Western South China Sea Research Articles

Distribution and factors influencing on structure of reef fish communities in Nha Trang Bay Marine Protected Area, South-Central Vietnam

Visual censuses of coral reef fishes in Nha Trang Bay Marine Protected Area (MPA) were conducted during September–October 2005. Nha Trang Bay MPA is relatively rich in reef fishes compared to other areas in Vietnam and the Pacific Ocean outside the ‘Coral Triangle,’ consistent with its biogeographic location in the western South China Sea. A total of 266 species of 40 families of coral reef fishes formed five distinct assemblages. Spatial variations in distribution and structure of the assemblages were associated with eight significant biological and physical variables which were cover of living hard corals, encrusting corals, branching corals, Acropora, Millepora, Montipora, depth and distance from the coast of the mainland. The six factors in front are likely related to provision of shelter and nutrition, while the distance factor is likely to represent a gradient in disturbance and impacts from various mainland sources including sedimentation and pollution discharge from nearby rivers. Local species richness ranged from 35 to 70 species 500 m−2 (mean: 51 ± 2 SE) for reef flat stations and from 23 to 68 species 500 m−2 (mean: 48 ± 4 SE) for reef slope stations. Total species richness at each site averaged 76 species (±4 SE), ranging from 56 to 110 species, dominated by wrasses, damselfishes, butterflyfishes, parrotfishes, surgeonfishes, groupers and goatfishes. Density of total fishes at each station ranged from 348 to 1,444 individuals 500 m−2 (mean: 722 ± 302 SE) for the reef flat stations and from 252 to 929 individuals 500 m−2 (mean: 536 ± 215.7 SE) for the reef slope stations. Overall mean density at each site averaged 628.9 (±238.4 SE) individuals 500 m−2. The highly protected sites supported higher mean density of fishes per site (ranged: 904.5–1,213 individuals 500 m−2 for Hon Mun and 1,167.5 individuals 500 m−2 for Hon Cau) compared to other sites (<800 individuals 500 m−2). Of the families included in the census, densities were dominated throughout the MPA by damselfishes and wrasses. Many target species, particularly groupers, snappers and emperors, were rare or absent and the low abundance of big fishes was consistent with over-harvesting. Similarly a low density of butterfly fishes and angelfishes is likely related to the supply for marine aquaria in Vietnam and overseas. This study provides an important baseline against which the success of present and future MPA management initiatives may be assessed.

Calcareous nannofossils in surface sediments of the eastern and western South China Sea

A detailed interbasinal study of calcareous nannofossils in surface sediments and their environmental preferences was conducted across the South China Sea, from the upwelling area off Vietnam to the non-upwelling area off Luzon, Philippines. Emiliania huxleyi, Florisphaera profunda and Gephyrocapsa oceanica are the dominant taxa in all the stations, comprising 72%–100% of the total nannofossil assemblage. In the shallow coastal waters and nearshore environments in the western portion, the dominant taxa are E. huxleyi and G. oceanica. Offshore, towards the central (abyssal) portion of the SCS, F. profunda dominates the nannofossil assemblage. Distribution patterns observed in this study, as well as the results of the principal components analysis, confirmed and established environmental preferences not only of the three major taxa, but of several minor taxa as well. Based on these results and cluster analysis of the samples, three assemblages were recognized in the study area: (a) upwelling, (b) oceanic and (c) deep basin assemblages. The upwelling assemblage is associated with the Vietnam and Sunda Shelves upwelling areas and is dominated by E. huxleyi and G. oceanica. The oceanic assemblage encompasses the outer shelf to slope environments and is characterized by the common occurrence of F. profunda and E. huxleyi (western study area) and F. profunda and G. oceanica (eastern study area). The deep basin assemblage includes the abyssal portion of the SCS (below the inferred lysocline and calcite compensation depth) and is dominated by F. profunda (> 80%). It is proposed in the present study that F. profunda is resistant to dissolution.

Anti‐cyclonic eddies northwest of Luzon in summer–fall observed by satellite altimeters

Anti‐cyclonic eddies northwest of Luzon of the Philippines in summer‐fall are identified in the merged data products of satellite altimeters of Topex/Poseidon, Jason‐1 and European Research Satellites. The generation and propagation of the anti‐cyclonic eddies, which are confirmed by satellite ocean color data, are found to be a seasonal phenomenon that is phase‐locked to the onset of the southwesterly monsoon and the relaxation of the cyclonic wind curl in the northeastern South China Sea. The eddies originate from northwest of Luzon in summer, move across the northeastern South China Sea to reach the China continental slope in fall, and propagate southwestward along the continental slope in fall–winter, inducing shelfbreak current variations in the western South China Sea in fall–winter. The anti‐cyclonic eddy discovered by Li et al. (1998) in the northern South China Sea is found to originate from northwest of Luzon and carry primarily the South China Sea waters. It does not appear to be an eddy shed from the Kuroshio in the Luzon Strait area as alluded by Li et al. (1998) and others.

Effect of 1998 El Niño on the distribution of phytoplankton in the South China Sea

The present study analyzed interannual variation of phytoplankton/Chlorophyll‐a (Chl‐a) distribution in the South China Sea (SCS) for the period from 1997 to 2005 using SeaWiFS‐derived Chl‐a data and other oceanographic data. The results show high spatial variation of Chl‐a concentrations in the SCS and revealed an anomalous event in 1998. High Chl‐a concentrations in the southwestern SCS in the summer season (June to August) may be related with strong Ekman Pumping and strong wind stress, whereas a jet‐shape high Chl‐a region offshore in western SCS was associated with coastal upwelling driven by offshore Ekman transport and Vietnamese offshore current. Chl‐a concentrations in 1998 summer in the SCS were the lowest among the 7 years and were particularly low in the western SCS. At the same season, the jet‐shape Chl‐a region offshore of southeast Vietnam almost disappeared, and southwesterly monsoon winds and offshore current were relatively weaker in 1998. This anomalous event of low phytoplankton biomass in the SCS coincided with an El Niño year in 1998.

Variability of surface circulation in the South China Sea from satellite altimeter data

11-year satellite altimeter sea surface height (SSH) anomaly data from January 1993 to December 2003 are used to present the dominant spatial patterns and temporal variations of the South China Sea (SCS) surface circulation through Empirical Orthogonal Function (EOF) analysis. The first three EOF modes show the obvious seasonal variations of SSH in the SCS. EOF mode one is generally characterized by a basin-wide circulation. Mode two describes the double-cell basin scale circulation structure. The two cells were located off west of the Luzon Island and southeast of Vietnam, respectively. EOF mode three presents the mesoscale eddy structure in the western SCS, which develops into a strong cyclonic eddy rapidly from July to September. EOF mode one and mode three are also embedded with interannual signals, indicating that the SCS surface circulation variation is influenced by El Nino events prominently. The strong El Nino of 1997/98 obviously changed the SCS circulation structure. This study also shows that there existed a series of mesoscale eddies in the western SCS, and their temporal variation indicates intra-seasonal and interannual signals.

Low frequency variability of South China Sea surface circulation from 11 years of satellite altimeter data

Eleven years of sea surface height from satellite altimetry data are used to describe the low frequency variability of South China Sea (SCS) surface circulation through Empirical Orthogonal Function (EOF) analysis. The annual variability clearly agrees with previous studies, revealing the SCS response to seasonal fluctuations of the overlaying monsoon winds. The summer eastward jet off west coast of SCS is shown from the second seasonal EOF mode. Two new patterns of year‐to‐year and decadal variability are presented for the first time from measurements. Long term positive trend appears from 1993 to 2001 and reverses the sign of its trend again from 2001 to 2003. Abnormal events occur in 1994–1995, in 1997–1998 and in 2002 with different spatial scales regarding both basin‐scale distribution and meso‐scale variability located off the western SCS. Association of these events with the El Niño‐Southern Oscillation is discussed.

Southeastern extension of the Red River fault zone (RRFZ) and its tectonic evolution significance in western South China Sea

Recent geophysical surveys and basin modeling suggest that the No. 1 fault in the Ying-gehai basin (YGHB) is the seaward elongation of the Red River fault zone (RRFZ) in the South China Sea (SCS). The RRFZ, which separates the South China and Indochina block, extends first along the Yuedong fault, offshore of Vietnam, and then continues southward and breaks off into two branches: the Lupar fault and the Tinjia fault. The southern extension of the Lupar fault dies out beneath the NW Borneo while the Tinjia fault extends southeast and reaches the Brunei-Sabah area. According to the gravity and geomagnetic data, and the tectonic evolution of the basins, there are different evolution histories between the Wan’an basin (WAB) and the basins in the Nansha block. The Tinjia fault may be the boundary between the Balingian block and the Nansha block. Hence, the line linking the Yuedong fault and the Tinjia fault, which both are continental margin faults and strike-slip ones in the geological evolution histories, constitute the boundary between the Indochina and Nansha block. The Lupar fault, in contrast, is an intraplate fault within the Indochina block. The results provide new hints for reconstructing the tectonic evolution history of the RRFZ and the opening of the SCS, and also a framework for hydrocarbon prospecting in the region.

Prototypical solitons in the South China Sea

Surface signatures associated with non‐linear internal waves are often seen in satellite images of the western South China Sea (SCS) slope and shelf. Observation in the deep sea, to the east, are rare. Here we report on the evolution of an energetic packet as it propagated through the deep central basin of the SCS, toward the western slope and shelf. The waves have amplitudes estimated at 170 m, half widths of 3 km, and phase speeds of 2.9 ± 0.1 m/s, faster than the mode‐1 linear phase speed of 2.6 m/s. The shape and observed phase speed were consistent with the Korteweg‐deVries (KdV) model over the 65‐km path that they were tracked. The intrinsic velocity shear of the waves is small compared to pre‐existing shears, and the waves exhibit weak turbulence. The KdV fit and a satellite‐derived estimate of horizontal wave extent imply a westward energy flux of 4.5 GW for each crest.

Late Quaternary East Asian Monsoon variability in the South China Sea: Evidence from planktonic foraminifera faunal and hydrographic gradient records

The surface hydrographic conditions of the South China Sea (SCS) are influenced strongly by fluctuations of the East Asian Monsoon (EAM). Long -term fluctuations of the EAM have been previously investigated through analysis of several proxy records obtained from SCS sediment cores. However, in this marginal sea, interpretation of these records is complicated by ice volume/sea level effects. Here, we have extracted east–west hydrographic gradients from two IMAGES core records in the SCS, which are proposed to indicate monsoon variability. This strategy minimizes the effect of ice volume/sea level on the monsoon signal. High -resolution records of planktonic foraminifer faunal assemblages and sea surface temperature (SST) of the last ∼ 135 kyr have been generated for two cores, taken off the Vietnam margin (MD012394) in the western SCS and near Palawan Island (MD972142) in the southeastern SCS. These core sites represent contrasting hydrographic conditions in modern monsoon climates, where the winter EAM drives cold and strong mixing conditions in the western SCS, but maintains warm and deep-thermocline conditions in the southeastern SCS. A Planktonic Foraminifera Hydrographic Index (PFHI) is established based on the abundance ratios of warm/thermocline and cold/mixing species assemblages and SST estimates from a Modern Analogue Technique (MAT). The records of the east–west gradients for PFHI (ΔPFHI) and for SST (ΔSST) between the two cores (MD012394 and MD972142) show a dominance of 23 -kyr precession cycles. In the precession cycles, the ΔPFHI and ΔSST increased at the December 21 perihelion, which is associated with seasons of minimum Northern Hemisphere winter and maximum Southern Hemisphere summer insolation. Cross -spectral analysis between ΔPFHI/ΔSST and the precession cycles indicates coherent and in -phase relationships. Considering that the December 21 perihelion of the cycle of precession accorded with the time of maximum summer insolation in the Southern Hemisphere, we suggest that the SCS variability of hydrographic gradients for the last 135 kyr has been dominated by winter monsoon dynamics in response to geographic forcing controlled by the strength of the Australian summer monsoon.

Variations of the Last Glacial Warm Pool: Sea surface temperature contrasts between the open western Pacific and South China Sea

High‐resolution paleoceanographic records from the southern South China Sea (SCS) and the tropical open western Pacific reveal multicentennial‐ to millennial‐scale variations of the Western Pacific Warm Pool (WPWP) over the last 40 kyr. During the Last Glacial Maximum (LGM), in marine isotope stage (MIS) 2, planktonic Globigerinoides sacculifer δ18O in core NS93‐5 from the southern SCS and in core WP92‐5 from the central WPWP registered a similar average value of about −1.5‰. However, the average δ18O value was about 0.5‰ more negative in the SCS than in the central WPWP, as was their difference before and after the LGM in the later part of MIS 3 and early portion of MIS 1. These results indicate weaker monsoonal and fluvial effects on the southern SCS during the last glacial than during pleniglacial and postglacial periods owing to different oceanic circulation modes and regional topography. The calculated seasonal temperature differences of summer and winter sea surface temperatures (SSTs) between the open WPWP and the southern SCS were up to 3–4.5°C during the LGM but only slightly over 3°C in the southern SCS and less than 2°C in the open WPWP during other periods. From about 29 to 25 ka the MIS 2/3 transition was marked by a sudden SST increase in the southern SCS but a decrease in the open WPWP. Over this 4 kyr period the average annual SST appeared to be only 28°C in the central WPWP but more than 28°C in the southern SCS, indicating a shift of the central WPWP toward the far west during that time. Furthermore, our results imply a period of nearly 6 kyr from 22.5 to 16.5 ka without an apparent warm pool of 28°C in the western Pacific.

Kuroshio intrusion and the circulation in the South China Sea

The Princeton Ocean Model is used to study the circulation in the South China Sea (SCS) and its seasonal transition. Kuroshio enters (leaves) the SCS through the southern (northern) portion of the Luzon Strait. The annually averaged net volume flux through the Luzon Strait is ∼2 Sv into the SCS with seasonal reversals. The inflow season is from May to January with the maximum intrusion of Kuroshio water reaching the western SCS during fall in compensation of summertime surface offshore transport associated with coastal upwelling. From February to April the net transport reverses from the SCS to the Pacific. The intruded Kuroshio often forms an anticyclonic current loop west of the Luzon Strait. The current loop separates near the Dongsha Islands with the northward branch continuously feeding the South China Sea Warm Current (SCSWC) near the shelf break and the westward branch becoming the South China Sea Branch of Kuroshio on the slope, which is most apparent in the fall. The SCSWC appears from December to February on the seaward side of the shelf break, flowing eastward against the prevailing wind. Diagnosis shows that the onshore Ekman transport due to northeasterly monsoon generates upwelling when moving upslope, and the particular distributions of the density and sea level associated with the cross shelf motion supports the SCSWC.

Offshore phytoplankton biomass increase and its oceanographic causes in the South China Sea

High phytoplankton biomass often occurs in plumes near river mouths or in eutrophic coastal waters for short time periods. However, we observed an increased phytoplankton biomass in a narrow jet-shaped protrusion into the western South China Sea (SCS) using satellite chlorophyll a (chl a) data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and concurrent ship measurements in 1999. In June, regional phytoplankton biomass appeared as a large jet shape extending from the coastal waters of Vietnam eastward towards the SCS, about 200 km northeast of the mouth of the Mekong River; this feature intensified in the form of a large jet or gyre from July to September, decayed in October, and disappeared entirely in November. The gyre was about 400 km in diameter with chl a concentrations from 0.5 to 2 mg m(-3). Data on sea surface temperature (SST), winds, and sea surface height anomalies indicated a strong offshore upwelling during a period of strong southwesterly winds alongshore. The upwelling coincided with the regional increase in phytoplankton biomass in terms of shape, timing, and location. We observed this phenomenon during every southwesterly monsoon season from 1997 to 2002 using chl a images from 3 satellite ocean color sensors-SeaWiFS, Ocean Color and Temperature Scanner (OCTS), and Ocean Color Imager (OCI)-and it appeared to be related to the cross-shelf upwelling that delivers nutrients to surface waters. The phytoplankton then flowed with a large anticyclonic gyre into the western SCS.

3-D baroclinic numerical simulation of the South China Sea I. Upper circulation

A three-dimensional baroclinic shelf sea model was employed to simulate the seasonal characteristics of the South China Sea (SCS) upper circulation. The results showed that: in summer, an anticyclonic eddy, after its formation between the Bashi Channel and Dongsha Islands in the northeastern SCS, moves southwestward until it disperses slowly. There exists a northward western boundary current along the east shore of the Indo-China Peninsula in the western SCS and an anticyclonic gyre in the southern SCS. But at the end of summer and beginning of autumn, a weak local cyclonic eddy forms in the Nansha Trough, then grows slowly and moves westward till it becomes a cyclonic gyre in the southern SCS in autumn. At the beginning of winter, there exists a cyclonic gyre in the northern and southern SCS, and there is a southward western boundary current along the east shore of the Indo-China Peninsula. But at the end of winter, an anticyclonic eddy grows and moves toward the western boundary after forming in the Nansha Trough. The eddy's movement induces a new opposite sign eddy on its eastern side, while the strength of the southward western boundary current gets weakened. This phenomenon continues till spring and causes eddies in the southern SCS.

EA WM-Related Air-Sea-Land Interaction and the Asian Summer Monsoon Circulation

Based on the data analysis, this study further explores the characteristics of East Asian winter monsoon (hereafter, EAWM, for brevity) as well as the related air-sea-land system, and illustrates how and to what degree anomalous signals of the subsequent Asian summer monsoon are rooted in the preceding EAWM activity. We identified an important air-sea coupled mode, i.e., the EAWM mode illustrated in Section 3. In cold seasons, strong EAWM-related air-sea two-way interaction is responsible for the development and persistence of the SSTA pattern of EAWM mode. As a consequence, the key regions, i.e., the western Pacific and South China Sea (hereafter, SCS, for brevity), are dominated by such an SSTA pattern from the winter to the following summer. In the strong EAWM years, the deficient snow cover dominates eastern Tibetan Plateau in winter, and in spring, this anomaly pattern is further strengthened and extended to the northwestern side of Tibetan Plateau. Thus, the combined effect of strong EAWM-related SSTA and Tibetan snow cover constitutes an important factor in modulating the Asian monsoon circulation. The active role of the EAWM activity as well as the related air-sea-land interaction would, in the subsequent seasons, lead to: 1) the enhancement of SCS monsoon and related stronger rainfall; 2) the northward displacement of subtropical high during Meiyu period and the related deficient rainfall over Meiyu rainband; 3) above-normal precipitation over the regions from northern Japan to northeastern China in summer; 4) more rainfall over the Arabian Sea and Northeast India, while less rainfall over southwest India and the Bay of Bengal. The strong EAWM-related air-sea interaction shows, to some degree, precursory signals to the following Asian summer monsoon. However, the mechanism for the variability of Indian summer monsoon subsequent to the strong EAWM years remains uncertain.

A sea surface radiation data set for climate applications in the tropical western Pacific and South China Sea

The sea surface shortwave and longwave radiative fluxes have been retrieved from the radiances measured by Japan's Geostationary Meteorological Satellite 5. The surface radiation data set covers the domain 40°S‐40°N and 90°E–170°W and a period starting from January 1998. The temporal resolution is 1 day, and the spatial resolution is 0.5°×0.5° latitude‐longitude. The retrieved surface radiation has been validated with the radiometric measurements at the Atmospheric Radiation Measurement (ARM) site on Manus Island in the equatorial western Pacific. It has also been validated with the measurements at the radiation site on Dungsha Island during the South China Sea Monsoon Experiment (SCSMEX) (May and June 1998). The data set is used to study the effect of El Niño and East Asian summer monsoon on the heating of the ocean. Interannual variations of clouds associated with El Niño and the East Asian summer monsoon have a large impact on the radiative heating of the ocean, exceeding 40 W m−2 in seasonal mean over large areas. Together with the Clouds and the Earth's Radiant Energy System (CERES) shortwave fluxes at the top of the atmosphere and the radiative transfer calculations of clear‐sky fluxes, this surface radiation data set is also used to study the impact of clouds on the solar heating of the atmosphere. It is found that clouds enhance the atmospheric solar heating by ∼21 W m−2 in the tropical western Pacific and the South China Sea.

Satellite Observation of Upwelling along the Western Coast of the South China Sea

We observed the evolution of upwelling along the western coast of the South China Sea (SCS). The data we used are NOAA (National Oceanic and Atmospheric Administration) satellite AVHRR (Advanced Very High Resolution Radiometer) IR (infrared) images taken in 1996 and 1997 summer at the HRPT (High Resolution Picture Transmission) receiving station built on Tai-Ping Island, which is located in the central SCS. An upwelling intensity, defined by the total heat loss in the upwelling cold water region, is used to determine the relationship between coastal upwelling and the wind stress derived from the ERS-2 (European Remote Sensing Satellite) data. The results show that the upwelling intensity has a good linear relationship with the total alongshore wind stress while it has a low correlation with the cross-shore component of wind stress. These results imply that the alongshore wind stress is the main factor to pump the cold water up to the sea surface layer. Meanwhile, the satellite infrared images also indicate that the centroid of cold water moved southward from 15°N to 11°N during the observation period. The size of upwelling area changed as well, and finally evolved into a cold jet stretching offshore along 11°N–12°N in the mid-August 1997. Satellite infrared and altimeteric data show that the evolution of upwelling region is closely associated with the development of two anticyclonic circulations in the western SCS.

Interannual Variation in Indochina Summer Monsoon Rainfall: Possible Mechanism

Indochina is located between two extensively researched components of the Asian monsoon system: the Indian subcontinent and southeast–east Asia. Highly correlated with the National Oceanic and Atmospheric Administration Niño-3 sea surface temperatures, the interannual variation of Indochina monsoon rainfall is caused by a mechanism different from the two aforementioned regions. This mechanism consists of two elements: 1) the interannual modulation of the occurrence frequency of westward-propagating weather disturbances in the South China Sea–western tropical Pacific by an anomalous short-wave train emanating from the western tropical Pacific, and 2) an east–west interannual seesaw of the global divergent water vapor flux induced by the interannual variation in the global divergent circulation. An effort is made in this study to illustrate this mechanism.

Some Effects of the Intraseasonal Oscillation on the Equatorial Waves over the Western Tropical Pacific–South China Sea Region during the Northern Summer

Previous studies have shown that the life cycle of the Asian summer monsoon is modulated by the 30–60-day oscillation; the arrival of the 30–60-day monsoon trough (ridge) at 15°–20°N intensifies (weakens) the monsoon flow over the South China Sea–western tropical Pacific region and, in turn, extends eastward (west-ward) the monsoon westerlies (trade easterlies) over this region. Since equatorial waves are important synoptic disturbances to the weather of this region, an effort was made in this study to explore the possible impact of the 30–60-day mode an the occurrence frequency and westward propagation of these waves. The major findings of this study are as follows: 1) the westward intrusion of trade easterlies during the break monsoon facilitates the occurrence and westward propagation of equatorial waves along trade easterlies in the southern flank of the North Pacific anticyclone, and 2) the eastward expansion of monsoon westerlies during the active monsoon varies relative to the activity of equatorial waves in a way opposite to the break monsoon condition. Since this is a pilot study, some future efforts are suggested.

Sphaeromatidae (Crustacea : Isopoda : Flabellifera) from the South China Sea

Collections of sphaeromatids from the northern and western South China Sea are reported. As a result of this study, the number of sphaeromatid species known from this sea has been more than tripled. Descriptions and illustrations of four new species (Dynoides harrisoni, Cerceis sinensis, Paracerceis holdichi and Paraleptosphaeroma brucei) from the South China Sea are presented, a new genus, Chitonosphaera, is erected for Gnorimosphaeroma lata Nishimura, 1968, and records are presented for a further 24 species.

Actions of typhoons over the Western Pacific (including the South China Sea) and El Nino

According to the time cross-section of SST in the equatorial eastern Pacific and the historical data on typhoon actions over the western Pacific (including the South China Sea), a composite analysis of the actions of typhoon over the western Pacific in El Nino year (SST in the equatorial eastern Pacific are continuously higher than normal) and in the inverse El Nino year (there are continuative negative anomalies of SST in the equatorial eastern Pacific) is carried out. The results show that the actions of typhoon are in close relation with El Nino: The annual average number of typhoons over the western Pacific and South China Sea is less than normal in El Nino year and more in the inverse El Nino year; The annual average number of the landing typhoon on the continent of China bears the same relationship with El Nino; The anomalies of typhoon actions mainly occur during July-November and their starting are behind the anomaly of SST in the equatorial eastern Pacific.