Effects Of Internal Waves Research Articles

Time-reversal focusing stability in the presence of background internal waves in shallow water

Effect of background internal waves on spatial and temporal low frequency sound focusing stability is investigated for an open oceanic shelf by means of numerical simulations. Focusing is achieved with the time-reversal procedure at a single source-receiving element at 10km range. Calculations are performed in terms of normal mode coupling theory. Internal wave field modeling is carried out using an averaged experimental power spectrum of vertical thermocline displacements measured in the Shallow Water'06 experiment. The results of numerical experiments show that the focal spot is stable for about 1 hour in the presence of typical internal waves on an open shelf. Two adaptive time-reversal algorithms are proposed to increase this period up to 12 hour. [Work supported by RFBR 11-02-00779.]

Analysis of acoustic fluctuation of the different tracks by the internal waves

Internal waves in shallow-water cause sound speed profiles variations and acoustic transmission abnormity. The acoustic data show arrival time variations and mode coupling phenomena. The 2009 experiment collected high quality environmental and acoustic data and were used to analyse the effect of internal waves on the acoustic transmission. One notable feature of the experiment is that internal crossed two tracks at different incidence angles. The paper compare acoustic fluctuation character at two different tracks and use mode filter method to investigate the mode coupling caused by internal wave. The correspondence between data and simulation results show that the alternate of mode 2 and mode 1 were observed at the track which is vertical with the internal wave direction. The alternate of Mode 3 and first two modes occur at the another track.

Thermal effects of internal gravity waves in the Martian upper atmosphere

For the first time, gravity wave‐induced heating and cooling effects were fully and interactively incorporated into a Martian general circulation model (GCM). Simulations with a comprehensive GCM with an implemented spectral nonlinear gravity wave (GW) parameterization revealed significant thermal effects of GWs in the mesosphere and lower thermosphere (MLT) between 100 and 150 km. Wave‐induced heating and cooling rates are comparable with those due to near‐IR CO2 heating and IR CO2cooling, correspondingly. Accounting for thermal effects of GWs results in a colder simulated MLT, with the most of cooling taking place in middle‐ and high‐latitudes. In the winter hemisphere, the temperature decrease can exceed 45 K. The colder simulated MLT is in a good agreement with the SPICAM stellar occultation measurements and Mars Odyssey aerobraking temperature retrievals. Our experiments suggest that thermal effects of GWs are probably a key physical mechanism in the MLT missing in contemporary Martian GCMs.

Dynamical effects of internal gravity waves in the equinoctial thermosphere

Using a recently developed spectral nonlinear gravity wave (GW) parameterization implemented into a 3-D coupled general circulation model, the effects of a broad spectrum of small-scale internal GWs of lower atmospheric origin on the equinoctial thermosphere are studied for the first time. GWs propagate to F region altitudes in both hemispheres, producing appreciable drag on the mean zonal wind. Some modifications of the two-cell equinoctial mean circulation by GWs are simulated, too. The mean zonal GW drag is comparable to the ion drag up to ∼260km in the middle- and high-latitudes. While the mean dynamical effect of GWs is the deceleration of the mean flow, the instantaneous GW body force can have both signs. In the Southern Hemisphere high-latitude, GWs are found to produce large torque of more than 1000ms−1 day−1 the mechanism of which is investigated in detail. GW anisotropy plays a crucial role in offsetting and modulating wave filtering, introducing increased favourable conditions for westerly harmonics in the high-latitudes. This leads to a very large localized eastward GW drag reaching a maximum in the upper thermosphere as a consequence of enhanced molecular viscosity, thermal conduction, and ion drag. Finally, the high-latitude distribution of the GW body force is presented in the upper thermosphere along with the comparison with ion drag. It demonstrates significant interhemispheric differences and large longitudinal variations in GW momentum deposition.

Transformations of the turbulent viscosity distribution under the effect of the internal waves in the system of stratified currents

Turbulent viscosity distributions in stratified currents are investigated. Relationship between the internal-wave-induced transformations of the temperature and turbulent exchange coefficient distributions is established. Dependences of turbulent viscosity on the phase velocity of the internal wave, water level variation, and velocity and thickness of the drift current are revealed. The contribution from the internal wave effect to the turbulent exchange coefficient value is shown to be 30% on average. Dependence of the turbulent exchange coefficient on the wave and current parameters is obtained.

Spectral effects of nonlinear internal waves on narrowband shallow-water signal propagation

Water column, bathymetry, and acoustical sediment properties collected during the transverse acoustic variability eXperiment (TAVEX) of 2008 are used to create a 30 km computational environment for propagation models. These models are used to display internal-wave-induced acoustic variability which is characteristic of the shallow-water northern East China Sea environment where the experimental work took place. Analyses of computational models are then compared to narrowband acoustic recordings made during the experiment using a spectral and time-of-arrival analysis approach. The expected acoustic effects and computational impact of specific internal wave characteristics on acoustic arrivals will be discussed along with recorded data comparison. [Work supported by the Office of Naval Research.]

A Novel Empirical Orthogonal Function (EOF)-Based Methodology to Study the Internal Wave Effects on Acoustic Propagation

This paper presents a novel approach to synthesize realistic environment for ocean-acoustic parametric studies. In its current form, this methodology applies to internal waves and tides. Empirical orthogonal function (EOF) decomposition is applied to a temporal series of temperature profiles. It can be observed that the first two time-dependent expansion coefficients are dynamically linked. When they are plotted one versus another in a scatter diagram, the cloud of points consists of a crescent shape that can easily be represented by a polynomial fit. If the first two expansion coefficients capture enough variability in the temperature profiles, the EOF modes plus the polynomial can be used to reconstruct temperature profiles independently from the set of data. This realistic synthesized environment can then be input to acoustic propagation models. This approach is applied to the case of the Messina Strait in which internal waves are known to be intensive. From a short-term series of temperature profiles collected on a thermistor string, range-dependent profiles along and across the strait are reconstructed. The acoustical impact study is conducted with the range-dependent acoustic model (RAM) parabolic equation (PE) model. The methodology presented in this paper is simple to run and requires a very affordable set of data. It could be used as an efficient alternative to ocean and acoustic model coupling for process studies or for regional studies especially in poorly known areas or highly variable areas, where it is difficult to obtain good sound-speed profile prediction from ocean models.

Comparisons of internal solitary wave and surface wave actions on marine structures and their responses

In this paper, a time-domain numerical model is established for computing the action of internal solitary wave on marine structures and structure motion responses. For a cylindrical structure, its side and bottom are discretized by pole and surface elements, respectively. The drag and inertial forces in the perpendicular direction of the structure are computed by the Morison equation from the pole elements, and the Froude–Krylov force in the axial direction of the structure due to internal wave motion is computed by integration of the dynamic pressure over the surface elements. The catenary theory is used to analyze the reaction force due to mooring lines, and the motion equation of the marine structure is solved by the fourth-order Runge–Kutta method in the time domain. The model is used to calculate the interaction of the internal solitary wave with a Spar platform with mooring system, and the surface wave action with the platform has also been computed by a frequency-domain boundary element method for comparison. Through the comparison based on a practical internal wave and surface wave states, it can be concluded that the internal wave force on the structure is only 9% of the one due to surface waves. However, the motion response due to the internal wave is much greater than the one due to the surface waves. It shows that the low-frequency effect of internal solitary waves is a great threat to the safety of marine structures.

Editorial: Special Issue on the 2006 Shallow Water Experiment (Part II)

The seven papers in this special issue deal with the effects of internal waves and internal tides on acoustic propagation and communication in shallow water, and geoacoustic characterization of the seabed.

Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves

During a 12-h period in the 2006 Shallow Water Experiment (SW06), binary phase shift keying (BPSK) signals at the carrier frequencies of 813 and 1627 Hz were propagated over a 19.8-km source-receiver range when a packet of strong internal waves passed through the acoustic track. The communication data are analyzed by time reversal processing followed by a single-channel decision feedback equalizer. Two types of internal wave effects are investigated in the context of acoustic communications. One is the rapid channel fluctuation within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the internal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the receiver to counteract such fluctuation. The other one is the long-term (in the scale of hours) performance degradation in the depressed waveguide when the internal waves passed through the acoustic track. Even with channel tracking, the time reversal receiver experiences average 3-4-dB decrease in the output signal-to-noise ratio (SNR). Such long-term performance degradation is explained by the ray approximation in the depressed waveguide.

Low‐frequency normal mode propagation in shallow water and its application to the acoustic inverse problem.

Acoustic normal mode theory is useful to describe low‐frequency sound propagation in shallow water. In this talk, normal‐mode propagation effects will be reviewed, as well as acoustic inverse approaches based on modal propagation theory. The normal modes of low frequency sound in shallow water are geometrically dispersive due to the presence of surface and bottom boundaries. One can exploit this modal dispersion to solve some acoustic inverse problems, such as passive acoustic localization and geoacoustic inversion. The approaches of matched field processing and backpropagation are commonly used. In addition, the modal wavenumber spectrum contains information on seabed properties that can be used to infer the geoacoustic parameters. Underwater sound propagation is strongly affected by water‐column fluctuations, and internal waves are one of the significant effects in shallow water environment. An overview of internal wave effects on normal mode propagation will be presented here, along with their consequence on acoustic inversions.

Effects of internal waves on signal temporal correlation length in the South China Sea

We simulated the temporal correlation of sound transmission using a two-dimensional advective frozen-ocean model with temperature data from a temperature sensor array on a propagation path in the South China Sea (SCS) Experiment 2009, and investigated the relationships of temporal correlation length, source-receiver range, and maximal sound speed fluctuation mainly caused by the solitary internal waves. We found that the temporal correlation length is −1.2-power dependent on source-receiver range and −0.9-power dependent on maximal sound speed fluctuation. The empirical relationship is deduced from one-day environmental measurements in a limited area, needing more works and verification in the future with more acoustic data. But the relationship is useful in many applications in the area of SCS Experiment 2009.

Effect of intense internal waves on the sound field interference structure

The effect of a train of internal-wave solitons on the formation of space-frequency interference pattern of sound field in an oceanic waveguide has been analyzed. Numerical calculations have been performed for the shallow-water channel parameters corresponding to the conditions of the SWARM’95 natural experiment.

Impacts of internal waves on chlorophyll a distribution in the northern portion of the South China Sea

Internal waves can bring nutrients to the upper level of water bodies and facilitate phytoplankton photosynthesis. Internal waves occur frequently in the northern portion of the South China Sea and inflict an important effect on chlorophyll a distribution. In this study, in-situ observation and satellite remote sensing data were used to study the effects of internal waves on chlorophyll a distribution. Based on the in-situ observations, lower chlorophyll a concentrations were present in the middle and bottom level in areas in which internal waves occur frequently, while the surface chlorophyll a distribution increased irregularly, and a small area with relatively higher chlorophyll a concentrations was observed in the area around the Dongsha Island. Satellite remote sensing showed that the chlorophyll a concentration increased in the area near Dongsha Island, where internal waves frequently occurred. The results of the increased chlorophyll a concentration in the surface water near Dongsha Island in the northern portion of the South China Sea indicated that internal waves could uplift phytoplankton and facilitate phytoplankton growth.

Boundary roughness and the effect of internal waves on signal coherence for shallow water transmission.

Broadband acoustic propagation experiments at three shallow sites allow for comparison of coherency of individual surface‐reflected bottom‐reflected modes of propagation. There appears to be a dependence of the correlation parameters of times and length on frequency and mode number that cannot be attributed to internal waves alone and likely depends on bottom and surface roughness. At low frequencies, f<100 Hz, during periods of quiescence internal waves, all modes of propagation have equally long coherence parameters. The coherency decrease equally for all modes as internal wave activity increases. For higher frequencies, 200 Hz <f<1000 Hz, the coherence parameters depend on mode number, with the lower order modes always more coherent than successive higher order modes. At still higher frequencies, f>1000 Hz, identifiable modes are not always observed; instead there is a continuum of arriving pulse energy with very low coherency even with minimal internal waves. Apparently, the randomizing effect of internal waves depends on bottom and surface roughness and frequency. At low frequencies, the boundaries appear flat and internal waves have a minimal effect. At the highest frequencies, phase coherence is already degraded by boundary roughness so that the slightest of internal wave activity completely randomizes the signals.

Observed space-time scales of internal waves and finescale intrusions on the New Jersey Shelf and their likely acoustic implications.

It has been known for some time that ocean density surfaces possess multi-scale variations in temperature (T) and salinity (S), termed spice (hot and salty, cold and fresh) by Munk in 1981. Compensating T and S variations along a surface of constant density have reinforcing sound speed anomalies and thus have important acoustic implications. With the advent of new technologies to observe small scale, rapidly changing ocean thermohaline structure, spice variations, and their acoustic effects are just starting to be understood. Better known are the impacts of internal waves which vertically advect ocean sound speed structure. This talk will present analysis of moored T and S observations on the New Jersey Shelf region during the SW06 experiment in which the sound speed effects of internal waves and spice can be approximately separated. These results will be contrasted with similar measurements made in the deep waters of the Philippine Sea. Using recent results from coupled mode theory, some discussion of the acoustic implications of the internal wave and spice structure will be presented. In particular, the phase randomizing effects of linear internal waves and spice will diminish the mean acoustical influence of intense but localized nonlinear internal waves.

Effects of anisotropic internal waves on acoustic propagation within the East China Sea.

Using environmental and acoustic data from the TAVEX 2008 Korean/U.S. cooperative experiment, the anisotropic spatial properties of internal waves recorded during CTD tows and their connection to temporal variations in recorded acoustic data from a horizontal line array will be discussed. Structure of internal waves, wave travel speed and trajectory, as well as computational modeling of the acoustic field and its comparison with recorded data will also be covered. [Work supported by the Office of Naval Research Contract No. N00014-08-1-0455.]

Acoustic ducting and refraction by sea bottom relief in shallow water.

Observations show that shallow water bottom relief often has a band‐limited directional spectrum produced by various oceanographic and geological processes. This directional bottom feature is shown to have a noticeable effect on three‐dimensional low‐frequency acoustic propagation. An analytical study with an idealized model of straight sea bottom ripples has shown that acoustic energy can be partially ducted between neighboring ripples, and this ducting will affect acoustic propagation in shallow water. In our work, we also study ducting and refracting due to idealized curved sea bottom ripples. Previous research has shown that non‐linear internal waves can also create acoustical ducts. Comparative analysis of these two different ducts is performed using our idealized model. The combined effects of internal waves and bathymetry are studied for various relative directions of internal wave front and bottom ripples. A numerical simulation of three‐dimensional sound propagation across realistic bathymetry and internal wave fluctuations is performed. In conclusion, both water column fluctuations and bathymetry variability need to be taken into account when studying three‐dimensional acoustic propagation in shallow water.

Editorial: Special Issue on the 2006 Shallow Water Experiment

The seven papers in this special issue deal with the effects of internal waves and internal tides on acoustic propagation and communication in shallow water, and geoacoustic characterization of the seabed.

Dynamics of Rotating Shallow Gravity Currents Passing through a Channel. Part I: Observation of Transverse Structure

Abstract A detailed dataset describing a quasi-stationary bottom gravity current, approximately 10 m thick and 10 km wide, passing through a channel-like constriction in the western Baltic Sea is presented. The data include full-depth, synoptic, and highly resolved transects of stratification and turbulence parameters, as well as detailed velocity transects across the gravity current at different down-channel locations. The velocity data reveal a persistent transverse circulation, creating a characteristic wedge-shaped density structure in the interface. A strong asymmetry was also found in the interior of the gravity current, where the evolution of a dynamically significant transverse density gradient to the right of the down-channel flow was observed. Spectral analysis of the near-bottom velocities showed a surprisingly strong contribution to the bottom stress from low-frequency motions with periods up to 30 min that are possibly related to internal wave effects. Cross-channel transects of shear microstructure were used to investigate the transverse variation of local entrainment rates and bottom stresses. These data indicate that frictional control is essential for this class of gravity currents that are characterized by subcritical Froude numbers, small entrainment, strong rotational effects, and small thickness compared to the bottom Ekman layer.