Nonlinear Internal Waves From the Luzon Strait (original) (raw)

Speed and evolution of nonlinear internal waves transiting the South China Sea

Journal of Physical …, 2010

In the South China Sea (SCS), 14 nonlinear internal waves are detected as they transit a synchronous array of 10 moorings spanning the waves' generation site at Luzon Strait, through the deep basin, and onto the upper continental slope 560 km to the west. Their arrival time, speed, width, energy, amplitude, and number of trailing waves are monitored. Waves occur twice daily in a particular pattern where larger, narrower ''A'' waves alternate with wider, smaller ''B'' waves. Waves begin as broad internal tides close to Luzon Strait's two ridges, steepening to O(3-10 km) wide in the deep basin and O(200-300 m) on the upper slope. Nearly all waves eventually develop wave trains, with larger-steeper waves developing them earlier and in greater numbers. The B waves in the deep basin begin at a mean speed of '5% greater than the linear mode-1 phase speed for semidiurnal internal waves (computed using climatological and in situ stratification). The A waves travel '5%-10% faster than B waves until they reach the continental slope, presumably because of their greater amplitude. On the upper continental slope, all waves speed up relative to linear values, but B waves now travel 8%-12% faster than A waves, in spite of being smaller. Solutions of the Taylor-Goldstein equation with observed currents demonstrate that the B waves' faster speed is a result of modulation of the background currents by an energetic diurnal internal tide on the upper slope. Attempts to ascertain the phase of the barotropic tide at which the waves were generated yielded inconsistent results, possibly partly because of contamination at the easternmost mooring by eastward signals generated at Luzon Strait's western ridge. These results present a coherent picture of the transbasin evolution of the waves but underscore the need to better understand their generation, the nature of their nonlinearity, and propagation through a time-variable background flow, which includes the internal tides.

The Generation and Evolution of Nonlinear Internal Waves in the Deep Basin of the South China Sea

Time series observations of nonlinear internal waves in the deep basin of the South China Sea are used to evaluate mechanisms for their generation and evolution. Internal tides are generated by tidal currents over ridges in Luzon Strait and steepen as they travel west, subsequently generating high-frequency nonlinear waves. Although nonlinear internal waves appear repeatedly on the western slopes of the South China Sea, their appearance in the deep basin is intermittent and more closely related to the amplitude of the semidiurnal than the predominant diurnal tidal current in Luzon Strait. As the internal tide propagates westward, it evolves under the influence of nonlinearity, rotation, and nonhydrostatic dispersion. The interaction between nonlinearity and rotation transforms the internal tide into a parabolic or corner shape. A fully nonlinear two-layer internal wave model explains the observed characteristics of internal tide evolution in the deep basin for different representative forcing conditions and allows assessment of differences between the fully and weakly nonlinear descriptions. Matching this model to a wave generation solution for representative topography in Luzon Strait leads to predictions in the deep basin consistent with observations. Separation of the eastern and western ridges is close to the internal semidiurnal tidal wavelength, contributing to intensification of the westward propagating semidiurnal component. Doppler effects of internal tide generation, when combined with a steady background flow, suggest an explanation for the apparent suppression of nonlinear wave generation during periods of westward intrusion of the Kuroshio.

Large-Amplitude Internal Waves in the South China Sea

One of the most spectacular phenomena recently discovered in the South China Sea is that of very large internal waves. Field observations and satellite images show that these internal waves are over 200 meters in amplitude and their crests extend more than 200 km (Fig. 3). These fast, transient, large-amplitude internal waves can push water up or down 200 meters in 10 minutes and seriously impact the safe operation of submerged vessels, particularly the less powerful unmanned undersea vessels, or UUVs. The largeamplitude internal waves can also have a strong effect on underwater sound propagation as reported by the Office of Naval Research (ONR) Asian Seas International Acoustics Experiment. 1 Several NRL scientists from the Acoustics and Oceanography Divisions participated in this experiment. In 2005, ONR launched the Nonlinear Internal Waves Initiative (NLIWI) to better understand the large-amplitude internal waves in the South China Sea. NRL teamed with university scientists to participate in the NLIWI to conduct internal wave studies using computer ocean models and observations.

The formation and fate of internal waves in the South China Sea

Nature, 2015

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variabilit...

Energy flux of nonlinear internal waves in northern South China Sea

Geophysical Research Letters, 2006

1] We analyze three sets of ADCP measurements taken on the Dongsha plateau, on the shallow continental shelf, and on the steep continental slope in the northern South China Sea (SCS). The data show strong divergences of energy and energy flux of nonlinear internal waves (NLIW) along and across waves' prevailing westward propagation path. The NLIW energy flux is 8.5 kW m À1 on the plateau, only 0.25 kW m À1 on the continental shelf 220 km westward along the propagation path, and only 1 kW m À1 on the continental slope 120 km northward across the propagation path. Along the wave path on the plateau, the average energy flux divergence of NLIW is 0.04WmAˋ2,whichcorrespondstoadissipationrateofO(10Aˋ7Aˋ10Aˋ6)WkgAˋ1.Combiningthepresentwithpreviousobservationsandmodelresults,ascenarioofNLIWenergyfluxintheSCSemerges.NLIWsaregeneratedeastoftheplateau,propagatepredominantlywestwardacrosstheplateaualongabeamof0.04 W m À2 , which corresponds to a dissipation rate of O(10 À7 À10 À6 ) W kg À1 . Combining the present with previous observations and model results, a scenario of NLIW energy flux in the SCS emerges. NLIWs are generated east of the plateau, propagate predominantly westward across the plateau along a beam of 0.04WmAˋ2,whichcorrespondstoadissipationrateofO(10Aˋ7Aˋ10Aˋ6)WkgAˋ1.Combiningthepresentwithpreviousobservationsandmodelresults,ascenarioofNLIWenergyfluxintheSCSemerges.NLIWsaregeneratedeastoftheplateau,propagatepredominantlywestwardacrosstheplateaualongabeamof100 km width that is centered at $21°N, and dissipate nearly all their energy before reaching the continental shelf.

Episodes of nonlinear internal waves in the northern East China Sea

Geophysical Research Letters, 2006

Episodes of high-frequency internal waves, which lasted approximately 3 hours, were detected in the northern East China Sea during a specific phase of the barotropic tide (i.e., low tide at 32°N, 125°E). The observed internal waves influenced the whole water column. The wave packets were presumably generated near the ocean shelf break, approximately 200 km to the southeast of the test site. During several internal wave episodes, which coincided with the neap tide, large-amplitude solitary wave-like features emerged preceding higher frequency internal waves. Shear instability of tidal currents is explored as a possible mechanism for sustaining or regenerating internal waves in the packets during the course of their propagation. It is suggested that rotating velocity field of tidal current supports sufficient vertical shear within wave packets to cause outbreaks of K-H instability. These instabilities may gradually transition to more symmetric Hölmböe waves, following the increase of the bulk Richardson number.

Nonlinear internal wave spirals in the northern East China Sea

Scientific reports, 2018

Oceanic internal waves are known to be important to the understanding of underwater acoustics, marine biogeochemistry, submarine navigation and engineering, and the Earth's climate. In spite of the importance and increased knowledge of their ubiquity, the wave generation is still poorly understood in most parts of the world's oceans. Here, we use satellite synthetic aperture radar images, in-situ observations, and numerical models to (1) show that wave energy (having relatively high amplitude) radiates from a shallow sill in the East China Sea in all directions, but with a significant time lag dependent on background conditions, (2) reveal that wave fronts are locally formed with often favorable conditions for re-initiation, and (3) demonstrate the resulting variety of wave patterns. These findings would be the case for any broad shelf having shallow sills with time-varying conditions, and therefore have significant implications on the redistribution of energy and materials ...

On the generation and evolution of nonlinear internal waves in the South China Sea

Journal of Geophysical Research, 2010

1] The nonhydrostatic Regional Ocean Modeling System is applied to the nonlinear internal waves, or solitons, that are generated at the Luzon ridge in the South China Sea. The Luzon ridge near the Batan islands is represented by an idealized ridge with a height of 2.6 km on a flat bottom. Model runs are performed for various ridge shapes and (a)symmetric tidal forcings. The model is in the mixed tidal lee wave regime. The barotropic tide over the ridge generates first-mode waves through the internal tide release mechanism. Westward-traveling solitons emerge from these first-mode waves through nonlinear steepening. In the internal tide release mechanism, asymmetric tides with strong eastward currents can generate strong westward solitons. The eastward current creates an elevation wave with a higher energy density west of the ridge, and as soon as the current slackens, the wave is released westward. On its backslope strong solitons develop. The energy density is further enhanced by nonlinearities, such as differences in phase speeds and energy fluxes related to lee waves. A modal and harmonic decomposition shows the generation of vertical modes and higher temporal harmonics and indicates significant wave-wave interaction (e.g., triads). In the mixed tidal lee wave regime, more energy is contained in the first mode compared to the higher modes. Hence, linear internal tide beams are less well defined and strong solitons develop.

NONLINEAR INTERNAL WAVES IN THE SOUTH CHINA SEA

2000

The Kuroshio moving north from Philippine Basin branches out near the south tip of Taiwan and part of the Kuroshio intrudes into the South China Sea through the Bashi Channel and the Luzon Strait. The internal tides and internal waves have been generated by the shallow ridges (200-300 m) in the Luzon Strait. Surface signature of huge internal wave packets