The research on typhoon wave spectrum in northwestern South China Sea

Based upon the one-year wind wave measurement data, collected from the South China Sea (SCS) at coordinates 20° 36.298′N, 110°45.433′E. by Acoustic Wave And Current (AWAC), we analyzed the wave characteristics and concluded that the most common wave direction was E and the second most common direction was ENE, the mean and the maximum values of significant height was 1.2 m and 4.36 m, respectively. The mean period was 4.0 s. We also evaluated the wave spectrums under conditions existing in three typhoons: Rumbi, Jeti and Utor. We found that unimodal spectrums occurred more often than others, and the maximum spectrum peak was 30.7911 m² s. The minimum peak frequency was 0.0625 Hz, and the mean peak frequency was 0.126 Hz. The wave period is important for the design of marine structures, especially the position of peak frequency had a great influence on the stress calculation. Spectral analysis showed that the values of peak frequency distributed between 0.063 Hz and 0.217 Hz, with the mean value 0.114 Hz. We fit the normalized spectrum with 6 theoretical spectral models, out of which, the Wen spectrum, JONSWAP spectrum and Wallops spectrum were proved to give the best fit. What distinguished the Wen Spectrum from the rest was that it does not rely on the measured spectrum for parameter estimation. Hence, we recommend that the Wen spectrum should be widely used in marine construction.     

Currents and mixing in the northern South China Sea

We investigated the vertical distribution of current velocity data of the entire water column at a site on the continental shelf of the northern South China Sea (SCS) from August 4 to September 6, 2007, and found that the characteristics of barotropic and baroclinic tides are mainly diurnal. During the observation period, we also estimated the mixing before and after the passage of Typhoon Pabuk. We found that the internal-wave-scale dissipation rate, the turbulent dissipation rate, and the mixing rate in every water layer increased by about an order of magnitude after the typhoon passage. We analyzed a case of abrupt strong current and calculated the mixing rate before, during, and after the typhoon event. The results show that the internal-wave-scale dissipation rate and the mixing rate in every water layer increased by about two orders of magnitude during the event, while the turbulent dissipation rate increased by about an order of magnitude. Passage of the abrupt strong current could also have increased the mixing rate of affected seawater by more than an order of magnitude. However, the passage of the typhoon differed in that there was an increase in mixing only in the lower layer where the abrupt strong current was particularly strong. The variation of the mixing rate may help us to understand the effects of typhoons and abrupt strong currents on the mixing of seawater.     

Model-simulated coastal trapped waves stimulated by typhoon in northwestern South China Sea

In this paper, we apply an unstructured grid coastal ocean model to simulate variations in the sea level and currents forced by two typhoons in the northwestern South China Sea (SCS). The model simulations show distinct differences for the two cases in which the typhoon paths were north and south of the Qiongzhou (QZ) Strait. In both cases, coastal trapped waves (CTWs) are stimulated but their propagation behaviors differ. Model sensitivity simulations suggest the dominant role played by alongshore wind in the eastern SCS (near Shanwei) and southeast of Hainan Island. We also examine the influence of the Leizhou Peninsula by changing the coastline in simulation experiments. Based on our results, we can draw the following conclusions: 1) The CTWs stimulated by the northern typhoon are stronger than the southern CTW. 2) In the two cases, the directions of the current structures of the QZ cross-transect are reversed. The strongest flow cores are both located in the middle-upper area of the strait and the results of our empirical orthogonal function analysis show that the vertical structure is highly barotropic. 3) The simulated CTWs divide into two branches in the QZ Strait for the northern typhoon, and an island trapped wave (ITW) around Hainan Island for the southern typhoon. 4) The Leizhou Peninsula plays a significant role in the distribution of the kinetic energy flux between the two CTW branches. In the presence of the Leizhou Peninsula, the QZ branch has only 39.7 percent of the total energy, whereas that ratio increases to 72.2 percent in its absence.     

Observed characteristics of atmospheric ducts over the South China Sea in autumn

The observed characteristics of lower atmospheric ducts over the South China Sea(SCS) were analyzed based on Global Position Systerm(GPS) radiosonde data collected four times daily during autumn open cruises from 2006 to 2012.Duct occurrence,thickness,and strength over the SCS were about 40%,150-m thick,and 8 M units,respectively,which were larger than during the summer monsoon period.Most ducts occurred at heights 1 500 m and these ducts easily trap electromagnetic wave clusters with wavelengths 2 m.Diurnal variation of the SCS ducts appeared evident.They occurred more often at midnight at higher altitudes(about 1 100 m),with a thickest layer of about 145 m and less frequently during the evening at lower altitudes(about 800 m),with a thinnest layer of about 125 m.Moreover,ducts during the daytime at a mean height of about 900 m,with the greatest strength of about 10 M units.Furthermore,all duct variables observed over the SCS in autumn decreased from north to south.These findings are useful not only in the design of radar and communication systems,but also for evaluating possible effects of anomalous propagation on meteorological radar and military applications.     

Variation in downwelling diffuse attenuation coefficient in the northern South China Sea

The diffuse attenuation coefficient for downwelling irradiance （Kd（λ）） is an important parameter for ocean studies. Based on the optical profile data measured during three cruises in the northern South China Sea in autumn from 2003 to 2005, variations in the Kd（λ） spectra were analyzed. The variability of Kd（λ） shows much distinct features in both magnitude and spectra pattern, it is much higher in coastal waters than that of open oceanic waters; and the blue-to-green （443/555） ratio of Kd（λ） tends to increase with chlorophyll a concentration （[Chl-a]） from open ocean to coastal waters. These characteristics can be explained most by the increase of aw＋p（443）/aw＋p（555） with [Chl-a]. In short waveband, the relation between Kd（λ）-Kw（λ） and [Chl-a] can be well described by a power law function, indicating the large contribution of phytoplankton to the variations in Kd（λ）. As for the spectral model of the diffuse attenuation coefficient, there are good linear relationships between Kd（490） and Kd（λ） in other wavelengths with own slope and intercept of a linear functions in the spectral range 412-555 nm. Kd（490） is well correlated with the spectral ratio of remote sensing reflectance; and should enough measurement data are given, this empirical algorithm would be used in the Kd（λ） retrieval from ocean color satellite data. The variation in Kd（λ） provides much useful information for us to study the bio-optical property in the northern South China Sea.     

Spatio-temporal variations in the siphonophore community of the northern South China Sea

To understand how hydrological and biological factors affect near-to off-shore variations in the siphonophore community,we sampled zooplankton at 82 stations in the northern South China Sea during summer,winter,and spring.Forty-one species of siphonophore were collected by vertical trawling.The species richness of siphonophores increased from the nearshore to offshore regions in all three seasons of investigation,with maximum richness in summer and minimum richness in winter.The abundance of siphonophores was also higher in summer than in spring and winter,concentrated in the nearshore region in the warm season and scattered in the offshore region in the cold season.Four siphonophore groups were classified according to the frequency of occurrence:nearshore,near-offshore,offshore,and tropical pelagic.Among them,the nearshore group had higher abundance nearshore compared with the offshore.The tropical pelagic group had higher species number offshore than nearshore.Spatial and temporal fluctuations in taxonomic composition and abundance of siphonophores were due to the influence of the coastal upwelling and surface ocean currents of the South China Sea,driven by the East Asia monsoonal system.     

Characteristics of nonlinear internal waves observed in the northern South China Sea

The South China Sea (SCS) is one of the most active areas of internal waves. We undertook a program of physical oceanography in the northern South China Sea from June to July of 2009, and conducted a 1-day observation from 15:40 of June 24 to 16:40 of June 25 using a chain of instruments, including temperature sensors, pressure sensors and temperature-pressure meters at a site (117.5°E, 21°N) northeast of the Dongsha Islands. We measured fluctuating tidal and subtidal properties with the thermistor-chain and a ship-mounted Acoustic Doppler Current Profiler, and observed a large-amplitude nonlinear internal wave passing the site followed by a number of small ones. To further investigate this phenomenon, we collected the tidal constituents from the TPXO7.1 dataset to evaluate the tidal characteristics at and around the recording site, from which we knew that the amplitude of the nonlinear internal wave was about 120 m and the period about 20 min. The horizontal and vertical velocities induced by the soliton were approximately 2 m/s and 0.5 m/s, respectively. This soliton occurred 2–3 days after a spring tide.     

A new radiolarian species Clathrocorys gracilis from northern South China Sea

A new species, Clathrocorys gracilis nov. spec., of Tripocyrtidae （Radiolarians： Nassellaria） from surface water in northern South China Sea, is described in this paper. The new species has four radial beams arising from the cephalic base centre, each foot with cup-shaped structure and ramified bar in the proximal section （near cephalic base）, and each wing with a large mesh in the middle, a medium mesh and a small mesh on each side respectively.     

Estimation of potential distribution of gas hydrate in the northern South China Sea

Gas hydrate research has significant importance for securing world energy resources, and has the potential to produce considerable economic benefits. Previous studies have shown that the South China Sea is an area that harbors gas hydrates. However, there is a lack of systematic investigations and understanding on the distribution of gas hydrate throughout the region. In this paper, we applied mineral resource quantitative assessment techniques to forecast and estimate the potential distribution of gas hydrate resources in the northern South China Sea. However, current hydrate samples from the South China Sea are too few to produce models of occurrences. Thus, according to similarity and contrast principles of mineral outputs, we can use a similar hydrate-mining environment with sufficient gas hydrate data as a testing ground for modeling northern South China Sea gas hydrate conditions. We selected the Gulf of Mexico, which has extensively studied gas hydrates, to develop predictive models of gas hydrate distributions, and to test errors in the model. Then, we compared the existing northern South China Sea hydrate-mining data with the Gulf of Mexico characteristics, and collated the relevant data into the model. Subsequently, we applied the model to the northern South China Sea to obtain the potential gas hydrate distribution of the area, and to identify significant exploration targets. Finally, we evaluated the reliability of the predicted results. The south seabed area of Taiwan Bank is recommended as a priority exploration target. The Zhujiang Mouth, Southeast Hainan, and Southwest Taiwan Basins, including the South Bijia Basin, also are recommended as exploration target areas. In addition, the method in this paper can provide a useful predictive approach for gas hydrate resource assessment, which gives a scientific basis for construction and implementation of long-term planning for gas hydrate exploration and general exploitation of the seabed of China.     

Estimation of potential distribution of gas hydrate in the northern South China Sea

Gas hydrate research has significant importance for securing world energy resources, and has the potential to produce considerable economic benefits. Previous studies have shown that the South China Sea is an area that harbors gas hydrates. However, there is a lack of systematic investigations and understanding on the distribution of gas hydrate throughout the region. In this paper, we applied mineral resource quantitative assessment techniques to forecast and estimate the potential distribution of gas hydrate resources in the northern South China Sea. However, current hydrate samples from the South China Sea are too few to produce models of occurrences. Thus, according to similarity and contrast principles of mineral outputs, we can use a similar hydrate-mining environment with sufficient gas hydrate data as a testing ground for modeling northern South China Sea gas hydrate conditions. We selected the Gulf of Mexico, which has extensively studied gas hydrates, to develop predictive models of gas hydrate distributions, and to test errors in the model. Then, we compared the existing northern South China Sea hydrate-mining data with the Gulf of Mexico characteristics, and collated the relevant data into the model. Subsequently, we applied the model to the northern South China Sea to obtain the potential gas hydrate distribution of the area, and to identify significant exploration targets. Finally, we evaluated the reliability of the predicted results. The south seabed area of Taiwan Bank is recommended as a priority exploration target. The Zhujiang Mouth, Southeast Hainan, and Southwest Taiwan Basins, including the South Bijia Basin, also are recommended as exploration target areas. In addition, the method in this paper can provide a useful predictive approach for gas hydrate resource assessment, which gives a scientific basis for construction and implementation of long-term planning for gas hydrate exploration and general exploitation of the seabed of China.     

Wind wave characteristics and engineering environment of the South China Sea

Wave simulation was conducted for the period 1976 to 2005 in the South China Sea (SCS) using the wave model, WAVEWATCH-III. Wave characteristics and engineering environment were studied in the region. The wind input data are from the objective reanalysis wind datasets, which assimilate meteorological data from several sources. Comparisons of significant wave heights between simulation and TOPEX/Poseidon altimeter and buoy data show a good agreement in general. By statistical analysis, the wave characteristics, such as significant wave heights, dominant wave directions, and their seasonal variations, were discussed. The largest significant wave heights are found in winter and the smallest in spring. The annual mean dominant wave direction is northeast (NE) along the southwest (SW)-NE axis, east northeast in the northwest (NW) part of SCS, and north northeast in the southeast (SE) part of SCS. The joint distributions of wave heights and wave periods (directions) were studied. The results show a single peak pattern for joint significant wave heights and periods, and a double peak pattern for joint significant wave heights and mean directions. Furthermore, the main wave extreme parameters and directional extreme values, particularly for the 100-year return period, were also investigated. The main extreme values of significant wave heights are larger in the northern part of SCS than in the southern part, with the maximum value occurring to the southeast of Hainan Island. The direction of large directional extreme H _s values is focus in E in the northern and middle sea areas of SCS, while the direction of those is focus in N in the southeast sea areas of SCS.     

Statistical analysis of surface hydrography and circulation variations in northern South China Sea

1 INTRODUCTIONThe South China Sea (SCS) is a semi-enclosedmarginal sea in western North Pacific Ocean withvery complex topography and is the important pas-sage connecting the Pacific and Indian Oceans. Ithas great impact to the global climate and a greatinterest of many oceanography researchers. Twodominant surface hydrographic and circulation fea-tures in the northern SCS are a strong fresh waterexpansion and a warm and high-salinity seawaterintrusion such as the SCS Diluted Water…     

Sea surface height oscillation with quasi-four-month period along the continental slope in the northern South China Sea

The sea surface height oscillation with a quasi-four-month period (SSHO4) along continental slope in the northern South China Sea (NSCS) is detected using satellite altimeter data and an ocean model simulation. The SSHO4 is at southwest of Dongsha Island, and is characterized by a wavelength of ~600 km and a southwestward phase speed of ~0.1 m/s. Crossing the climatological background SST front, geostrophic currents corresponding to the SSHO4 generally induce sea surface temperature (SST) "tongues" during January-March. The cold and warm SST tongues appear southwest of cyclonic and anticyclonic eddies, respectively. The distance between the warm and cold SST tongues is about half the wavelength of the SSHO4. The geostrophic currents play an important role in lateral mixing, as manifested by the SST tongue phenomena in the NSCS.     

Methane seepages in the northern South China Sea: evidence from in situ measurements

Continental margins in world oceans contain large amounts of marine gas hydrates. Changes in the temperature and pressure of sediment may destabilize the methane hydrate, leading to its release and seepage into the sea. This process would increase the dissolved methane in the ambient seawater. In this study, a methane sensor was used to detect methane anomalies in the water column in southwestern Dongsha and Shenhu, northern slope of the South China Sea. Methane plumes were detected at stations SCS001 and T001 in the southwestern Dongsha area, and station SCS002 in the Shenhu area, respectively. The maximum methane concentrations were 8.8 nmol/L in southwestern Dongsha and 10.1 nmol/L in Shenhu, which are about 4–5 times higher than the background methane concentration. This indicates that there are active methane seepages present in both the southwestern Dongsha and Shenhu areas, which are likely related to a methane hydrate reservoir beneath the seafloor. A methane sensor with a low detection limit is a practical instrument with which to detect methane plumes in oceanic environments and to monitor methane leakage from the seafloor.     

Numerical simulation of wave field in the South China Sea using WAVEWATCH III

Wave fi elds of the South China Sea(SCS) from 1976 to 2005 were simulated using WAVEWATCH III by inputting high-resolution reanalysis wind fi eld datasets assimilated from several meteorological data sources. Comparisons of wave heights between WAVEWATCH III and TOPEX/Poseidon altimeter and buoy data show a good agreement. Our results show seasonal variation of wave direction as follows: 1. During the summer monsoon(April–September), waves from south occur from April through September in the southern SCS region, which prevail taking about 40% of the time; 2. During the winter monsoon(December–March), waves from northeast prevail throughout the SCS for 56% of the period; 3. The dominant wave direction in SCS is NE. The seasonal variation of wave height H s in SCS shows that in spring, H s ≥1 m in the central SCS region and is less than 1 m in other areas. In summer, H s is higher than in spring. During September–November, infl uenced by tropical cyclones, H s is mostly higher than 1 m. East of Hainan Island, H s 2 m. In winter, H s reaches its maximum value infl uenced by the north-east monsoon, and heights over 2 m are found over a large part of SCS. Finally, we calculated the extreme wave parameters in SCS and found that the extreme wind speed and wave height for the 100-year return period for SCS peaked at 45 m/s and 19 m, respectively, SE of Hainan Island and decreased from north to south.     

Marine biofouling in offshore areas south of Hainan Island, northern South China Sea

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