individual waves required just 2 h to traverse the array and
could thus easily be tracked from mooring to mooring. In general, the
amplitude of the incoming NLIWs tracked the fortnightly tidal envelope in
the Luzon Strait; they lagged by 48.5 h and were smaller than the waves
previously observed to the southwest near the Dongsha Plateau. Two types of waves,
a waves and b waves, were observed, with the b waves always leading the
a waves by 6–8 h. Most of the NLIWs were remotely generated, but a few
of the b waves formed locally via convergence and breaking at the leading
edge of the upslope-propagating internal tide. Waves incident upon the
moored array with amplitude less than 50 m and energy less than 100 MJ m−1 propagated adiabatically upslope with little change of form. Larger
waves formed packets via wave dispersion. For the larger waves, the kinetic
energy flux decreased sharply upslope between 342 and 266 m, while the
potential energy flux increased slightly, causing an increasing ratio of
potential-to-kinetic energy as the waves shoaled. None of the waves met the
criteria for convective breaking. The results are in rough agreement with
recent theory and numerical simulations of shoaling waves.
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 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" /spl alpha/ 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.
To clarify the acoustic propagations and seabed properties in the northern South China Sea, the South China Sea Upper-Slope Sand Dunes Experiments were conducted in 2013-2014. To estimate the ...geacoustic properties of the surficial sediments in the experimental area, geoacoustic inversion with chirp sonar data, predictive model, and matched field technique were applied and compared with one another. Firstly, a predictive model based on sediment samples and the Biot theory with fluid approximation, the effective density fluid model (EDFM), was presented and utilized. The sediment samples were collected using a Shipek sediment sampler and were analyzed using a GEOTEK Multi-Sensor Core Logger and a Beckman Coulter LS particle size analyzer. The relationship between porosities and mean grain sizes in this area with shoaling internal waves has been discussed and compared to the Bachman's regression equation for SAX99. Propagating shoaling internal waves with huge amplitudes observed in the medium over the slope, shelf break, plateau, and continental shelf in the South China Sea are expected to cause strong amplification of near - bottom velocities. Hence, it may cause movement of an exposed bed particle on the bottom sediment, resulting in different grain-packing configurations. The measured porosities and mean grain sizes are inputted to the geoacoustic model to estimate the properties of the surficial sediments. The estimated bulk density is verified by core sample analysis, and the estimated sound speed and attenuation are verified by the match field geoacoustic inversion using the acoustic propagation data. Furthermore, the regression between the measured porosities and mean grain sizes is utilized in the geoacoustic inversion model, and the geoacoustic properties along the chirp sonar survey track are obtained.
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.
Observations are presented of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study conducted between July 26 and August 5, 1996. Current and ...temperature measurements were made with an upward looking acoustic Doppler current profiler (ADCP) located on the 147 m isobath near the shelfbreak and three vertical thermistor moorings located upshelf. Data from the ADCP and two nearby thermistor chains show energetic internal tides propagating at roughly 0.9 m s−1 to the north‐northwest, nearly perpendicular to the local topography with 10–15 cm s−1 horizontal currents and 15–30 m vertical displacements. These waves evolve rapidly within a 5.8 km range into an undular internal tidal bore. Cross‐isobath barotropic tidal currents, responsible for generating the internal tides are in the 5–12 cm s−1 range. The bore formation is highly variable. There is evidence of a correlation between internal tide steepening and a shelfbreak front jet orientation that is oppositely directed to the internal tide propagation. There is no correlation between steepening and the jet's vertical shear. Statistics of the undular bores show rms travel time fluctuations from 0.8 to 1.7 hours and average tidal bore durations from 12 to 9 hours. The average undular bore speed is 0.9 m s−1, with an rms fluctuation of 0.4 m s−1. The number of high‐frequency waves in the bore varies from 0 to 8 near the shelfbreak and increases to 30 waves 26.7 km upshelf. The observed distribution function of temporal spacing between high‐frequency internal waves is spread between 4 and 20 min.
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.
A low off-state current of 1.6×10−14A/μm and a small subthreshold gate swing of 152mV/decade were achieved in a novel thin film transistor using a c-axis crystallized InGaZnO semiconductor that could ...be obtained at a low substrate temperature range of 150°C. From experimental results, we found that the lowered off-state current is mainly attributed to the formation of rich Ga–O bonds to reduce oxygen vacancies, and the c-axis crystallized structure of IGZO to increase the potential barrier on the source side due to the increase of local trap states at the grain boundary.
Ocean acidification has been observed since the beginning of the industrial era and is expected to further reduce ocean pH in the future. A significant increase in ocean noise has been suggested ...based upon the percentage change in acoustic absorption coefficient at low frequencies. Presented here is an analysis using transmission loss models of all relevant loss mechanisms for three environments experiencing a significant near-surface pH reduction of 8.1-7.4. Results show no observable change in the shallow water and surface duct environments, and a statistically insignificant change of less than 0.5 dB for all frequencies in the deep water environment.
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