Zhongsha Islands Research Articles

Spatiotemporal Variation and Predictors of the Purpleback Flying Squid (Sthenoteuthis oualaniensis) Distribution Surrounding the Xisha and Zhongsha Islands during a Fishing Moratorium

As an economic species widely distributed in the South China Sea (SCS), the purpleback flying squid (Sthenoteuthis oualaniensis) still has a large potential for exploitation, and the variations in its use as a resource are highly correlated with environmental and other factors. In this study, using a generalized additive model (GAM) and gradient forest analysis (GFA), in conjunction with environmental factors, the distribution of purpleback flying squid surrounding the Xisha and Zhongsha islands during the fishing moratorium period was investigated. The results indicated that catch per unit effort (CPUE) had a gradual increase from May to July 2023 in the primary fishing area surrounded the Xisha Islands during May to June, then moved southward towards 13–15° N after July. CPUE is used as an important indicator to reflect the abundance of the fishery, while the GFA results show that CPUE has a better fit than catch in this study. Therefore, the subsequent analysis focused on CPUE. Longitude and sea surface temperature (SST) were of relative higher importance, followed by sea surface salinity (SSS), latitude, chlorophyll a concentration (Chla), sea surface height (SSH), and mixed layer depth (MLD). Longitude and CPUE had a significant, positive correlation. The CPUE gradually increased with latitude within 14–16° N. The CPUE increased slowly as SST increased from 29.5 to 30.5 °C in the primary fishing area. The Chla in this fishing zone was 0–0.2 mg/m3 and displayed a significant positive association with CPUE. Conversely, SSS, SSH, and MLD had negative correlations with CPUE. These findings will promote the sustainable utilization of purpleback flying squid in the SCS.

Biodiversity of Nematodes from Coral Reef Sediments in the South China Sea Based on eDNA Metabarcoding

Coral reef ecosystems in the South China Sea are one of the globally important marine biodiversity hotspots. However, there are few studies on nematode biodiversity in coral reef sediments. Here, we assessed nematode biodiversity in coral reef sediments in the South China Sea using eDNA metabarcoding. Eight sampling stations were set up in the region north of the South China Sea Basin (Xisha and Zhongsha Islands) and south of it (Nansha Islands), respectively. We also compared and analysed the nematode biodiversity and community structure in different regions, as well as the effects of environmental factors on the community structure. The results showed that a total of 503 operational taxonomic units (OTUs) were detected in 16 sediment samples. The nematodes identified belong to two classes, 10 orders, 37 families, 51 genera and 61 species, and all of them were Enoplea and Chromadorea, except for the unidentified taxa, which accounted for 97.26% of the total abundance. The analyses showed that the northern region had higher nematode abundance and diversity relative to the southern region. However, there was no significant difference in the nematode community structure between the two regions, which was less affected by geographic location. Similarly, environmental factors (total nitrogen, total phosphorus, copper, zinc, cadmium, lead and arsenic) have an effect on the abundance of specific nematode groups but not on the overall community structure. In summary, this study initially reveals the composition and diversity of nematode communities in coral reef sediments in the South China Sea and provides an important reference for further in-depth study of the South China Sea ecosystem.

Fractal modeling for geochemical data of deep-sea surface sediments: A case study from Zhongsha Island, Southern China Sea

This study applied principal component analysis (PCA), concentration-area (C-A) and spectrum-area (S-A) fractal models to identify geochemical element combination anomalies related to heavy metals from the data for 43 geochemical elements in surface sediments of the Zhongsha Islands. The relationships between these 43 elements were first examined by applying correlation and cluster analysis to the surface sediment sample dataset. The multifractal spectral functions of the 43 elements were then calculated and all elements were classified using the curve shape of the spectral function. The results of classification were consistent with that of cluster analysis, thereby verifying the reliability of fractal analysis of the study area. PCA was then used to identify element combination anomalies and to assess high-value areas of element contents, and the S-A model was used to distinguish between the anomaly and background signal. The results of this multiple-analysis approach show that the element combination anomaly is mainly located in the deep-sea plain area, with a few elevated values in the Zhongsha South Basin. This study showed that quantitative evaluation of anomalies of multi-element associations can effectively delineate a target area and that anomalies of element associations extracted by the S-A fractal method can provide a more accurate outline areas containing high contents of combined elements. This study can act as a theoretical reference for the environmental assessment of the Zhongsha sea area.

Experimental Study on Damage Behavior of Porous Coral Limestone from the Zhongsha Islands, South China Sea

Experimental Study on Damage Behavior of Porous Coral Limestone from the Zhongsha Islands, South China Sea

Application of Singularity Theory to the Distribution of Heavy Metals in Surface Sediments of the Zhongsha Islands

This research aimed to use nonlinear theory and technology to describe the spatial distribution of heavy metals in the surface sediments of the Zhongsha Islands Sea region. The goal of this study is to explore the spatial distribution characteristics of heavy metals in the surface sediments of the Zhongsha Islands. The singularity theory and method were used to delineate heavy metal geochemical anomalies and the generalized self-similarity analysis method was used to decompose heavy metal geochemical anomalies and background concentrations. The results showed that there were abnormally high concentrations of heavy metals in the deep-sea plain area and in the western central sand trough area. The results of this study can inform priority areas for environmental monitoring. The element anomalies extracted by the singularity analysis method in this paper can be a guide to the next step in the investigation and provide the basis for the regional environmental assessment.

Biodiversity and distribution of benthic dinoflagellates in tropical Zhongsha Islands, South China Sea

Biodiversity and distribution of benthic dinoflagellates in tropical Zhongsha Islands, South China Sea

The Application of DNA Barcoding in Crustacean Larvae Identification from the Zhongsha Islands, South China Sea

Marine crustaceans are known as a group with high morphological diversity and great economic value. Most species have planktonic larval stages that are difficult to identify to species level using traditional approaches because of insufficient morphological diagnostic characters or taxonomic descriptions. We used DNA barcoding and molecular species identification to investigate the species diversity and distribution of crustacean larvae in the Zhongsha Islands waters, South China Sea. In total, 108 sequences were obtained from crustacean larvae collected in the Zhongsha Islands waters in 2019 using vertical hauls between the depths of 5 and 200 m. The molecular classification approach confirmed that 108 sequences represented crustaceans typical to the South China Sea, with 70 species identified, representing 43 genera, 23 families, and 4 orders. However, the cytochrome c oxidase subunit I gene sequences of only 27 species identified from the larval samples matched with available sequences taken from adults in GenBank. The comparison of K2P distances yielded a notable gap of 3.5–10.7% between intraspecific and interspecific distances across the sequence dataset. More than 80% of the crustacean larvae species belonged to the order Decapoda, and they displayed marked differences in their distributionsin the Zhongshan Islands waters. The orders Calanoida and Amphipoda were represented by the fewest species, which were recorded only at the edge of the Zhongsha Atoll.

Spatio-temporal Variations of Sea Surface Wind in Coral Reef Regions over the South China Sea from 1988 to 2017

The seasonal and interannual variabilities of sea surface wind (SSW) in the South China Sea (SCS), especially in coral reef regions such as Nansha Islands, Xisha Islands, Zhongsha Islands and Dongsha Islands were investigated in detail using the Blended Sea Winds dataset (1988–2017). Annual and monthly variations of SSW and sea surface temperature (SST) in the four zones were investigated. Empirical Orthogonal Function (EOF) analysis of wind field was performed to aid in better understanding the different spatial patterns. The results indicate that, as observed in the spatial distribution of the first mode of monthly mean wind speed anomaly, the magnitudes in the four island zones are all negative and are similar to each other, showing that the variations of SSW in the four island zones are consistent. In the second mode, the magnitudes in Nansha Islands are opposite to those in the other three zones. The spatial distribution of the third mode reflects regional differences. The maximum annual SSW appears in Dongsha Islands, and the minimum appears in Nansha Islands. The interannual variations of SSW in all island zones are basically concurrent. The island zones with high SSW mostly have low SST, and vice versa. There may be an inverse relationship between SSW and SST in coral reef regions in the SCS. The multi-year monthly variations of SSW in the island zones present a ‘W’-shaped structural variation. Each island undergoes two months of minimum SSW every year, one during March–May (MAM) and the other during September–November (SON). Both months are in monsoon transition periods. During the months with low SSW, high SST appears. The SST peaks almost correspond to the SSW troughs. This further indicates that SSW and SST may have opposite changes in coral reef regions. Coral bleaching events often correspond to years of high SST and low SSW.

Distribution, fate and sources of polycyclic aromatic hydrocarbons (PAHs) in atmosphere and surface water of multiple coral reef regions from the South China Sea: A case study in spring-summer

Our previous study revealed PAHs' wide occurrence in corals from multiple coral reef regions (CRRs) in the South China Sea. However, little is known about their occurrence, distribution, fate, and sources in the ambient environment of these CRRs. This study aimed to resolve these research gaps. The results showed ∑15PAHs (total concentrations of 15 US EPA priority controlled PAHs exclude naphthalene) in the atmosphere (gas-phase: 0.31–49.6 ng m−3; particle-phase: 2.6–649 pg m−3) were mainly influenced by air mass origins. Southwesterly wind caused higher ∑15PAHs than the southeasterly wind. The ∑15PAHs in seawater from the nearshore (462 ± 244 ng L−1) was higher than that from offshore Zhongsha Islands (80.5 ± 72.1 ng L−1) because of the effect of terrigenous pollution and ocean current. Source apportionment indicated that the mixed sources of spilled oil and combustion from neighboring countries were the main contributors to PAHs in these CRRs. The total deposition fluxes showed that PAHs tended to migrate from the atmosphere to seawater. Global warming may inhibit this process, but PAHs still have a migration pattern of atmosphere-ocean-corals, which will further increase the environmental pressure on coral reef ecology.

Summer phytoplankton assemblages and carbon biomass in the northern south China sea

Phytoplankton assemblage composition, abundance, carbon biomass and distribution were studied in summer 2004 in the northern South China Sea (NSCS; 18°-22°N, 110°-117°E). This is the first report of natural phytoplankton assemblages which were analyzed by Utermöhl method and their biovolume conversion carbon biomass in the NSCS. A taxa of phytoplankton was identified, which belong to 4 classes and 76 genera, and mainly composed of Bacillariophyceae. Most of them belong to nanophytoplankton (Equivalent Sphere Diameter is 10–15 μm). Associated phytoplankton, with a share of 3% UPA (Utermöhl phytoplankton assemblage cell abundance), primarily consisted of Dinophyceae. Chrysophyceae and Cyanophyceae occurred in appreciable amounts only at upper layers waters in the NSCS. Combined the UPC (Utermöhl phytoplankton carbon) with PPC (picophytoplankton carbon) into the BPC (bulk phytoplankton carbon biomass), it ranged from 0.117 to 1467.172 μg C L−1, its average was 54.848 ± 197.578 μg C L−1. The ratios of PPC contribution to BPC ranged from 0.001% to 6.567%, its average was 0.667 ± 1.215%. This indicates that picophytoplankton contribution to phytoplankton carbon pool is much low than that of chlorophyll a partitioning. Two carbon biomass provinces, one was higher biomass province along continental surface and bottom layers, and the other one was lower biomass province in open sea waters, were clearly discerned along the isoline of 20 μg C L−1. The calculated budget amount BPC pool in the NSCS was 1.21 × 104 tC. The calculated Carbon: Chlorophyll a ratio (C: Chl a) in study area ranged from 0.21 to 217.648, its average was 37.31 ± 46.53. Several typical phytoplankton assemblages can be discerned based on Shannon-Wiener diversity index, Pielou's evenness and C: Chl a. Examination of phytoplankton abundances, biomass and environmental factors by multivariate techniques PCA (Principal Component Analysis) suggests BPC distribution in the NSCS was mainly controlled by salinity at Section along PRE (Pearl River estuary) to Dongsha Islands, and nitrate concentration at Section along the Yangjiang to Zhongsha Islands. The keystone phytoplankton species in the NSCS were mainly composed of large or chain form Bacillariophyceae, such as Pseudo-nitzschia delicatissima, Chaetoceros spp. and Asterionellopsis glacialis, and some Dinophyceae also take an important role in the NSCS, such as Gyrodinium spirale and Prorocentrum dentatum. In addition, the features of phytoplankton distribution were according to functional morphology.

The composition and geochemical significance of organic matters in surface sediments from the Southwest Sub-basin of the South China Sea

Information about ocean evolution and material-energy exchange between the ocean and surrounding continents can be preserved in marine sediments. The Southwest Sub-basin, located among the Xisha Islands, the Zhongsha Islands, and the Nansha Islands, is an ideal tectonic unit to analyze the sedimentary environments of the South China Sea. In this paper, the distribution patterns of lipid biomarkers and their compound-specific stable carbon isotopes in surface sediments from the Southwest Sub-basin of the South China Sea were analyzed. Lipid biomarkers are composed of different proportions of aliphatic hydrocarbons, carboxylic acids, alcohols and series of sterols, indicating that the organic matter of the Southwest Sub-basin was derived from marine bacteria, algae and terrestrial higher plants. The average concentration of total organic carbon (TOC) in the study samples was 0.5 ± 0.16%. TOC, total n-alkanes, and total carboxylic acids (TFA) decreased gradually from the margins to the center of the sub-basin, whereas the content of total n-alkanols increased. The spatial distribution trends resulted from varying water depths and the contribution of terrestrial organic matter. The concentration of TFA and the TFA/TOC revealed that the accumulation of organic matter in this area was generally low. The strong correlation between TOC and TFA of surface sediments in the study area may be related to a balance between the ocean production and ocean deposition rates.

Deep water characteristics and circulation in the South China Sea

This study investigates the deep circulation in the South China Sea (SCS) using oceanographic observations combined with results from a bottom layer reduced gravity model. The SCS water, 2000 m below the surface, is quite different from that in the adjacent Pacific Ocean, and it is characterized by its low dissolved oxygen (DO), high temperature and low salinity. The horizontal distribution of deep water properties indicates a basin-scale cyclonic circulation driven by the Luzon overflow. The results of the bottom layer reduced gravity model are consistent with the existence of the cyclonic circulation in the deep SCS. The circulation is stronger at the northern/western boundary. After overflowing the sill of the Luzon Strait, the deep water moves broadly southwestward, constrained by the 3500 m isobath. The broadening of the southward flow is induced by the downwelling velocity in the interior of the deep basin. The main deep circulation bifurcates into two branches after the Zhongsha Islands. The southward branch continues flowing along the 3500 m isobath, and the eastward branch forms the sub-basin scale cyclonic circulation around the seamounts in the central deep SCS. The returning flow along the east boundary is fairly weak. The numerical experiments of the bottom layer reduced gravity model reveal the important roles of topography, bottom friction, and the upwelling/downwelling pattern in controlling the spatial structure, particularly the strong, deep western boundary current.

A newly-discovered historical map using both national boundary and administrative line to represent the U-boundary in the South China Sea

A New Map of the People’s Republic of China was discovered through investigating a large number of historical maps. This map was published in April 1951, both continuous national boundary line and administrative region line were applied in this map to represent China’s national U shaped boundary line (U-boundary) in the South China Sea (SCS). It is an important historical material that supports the U-boundary line is the border of China’s South China Sea. This is the first research of this newly-discovered map. In the map, the U-boundary in the SCS (SCS U-boundary) is not only presented by a continuous boundary map symbol (·>—— —— This New Map of the People’s Republic of China was edited by the Guanghua and Geosciences Club and was published by SDX Joint Publishing Company. The continuous U-line in this map clearly shows the accurate boundaries of China’s border in the SCS. It is an important historical map that clearly proves the U-boundary line is the border of China’s territorial sea in the SCS. Moreover, the red solid administrative-line overlaying the U-boundary definitely indicated that the sovereignty of the sea within the U-boundary belonged to China. The regions of the SCS U-boundary include Dongsha Islands, Xisha Islands, Nansha Islands, Zhongsha Islands, Taiwan Island, Beibu Gulf and the Zengmu Reef. This study compared maps of historical U-line which were published before 1951, and also analyzed the domestic and international maritime situation, and drawing methods of the maritime boundary lines in the history. The results show that “the continuous line” was recognized by the international community in the corresponding historical period, and both the “intermittent (dotted) line” and “the continuous line” are appropriate descriptions and drawing-methods for the sea boundary in historical period. Analysis of six maps, including the historical maps and the latest vertical world map, showed that the territory of U-boundary line inherited the territory of China’s SCS in the “South China Sea Island Map” that was officially published and recognized worldwide in 1948. This study recommends using “the South China Sea U-Shape boundary line” (SCS U-boundary) to describe the border of China’s sea in SCS. “SCS U-boundary” not only emphasizes the U shape borderline of China’s sea in the SCS, but also can be easily expressed and remembered worldwide. Using “SCS U-boundary” is a better way to avoid problems of “number” and “number of segments”, and can further express the certainty of the integrity, continuity and border of the China’s seas in the SCS as well. In summary, the study points out: (1) SCS U-boundary is the border of China’s sea in the SCS, and its sovereignty belongs to China; (2) both “continuous U-boundary line” and “intermittent U-boundary line” are internationally recognized to describe sea borders; (3) “SCS U-boundary” is more vividly, accurately, completely and scientifically to characterize China’s sea border in the SCS.

Lithospheric structure of the South China Sea and adjacent regions: Results from potential field modelling

This work aims to investigate the crustal and lithospheric mantle thickness of the South China Sea (SCS) and adjacent regions. The crust-mantle interface, average crustal density, and lithospheric mantle base are calculated from free-air gravity anomaly and topographic data using an iterative inversion method. We construct a three-dimensional lithospheric model with different hierarchical layers. The satellite-derived gravity is used to invert the average crustal density and Moho (crust-mantle interface) undulations. The average crustal density and LAB (lithosphere–asthenosphere boundary) depths are further adjusted by topographic data under the assumption of local isostasy. The average difference in Moho depths between this study and the seismic measurement results is <1.5 km. The results show that in oceanic regions, the Moho depths are 7.5–30 km and the LAB depths are 65–120 km. The lithospheric thickness of the SCS basin and the adjacent regions increases from the sea basin to the continental margin with a large gradient in the ocean-continent transition zones. The Moho depths of conjugate plots during the opening of SCS, Zhongsha Islands and Reed Bank, reveal the asymmetric spreading pattern of SCS seafloor spreading. The lithospheric thinning pattern indicate two different spreading directions during seafloor spreading, which changed from N-S to NW-SE after the southward transition of the spreading axis. The lithosphere of the SCS basin and adjacent regions indicate that the SCS basin is a young basin with a stable interior lithosphere.

Deep Western Boundary Current in the South China Sea

Deep western boundary current (DWBC) was observed for the first time by an array of 6 current meter moorings southeast of the Zhongsha Islands in the South China Sea (SCS) deep basin during the period from August 2012 to January 2014. In the mean, the DWBC in the SCS flows southwestward with core velocity of 2.0 cm/s and a volume transport of 1.65 Sv (1 Sv = 1 × 106 m3/s). Its temporal variability is dominated by intraseasonal fluctuations with period around 90 days. The main axis of the DWBC, characterized by a low temperature core, tends not to shift with the 90-day fluctuation.

Comparison of crustal structures in the fossil spreading center of South China Sea basins and the tectonic significance

This study focuses on the magmatic activities and seamounts in the fossil spreading center of three sub-basins in the South China Sea (SCS). The northwest sub-basin (NWSB), southwest sub-basin (SWSB), and east sub-basin (ESB) are of great importance for understanding the tectonic evolution and post-spreading magmatic activities of the SCS. Deep crustal structures beneath the spreading center of these basins have a significant bearing on the magmatic process. For this study, on the basis of a previous velocity model, we used two-dimensional ray-tracing methods and high-quality ocean-bottom seismometer (OBS) data collected in 2006 to image the NWSB deep P-wave velocity structures. We then performed travel-time analysis and estimated uncertainties of the final model parameters to prove that the velocity model was convincing. Finally, we compared the one-dimensional velocity structures among OBS profiles 2006-p1 in the NWSB, G8G0 in the ESB, and T1 in the SWSB. Our study of crustal structure yielded the following results: (1) the NWSB is confirmed to be oceanic crust compared with typical Atlantic oceanic crust. The crustal velocity is 5.7–7.0 km/s, whereas the crustal thickness is 6–8 km in the basin (90–130 km). There is no high-velocity layer. The Mohorovicic discontinuity (Moho) is distributed symmetrically along the spreading axis and gradually increases from the basin center to the edge. The depth of the Moho is 11–12 km in the basin (55–70 km) and changes dramatically to 16 km depth under the north margin (170–200 km) and the Zhongsha Islands (0–55 km). The depth of the Moho in all three sub-basins increases by 1 km in the spreading center. This suggests that crustal structure in the SCS is greatly influenced by magmatic activities. The Shuangfeng seamount, with a 6.5-km-thick crust, located in the center of the spreading ridge, is a post-spreading volcanic seamount. The 6.4-km/s velocity contour beneath the Shuangfeng seamount gradually rises toward the northwest part, and the velocity at the seamount bottom is lower than 7.4 km/s. Therefore, we speculate that the magma underplating does not exist. Consistent with multichannel seismic date, we conclude that the Shuangfeng seamount was mainly formed by an intrusive process: The magma intruded along the former extinct spreading axis. The Longnan seamount in the SWSB and the Jixiang seamount in the ESB were mainly formed by an eruption process, supposedly the result of the buoyant decompression melting mechanism. (2) The detailed velocity model of the NWSB shows that the oceanic crust exhibits obvious across-axis velocity variations. The lateral variations in crustal thickness and velocity are also found in the ESB and SWSB. This shows that the previously formed oceanic crust in all three SCS sub-basins are reformed by post-spreading magma. The difference in crustal velocity of the NWSB indicates that the magma supply varied in space during or even after the seafloor spreading. The OBS 2006-p1 velocity model indicates that the velocity contours gradually rise from the southeast to northwest (140–160 km) direction. The crust layer may have experienced asymmetric basalt magma flow, and the northwest part of the basin may have received more magma than the southeast part during the process of seafloor spreading. Low crustal velocities were found in the SWSB and ESB but not in the NWSB. This indicates an inadequate supply of magma in the NWSB. Compared with the other two sub-basins, the Cenozoic tectonic activities in the NWSB were less active. The residual magma might have been consumed after the NWSB spreading. Either there are no magma chambers in the NWSB spreading center, or the magma chambers are too small to detect.

Evidence of China’s sea boundary in the South China Sea

According to a series of important historical maps, i.e., the Location Map of the South China Sea Islands, the Nansha Islands, Zhongsha Islands, Xisha Islands, Yongxing Island and Shidao Island, and Taiping Island (archived by the Territorial Administration Division of the Ministry of Interior of Republic of China in 1946), and the Administration District Map of the Republic of China published in 1948, the dashed line surrounding the South China Sea Islands represents China’s sea boundary in the South China Sea at that time. It was both connected with, and an extension of, the land boundary of China. At that time the dashed line was used to represent the waters boundaries while the solid line was used to represent the land boundary—a universal method used in maps that was then recognized internationally. The above observation provides historical and scientific evidence of China’s sea boundary in the South China Sea that is useful for the international maritime delimitation over the South China Sea area.

Validation and application of multi-source altimeter wave data in China’s offshore areas

Studies of offshore wave climate based on satellite altimeter significant wave height (SWH) have widespread application value. This study used a calibrated multi-altimeter SWH dataset to investigate the wave climate characteristics in the offshore areas of China. First, the SWH measurements from 28 buoys located in China’s coastal seas were compared with an Ifremer calibrated altimeter SWH dataset. Although the altimeter dataset tended to slightly overestimate SWH, it was in good agreement with the in situ data in general. The correlation coefficient was 0.97 and the root-mean-square (RMS) of differences was 0.30 m. The validation results showed a slight difference in different areas. The correlation coefficient was the maximum (0.97) and the RMS difference was the minimum (0.28 m) in the area from the East China Sea to the north of the South China Sea. The correlation coefficient of approximately 0.95 was relatively low in the seas off the Changjiang (Yangtze River) Estuary. The RMS difference was the maximum (0.32 m) in the seas off the Changjiang Estuary and was 0.30 m in the Bohai Sea and the Yellow Sea. Based on the above evidence, it is confirmed that the multialtimeter wave data are reliable in China’s offshore areas. Then, the characteristics of the wave field, including the frequency of huge waves and the multi-year return SWH in China’s offshore seas were analyzed using the 23-year altimeter wave dataset. The 23-year mean SWH generally ranged from 0.6–2.2 m. The greatest SWH appeared in the southeast of the China East Sea, the Taiwan Strait and the northeast of the South China Sea. Obvious seasonal variation of SWH was found in most areas; SWH was greater in winter and autumn than in summer and spring. Extreme waves greater than 4 m in height mainly occurred in the following areas: the southeast of the East China Sea, the south of the Ryukyu Islands, the east of Taiwan-Luzon Island, and the Dongsha Islands extending to the Zhongsha Islands, and the frequency of extreme waves was 3%–6%. Extreme waves occurred most frequently in autumn and rarely in spring. The 100-year return wave height was greatest from the northwest Pacific seas extending to southeast of the Ryukyu Islands (9–12 m), and the northeast of the South China Sea and the East China Sea had the second largest wave heights (7–11 m). For inshore areas, the 100-year return wave height was the greatest in the waters off the east coast of Guangdong Province and the south coast of Zhejiang Province (7–8 m), whereas it was at a minimum in the area from the Changjiang Estuary to the Bohai Sea (4–6 m). An investigation of sampling effects indicates that when using the 1°×1°grid dataset, although the combination of nine altimeters obviously enhanced the time and space coverage of sampling, the accuracy of statistical results, particularly extreme values obtained from the dataset, still suffered from undersampling problems because the time sampling percent in each 1°×1°grid cell was always less than 33%.

Conjugate margin pattern of the Southwest Sub‐basin, South China Sea: insights from deformation structures in the continent‐ocean transition zone

AbstractEight cross‐sections from the continent‐ocean transition zone of the Southwest Sub‐basin, South China Sea, have been used to compare conjugate rifted margins that are typical of magma‐poor rift systems. The geological timing of the break‐up unconformity in the continental margins shows that the Reed Bank and the Dangerous Grounds in the south continental margin connect with the Zhongsha Islands and the western Phu Khanh Basin, which experienced northwest‐southeast oriented movement after the opening of the Southwest Sub‐basin. Detailed interpretations identify a range of deformational and sedimentary features in the continent‐ocean transition zones with and without rigid blocks. Remarkable structural similarities appear in the transition zone lying within the convex‐shaped area shaped by rigid blocks of the conjugate margin (e.g. the Zhongsha Islands and the Reed Bank) or within concave‐shaped areas sandwiched by rigid blocks. Rigid tectonic blocks such as the Zhongsha Islands and the Reed Bank played important roles in both the shaping of the continent‐ocean boundary and the initial continental break‐up. Copyright © 2015 John Wiley &amp; Sons, Ltd.

An observed cyclonic eddy associated with boundary current in the northwestern South China Sea

A cyclonic eddy detected in the northwestern South China Sea during the summer of 2011 is investigated using both satellite data and several sets of in situ observations including cruise measurements and a mooring line. The eddy was generated to the west of the Zhongsha Islands in early June and then propagated northwestward. In July and August, when the cyclonic gyre originating from the offshore jet recirculation east of Vietnam matured, the eddy was absorbed into the gyre and strengthened by it. The eddy regulated the thermohaline characteristics and the currents of the entire water column. Along-slope boundary currents (toward the northeast in summer) were completely reversed from the surface to about 1300 m below. The thermohaline in deep water (660–950 m) showed a significant response to the presence of the eddy, with the isotherms and isohalines rising up to 100 m. The recurrence of the phenomenon is further explored by a statistical analysis of satellite observations. Results suggest the presence of a cyclonic eddy and abnormal offshore jets are closely associated with the El Nino Southern Oscillation events.