Progress of regional oceanography study associated with western boundary current in the South China Sea

Recent progress of physical oceanography in the South China Sea (SCS) associated with the western boundary current (WBC) and eddies is reviewed in this paper. It includes Argo observations of the WBC, eddy detection in the WBC based on satellite images, cross-continental shelf exchange in the WBC, eddy-current interaction, interannual variability of the WBC, air-sea interaction, the SCS throughflow (SCSTF), among others. The WBC in the SCS is strong, and its structure, variability and dynamic processes on seasonal and interannual time scales are yet to be fully understood. In this paper, we summarize progresses on the variability of the WBC, eddy-current interaction, air-sea interaction, and the SCSTF achieved in the past few years. Firstly, using the drifting buoy observations, we point out that the WBC becomes stronger and narrower after it reaches the central Vietnam coast. The possible mechanisms influencing the ocean circulation in the northern SCS are discussed, and the dynamic mechanisms that induce the countercurrent in the region of northern branch of WBC in winter are also studied quantitatively using momentum balance. The geostropic component of the WBC was diagnosed using the ship observation along 18°N, and we found that the WBC changed significantly on interannual time scale. Secondly, using the ship observations, two anti-cyclonic eddies in the winter of 2003/2004 in the northern SCS, and three anti-cyclonic eddies in the summer of 2007 along 18°N were studied. The results show that the two anti-cyclonic eddies can propagate southwestward along the continental shelf at the speed of first Rossby wave (～0.1 ms-1 ) in winter, and the interaction between the three anti-cyclonic eddies in summer and the WBC in the SCS is preliminarily revealed. Eddies on the continental shelf of northern SCS propagated southeastward with a maximum speed of 0.09 ms-1 , and those to the east of Vietnam coast had the largest kinetic energy, both of which imply strong interaction between eddy activity and WBC in the SCS. Thirdly, strong intraseasonal variability (ISV) of sea surface temperature (SST) near the WBC regions was found, and the ISV signal of SST in winter weakens the ISV signal of latent heat flux by 20%. Fourthly, the long-term change of SCSTF volume transport and its connection with the ocean circulation in the Pacific were discussed.     

Pulleniatina Minimum Event during the last deglaciation in the southern South China Sea

The planktonic foraminiferal faunal census of core MD 05-2894 （7°2.25′N, 111°33.11′E, water depth 1982 m）, retrieved from the southern South China Sea （SCS） during the ＂Marco Polo＂ cruise in 2005, was performed to investigate the abundance changes of a subsurface dweller, Pulleniatina obliquiloculata. The results display that the abundance of P. obliquiloculata nearly declines to zero during 16.0--14.9 ka, corresponding to the Heinrich 1 （H1） cold interval. The unexpected decrease of P. obliquiloculata occurs in the adjacent cores, roughly between 17 and 14.8 ka based on the previous studies. Accordingly, the Pulleniatina Minimum Event in the last deglaciation can serve as a good stratigraphical indicator, at least in the southern SCS. To further explore the changes of sea surface temperature （SST） and subsurface seawater temperature （SSST）, we made parallel Mg/Ca measurements on surface dweller Globigerinoides tuber and subsurface dweller P. obliquiloculata tests. Since the last deglaciation, the SSTs show a continuous increasing trend towards the late Holocene, while the warming of the subsurface water is punctuated by a 2℃-cooling interval across the deglacial Pulleniatina Minimum Event. Both increased 5180 differences between G. ruber and P. obliquiloculata, and increased temperature differences between surface and subsurface water suggest a shoaling of the mixed layer during the deglacial Pulleniatina Minimum Event. Therefore, we consider that the significant changes in the upper ocean structure are responsible for the Pulleniatina Minimum Event during the last deglaciation in the southern SCS.     

Coupling and propagating of mesoscale sea level variability between the western Pacific and the South China Sea

The coupling and propagating features of mesoscale sea level variability between the western Pacific (WP) and the South China Sea (SCS) were studied based on time series analyses of satellite altimeter measured, along-track sea surface height anomalies (SSHA) along 21°N and the slope of the northern SCS. The analyses show that mesoscale sea level fluctuations in the WP have rather limited coherent effects on the SCS, and no statistically significant propagation of mesoscale variability through the Luzon Strait was observed except in the 45-day band. Evidence suggests that the 45-day fluctuation is very much likely a character of the Kuroshio originating from its low-latitude beginning. It is also sug- gested that the westward propagating Rossby waves will deform when they encounter the dynamical barrier of the Kuroshio. The Kuroshio will then acts on the SCS in its own way. The SCS is a relatively isolated system in the sense of coupling and propagating of oceanic mesoscale waves.     

Evaluation of a 3dVAR system for the South China Sea

The authors evaluate a three-dimensional variational (3dVAR) system for the South China Sea (SCS) in this study. The assimilation method applied in the system takes into consideration error correlation along each ground track and uses recursive filter for optimization. Data from three R/V cruises during the spring and summer of 1998 and the summer of 2000 are used to evaluate the system. The root-mean-square error and bias are reduced significantly and when the altimeter data is assimilated, the distribution of the error is much closer to the Gaussian distribution. Precipitation and river discharge in the southwestern SCS are reproduced, and the variability of sea surface height is efficiently transferred to the subsurface. The 3dVAR system performs well for each of the three cruises, suggesting that it is steady for routine usage.     

Evaluation of a 3dVAR system for the South China Sea

The authors evaluate a three-dimensional variational （3dVAR） system for the South China Sea （SCS） in this study. The assimilation method applied in the system takes into consideration error correlation along each ground track and uses recursive filter for optimiza- tion. Data from three R/V cruises during the spring and summer of 1998 and the summer of 2000 are used to evaluate the system. The root-mean-square error and bias are reduced significantly and when the altimeter data are assimilated, the distribution of the error is much closer to the Gaussian distribution. Precipitation and river discharge in the southwestern SCS are reproduced, and the variability of sea surface height is efficiently transferred to the subsurface. The 3dVAR system performs well for each of the three cruises, suggesting that it is steady for routine usage.     

Simulation or meridional overturning in the upper layer of the South China Sea with an idealized bottom topography

The large-scale upper circulations and meridional overturning in the upper layer of the South China Sea (SCS) with idealized bottom topography in winter and summer are investigated. Simulations with the GFDL general circulation model are carried out under the conditions of open or enclosed boundary regarding transport in the Luzon Strait. The intrusion area of Kuroshio, its impact on the meridional overturning in the upper layer of the SCS and seasonal characteristic of this impact are explored, respectively. The model is forced by climatological wind stress and relaxed to monthly mean climatological temperature and salinity. The resultant meridional overturning is non-enclosed, wit htransporting from north to south in the surface and returning to north at the depth of about 500 m in winter, about 200 m in summer, with amplitudes of 10^5 m^3/s. It shows the transporting path of intermediate water of the SCS and offers an idealized reference for further study on dynamics of wind-driven and thermohaline circulation of the SCS.     

The wavelet analysis of satellite sea surface temperature in the South China Sea and the Pacific Ocean

Using wavelet transform, the sea surface temperature (SST) during the period of 1982–1999 of the South China Sea and the equatorial Pacific, from datasets of NOAA/AVHRR, was analyzed. It is shown that there are 4- and 8-year interannual oscillations in the eastern equatorial Pacific and 8-year interannual oscillation in the western equatorial Pacific. In terms of attractive time-frequency localization and multi-scale properties of wavelet transform, as shown by the Morlet wavelet, it is found that an in-phase coupling oscillation occurs between the SCS and the equatorial Pacific. The SST changes of SCS will have echoed every event of EI Niño (abnormally warm) and La Niño (abnormally cold) in the equatorial Pacific. There is a positive correlation between the SCS and the western equatorial Pacific in the 8-year time-scale. Evidence is presented that the SST anomalies of the equatorial Pacific influence the SST of the SCS.     

Large-scale low-frequency rainfall regimes and their transition modes in summertime over China

It is well known that the predictable period of daily weather is generally within 2-3 weeks, so spatio-temporal mean predictands have to be employed in the monthly, seasonal, and annual predictions. At present, rainfall patterns (RPs)[1-5] are the key for…     

Circulation in the South China Sea in summer of 1998

Based on the CTD and meteorological data obtained by R/VXiangyanghong No. 14 in the South China Sea (SCS) in the summer of 1998, both current velocity and volume transport are calculated by using a modified inverse model. Circulation in the SCS is analyzed by combining the calculated results with ADCP data. The following results are obtained, (i) The most important feature of the circulation in the northeastern SCS is that a branch of the Kuroshio intrudes into the SCS with a small volume transport. It flows anticyclonically through the Bashi Strait and towards the southwest off the Taiwan Island, and it does not intrude into the inner SCS. (ii) The northern SCS is dominated mainly by a cyclonic circulation system with two cold eddies, (iii) The central and southwestern SCSs are mainly occupied by anticyclonic circulation systems, including three anticyclonic and one stronger cyclonic eddies. (iv) In the southeastern SCS, there is a large scope of cyclonic circulation extending in the SW-NE direction, (v) There is a “multi-eddy phenomenon” in the SCS, and a larger eddy contains several small eddies. (vi) There is western intensifying of the currents and eddies in the SCS.     

Simulation of meridional overturning in the upper layer of the South China Sea with an idealized bottom topography

The South China Sea (SCS) is the largest marginal sea in the northwestern Pacific. Several important water gateways exist around the SCS, such as Luzon Strait, Taiwan Strait, etc., that lie in the northeastern SCS, and others like Kalimantan Strait, Balawan Strait, etc., lie in the south. Luzon Strait connects the SCS and Pacific via the Philippine Sea, and its water exchange has significant impact on the interior circulations of the SCS[1]. Since direct observation data about the …     

The onset and advance of the Asian summer monsoon

The transition from the winter monsoon to summer monsoon is characterized by the abrupt change of the atmospheric circulation. Although many studies on the intraseasonal variation of the Asian summer monsoon (ASM) have been made, there are controversial v…     

Seasonal features of the Sverdrup circulation in the South China Sea

Based on the Sverdrup relation, using climatological wind stress data, the basin scale Sverdrup transport in the South China Sea(SCS) is calculated and the basic seasonal features of the Sverdrup circulation are obtained. A comparison of these calculated features with observations proves that the wind-driven circulation in the SCS is very important for the formation of the SCS upper oceanic circulation in winter, summer and fall. It is shown that the non-uniform sea surface wind is one of the causes to form multi circulation centers in the basin of the SCS. The westward current at 18°N is caused by the local wind, which is stronger in fall and winter. The seasonal variation of circulation in the southern SCS is much more remarkable than that in the north. The wind in spring is helpful to the seasonal reversal of the circulation in the central SCS. The northward transport of the cyclonic circulation reaches the maximum in fall.     

Short-term dynamics of nutrients influenced by upwelling in a small oligotrophic coastal ecosystem, Gan Bay, in the northwest Philippines

We present a time-series analysis of nutrient and pCO2 (partial pressure of CO2) levels in an oligotrophic coastal ecosystem (Gan Bay), which was likely to be influenced by upwelled subsurface water. Gan Bay is off Currimao Harbor, in the northwest Philippines and is located at the boundary of the South China Sea (SCS). This 42-h time-series observation was conducted in December 2006. In addition to continuous observations of dissolved oxygen (DO) and pCO2, discrete samples were collected at a depth of 5 m every 3 h for measurements of nutrients, including soluble reactive phosphorus (SRP) and inorganic nitrogen (NO3 +NO2 ) in order to examine their dynamics and possible physical and biological controls. We observed remarkably large short-term variations in the surface water,spanning a 10-fold change for SRP (32–330 nM) and from <0.3 μM to 4.3 μM for NO3 +NO2 . DO also varied substantially from a lower end of 171 to 205 μM O2. Surface water pCO2 changed from an equilibrium stage with the atmosphere ( 386 μatm) to a stage where it was a significant source for the atmospheric CO2 (seawater pCO2 469 μatm). We found that the variation of nutrients was driven neither by tidal mixing nor by biological activities, as was suggested by the variations in the total bacterial abundance and chlorophyll a. Instead, our inverse T–S relationship suggested a two end-member mixing process during the observation period. The N:P ratio throughout the observation period was 13.2, which is characteristic of SCS subsurface and deep waters. Moreover, pCO2 was correlated inversely with the sea surface temperature. It is likely, therefore, that an upwelled subsurface cold water with high nutrients, low-temperature and high-pCO2 existed. It should be noted that this upwelled cold water did not appear to impact the entire observation period (approximately 35 h of 42 h), which might suggest an extremely dynamic nature for this upwelled cold water mass.     

Opening and evolution of the South China Sea constrained by studies on volcanic rocks: Preliminary results and a research design

The South China Sea (SCS) is characterized by abundant seamounts, which provide important information about the evolution of the SCS and related deep processes. Cenozoic volcanism in the SCS and its surroundings comprises three stages relative to the spreading of the SCS:prespreading (>32 Ma), syn-spreading (32-16 Ma), and post-spreading (<16 Ma). The pre-spreading magmatism predominantly occurs on the northern margin of the SCS and in South China coastal areas and shows a bi-modal affinity. The syn-spreading magmatic activity was very limited on the periphery of the SCS, but may be concentrated in the SCS. However, seafloor samples of this stage are not available yet because of overlying thick sedimentary deposits. Post-spreading magmatism is widespread in the central and southwest sub-basins of the SCS, Hainan Island, Leizhou Peninsula, Thailand, and Vietnam. These are mainly alkali basalts with subordinate tholeiites, and display OIB-type geochemical characteristics. The Dupal isotope anomaly and presence of high-magnesian olivine phenocrysts suggests their possible derivation from the Hainan mantle plume. The temporal and spatial distribution of Cenozoic volcanism in the SCS and its surroundings may be accounted for either by plate stress re-organization before and after SCS spreading, or by ridge suction of plume flow during opening of the SCS. If the latter is the case, the volcanic rocks within the SCS basin may not be typical mid-ocean ridge basalts (MORB). It remains puz-zling, however, that the transition between the South China continental margin and the SCS basin does not have features typical of a volcanic rifted margin. Clearly, the relationship between mantle plume and SCS opening needs further evaluation. A better un-derstanding of the link between deep processes and opening of the SCS not only requires enhanced studies on igneous petrogene-sis, but also is heavily dependent on systematic sampling of seafloor rocks.     

Evolution of the South China Sea and monsoon history revealed in deep-sea records

As the third summary report of ODP Leg 184 to the South China Sea (SCS), this paper discusses the evolution of the East Asian monsoon and the SCS basin. A multi-proxy approach, involving geochemistry, micropale-ontology, pollen and other analyses, was adopted for reconstructing the evolutionary history of the East Asian monsoon, which was characterized by a series of paleo-climate events especially at 8, 3.2, 2.2 and 0.4 Ma. The new record indicates similar stages in the development of the East and South Asian monsoons, with an enhanced winter monsoon over East Asia being the major difference. The rich spectrums of monsoon variability from the southern SCS also reveal other characteristic features of the low latitude ocean. Evidence for the evolution of the SCS includes the hemipelagic Oligocene sediments, implying the existence of deep water environments during the early seafloor spreading stage of the SCS basin. The four major unconformities and some remarkabl ediagenetic features in upper Oligocene deposits indicate the strongest tectonic events in the region. From a careful comparison of lithologies and sedimentation rates, we conclude that the prominent differences in sedimentary environments between the southern and northern SCS were established only by ～3 Ma.     

南海夏季风建立日期的预报研究

利用1958-2001年ECMWF和1948-2006年NCEP的全球再分析气象资料,研究南海夏季风爆发迟早与前期气象要素场之间的关系。结果表明,南海夏季风建立日期与当年2月份全球风场(u、v、ω分量)、位势高度场、气温场等气象要素场之间都存在显著相关区。把各要素场中的显著高相关区视为影响季风建立日期的关键区,并定义关键区内的要素平均值为建立日期的预报因子,从中选取10个与建立日期关系最为紧密的预报因子建立预报方程,对季风建立日期进行预测。分析发现,有10个预报因子与季风建立日期有密切联系,对季风爆发迟早有一定的指示能力和预测作用。最后,经过SAS系统的分析和试验选择最好的回归预测方案,利用10个预测因子建立南海夏季风建立日期回归预报方程(简称回归预报方程)。对回归预测方程进行回报实验发现,拟合得到的南海夏季风建立日期与许多学者确定的建立日期之间的相关系数都达到95%以上的置信度,拟合误差基本控制在2候以内,有一定的预报能力。可以说,该方法是作为预报南海夏季风爆发迟早的一种新的尝试。     

南海表层环流和热结构特征的数值模拟与影响因素分析

 采用普林斯顿海洋模式,在真实的地形数据、计算区域右边界上半部分设为开边界的条件下,对南中国海98°-126°E,3°S -26°N的范围进行了环流和温度结构的模拟。模拟从静止的海洋开始,以1月份的月平均温盐数据为初始场,在12个不同的月平均风场驱动下,模式稳定地模拟了4个模式年。从第3年开始进行数据分析。首先从数值模拟的角度给出了南海表层环流和热结构的时空演变过程,继而详细分析了气候和环境因素的影响。结果表明：冬季南海主要被一个大的气旋式环流占据,夏季主要呈大的反气旋式环流。春季和秋季是季风转换季节,南海环流在受到上一个季节影响的同时也向下一个季节的典型流态转换,并由多个涡旋组成。此外,气候和环境条件的设置,都会影响到南海的环流和热结构特征。     

Changes in sea surface temperature in western South China Sea over the past 450 ka

Sea surface temperature over the past 450 ka was obtained by the unsaturation of molecular fossil-long chain alkenone with a resolution of about 1 ka from the western South China Sea. This is the longest temperature profile in the South China Sea at such high resolution. The U37^κ-SST results revealed similar glacial-interglacial cycles as the δ^18O profile of planktonic foraminifera, with SST variability of 23-25.5℃ for glacial and 25-28℃ for interglacial periods. The highest SST （28.4℃） was recorded at MIS5.5 and lowest SST （22.6℃） during MIS2. The SST record preceded the planktonic foraminiferal δ^18O on five glacial-interglacial transitions. Comparison of temperature records from the Southern and Northern Hemispheres indicated a more Southern Hemisphere-like pattern for the temperature variation in the SCS. Strong precession and semiprecession signals in the spectra of our SST record manifest the tropical phenomena.     

白洋淀次表层水温分布研究

白洋淀是华北地区重要的淡水湖泊,对该地区生态环境的影响巨大.利用开发的数字化软件对白洋淀进行数字化,计算出了3-9月白洋淀内次表层水温的分布及其变化规律,从而为白洋淀的管理及相关学科的发展提供借鉴和参考.结果表明,白洋淀次表层水温变化与季节变化密切相关,3月份次表层开始出现温度差异,温差为3℃;4-7月,随着气温的回升,水温也逐渐上升;次表层水温的年最高值出现在8月,其值为14℃;9月份温度开始回落,温差也减小到2℃.     

Factors affecting the abundance and community structureof the phytoplankton in northern South China Sea in the summerof 2008： a biomarker study

Phytoplankton biomarkers were analyzedusing suspended particles collected from the northernSouth China Sea （SCS） during the summer cruise of2008, with the goal of understanding the algal communitystructure and biomass distribution pattern in the summerseason. The results indicated that the distribution of algalbiomarkers in surface water of SCS was impacted andconstrained by the local hydrological settings： the highbiological community generally appeared in Pearl Riverestuaries, southwestern off Taiwan island and southeast-em off Hainan Island, while the relatively lower biomasswas found in the deep basin and strait areas. Diatomswere the dominant phytoplankton species, which werefollowed by dinoflagellates. Coccolithophorid biomassgradually increased toward the open ocean. The presentwork indicated that the algal biomarkers effectively doc-umented the variability of the phytoplankton biomass andcommunity structure as well as their linkage with theoceanic dynamics in SCS during summer 2008. Thisresearch provided not only the foundation for the appli-cation of algal lipids in the modem ocean ecosystem, butalso the basis for the reconstruction of the past oceanicalgal community structures.