Poleward Propagation of Typhoon-Induced Near-Inertial Waves in the Northern South China Sea (original) (raw)
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Bulletin of Marine Science, 2018
A mooring and a real-time buoy allowed observation of oceanic and atmospheric variation in the wake of the 2008 Typhoon Hagupit in the South China Sea (SCS). In the present study, a regional ocean modeling system (ROMS) was used to explore the propagation characteristics of near-inertial waves (NIWs) along the continental shelf. The simulated NIWs were similar to the observation results. There were strong NIWs after the passage of Typhoon Hagupit, and the NIWs presented dual peaks, with one located in the mixed layer and the other at the thermocline. In the upper layer, the near-inertial kinetic energy on the left of the typhoon track was slightly weaker than that on the right. The responses of the ocean to the typhoon in shallow (<100 m) vs deep (>200 m) areas were different. The thermocline was significantly enhanced and deepened in shallow areas. The near-inertial kinetic energy of the thermocline in shallow areas occurred earlier and more intensely than that in the deep areas. The shallower mixed layer and the stronger thermocline in shallow areas are the main reasons for the difference in response. When the typhoon arrived, the mixed layer in shallow areas sank rapidly, causing the thermocline to sink, and the thermocline strength to increase and the thickness to decrease. The thermocline triggered NIWs during its compression. High compression and a strong thermocline result in powerful NIWs.
Upper ocean near-inertial response to 1998 Typhoon Faith in the South China Sea
Acta Oceanologica Sinica, 2012
During the South China Sea monsoon experiment (SCSMEX), three autonomous temperature line acquisition system (ATLAS) buoys with acoustic Doppler current profiler (ADCP) were moored in the South China Sea to measure temperature, salinity and current velocity. Typhoon Faith passed through about 250 km south to one of the mooring buoys located at 12 • 58.5 N, 114 • 24.5 E from December 11 to 14, 1998. The data analysis indicates that the typhoon winds induce a great increase in the kinetic energy at near-inertial frequencies with two maxima in the mixed layer and thermocline. The near-inertial oscillations were observed at the upper 270 m in the wake of Typhoon Faith. The oscillations were originally excited in the sea surface layer and propagated downward. The amplitudes of the oscillations decrease with depth except in the thermocline. The near-inertial oscillation signals are also remarkable in temperature and salinity fields.
Frequency Shift of Near-Inertial Waves in the South China Sea
Journal of Physical Oceanography, 2020
Despite sufficient wind forcing, internal waves in the South China Sea do not exhibit the strong near-inertial wave (NIW) peak that is typical in most of the world oceans. Using data from 10 contemporaneous moorings deployed in summer 2011, we show that strong isopycnal vertical tidal displacements transfer most of the near-inertial (NI) kinetic energy (KE) to frequencies higher than the inertial frequency in an Eulerian reference frame. Transforming to an isopycnal-following reference frame increases the KE at NI frequencies, suggesting the presence of NIWs. However, the projection onto a semi-Lagrangian coordinate system still underestimates the expected NI peak. To fully resolve NIWs requires the use of time-dependent vertical wavenumber–frequency spectra because the intrinsic frequency of the NIWs varies substantially, owing to Doppler shifting by lateral mesoscale flows. Here, we show NIW intrinsic frequency variations of ±0.2 cpd within few days, of similar magnitude as the ob...
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.
Typhoon Effects on the South China Sea Wave Characteristics During Winter Monsoon
2006
Ocean wave characteristics in the western Atlantic Ocean (Hurricane Region) to tropical cyclones have been well identified, but not the regional seas in the western Pacific, e.g., the South China Sea (Typhoon Region). This is due to the lack of observational and modeling studies in the regional seas of the western Pacific. To fill this gap, Wavewatch-III (WW3) is used to study the response of the South China Sea (SCS) to Typhoon Muifa (2004). The major purposes are to find the similarity and dissimilarity of wave characteristics between the two regions, and to evaluate the WW3 capability to typhoon forcing. The WW3 model is integrated from the JONSWAP wave spectra with a tropical cyclone wind profile model, simulating Typhoon Muifa, from 16 to 25 November 2004. This study shows strong similarities in the responses between Hurricane and Typhoon Regions, including strong asymmetry in the significant wave height (H s) along the typhoon translation track with the maximum H s in the right-front quadrant of the typhoon center, and asymmetry in the directional wave spectra at different locations (frontward, backward, rightward, and leftward) around the typhoon center. The unique features of the SCS wave characteristics to Muifa are also discussed.
Continental Shelf Research, 2016
This study deals with winter storm-induced continental shelf waves (CSWs) in the northern South China Sea in winter 2009 using tidal gauge data and along-track satellite altimeter data. The results show that the periods of CSWs propagating along the coast from Kanmen to Shuidong are in bands of 62 and 133 h. The phase speeds are in a range from 8 to 13 m s-1 between Kanmen and Shantou, from 9 to 11 m s-1 between Shantou and Huizhou and from 10 to 15 m s-1 between Huizhou and Shuidong, respectively. Satellite altimeter captured along-track sea level anomaly during the CSW events. Further analysis using the theoretical cross-shore CSW modes to fit the along-track sea level anomaly data indicates that the first three wave modes play important roles during the CSW events and the first mode is a dominant component.
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.
NONLINEAR INTERNAL WAVE STUDY IN THE SOUTH CHINA SEA
Advances in Engineering Mechanics - Reflections and Outlooks - In Honor of Theodore Y.-T. Wu, 2005
The internal wave distribution map in the northeast part of South China Sea (SCS) has been compiled from hundreds of ERS-1/2, RADARSAT and Space Shuttle Synthetic Aperture Radar (SAR) images. Based on the map compiled from satellite data, the wave crest can be as long as 200 km with amplitude of 100 m. In recent Asian Seas International Acoustics Experiment (ASIAEX), extensive moorings have been deployed around the continental shelf break area in the northeast of South China Sea. Simultaneous RADARSAT SAR images have been collected during the field test to integrate with the in-situ measurements from moorings, ship-board sensors, and CTD casts. Besides it provides synoptic information, satellite imagery is very useful for tracking the internal waves, and locating surface fronts and mesoscale features. Environmental parameters have been calculated based on extensive CTD casts data near the ASIAEX area. Nonlinear internal wave models have been applied to integrate and assimilate both SAR and mooring data. The shoaling, turning, and dissipation of large internal waves on the shelf break, elevation solitons, and wave-wave interaction have been studied.
Journal Of Geophysical Research: Oceans, 2019
For investigation of internal solitary waves (ISWs) in the South China Sea (SCS), most cruise observations are concentrated from Luzon Strait to Dongsha Atoll in the northeastern SCS but few on the continental slope far away from the west of Dongsha Atoll. In this study, we use 1-year long mooring data to determine dynamic and statistical features of the ISWs on the shelf slope of the northwestern SCS. The analysis results of the mooring data reveal that the ocean internal waves on the shelf slope of the northwestern SCS have physical properties of highly nonlinear waves, which are well described by the solutions of the Korteweg-de Vries equation. The mean nonlinear phase speeds of mode-1 and mode-2 ISWs are 1.38 ± 0.14 and 0.66 ± 0.12 m/s, respectively. The major direction of mode-1 ISWs is northwestward 305°± 21°. Strong ISW currents force the major direction of total current velocities to turn 67.5°in the upper layer and 135°in the lower layer. The monthly occurrence frequency distribution of ISWs shows a peak in July with a maximum frequency of 16.2% and a trough in March with a minimum frequency of 3.3%. Mode-2 ISWs appear most in December, accounting for 50% of total mode-2 ISWs. The largest mode-2 ISW on record up today with the depressed amplitude as large as 91 m, and the elevated amplitude of 73 m was observed at mooring station. These new findings and new data are of significance to local internal wave prediction model development. Plain Language Summary Unlike the sea surface waves, the ocean internal waves are a sort of wave motions occurring in the ocean interior. Strong internal waves carry strong currents and huge power, which may greatly impact the normal ocean and cause harm to underwater navigation and ocean engineering facilities. Thus, it is a necessary work to clarify the internal wave activities in an area of interest. We deployed a chain of instrument system moored at the sea floor on the continental slope of the northwestern South China Sea, where is rich in oil and gas resources, to observe the internal waves for 1 year from June 2016 to July 2017. Using the data of observed 684 internal waves, we analyze their physical properties and statistical features. The results indicate that (1) the observed waves have typical features of internal solitary waves, (2) the maximum amplitude reaches near 100 m, and (3) strong internal wave currents force the major direction of normal current to turn about 70°and 135°in the upper and lower layers, respectively, and March and July are trough and peak months for internal wave occurrence. These data and results are of significance for underwater navigation safety and ocean engineering designs.