Large Nonlinear Internal Waves Research Articles

Seasonal Variability and Generation Mechanisms of Nonlinear Internal Waves in the Strait of Georgia

AbstractSeasonal variability and generation mechanisms of large nonlinear internal waves are reported, based on an analysis of 9 years of continuous observational data collected at nodes of the Ocean Networks Canada coastal observatory in the Strait of Georgia, Canada. About one thousand large nonlinear internal wave packets were identified. The timing of these packets suggests a very strong correlation with daily tides and fortnightly cycles. More waves are seen during the stronger tides of the fortnightly cycle but overall waves are seen in less than 40% of all days. Other wave characteristics show seasonal variations. In winter, when the pycnocline is weaker, both the spatial and temporal scales of internal waves are several times larger than summertime waves and wave amplitudes can approach 30 m. These waves are large enough vertically but small enough horizontally that standard acoustic Doppler current profiler (ADCP) processing algorithms present a strongly distorted view of wave shape and features. A specialized reprocessing of the beam velocities is used to correctly measure the wave velocity field and to determine wave propagation speed and direction. In addition to the previously known northward‐propagating waves in the Strait, we also found waves propagating in other directions and a noticeable correlation with winds, which may be a proxy for spatial variations in density and/or velocity shear. By considering wave propagation direction, relationship with tidal phase, and relationship with wind, three generation mechanisms were found. Two particularly interesting generation mechanisms involve waves radiating from the Sand Heads area.

Impact of fine-scale sound-speed fluctuations on acoustic autocorrelation times in the East China Sea

Autocorrelation times of acoustic signals propagating through shallow oceans are largely driven by sound-speed fluctuations. In August 2008, the Transverse Acoustic Variability Experiment obtained measurements in the East China Sea (65–80 m water depth) of fluctuating signals propagating 33 km from a moored source to a bottomed line array. Supporting environmental measurements were obtained by a towed conductivity-temperature-depth chain. For time periods without large nonlinear internal waves, the measured internal-wave power spectrum shows excess energy at high wavenumbers in comparison with the shallow-water internal-wave model of Levine. Likewise the associated sound-speed fluctuation spectrum exhibits high-wavenumber components in excess of a simple power law. Autocorrelation times of measured 300 Hz acoustic signals were compared with simulated times obtained using a parabolic-equation model to propagate acoustic fields through sound-speed fluctuations driven by linear internal-wave displacements. Results of the comparison are that median measured autocorrelation times (115 s) are shorter than simulations (300 s) driven by spectra with depleted high-wavenumber components. Simple frozen ocean simulations that translate the environment at 0.6 m/s produce autocorrelation times close to the data. [Work supported by the Office of Naval Research.]

EXPERIMENTAL EVIDENCE OF HORIZONTAL REFRACTION BY NONLINEAR INTERNAL WAVES OF ELEVATION IN SHALLOW WATER IN THE SOUTH CHINA SEA: 3D VERSUS Nx2D ACOUSTIC PROPAGATION MODELING

A joint Taiwanese-U.S. field experiment was conducted in the South China Sea (SCS), entitled the South China Sea Oceanic Processes Experiment (Taiwan)/Non-Linear Internal Waves Initiative (US) (SCOPE/NLIWI), the ocean acoustics portion of which occurred during April 12–22, 2007. The acoustics objective was to quantify the temporal and spatial variability in acoustic propagation characteristics on the continental shelf in the presence of locally-generated and trans-basin nonlinear internal waves (NLIW). Broadband (400 Hz center frequency) m-sequence signals transmitted nearly continuously by a source moored near the seabed were received by vertical line arrays at 3 and 6 km range. The acoustic transect was oriented approximately parallel to the wave fronts of the shoaling trans-basin NLIW's which had crossed the deep basin from their origin in the Luzon Strait. The acoustic propagation variability due to strong vertical and horizontal refraction induced by the very large NLIW's creates an extremely complex acoustic field as a function of time and space. Experimental data and numerical acoustic propagation modeling results are presented to (1) examine and estimate the contribution of internal wave induced horizontal refraction to the received acoustic field; and (2) to quantify the range of propagation angles relative to the internal wave fronts within which strong horizontal refraction occurs and 3D propagation models are required to accurately predict the range- and depth-dependent acoustic propagation.

Single-path acoustic scintillation results from the Shallow Water 2006 Experiment.

In “sound transmission through a fluctuating ocean,” Flattxe et al. described saturation of a single acoustic path as that path becoming a number of interfering uncorrelated micropaths due to refraction by internal waves. The probability density function of intensity becomes exponential with a scintillation index of 1.0. In deep water, however, full saturation is not achieved due to weak scattering and absorption. Mid‐frequency (1–10 kHz) data from the Shallow Water 2006 Experiment are used to determine single‐path intensity statistics. At a range of 1 km in water 80 m deep, an acoustic path is isolated that went through two upper turning points separated by a single bottom reflection. The data were collected during a period when large nonlinear internal waves were absent. The scintillation index calculated from the data increases with frequency until reaching a maximum of 1.2 around 6 kHz. It then decreases to 1.0, suggesting that single‐path saturation can be achieved at mid‐frequencies in shallow water. The probability density functions of intensity at various frequencies show a trend toward exponential. Because shallow water internal waves are dominated by the first mode, uncorrelated micropaths are an unlikely mechanism for producing the observed saturation. [Work supported by ONR.]

Simultaneous nearby measurements of acoustic propagation and high‐resolution sound speed structure containing internal waves

On two occasions during the SW06 experiment, towed CTD chain measurements were made close to an acoustic propagation path. The acoustic path was 1 km long, oriented roughly in the direction of propagation of large nonlinear internal waves. On the first occasion, large nonlinear internal waves were absent, and on the second occasion, they were present. The CTD chain was towed in loops around the acoustic path, roughly 200 m on either side of the path. On the first occasion, 17 loops were made in about 5.5 hr, and on the second occasion, 7 loops were made in about 2.5 hr. Throughout these time periods, acoustic transmissions between 2 kHz and 10 kHz were carried out. The acoustic environment on the path is estimated by space and time interpolation between the tows on the two sides of the path. The acoustic data is compared with propagation modeling in this environment.

Physical and ecological processes of internal waves on an isolated reef ecosystem in the South China Sea

Field measurements at Dongsha Atoll in the northern South China Sea showed frequent drops in daily water temperature up to 8°C during intrusions of large nonlinear internal waves. The internal waves had a period of about 4 hours and velocity amplitude of 0.3 m s−1. Large variations of dissolved oxygen were observed a week after cold‐water intrusion, which was followed in another 2 days by a significant increase of chlorophyll content. The 7–8‐day delay suggests that phytoplankton blooms occurred after the nutrients were released through microbial decomposition of organics brought up by the internal waves. This study highlights the effects of internal waves and the possible role of the microbial food web on nutrient regeneration in a tropical reef ecosystem.

Nonhydrostatic and nonlinear contributions to the energy flux budget in nonlinear internal waves

We use high‐resolution two‐dimensional simulations to model the generation and evolution of nonlinear internal waves formed as a result of the interaction of a first‐mode internal wave field with idealized slopes. The derivation of the energy equation and the energy flux terms are presented. By employing an analysis of the distribution of the energy flux across the shelf break, we quantify the contributions to the energy flux budget from nonhydrostatic as well as nonlinear effects in comparison to the contribution from the baroclinic pressure anomaly term used widely for linear internal waves. Our results show that the contributions to the total energy flux from these additional terms are significant in these large nonlinear internal waves, consistent with recent field observations.

Analysis and modeling of broadband airgun data influenced by nonlinear internal waves

To investigate acoustic effects of nonlinear internal waves, the two southwest tracks of the SWARM 95 experiment are considered. An airgun source produced broadband acoustic signals while a packet of large nonlinear internal waves passed between the source and two vertical linear arrays. The broadband data and its frequency range (10-180 Hz) distinguish this study from previous work. Models are developed for the internal wave environment, the geoacoustic parameters, and the airgun source signature. Parabolic equation simulations demonstrate that observed variations in intensity and wavelet time-frequency plots can be attributed to nonlinear internal waves. Empirical tests are provided of the internal wave-acoustic resonance condition that is the apparent theoretical mechanism responsible for the variations. Peaks of the effective internal wave spectrum are shown to coincide with differences in dominant acoustic wavenumbers comprising the airgun signal. The robustness of these relationships is investigated by simulations for a variety of geoacoustic and nonlinear internal wave model parameters.

Mass transport by non-linear internal waves on the Malin Shelf

Packets of non-linear internal waves (NIWs) were observed with a moored ADCP in the seasonal thermocline at the edge of the Malin Shelf, to the west of Scotland, during August 1995. A wide range of wave sizes was encountered including some that were strongly non-linear. The largest waves had a vertical displacement of about 25 m (in 145 m water) and a vertical shear greater than 0.4 m s −1 over a depth of 65 m across the thermocline. In general they propagated toward the ESE, which is south of a line normal to the shelf break, with a phase speed of between 0.54 and 0.60 m s −1. Within each packet the direction of the current flow in successive waves veered at a similar rate to that of waves with a tidal frequency. However, large NIWs were only encountered during neap rather than spring tides, so their relationship with the tide is not entirely clear. The first empirical mode dominated the motion, but its shape was often more convoluted than an equivalent baroclinic mode. A two-layer KdV theory for the mass transport due to the waves, which relates transport to the elevation of the interface and the linear long wave phase speed, is presented. It compares well with the observed transport in the lower layer. None of the waves possessed significant dispersion, so they did not conform to the KdV soliton solution. In a typical NIW packet a lower layer transport of about 5 m 2 s −1 offshore was maintained over a period of about 1.5 h, with a peak of about 20 m 2 s −1. A sustained transport of about 0.3 m 2 s −1 can be attributed to NIWs at the Malin Shelf edge in summer.

Large-amplitude internal soliton packets in the central Bay of Biscay

In the Bay of Biscay, large-amplitude internal tides result from the interaction of surface tides with steep bottom topography. Near the shelf break, the troughs of the internal tides are associated with packets of large nonlinear internal waves or “solitons”. By using an Acoustic Doppler Current Profiler, we here demonstrate that a similar phenomenon occurred at a considerable distance (150 km) from the shelf break, and make a detailed examination of the dramatic changes in velocity structure which accompanied the passage of the soliton packets. In particular, horizontal and vertical velocities exceeding, respectively, 60 and 16 cm s −1 were observed. A novel mechanism is proposed for the possible local generation of such large-amplitude solitons so far from topography.