A case study of continental shelf waves in the northwestern South China Sea induced by winter storms in 2021

This study aims to investigate characteristics of continental shelf wave(CSW)on the northwestern continental shelf of the South China Sea(SCS)induced by winter storms in 2021.Mooring and cruise observations,tidal gauge data at stations Hong Kong,Zhapo and Qinglan and sea surface wind data from January 1 to February 28,2021 are used to examine the relationship between along-shelf wind and sea level fluctuation.Two events of CSWs driven by the along-shelf sea surface wind are detected from wavelet spectra of tidal gauge data.The signals are triply peaked at periods of 56 h,94 h and 180 h,propagating along the coast with phase speed ranging from 6.9 m/s to18.9 m/s.The dispersion relation shows their property of the Kelvin mode of CSW.We develop a simple method to estimate amplitude of sea surface fluctuation by along-shelf wind.The results are comparable with the observation data,suggesting it is effective.The mode 2 CSWs fits very well with the mooring current velocity data.The results from rare current help to understand wave-current interaction in the northwestern SCS.

Measurement of the current and spectral analysis on the continental shelf in the East China Sea

Direct measurements of current velocity and water temperature were undertaken at the mooring Sta. M (125°29.38' N, 31°49.70' E) on the continental shelf area in the East China Sea in June 1999 by R/V Xiangyanghong 14. The relationship between various time series of oceanic fluctuations is calculated by spectral analysis. The major results are as follows: (1) an average (u,v) of (6.9,-3.0 cm/s) at the 30 m depth is obtained during the 9-day observation, and that at the 45 m depth is (3.7,-1.1 cm/s), i. e., the mean flows are southeastward at both the 30 and 45 m depths;the currents become stronger gradually during the observation period; this may be mainly attributed to the transition of the tidal currents from neap to spring; (2)semidiurnal fluctuation is the most dominant in the current fluctuations, and rotates mainly clockwise; in the next place, there is also diurnal fluctuation;the local inertial period is close to the period of diurnal fluctuation, and an inertial motion is clockwise; thus, local inertial motion combines with diurnal fluctuation, and makes the spectral peaks in clockwise components much higher than those in counterclockwise ones; except for the fluctuations of above main periods, there is also the peak at 3 d period for counterclockwise compo-nents in the upper and lower layers; (3) the calculation of cross spectra between two time series of current velocities at the 30 and 45 m depths shows that both the current fluctuations at the 30 and 45 m depths are much alike, i. e., they are synchro; this shows that the flow field here is rather vertically homogeneous; (4)power spectra of tempera-ture time series at both the 30 and 45 m depths show that the semidiurnal peak is the most predominant, and the second highest peak is the diurnal period; besides spectral peaks at above periods, there are also obvious spectral peaks at 6.8 h and 2 d; (5)plots of temperature time series at 16 , 30 , 35 , 45 and 50 m depths show that the temporal variations of temperature at these depths are synchro, which are like those in the velocity field; temperature records also show a gradual rise in temperature, which are also like those in the velocity field.

pCO_2 distribution and CO_2 flux on the inner continental shelf of the East China Sea during summer 2011

Measurements ofpH, total alkalinity （TA）, partial pressure of CO2 （pCO2） and air-sea CO2 flux （FCO2） were conducted for the inner continental shelf of the East China Sea （ECS） during August 2011. Variations in pCO2 distribution and FCO2 magnitude during the construction of the Three Gorges Dam （TGD） （2003-2009）, and the potential effects of the TGD on the air-sea CO2 exchange were examined. Results showed that the ECS acts as an overall CO2 sink during summer, with pCO2 ranging from 107 to 585 p.atm and an average FCO2 of -6.39 mmol/（m2·d）. Low pCO2 （〈350 μatm） levels were observed at the central shelf （28°-32°N, 123°-125.5°E） where most CO2-absorption occurred. HighpCO2 （〉420 μatm） levels were found in the Changjiang estuary and Hangzhou Bay which acted as the main CO2 source. A negative relationship between pCO2 and salinity （R2=0.722 0） in the estuary zone indicated the predominant effect of the Changjiang Diluted Water （CDW） on the seawater CO2 system, whereas a positive relationship （R2=0.744 8） in the offshore zone revealed the influence of the Taiwan Current Warm Water （TCWW）. Together with the historical data, our results indicated that the CO2 sink has shown a shift southwest while FC02 exhibited dramatic fluctuation during the construction of the TGD, which is located in the middle reaches of the Changjiang. These variations probably reflect fluctuation in the Changjiang runoff, nutrient import, phytoplankton productivity, and sediment input, which are likely to have been caused by the operations of the TGD. Nevertheless, the potential influence of the TGD on the CO2 flux in the ECS is worthy of further study.

Observation of continental shelf wave propagating along the eastern Taiwan Strait during Typhoon Meranti 2016

This study is the fi rst to depict typhoon-induced continental shelf wave(CSW)propagation in the eastern Taiwan Strait(TWS)during the passage of Typhoon Meranti in 2016 using tidal gauge data and along-track satellite altimeter data.The strong amplitude response of sea level oscillations(within the range of 0.30–0.54 m)as a free,barotropic CSW after Meranti,which impacted the TWS,was clearly detected in time and frequency(in bands of 64–81 h)using wavelet and cross-wavelet analyses.The measured group and phase speeds were consistent with the dispersion curves for CSW with the fi rst-mode derived from the cross-shelf sections of the eastern TWS,with the mean speeds reaching 3 and 5.6±0.7 m/s,respectively.Coincidentally,the typhoon-induced sea level anomaly(SLA)was also captured by the satellite altimeter before this CSW entered into the TWS.Using the theoretical cross-shore CSW modes to fi t the SLA data,the results indicated that the fi rst three wave modes can interpret this CSW event appeared in the southern TWS very well,with the fi rst mode being the dominant one.

Vertical Structure of the Tidal Currents on the Continental Shelf of the East China Sea

The available data on tidal currents spanning periods greater than six months for the continental shelf of the East China Sea (26°30.052′N, 122°35.998′E) were analyzed using several methods. Tidal Current Harmonic Analysis results demonstrated that semi-diurnal tides dominated the current movement. The tidal currents of the principal diurnal and semidiurnal rotated clockwise with depth, with the deflection of the major semi-axes to the right in the upper layer and to the left in the lower layer. The vertical structures of two principal semi-diurnal constituents-M2 and S2-were similar, which indicates that the tidal currents are mainly barotropic in this area. The main features of the variation of the four principal tidal constituents with depth demonstrate that the currents in this region are influenced by the upper and lower boundary layers. Therefore, the tidal constituents of the shallow water are similar. Different vertical modes were calculated based on the Empirical Orthogonal Function (EOF) analysis of the Eastern and Northern components of the tidal currents, with a variance contribution for the zero-order model of at least 90%. The variance contribution of the baroclinic model is minimal, which further reveals a strong barotropic character for the tidal currents of this region.

Application of the Three-Dimensional Shallow Sea and Continental Shelf Mode for Inversion of Undercurrent

Abstract. Due to the incapability of high frequency surface wave radar （HFSWR） to detect undercurrent parameters, a new algorithm is proposed to apply a three-dimensional （3D） nonlinear barotropic shallow sea and continental shelf model in coordinate system to the inversion of undercurrent. The calculation domain of this model is the area detected by HFSWR. Considering the benthal topography of the detected area and the ocean dynamic parameters, such as surface current, wind and wave detected by HFSWR, the relation between surface current and undercurrent is established in this model, Accordingly, the undercurrent parameters of corresponding area are obtained. The inversion results agree with the law of ocean dynamics and reproduce the movement of undercurrent.

Methods and procedures to determine the outer limits of the continental shelf beyond 200 nautical miles

This paper establishes techniques and methods to determine a variety of boundaries associated with 200 nautical miles beyond the continental shelf. The methods, based on topography, slope and second-deriv- ative profile integrated analysis, are now able to identify automatically the foot of the continental slope （FOS）. By analyzing the sedimentary profile, the points of 1% sediment thickness are recognized. Through the intersection, cut, deletion and mergence calculation of the extrapolated data set of fLxed-point series, the method succeeds in generating automatically the extrapolated boundaries, including the FOS＋60 M line, the 350 M line, and the 2500 m＋100 M line. In addition, based on the automatic analysis of the topographic profile, it can be applied to determine rapidly the points of maximum water depth. Taking the northern Okinawa Trough （OT） as an example, these methods are used to calculate and examine the boundaries in- cluded in the Submission by the People＇s Republic of China Concerning the Outer Limits of the Continental Shelf beyond 200 Nautical miles in Part of the East China Sea （ECS）; the boundaries thus derived have a solid scientific and rational basis.

Trend and Dynamic Cause of Sediment Particle Size on the Adjacent Continental Shelf of the Yangtze Estuary

Based on the measured data in recent 20 years, the variation trends of the median grain size of the surface sediment, the sand-silt boundary and the mud area on the adjacent continental shelf of the Yangtze Estuary were analyzed in depth, and the effects of natural mechanism and human activities were discussed. The results show that： （1） In recent years （2006-2013）, the median grain size of sediment and the distribution pattern of grouped sediments in the adjacent continental shelf area to the Yangtze Estuary have presented no obvious change compared with those before 2006; （2） The median diameter of the surface sediment in the continental shelf area displayed a coarsening trend with the decrease of sediment discharge from the basin and the drop in suspended sediment concentration in the shore area; （3） In 2004-2007, the sand-silt boundary in the north part （31 ~30＇N） of the continental shelf area presented no significant changes, while that in the south part （31~30＇S） moved inwards; In 2008-2013, both the sand-silt boundaries in the north and south parts of the continental shelf area moved inwards, mainly due to the fact that in the dry season, a relatively enhanced hydrodynamic force of the tides was generated in the Yangtze River, as well as a decreased suspended sediment concentration and a flow along the banks in North Jiangsu; （4） The mud area where the maximum deposition rate is found in the Yangtze Estuary, tends to shrink due to the drop in sediment discharge from the basin, and the decrease in suspended sediment concentration in the shore area and erosion in the delta. Moreover, it tended to shift to the south at the same time because the implement of the training works on the deep-water channel of the North Passage changed the split ratio between the North and South Passages with an increase in the power of the discharged runoff in the South Passage.

Predicting sediment flux from continental shelf islands,southeastern China

Continental shelf islands are contributors of terrestrial sediment supply to shelf regions,and the sediment flux from these islands shall be quantified.We calculated the sediment flux of continental shelf islands in the southeastern China using two empirical equations under two preconditions.The first,the sediment load/yield of the islands has the same pattern as the adjacent small,mountainous rivers along the coastline;and the second,each of the islands was treated as a single catchment.The results show that the sediment supply from these islands reached an order of magnitude of 1 Mt/a,which is comparable to the supply from the local smaller rivers.A sensitivity analysis indicates that this value represents the lower limit of estimate;if the accurate amount of sub-catchments of any island is considered,then this value will be enhanced slightly.This study demonstrates that the sediment supply from continental shelf islands to oceans is an important factor affecting the regional sedimentation and,therefore,should be paid with attention.

The characteristics of warm water inflowing and its temporal and spatial variation on the Prydz Bay continental shelf,Antarctic

Hydrographic observations collected by conductivity-temperature-depth（CTD） and instrumented elephant seals on the Prydz Bay continental shelf during 2012 and 2013 are used to characterize the intrusion of modified circumpolar deep water.As a regular occurrence,modified circumpolar deep water（MCDW） intrudes onto the shelf mainly between 150–300 m layer of 73°–75°E and then turns southeast affected by the cyclonic gyre of the Prydz Bay.The southernmost point of the warm water signal is captured on the east front of Amery Ice Shelf during March 2012.In terms of vertical distribution,MCDW occupies the central layer of 200 m with about 100 m thickness in the austral summer,but when to winter transition,the layer of MCDW deepens with time on the central shelf.

The continental shelf wave of the East China Sea and its effect on the Kuroshio

The free shelf wave theory is applied to the practical case of the continental shelf in the East China Sea to analyse the effects of the shelf wave on the Kuroshio. The results indicate that the shelf wave in lower frequency travels from north to south and its phase velocity is proportional to the Kuroshio's current velocity) the maximum current velocity of the Kuroshio lies at the continental margin. The analytical solutions obtained indicate that the hydrodynamic characters of the sea region over the shelf present band structure. The horizontal motion ( x -component) caused by the shelf wave at the margin may be one of the causes for generating wavy pattern of the Kuroshio's axis .

Transport patterns of micro nutrient elements from the continental shelf of the East China Sea to the Kuroshio area

On the basis of the in situ data of DO2, pH, SiO2. PO4-P, NO3-N and NO2-N collected in the north of the East China Sea during 1987-1988, the following points are mainly expounded.1.The inorgonic nutrients are obviously affected by continent runoff in the north of the East China Sea. Their distributions are characteristic of its distribution of terrigenous materials.2.There are three transport paths of nutrients from the shelf to the Kuroshio area. The first is mixing-diffusing-advec-tion and upwelling process, the process of biology and biochemistry belongs to the second, and the sinking process is the last one.3.The swing of the Kuroshio axis affectes both the range of the migration of substances through mixing-diffusing-advec-tion process and the upwelling degree of the subsurface Kuroshio water to the shelf.4.Most part of the substances sink as macroparticles to the deep layer before reaching the Kuroshio area.

Biocalcification crisis in the continental shelf under ocean acidification

Ocean acidification(OA)is a persistent challenge for humans and is predicted to have deleterious effects on marine organisms,especially marine calcifiers such as coral and foraminifera.Benthic foraminifera is an important component of sediment in the continental shelf,while little is known about the impact of ocean acidification on benthic foraminifera both at the community and individual level and associated calcium carbonate deposition.We conducted eight months continued culture experiment under the scenario of 400,800,1200 and 1600 ppm pCO_(2)gradients on living benthic foraminifera from four stations in the continental shelf of the West Pacific Ocean.Statistic results showed OA had a negative effect on the abundance of benthic foraminifera.In contrast,the diversity increased roughly under OA conditions implying OA might stimulate the emergence of rare species and promote community diversity to some extent.In addition,we confirmed that the offshore area wasn’t the refuge for benthic foraminifera while the nearshore one had more resistance to moderate acidification.Calcareous species Protelphidium tuberculatum was the dominant species occupying on average 75%in all treatments and its shell diameter,weight and thickness showed a decrease,indicating the decrease of calcification of benthic foraminifera.A relationship between the weight of P.tuberculatum and pCO_(2)(R^(2)=0.96)was established.Based on the present work,calcareous benthic foraminifera deposited 8.57104 t calcium carbonate per year and this might reduce by nearly half and 90%under 800 and 1200 ppm scenarios,which indicates a biocalcification crisis under ongoing OA.This work shows an analogy for palaeoceanic OA and also provides new insights into the sediment of calcium carbonate in the future.

Exotic origin of the Chinese continental shelf: new insights into the tectonic evolution of the western Pacific and eastern China since the Mesozoic

The effect of paleo-Pacific subduction on the geological evolution of the western Pacific and continental China is likely complex. Nevertheless, our analysis of the distribution of Mesozoic granitoids in the eastern continental China in space and time has led us to an interesting conclusion： The basement of the continental shelf beneath East and South China Seas may actually be of exotic origin geologically unrelated to the continental lithosphere of eastern China. By accepting the notion that the Jurassic- Cretaceous granitoids in the region are genetically associated with western Pacific subduction and the concept that subduction may cease to continue only if the trench is being jammed, then the termination of the granitoid magmatism throughout the vast region at -88±2 Ma manifests the likelihood of ＂sudden＂, or shortly beforehand （- 100 Ma）, trench jam of the Mesozoic western Pacific subduction. Trench jam happens if the incoming ＂plate＂ or portion of the plate contains a sizeable mass that is too buoyant to subduct. The best candidate for such a buoyant and unsubductable mass is either an oceanic plateau or a micro-continent. We hypothesize that the basement of the Chinese continental shelf represents such an exotic, buoyant and unsubductable mass, rather than seaward extension of the continental lithosphere of eastern China. The locus of the jammed trench （i.e., the suture） is predictably located on the shelf in the vicinity of, and parallel to, the arc-curved coastal line of the southeast continental China. It is not straightforward to locate the locus in the northern section of the East China Sea shelf because of the more recent （〈20 Ma） tectonic re-organization associated with the opening of the Sea of Japan. We predict that the trench jam at - 100 Ma led to the re-orientation of the Pacific plate motion in the course of NNW direction as inferred from the age-progressive Emperor Seamount Chain of Hawaiian hotspot origin （its oldest unsubdued Meiji and Detroit seamounts are -82 Ma）, making the boundary between the Pacific plate and the newly accreted plate of eastern Asia transform fault at the location east of the continental shelf of exotic origin. This explains the apparent-40 Myr magmatic gap from - 88 to - 50 Ma prior to present-day western Pacific subduction initiation. We propose that basement penetration drilling on well-chosen sites is needed to test the hypothesis in order to reveal the true nature of the Chinese continental shelf basement. This testing becomes critical and cannot longer be neglected in order to genuinely understand the tectonic evolution of the western Pacific and its effect on the geology of eastern China since the Mesozoic, including the cratonic lithosphere thinning, related magmatism/mineralization, and the mechanism of the subsequent South China Sea opening, while also offering novel perspectives on aspects of the plate tectonics theory. We also suggest the importance of future plate tectonic reconstruction of the western Pacific to consider the nature and histories of the Chinese continental shelf of exotic origin as well as the probable transform plate boundary from - 100 to -50 Ma. Effort is needed to reveal the true nature and origin of the - 88 ± 2 Ma granitic gneisses in Taiwan and the 110-88 Ma granitoids on the Hainan Island.

A theoretical model for the intrusion of the Kuroshio across the continental shelf of East China Sea

Here we use a two-layer model to study the dynamics of the intrusion of the Kuroshio onto the continental shelf. Results show that the interaction of the Kuroshio water and shelf water produces a stable upwelling zone above 100 m depth northeast of Talwan, which provides a dynamical explanation to the presence of the cold core previously observed by satellite. The affected shelf water from the interaction has an onshore portion, which turns right and becomes a northward alongshore flow when it moves closer to shore. This implies that the Kuroshio water cannot penetrate deep onto the inner part of the continental shelf, but it generates a strong northward jet that is formed mainly by the shelf water.

Tropical storm-forced near-inertial energy dissipation in the southeast continental shelf region of Hainan Island

Near-inertial motion is an important dynamic process in the upper ocean and plays a significant role in mass, heat, and energy transport across the thermocline. In this study, the dissipation of wind-induced near-inertial energy in the thermocline is investigated by using observation data collected in July and August 2005 during the tropical storm Washi by a moored system at(19°35′N, 112°E) in the continental shelf region off Hainan Island. In the observation period, the near-inertial part dominated the observed ocean kinetic energy and about 80% of the near-inertial energy dissipated in the upper layer. Extremely strong turbulent mixing induced by near-inertial wave was observed in the thermocline, where the turbulent energy dissipation rate increased by two orders of magnitude above the background level. It is found that the energy loss of near-inertial waves in the thermocline is mainly in the large-scales. This is different from the previous hypothesis based on "Kolmogorov cascade" turbulence theory that the kinetic energy is dissipated mainly by small-scale motions.

Continental shelf waves forced by nonlinear continental shelf topography

The characteristics of continental shelf waves forced by nonlinear continental shelf topography are studied with a shallowwater model.Results show that there are two topographic Rossby waves and two inertia gravity waves.The northward propagating topographic Rossby wave couples with the inertia gravity wave into an unstable wave in the long wave band.When the continental slope is increased,the topographic Rossby wave still couples with the inertia gravity wave into an unstable wave,but the frequency decreases.In the South China Sea(SCS),the western boundary can develop an unstable wave because of topographic forcing,nonlinear conditions,and increasing amplitude.It is possible that the unstable wave develops into a vortex.In observations,the SCS has a strong western boundary current and abundant mesoscale vortice.There is a strong relationship between the emergence,disappearance,and movement of the circulation's multi-eddy structure and the seasonal evolvement of the SCS's circulation.This article shows a possible mechanism for the formation of vortices in the SCS.

Sedimentary Features of Shallow Ancient River Channels on the Northern Shelf of the South China Sea

Quaternary buried ancient river channels are widespread in the shallow-level sediments of the northern shelf of the South China Sea. The sedimentary sequence mainly of fluvial deposits comprise an important component part of the low-stand system tract and transgressive system tract in the study region. The plannar variation and spatial association of the sedimentary features such as incised valley fillings, deltaic foreset wedges and block slides of shelf-marginal fans reflect the palaeogeographic environment during the fall of the regional sea level in the northern part of the South China Sea. Based on the high-resolution seismic reflection data and gelogical data from boreholes, the present paper makes an integrated interpretation of the Quaternary ancient river channels in the shallow sediments of the study area, studies the sedimentary features of the ancient channels such as their spatial distribution, seismic facies reflection indicators, sedimentary facies and sand -body types, and discusses their formational setting and evolutionary model, with the main purpose to render a service to the hydrocarbon resources exploration and development and marine engineering in the northern shelf of the South China Sea.

Sedimentary Environment Analysis of Ancient Sand Ridges from Zk23 Hole in the East China Sea

Abstract： Based on the analysis of core samples from the hole of Zk23 in the East China Sea Continental Shelf and by means of sedimentary stratigraphy, biostratigraphy and chronostratigraphy, the authors consider that the fine-sand deposition in borehole was part of buried ancient estuary sand ridges of the Yangtze River. The deposition history of study area around the hole before and after the glacial period as well as postglacial period is made clear after our research： （1） the estuarine sublayer -undersea delta facies strata was deposited under the fast sea level rise about 15 kaB.P; （2） sand ridges mostly consisting of fine-sand, were formed when the sea level was the fluctuant range of 60 - 80 m of isoba during the deglacial period around 15 - 12 kaB.P; （3） first silty clay and clay silt strata above the sand ridges were deposited during the period when the sea level rose fast from 12 to 7 kaB.P, and then it keeps stable to the present.

Late Quaternary sedimentary environmental evolution offshore of the Hangzhou Bay,East China——implications for sea level change and formation of Changjiang alongshore current

This study focuses on sedimentary environmental changes offshore of Hangzhou Bay, East China, since the Late Quaternary. AMS ^14C ages from core CJK10, lithologies, distribution of foraminifera, heavy minerals, and S and C1 elements show a fluvial terrace environment during -23.2-11.0 cal ka BP; a littoral to tidal-flat environment during 11.0-10.2 cal ka BP; and a shallow marine environment with a relatively low sedimentation rate （0.1-0.22 cm/a） since 4.3 cal ka BP. High depositional rates （-1.6 cm/a） from 10.9 to 10.2 cal ka BP resulted from sufficient accommodation space created by rapid sea level rise from -44 m to -33 m, from high sediment delivery by local rivers, and effective trapping of sediments by tidal-flat vegetation. The rate of sea level rise was variable; relatively high from 10.9 to 10.6 cal ka BP （2.1 cm/a）, and lower since 10.6 cal ka BP （1.2 cm/a）. The Changjiang alongshore current crossed the Hangzhou Bay to form the mud wedge on the inner shelf of the East China Sea later than 9.4 cal ka BP. The CJK10 site was a tide-dominated shelf environment and experienced erosion from approximately 9.4-9.2 cal ka BP to 4.3 cal ka BP. The depositional hiatus was caused by the Changjiang alongshore current, which was relatively weak during 9.4-7.5 cal ka BP and increased in strength during -7.5-4 cal ka BP. From -4.3 cal ka BP, a large amount of sediment from the Changjiang River was partly deposited on the continental shelf of Hangzhou Bay with some transported southward. Therefore, this study clarifies the history of Changjiang-derived sediment dispersal and deposition, although a detailed record of the changes in the Chang3iang alongshore current since 4.3 cal ka BP is difficult to obtain because of the scarcity of evidence.