24245327
Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea
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.
23989635
Acoustic intensity fluctuations induced by South China Sea internal tides and solitons
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.
129743564
Nonlinear Internal Waves in the South China Sea
RÉSUMÉ En se basant sur la récente compilation des cartes représentant la répartition des vagues internes, on constate que presque toutes les vagues internes de la Mer de la Chine du Sud se propagent vers l'ouest. La plupart de ces vagues internes sont générées dans les hauts fonds ou ses rebords du Détroit de Luzon. La crête de ses vagues peut faire jusqu'à 200 km de long avec une amplitude de 100 m. Grâce aux images collectées par RADARSAT ScanSAR les 26 avril et 4 mai 1998, de gigantesques solitons internes ont pu être observés près des îles Dongsha, ils avaient une crête de plus de 200 km de long et une vitesse pouvant aller jusqu'à 1,9 m/s. Les découvertes les plus intéressantes sont l'élévation des vagues internes dans les eaux peu profondes (220 m) et les dépressions des vagues sur les brisants (500 m de profondeur). Les deux phénomènes peuvent être observés sur les même images SAR (Radar ouverture synthétique) prises le 4 mai 1998. Des simulations numériques ont montré que le temps nécessaire à la conversion de vagues internes dépressionnaires en vagues internes élevées est d'environ 20 heures, et que la distance de propagation de la vague peut aller jusqu 'à 200 km. Donc, en se rappelant que les vagues sont générées par des marées semi-diurnes de 12,4 heures, une capacité de bande large telle celle du RADARSAT ScanSAR est requise pour pouvoir observer l'évolution des vagues à différentes profondeurs. De plus, sur les images ScanSAR prises près des îles Dongsha, les gigantesques solitons internes qui se propagent vers l'ouest sont souvent brisés sur les écueils que sont les récifs de corail. Dans certains cas, les vagues qui se sont brisées vont se mélanger après avoir passé les récifs de corail et interagir les unes avec les autres. Les interactions non-linéaires entre vagues seront aussi abordées dans cet article.
129309343
The Sulu Sea Internal Soliton Experiment
Abstract We present the results of a comprehensive study of large-amplitude internal solitons generated in the Sulu Sea in the Philippines by intense tidal flow over a sharp bathymetric feature. Surface signatures of these waves have been observed in images from the DMSP, Landsat, and Nimbus-7 satellites and the space shuttle SIR-A synthetic aperture radar. A two-week experiment was conducted in the Sulu Sea in May 1980 from R.V. Oceanograher, during which a three-mooring phased array of current meters and thermistors was implanted near the wave source and at 82 km and 200 km from the source. Seventeen soliton packets were observed whose properties underwent significant change as they propagated across the sea. Vertical profiles of density, temperature, and acoustic backscatter, as well as radar and photographic surface signatures, were obtained from the ship. Data from satellites, moorings, and the ship give unprecedented information on the spatial and temporal characteristics of solitary waves, which ha...
41171820
Internal solitons in the northeastern south China Sea. Part I: sources and deep water propagation
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.
128996656
Nonlinear internal waves in the South China Sea: Observation of the conversion of depression internal waves to elevation internal waves
[1] The conversion of depression nonlinear internal solitons to elevation internal waves has been observed twice at the South China Sea continental shelf break. Shipboard X-band radar, tow-yo CTD, ADCP and high frequency acoustic flow visualization of the process are presented. The data focuses on up slope propagation of depression internal solitons from a water depth of ∼264 m to a water depth of ∼110 m. Dissipation by large amplitude shear instabilities was observed. The kinetic energy density was found to be a decreasing monotonic function of range with an energy dissipation rate coefficient of 0.063 km−1. The rate of energy dissipation over the ∼16.5 km soliton propagation path was 0.17 W/m/m. Simple numerical models of the waveform evolution are qualitatively compared to the observations. The observations, extracted scale parameters and dissipation rates should be useful for testing a variety of evolution equation representations of internal soliton propagation.
121693186
The Fission and Disintegration of Internal Solitary Waves Moving over Two-Dimensional Topography
Abstract The propagation of long internal waves over two-dimensional topography is discussed. A two-dimensional version of the Korteweg-deVries equation is derived with variable coefficients which depend on the local fluid depth. The fission law for solitary waves propagating into shallower water for two density stratification models and the possible disintegration of a solitary wave into a dispersive packet are discussed on the basis of this equation.
26402369
Numerical Experiments on the Breaking of Solitary Internal Wavesover a Slope–Shelf Topography
Abstract A theoretical study of the transformation of large amplitude internal solitary waves (ISW) of permanent form over a slope–shelf topography is considered using as basis the Reynolds equations. The vertical fluid stratification, amplitudes of the propagating ISWs, and the bottom parameters were taken close to those observed in the Andaman and Sulu Seas. The problem was solved numerically. It was found that, when an intense ISW of depression propagates from a deep part of a basin onto the shelf with water depth Hs, a breaking event will arise whenever the wave amplitude am is larger than 0.4(Hs − Hm), where Hm is the undisturbed depth of the isopycnal of maximum depression. The cumulative effect of nonlinearity in a propagating ISW leads to a steepening and overturning of a rear wave face over the inclined bottom. Immediately before breaking the horizontal orbital velocity at the site of instability exceeds the phase speed of the ISW. So, the strong breaking is caused by a kinematic instability of t...
131703829
Numerical simulation of principal tidal constituents in the South China Sea, Gulf of Tonkin and Gulf of Thailand
The principal tidal constituents M-2, S-2, K-1 and O-1 in the South China Sea, Gulf of Tonkin and Gulf of Thailand are simulated simultaneously using the numerical scheme of Kwok et al. (1995 Proceedings of the 1st Asian Computational Fluid Dynamics Conference, pp. 16-19). The average differences between the computed and observed harmonic constants are mostly within 5 cm and 10 degrees for amplitudes and phase-lags, respectively. The simulated tidal regimes in the present model are believed to be more accurate than the previous numerical results. Our studies confirm that a clockwise rotating M-2 amphidromic system lies in the southeast of the Gulf of Thailand and an S-2 amphidromic system at the near-shore area of the northeast South China Sea. The linear tidal energy equation developed by Garrett (1975 Deep-Sea Research 22, 23-35) is generalized to the nonlinear case. Based on the numerical results, the energy budgets in the South China Sea and its subareas, namely the Taiwan Strait, the Gulf of Tonkin, the Gulf of Thailand and the remaining area are investigated. The tidal motion in the Taiwan Strait is maintained mainly by the energy fluxes from the East China Sea for both semidiurnal and diurnal species and partially from the Luzon Strait for semidiurnal species. For the other parts of the South China Sea, the tidal motion is mainly maintained by the energy fluxes through the Luzon Strait. The energy inputs from the tide-generating force are negative for semidiurnal species and positive for diurnal species. (C) 1999 Elsevier Science Ltd. All rights reserved.
15524500
Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE
A standard part of any oceanic pressure gauge or current meter analysis is the separation of tidal from non-tidal components of the signal. The tidal signal can either be discarded, or its characteristics described in some fashion useful for further analysis. Although tidal signals can be removed by standard high or bandpass filtering techniques, their relatively deterministic character and large amplitude make special techniques more effective. In classical harmonic analysis, the tidal signal is modelled as the sum of a finite set of sinusoids at specific frequencies related to astronomical parameters. A set of programs has been written in MATLAB to (a) perform classical harmonic analysis for periods of about 1 year or shorter, (b) account for (some) unresolved constituents using nodal corrections, and (c) compute confidence intervals for the analyzed components.
131598726
A Nonlinear Model of Internal Tide Transformation on the Australian North West Shelf
A numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide. The model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf. The model is run using observed hydrographic conditions from the Australian North West Shelf and results are compared to current meter and thermistor observations from the shelf-break region. It is found from observations that the coefficient of nonlinearity in the K-dV equation changes sign from negative in deep water to positive in shallow water, and this plays a major role in determining the form of the internal tide transformation. On the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide, which distorts the density profile over a wave period. Both the model and observations show the formation of an initial shock on the leading face of the internal tide. In shallow water, the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves. After further propagation a second shock forms on the back face of the wave, followed by a packet of solitons. The inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion. Friction is found to be important in limiting the amplitudes of the evolving waves.
17000342
Observations of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study
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 21 to the north-northwest, nearly perpendicular to the local topography with 10 -15 cm s 21 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 21 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 21 , with an rms fluctuation of 0.4 m s 21 . 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.
129739173
Evolution of nonlinear internal waves in the East and South
Synthetic Aperture Radar (SAR) images from ERS-I have been used to study the characteristics of internal waves northeast and south of Taiwan in the East China Sea, and east of Hainan Island in the South China Sea. Rank-ordered packets of internal solitons propagating shoreward from the edge of the continental shelf were observed in the SAR images. On the basis of the assumption of a semidiurnal tidal origin, the wave speed can be estimated and is consistent with the internal wave theory. By using the SAR images and hydrographic data, internal waves of elevation have been identified in shallow water by a thicker mixed layer as compared with the bottom layer on the continental shelf. The generation mechanism includes the influences of the tide and the Kuroshio intrusion across the continental shelf for the formations of elevation internal waves. The effects of water depth on the evolution of solitons and wave packets are modeled by the nonlinear Kortweg-deVries (KdV) type equation and linked to satellite image observations. The numerical calculations of internal wave evolution on the continental shelf have been performed and compared with the SAR observations. For a case of depression waves in deep water, the solitons first disintegrate into dispersive wave trains and then evolve to a packet of elevation waves in the shallow water area after they pass through a “turning point” of approximately equal layer depths that has been observed in the SAR image and simulated by the numerical model. The importance of the dissipation effect in the coastal area is also discussed and demonstrated.
129836617
A generalized Korteweg-de Vries model of internal tide transformation in the coastal zone
A nonlinear model is developed, based on the rotated-modified extended Korteweg-de Vries (reKdV) equation, of the evolution of an initially sinusoidal long wave in the coastal zone, representing an internal tide, into nonlinear waves including internal solitary waves. The coefficients of the basic equation are calculated using observed conditions for the north west shelf (NWS) of Australia. The roles of both quadratic and cubic nonlinearity, the Earth's rotation, and frictional dissipation are discussed. The combined action of nonlinearity and rotation leads to a number of intersting features in the wave form including solitons of both polarities, “thick” solitons, and sharp waves with steep fronts. It is shown that rotation is important for modelling the evolution of the internal tide, even for the relatively low latitude on the NWS of 20°S. Rotation increases the phase speed of the long internal tide, reduces the number of internal solitary waves that form from a long wave, and changes the form of the waves. The effects of nonlinearity on the vertical modal structure of the internal waves are also discussed. Results of numerical simulations are compared with current and temperature observations of the internal wave field on the NWS which show many of the features produced by the generalized KdV model.
131569059
A Simple Model for the Shear-induced Decay of an Internal Solitary Wave
Abstract Internal solitary waves (ISWs) are a common feature of the coastal zone and marginal seas, especially close to shelf breaks, and are observed to mix the water column at the depth of maximum density gradient. For a two-layer system separated by a thin interface with a finite density gradient, the Richardson number in the interface fails below 1/4 if, in the simplest case, the ISW amplitude exceeds 2(hH1)1/2, where h is the interface thickness and H1 the thickness of the upper layer. Assuming that mixing then thickens the interface and that the potential energy for this comes from the ISW, we derive formulas for the damping rate of the ISW. The model is generalized to allow for a stratified upper layer; a Richardson number of less than 1/4 now requires that the displacement of the base of the upper layer exceeds 0.82 times the thickness of the layer. The ISW damping rate is sensitive to the ratio of the mixing depths above and below the base of the upper layer but can be plausibly matched to field ...
122915102
Internal solitary wave breaking and run-up on a uniform slope
Laboratory experiments have been conducted to study the shoaling of internal solitary waves of depression in a two-layer system on a uniform slope. The shoaling of a single solitary wave results in wave breaking and the production of multiple turbulent surges, or boluses, which propagate up the slope. Significant vertical mixing occurs everywhere inshore of the breaking location. The kinematics of the breaking and bolus runup are described and a breaking criterion is found. The energetics of the breaking are investigated.
128512654
A Nonlinear Model of the Shoaling and Refraction of Interfacial Solitary Waves in the Ocean. Part II: Oblique Refraction across a Continental Slope and Propagation over a Seamount
Abstract The refraction of internal solitary waves can be due to variations in water depth, nonlinearity, stratification, or shear. This paper addresses the effects of the first two of these effects. Part I of this paper investigated how the amplitude and phase speed of an interfacial solitary wave varied as the wave propagated across a one-dimensional slope. Important differences between the predictions of Korteweg–de Vries (KdV) and extended KdV (EKdV) theory were found. In particular the EKdV limited the amplitude growth seen in KdV as the wave passed into shallow water, leading to an associated strong retardation in speed. In this second part of the paper these results are applied to two-dimensional bathymetry. The propagation of an oblique planar wave across a wedge-shaped slope is first studied. Whereas the linear internal-wave theory predicts small refraction of the wave front, the nonlinear EKdV model shows that the refraction of large amplitude internal waves is significantly greater. This is exp...
129028201
Energetics of borelike internal waves
The net integrated energy flux into a train of internal waves is evaluated in a two-layer model. The nonzero value for this integral results from the difference in the stratification between the initial and final state, similar to the energy supply to a surface bore. We apply this expression to waves measured by Wesson and Gregg [1988] in the Strait of Gibraltar and to waves measured by Farmer and Smith [1980] in Knight Inlet. We find the energy supply to be important to the energetics, but the data do not allow a definitive test of the conjecture that the primary energy balance is between this supply and dissipation. We contrast our conjecture to the solitary-wave considerations of Bogucki and Garrett [1993].
116672531
Atmosphere-Ocean Dynamics
How the Ocean--Atmosphere System Is Driven. Transfer of Properties between Atmosphere and Ocean. Properties of a Fluid at Rest. Equations Satisfied by a Moving Fluid. Adjustment under Gravity in a Nonrotating System. Adjustment under Gravity of a Density-Stratified Fluid. Effect of Rotation. Gravity Waves in a Rotating Fluid. Forced Motion. Effects of Side Boundaries. The Tropics. Mid-Latitudes. Instabilities, Fronts, and the General Circulation. Units and Their SI Equivalents. Useful Values. Properties of Seawater. Properties of Moist Air. A List of Atlases and Data Sources. References. Index.
140547212
On internal tide generation models
This paper discusses linear generation models for internal tides over large topographic features such as continental slopes, with the objects of making the procedures simpler and more adaptable to oceanic situations than heretofore, providing a unified summary of the mechanics involved, and estimating the global internal tide energy generation at all continental shelf-slope junctions for M2 and S2 frequencies. A ‘composite model’ involving wave generation on a shallow thermocline as well as on the continuously stratified fluid below is developed, together with a discussion of the approximations required. The simple case of a linear slope is discussed in detail; one conclusion is that linear internal tides generated on a shallow thermocline will be small unless T = hL/Lα < 2, where hL is the total depth on the continental shelf, 2πL is a representative wavelength (typically 20 to 30 km) and α is the slope of the continental slope. Such waves propagating seawards are expected to ‘leak’ into the deeper stratified water in one or two wavelengths, whilst those propagating shorewards on the shelf may steepen to form undular bores. The composite model has been applied to all relevant regions of coastline, and the five areas with the largest internal tidal energy production are the European northwest shelf, the East and South China seas, the Guiana Basin region, the Australian northwest shelf, and the Gulf of Maine-Grand Banks region. The summed global barotropic to baroclinic tidal energy conversions by this process in the coastal regions are estimated to be 1.45 × 1010 J s−1 for M2 and 2.73 × 109 J s−1 for S2, on an annual mean basis.
130795097
An overview of the 1995 SWARM shallow-water internal wave acoustic scattering experiment
An overview is given of the July-August 1995 SWARM shallow-water internal wave acoustic scattering experiment. This experiment studied both acoustic propagation through and scattering by the linear and nonlinear internal waves found on the Mid-Atlantic Bight continental shelf, as well as the physical oceanography of the internal wavefield. In order that their goal of explaining the nature of the acoustic scattering should not be hindered by incomplete environmental knowledge, numerous instruments, both ship-deployed and moored, measured the acoustics, geophysics, and oceanography. In this paper, the authors show some of the results from the first year's analysis of the environmental and acoustic data. The environmental measurements, which are a key input to the analyses of the acoustic data, are given slightly more emphasis at this point in time. Some of the more interesting oceanographic, geophysical, and acoustical results the authors present are: evidence for the dominance of the lee-wave mechanism for soliton production, evidence for the "solibore internal tide" the "dnoidal wave" description of solitons, the inversion of chirp sonar data for bottom properties, propagation loss extraction from air-gun data, and the intensity and travel-time fluctuations seen in propagating acoustic normal modes. Directions for future research are outlined.