Numerical study on gas production via a horizontal well from hydrate reservoirs with different slope angles in the South China Sea

It is important to study the effect of hydrate production on the physical and mechanical properties of low-permeability clayey–silty reservoirs for the largescale exploitation of hydrate reservoirs in the South China Sea.In this study,a multiphysical-field coupling model,combined with actual exploration drilling data and the mechanical experimental data of hydrate cores in the laboratory,was established to investigate the physical and mechanical properties of low-permeability reservoirs with different slope angles during 5-year hydrate production by the depressurization method via a horizontal well.The result shows that the permeability of reservoirs severely affects gas production rate,and the maximum gas production amount of a 20-m-long horizontal well can reach186.8 m3/day during the 5-year hydrate production.Reservoirs with smaller slope angles show higher gas production rates.The depressurization propagation and hydrate dissociation mainly develop along the direction parallel to the slope.Besides,the mean effective stress of reservoirs is concentrated in the near-wellbore area with the on-going hydrate production,and gradually decreases with the increase of the slope angle.Different from the effective stress distribution law,the total reservoir settlement amount first decreases and then increases with the increase of the slope angle.The maximum settlement of reservoirs with a 0°slope angle is up to 3.4 m,and the displacement in the near-wellbore area is as high as2.2 m after 5 years of hydrate production.It is concluded that the pore pressure drop region of low-permeability reservoirs in the South China Sea is limited,and various slope angles further lead to differences in effective stress and strain of reservoirs during hydrate production,resulting in severe uneven settlement of reservoirs.

Comparison of Microphysical Characteristics Between Warm-sector and Frontal Heavy Rainfall in the South of China

During the April-June raining season,warm-sector heavy rainfall(WR) and frontal heavy rainfall(FR) often occur in the south of China,causing natural disasters.In this study,the microphysical characteristics of WR and FR events from 2016 to 2022 are analyzed by using 2-dimensional video disdrometer(2DVD) data in the south of China.The microphysical characteristics of WR and FR events are quite different.Compared with FR events,WR events have higher concentration of D<5.3 mm(especially D <1 mm),leading to higher rain rates.The mean values of Dmand lgNwof WR events are higher than that of FR events.The microphysical characteristics in different rain rate classes(C1:R~5-20 mm h-1,C2:R~20-50 mm h-1,C3:R~50-100 mm h^(-1),and C4:R> 100 mm h^(-1)) for WR and FR events are also different.Raindrops from C3 contribute the most to the precipitation of WR events,and raindrops from C2 contribute the most to the precipitation of FR events.For C2 and C3,compared with FR events,WR events have higher concentration of D <1 mm and D~3-4.5 mm.Moreover,the shape and slope(μ-A) relationships and the radar reflectivity and rain rate(Z-R) relationships of WR and FR events are quite different in each rain rate class.The investigation of the difference in microphysical characteristics between WR and FR events provide useful information for radar-based quantitative precipitation estimation and numerical prediction.

Prediction of Petroleum Prospect in the South Yellow Sea Basin Using Satellite Remote Sensing

Using satellite remote sensing to measure the sea surface temperature of the East China Sea Shelf Basin can serve the purpose of predicting the petroleum prospect in the South Yellow Sea Basin. The satellite thermal infrared temperature anomaly area always repeats in the same position as the proved oil prospect area in the East China Sea Shelf Basin, and coincides well with both the CH4 content curve at the water-atmosphere interface and the seafloor geochemical anomaly. The sea surface temperature anomaly areas of the South Yellow Sea Basin in the satellite remote sensing image go banded along 123030, in the S-N direction, and naturally follow the aeromagnetically interpreted eastern nose-type uplift that shows a S-N strike. The north and south ends of the eastern nose are considered as good oil prospect areas because temperature anomaly often occurs there.

Seasonal Variation and Heat Preference of the South Asia High

By use of the NCEP/ NCAR reanalysis data, the seasonal variation of the South Asia high (SAH) is analyzed. The influences of temporal and spatial variations of the middle and upper level atmospheric temperatures, the visible heat sources, and the diabatic heating rates in the whole atmospheric column on the seasonal variation of the SAH are discussed. Results show that the SAH has two seasonal balancing modes, one of which is the land high in summer and the other the ocean high in winter. The land high itself can be divided into two patterns as well, that is the Tibetan high and the Iranian high. Heating fields have important impacts on the seasonal variation of the SAH. The SAH is a warm high and its center has the property of heat preference, usually locating over or moving to an area with relatively larger heating rates. The annual cycle of the SAH is mainly controlled by the seasonal process of the latent and sensible heating in South Asia. Strong shortwave radiative heating in the north at high latitudes and over the Tibetan Plateau also has an effects on the northward movement and maintenance of the SAH. The cooling effect of infrared radiation is an important cause in weakening the SAH.

Evidence for water exchange between the South China Sea and the Pacific Ocean through the Luzon Strait

An analysis of historical oxygen data provides evidence on the water exchange between theSouth China Sea (SCS) and the Pacific Ocean (PO). In the vicinity of the Luzon Strait (LS) , the dissolved oxygen concentration of sea water is found to be lower on the Pacific side than on the SCS side at depths between 700 and 1500 m (intermediate layer) , while the situation is reversed above 700 m (upper layer) and below 1 500 m (deep layer). The evidence suggests that water exits the SCS in the intermediate layer but enters it from the Pacific in both the upper and the deep layers, supporting the earlier speculation that the Luzon Strait transport has a sandwiched structure in the vertical. Within the SCS basin, the oxygen distribution indicates widespread vertical movement, including the upwelling in the intermediate layer and the downwelling in the deep layer.

Deep-water Fan Systems and Petroleum Resources on the Northern Slope of the South China Sea

The shallow shelf delta/strand arenaceous-pelitic deposit region in the north of the Pearl River mouth basin, sitting on the northern continental shelf of the South China Sea, has already become an important oil production base in China. Recent researched has revealed that a great deal of deep-water fans of great petroleum potentiality exist on the Baiyun deep-water slope below the big paleo Pearl River and its large delta. Based on a mass of exploration wells and 2-D seismic data of the shallow shelf region, a interpretation of sequence stratigraphy confirmed the existence of deep-water fans. The cyclic falling of sea level, abundant detrital matter from the paleo Pearl River and the persistent geothermal subsidence in the Baiyun sag are the three prerequisites for the formation and development of deep-water fans. There are many in common between the deep-water shelf depositional system of the northern South China Sea and the exploration hotspots region on the two banks of the Atlantic. For example, both are located on passive continent margins, and persistent secular thermal subsidence and large paleo rivers have supplied abundant material sources and organic matter. More recently, the discovery of the big gas pool on the northern slope of the Baiyun sag confirms that the Lower Tertiary lacustrine facies in the Baiyun sag has a great potentiality of source rocks. The fans overlying the Lower Tertiary source rocks should become the main exploration areas for oil and gas resources.

Discovery of Radiolarian Fossils from the Aiketik Group at the Western End of the South Tianshan Mountains of Chinaand Its Implications

The Aiketik Group, distributed at the western end of the South TianshanMountains, China, is an important lithostratigraphic unit involved in the South Tianshan orogen. Itis separated from the adjacent rocks by faults. Generally, the geologists ascribed it to the UpperCarboniferous according to Pseudostaffella sp., Profusulinella sp. and Fusulinella sp. found fromthe limestone and sandy limestone of Aiketik. Our radiolarian fossils were obtained from the chertsamples collected from the Haladaok section located at the upper Tuoshihan River. The fossils mainlyinclude Albaillella undulata Deflandre, Albaillella paradoxa Deflandre, Albaillella sp. aff. A.paradoxa Deflandre, Albaillella sp. cf. A. deflandrei Gourmelon, Albaillella sp., Albaillellaexcelsa Ishiga, Kito and Imoto (?), Belowea variabilis (Ormiston et Lane), Callella cf. C.parvispinosa Won, Entactinia cf. E. tortispina Ormiston et Lane, Entactinia aff. E. tortispinaOrmiston et Lane, Entactinia variospina Won, Entactinia sp., Eostylodictya rota (Won),Latentifistula impella (Ormistone et Lane) (?), Latentifistula turgita Omiston et Lane,Latentifistulidae gen. et. sp. indet. and Polyentactinia cf. aranea Gourmelon. Among them,Albaillella excelsa Ishiga, Kito and Imoto (?) is a Late Permian species with some elementsuncertain as there is only one poorly-preserved fossil of this species found so far. And tworadiolarian assemblages can be identified from the other fossils. One is the early EarlyCarboniferous assemblage represented by Albaillella undulata Deflandre, Albaillella paradoxa andAlbaillella sp. cf. A. deflandrei Gourmelon. And the other is the late Early Carboniferousassemblage represented by Eostylodictya rota (Won). This is the first discovery of radiolarianfossils in the Aiketik Group, also the first discovery of Late Permian radiolarian fossils in theSouth Tianshan Mountains. Meanwhile, this is the current westernmost sampling site of radiolarianfossils in the South Tianshan Mountains.

Types,Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area,Northern Continental Slope of the South China Sea

The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed the P-T conditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas-bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas-bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways（largescale faults, gas chimneys and mud diapirs） presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large-scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas-bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re-deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways（large-scale faults and mud diapirs） in the LW3-1 deep-sea oil/gas field, the migration efficiency of the vertical pathways（composed of gas chimneys） in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good-quality deep source rocks and shallow, mainly biogenicallyproduced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine-grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation ofthermogenic methane during long-distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.

Impact of the Thermal State of the Tropical Western Pacific on Onset Date and Process of the South China Sea Summer Monsoon

Since the early or late onset of the South China Sea summer monsoon （SCSM） has a large impact on summer monsoon rainfall in East Asia, the mechanism and process of early or late onset of the SCSM are an worthy issue to study. In this paper, the results analyzed by using the observed data show that the onset date and process of the SCSM are closely associated with the thermal state of the tropical western Pacific in spring. When the tropical western Pacific is in a warming state in spring, the western Pacific subtropical high shifts eastward, and twin cyclones are early caused over the Bay of Bengal and Sumatra before the SCSM onset. In this case, the cyclonic circulation located over the Bay of Bengal can be early intensified and become into a strong trough. Thus, the westerly flow and convective activity can be intensified over Sumatra, the Indo-China Peninsula and the South China Sea （SCS） in mid-May. This leads to early onset of the SCSM. In contrast, when the tropical western Pacific is in a cooling state, the western Pacific subtropical high anomalously shifts westward, the twin cyclones located over the equatorial eastern Indian Ocean and Sumatra are weakened, and the twin anomaly anticyclones appear over these regions from late April to mid-May. Thus, the westerly flow and convective activity cannot be early intensified over the Indo-China Peninsula and the SCS. Only when the western Pacific subtropical high moves eastward, the weak trough located over the Bay of Bengal can be intensified and become into a strong trough, the strong southwesterly wind and convective activity can be intensified over the Indo-China Peninsula and the SCS in late May. Thus, this leads to late onset of the SCSM. Moreover, in this paper, the influencing mechanism of the thermal state of the tropical western Pacific on the SCSM onset is discussed further from the Walker circulation anomalies in the different thermal states of the tropical western Pacific.

Features of eddy kinetic energy and variations of upper circulation in the South China Sea

The features of eddy kinetic energy (EKE) and the variations of upper circulation in theSouth China Sea (SCS) are discussed in this paper using geostrophic currents estimated from Maps of Sea Level Anomalies of the TOPEX/Poseidon altimetry data. A high EKE center is identified in the southeast of Vietnam coast with the highest energy level 1 400 cm2 ·s^(-2) in both summer and autumn. This high EKE center is caused by the instability of the current axis leaving the coast of Vietnam in summer and the transition of seasonal circulation patterns in autumn. There exists another high EKE region in the northeastern SCS, southwest to Taiwan Island in winter. This high EKE region is generated from the eddy activities caused by the Kuroshio intrusion and accumulates more than one third of the annual EKE, which confirms that the eddies are most active in winter. The transition of upper circulation patterns is also evidenced by the directions of the major axises of velocity variance ellipses between 10?and 14.5°N, which supports the model results reported before.

A numerical study of the South China Sea deep circulation and its relation to the Luzon Strait transport

A fine-resolution MOM code is used to study the South China Sea basin-scale circulationand its relation to the mass transport through the Luzon Strait. The model domain includes the South China Sea, part of the East China Sea, and part of the Philippine Sea so that the currents in the vicinity of the Luzon Strait are free to evolve. In addition, all channels between the South China Sea and the Indonesian seas are closed so that the focus is on the Luzon Strait transport. The model is driven by specified Philippine Sea currents and by surface heat and salt flux conditions. For simplicity, no wind-stress is applied at the surface.The simulated Luzon Strait transport and the South China Sea circulation feature a sandwich vertical structure from the surface to the bottom. The Philippine Sea water is simulated to enter the South China Sea at the surface and in the deep ocean and is carried to the southern basin by western boundary currents. At the intermediate depth, the net Luzon Strait transport is out of the South China Sea and is fed by a western boundary current flowing to the north at the base of the thermocline. Corresponding to the western boundary currents, the basin circulation of the South China Sea is cyclonic gyres at the surface and in the abyss but an anti-cyclonic gyre at the intermediate depth. The vorticity balance of the gyre circulation is between the vortex stretching and the meridional change of the planetary vorticity. Based on these facts, it is hypothesized that the Luzon Strait transports are determined by the diapycnal mixing inside the entire South China Sea. The South China Sea plays the role of a 'mixing mill' that mixes the surface and deep waters to return them to the Luzon Strait at the intermediate depth. The gyre structures are consistent with the Stommel and Arons theory (1960), which suggests that the mixing-induced circulation inside the South China Sea should be cyclonic gyres at the surface and at the bottom but an anti-cyclonic gyre at the intermediate depth. The simulated gyre circulation at the intermediate depth has been confirmed by the dynamic height calculation based on the Levitus hydrography data. The sandwich transports in the Luzon Strait are consistent with recent hydrographical observations.Model results suggest that the Kuroshio tends to form a loop current in the northeastern South China Sea. The simulated Kuroshio Loop Current is generated by the pressure head at the Pacific side of the Luzon Strait and is enhanced by the β-plane effects. The β - plane appears to be of paramount importance to the South China Sea circulation and to the Luzon Strait transports. Without the β-plane, theLuzon Strait transports would be greatly reduced and the South China Sea circulation would be complete-ly different.

Impact of the eastern water diversion from the south to the north project on the saltwater intrusion in the Changjiang Estuary in China

The south to the north project (WDP) on the saltwater intrusion in the Changjiang Estuary is studied by the improved three-dimensionai (3D) numerical model.The net unit width flux in the Changjiang Estuary as well as the sectional salt flux is calculated in the North Branch (NB),the South Branch (SB),the North Channel (NC),the South Channel (SC),the North Passage (NP) and the South Passage (SP),respectively.The net seaward water flux in the SB is reduced,and the net water flux spilling over from the NB to the SB is enhanced after the eastern WDP.Under the mean river discharge condition in the dry season,the net salt flux spilling over from the NB to the SB is increased by 2.09 t/s and 0.52 t/s during the spring and neap tides,respectively,due to the eastern WDP.The saltwater intrusion in the Changjiang Estuary is enhanced by the eastern WDP.Compared with that during the spring tide,the net water diversion ratio during the neap tide in the NC is smaller,and thus the enhancement of the saltwater intrusion by the eastern WDP is smaller in the NC,and larger in the NP and the SP.The tidally averaged surface salinity at the water intakes of the Dongfengxisha Reservoir,the Chenhang Reservoir and the Qingcaosha Reservoir rises both during the spring and neap tides.

Geoacoustic Inversion for Bottom Parameters in the Deep-Water Area of the South China Sea

Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss （TL） data in the shadow zone of deep water. We propose a multiple-step fill inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid hMf-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea （SCS） in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. Tile inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very we//.

Diagnostic Study of Apparent Heat Sources and Moisture Sinks in the South China Sea and its Adjacent Areas during the Onset of 1998 SCS Monsoon

The apparent heat sources (?Q1 ?) and moisture sinks (?Q2 ?) are calculated based on the reanalyzed data of the South China Sea Monsoon Experiment (SCSMEX) from May 1 to August 31, 1998. It is found that the formation and distribution of the atmospheric heat sources are important for the monsoon onset. The earlier onset of the SCS monsoon is the result of enduring atmospheric heating in the Indo–China Peninsula and South China areas. The atmospheric heating firstly appears in the Indo–China Peninsula area and the sensible heat is the major one. The 30–50 day periodic oscillation of atmospheric heat sources between the SCS area and the western Pacific warm pool has a reverse phase distribution before the middle of July and the low frequency oscillation of heat sources in SCS area has an obvious longitudinal propagation. The 30–50 day low frequency oscillation has vital modificatory effects on the summer monsoon evolution during 1998. Key words Apparent heat sources - Apparent moisture sinks - The South China Sea monsoon - Diagnostic Study Sponsored by the National Key Project of Fundamental Research “ SCSMEX” and the Research Fund for the Doctoral Program of Higher Education: “ Study of the Air-sea Interaction in the SCS Monsoon Region”.

Hydrological analysis of Kuroshio water intrusion into the South China Sea

Using CFD and ADCP data collected by four research vessels from both sides of the Taiwan Strait in the northeastern area of the forth China Sea in August - September 1994 and incorporating with CTD data collected in the same area in March 1992 and some historical hydrologic data, the water features and the intrusion of Kuroshio water into the South China Sea are discussed, which shows that the water mass distribution in the survey period was similar to that in the cruise of late winter and early spring (March 1992), that is, the water structure in the northeast of the sea and in the Northwest Pacific had their own independent features of temperature and salinity. Though the intrusion of Kuroshio water into the sea was found, it was very weak. Therefore there would be no direct Kuroshio branch into the sea in the late summer and early autumn. Even the Kuroshio water intrusion from the Bashi Channel into the Taiwan Strait, its influence was also very weak. Analysis of isopycnic surface and geretrophic current and the Observed ADCP data show that there was a rather strong northward flow in the southeast pat of the survey area which flowed northward along the west coast of Philippines, rounded the northwest corner of Luzon Island and then flowed northeastward. Some kind of mixing with Kuroshio water was shown in the Bashi Channel. The water mass was obviously of high temperature and low salinity in winter and comparatively low temperature and low salinity in summer.

Mass movements in small canyons in the northeast of Baiyun deepwater area, north of the South China Sea

The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea （SCS）, and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of -500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements. Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.

Water Masses in the South China Sea and Water Exchange between the Pacific and the South China Sea

Water masses in the South China Sea (SCS) were identified and analyzed with the data collected in the summer and winter of 1998. The distributions of temperature and salinity near the Bashi Channel (the Luzon Strait) were analyzed by using the data obtained in July and December of 1997. Based on the results from the data collected in the winter of 1998, waters in the open sea areas of the SCS were divided into six water masses: the Surface Water Mass of the SCS (S), the Subsurface Water Mass of the SCS (U), the Subsurface-Intermediate Water Mass of the SCS (UI),the Intermediate Water Mass of the SCS (I), the Deep Water Mass of the SCS (D) and the Bottom Water Mass of the SCS(B). For the summer of 1998, the Kuroshio Surface Water Mass (KS) and the Kuroshio Subsurface Water Mass (KU) were also identified in the SCS. But no Kuroshio water was found to pass the 119.5°E meridian and enter the SCS in the time of winter observations. The Sulu Sea Water (SSW) intruded into the SCS through the Mindoro Channel between 50-75 m in the summer of 1998. However, the data obtained in the summer and winter of 1997 indicated that water from the Pacific had entered the SCS through the nor-thern part of the Luzon Strait in these seasons, but water from the SCS had entered the Pacific through the southern part of the Strait. These phenomena might correlate with the 1998 El-Nio event.

Multi-scale variability of subsurface temperature in the South China Sea

Using Morlet wavelet transform and harmonic analysis the multi-scale variability of subsurface temperature in the South China Sea is studied by analyzing one-year (from April 1998 to April 1999) ATLAS mooring data. By wavelet transform, annual and semi-annual cycle as well as intrasea-sonal variations are found, with different dominance, in subsurface temperature. For annual harmonic cycle, both the downward net surface heat flux and thermocline vertical movement partially control the subsurface temperature variability. For semi-annual cycle and intraseasonal variability, the subsurface temperature variability is mainly linked to the vertical displacement of thermocline.

An Anomalous Seamount on the Southwestern Mid-Ridge of the South China Sea

Objective The development of continental rifting and seafloor spreading can be predominated by magmatic upwelling （magma-rich） or tectonic stretching （magma-poor）. Located in the terminal portion of propagating seafloor spreading of the South China Sea （SCS）, the southwestern mid-ridge of the southwest sub-basin （SWSB） of the SCS was found with much thin crust and numerous faulted blocks coupling with magma-poor continental margins along its flanks. Therefore, the southwestern part of the SWSB is most likely to be a tectonic-dominated basin. To justify the hypothesis with further evidence, we investigated the nature of seamounts on the southwestern mid-ridge of the SWSB which may inherit seafloor spreading.

Effects of Tidal Currents on Nonlinear Internal Solitary Waves in the South China Sea

The propagation and fission process of internal solitary waves （ISWs） with amplitudes of about 170 m are simulated in the northeast of the South China Sea （NSCS） by using the generalized Korteweg-de Vries （KdV） equation under continuous stratification. More attention is paid to the effects of the ebb and flood background currents on the fission process of ISWs. This kind of background current is provided by the composed results simulated in terms of monthly mean baroclinic circulation and barotropic tidal current. It is found that the obtained relation of the number of fission solitons to the water depth and stratification is roughly in accordance with the fission law derived by Djordjevic and Redekopp in 1978; however, there exists obvious difference between the effects of the ebb and flood background currents on the wave-lengths of fission solitons （defined as the distance between two neighboring peaks of ISWs）. The difference in nonlinearity coefficient a between the ebb and flood background currents is a main cause for the different wave-lengths of fission solitons.