Intercomparison of three South China Sea circulation models

1IntroductionTheSouthChinaSeaisthelargesttropicalmarginaldeepsealocatingbetweenthewesternPacificOceanandtheeasternIndianOcean.AsapartofAsia-Australiamaritimecontinent,monsoonisaprimaryfactorforcingtheSouthChinaSeaCurrent(SCSC)variation.Drivenbynortheasterlymonsooninwinterandsouth-westerlymonsooninsummer,respectively,theSCSCbehavesacyclonicgyreandananticy-clonicgyre,correspondingly(Wyrtki,1961;Xuetal.,1982).Owingtotheshortageandexpen-sivenessofdirectobservationsintheSCS,fur-therunder…     

Numerical study on the summer circulation of the upper South China Sea and its establishment

A coupled single-layer/two-layer model is employed to study the South China Sea (SCS) upper circulation and its response before and after the onset of summer monsoon. It is found that, in summer, due to the β effect and the first baroclinic mode of the wind-driven current, a northward western boundary jet current is formed along the Indo-China Peninsula coast, and it leaves the coast at about 13° N and diffuses towards northeast; next to the Indo-China Peninsula, a large anticyclonic     

Calculation of circulation in the South China Sea during summer of 2000 by the modified inverse method

On the basis of hydrographic data obtained in August 2000 cruise, the circulation in the South China Sea (SCS) is computed by the modified inverse method in combination with SSH data from TOPEX/ERS-2 analysis. For study of the dynamical mechanism, which causes the pattern of summer circulation in the SCS, the diagnostic model (Yuan et al. 1982. Acta Oceanologica Sinica,4(1):1-11; Yuan and Su. 1992. Numerical Computation of Physical Oceanography.474-542) is used to simulate numerically the summer circulation in the SCS. The following results     

P矢量方法在南海夏季环流诊断计算中的应用

基于1998年6～7月南海调查航次的CTD资料,对南海环流采用最近发展的P矢量方法进行诊断计算.计算结果:黑潮向西入侵南海,然后做反气旋弯曲向东北方向流动,最终有通过巴士海峡流出南海的趋势.在南海北部存在一个气旋性环流,这个环流的强度和范围随深度增加而减小.该环流的冷中心位置随深度增加稍向南移.南海中部、越南以东海域存在一个明显的气旋涡和反气旋涡,尤其在200m及其以上水层均相当稳定,反气旋涡位于越南以东,其中心位置在11°53'N,111°50'E,气旋涡的中心位置在13°17'N,112°55'E,两者的尺度皆约为250km.吕宋岛西侧存在一个反气旋涡.在计算海区南部、巴拉望岛西南海域,100m以上层存在一个反气旋式涡.从各层流场分布均可以显示海流在西部强化的现象.     

Seasonal and interannual variations in the velocity field of the South China Sea

A three-dimensional numerical model is used to simulate sea level and velocity variations in the South China Sea for 1992–1995. The model is driven by daily wind and daily sea surface temperature fields derived from the NCEP/NCAR 40-year reanalysis project. The four-year model outputs are analyzed using time-domain Empirical Orthogonal Functions (EOF). Spatial and temporal variations of the first two modes from the simulation compare favorably with those derived from satellite altimetry. Mode 1, which is associated with a southern gyre, shows symmetric seasonal reversal. Mode 2, which contributes to a northern gyre, is responsible for the asymmetric seasonal and interannual variations. In winter, the southern and northern cyclonic gyres combine into a strong basin-wide cyclonic gyre. In summer, a cyclonic northern gyre and an anticyclonic southern gyre form a dipole with a jet leaving the coast of Vietnam. Interannual variations are particularly noticeable during El Niño. The winter gyre is generally weakened and confined to the southern basin, and the summer dipole structure does not form. Vertical motions weaken accordingly with the basin-wide circulation. Variations of the wind stress curl in the first two EOF modes coincide with those of the model-derived sea level and horizontal velocities. The mode 1 wind stress curl, significant in the southern basin, coincides with the reversal of the southern gyre. The mode 2 curl, large in the central basin, is responsible for the asymmetry in the winter and summer gyres. Lack of the mode 2 contribution during El Niño events weakens the circulation. The agreement indicates that changes in the wind stress curl contribute to the seasonal and interannual variations in the South China Sea.     

南海大尺度动力场年循环和年际变化

应用COADS风应力、Levitus温度资料，描述南海上层海洋动力场的年循环及其与热力场之间的关系和南海大尺度动力场的年际变化。针对冬、夏2个季节，分析Sverdrup环流场与上层海温之间的关系。研究发现，上层海温变化与上层海洋环流基本结构非常相似，即上层海温变化在一定程度上反映了南海Sverdrup平衡，而且随着浓度的增加，平均海温场与流函数场之间的对应关系更好。本文还着重分析了El Nino期间和La Nina期间的南海异常流函数场。研究发现，异常流函数场在El Nino期间的夏季主要是强化南海自身的环流结构，即强化南部反气旋式涡流（gvre)和强化北部气旋式涡流；冬季则削弱整个南海的气旋式流场。LaNina期间对夏季环流态的影响主要集中在南海北部，即削弱北部气旋式涡流，而对于南海南部的影响甚微；冬季则强化整个南海的气旋式流场。     

Sea surface height anomaly and geostrophic circulation variations in the South China Sea from TOPEX/POSEIDON altimetry

The sea surface height anomaly (SSHA) and geostrophic circulation in the South ChinaSea (SCS) are studied using TOPEX/POSE1DON (T/P) altimetry data. The SSHA, which is obtained after tidal correction based on the tidal results from T/P data, is predominated by seasonal alternating monsoons. The results reveal that the SSHA in the central part of the SCS is positive in spring and summer, but negative in autumn and winter. It is also found that the SSHA in the SCS can be approached with the sum of tidal constituents SA and SSA. The geostrophic circulations in the SCS are calculated according to sea surface dynamic topography, which is the sum of SSHA and mean sea surface height. It is suggested that the circulation in the upper layer of the SCS is generally cyclonic and notably western intensified during autumn and winter, while the western intensification is weak during spring and summer. It is also indicated that the Kuroshio intrudes into the northeastern SCS throuth the Luzon Strait in winter. But ther     

Recent progress in studies of the South China Sea circulation

The South China Sea (SCS) is a semi-enclosed marginal sea with deep a basin. The SCS is located at low latitudes, where the ocean circulations are driven principally by the Asia-Australia monsoon. Ocean circulation in the SCS is very complex and plays an important role in both the marine environment and climate variability. Due to the monsoon-mountain interactions the seasonal spatial pattern of the sea surface wind stress curl is very specific. These distinct patterns induce different basin-scale circulation and gyre in summer and winter, respectively. The intensified western boundary currents associated with the cyclonic and anticyclonic gyres in the SCS play important roles in the sea surface temperature variability of the basin. The mesoscale eddies in the SCS are rather active and their formation mechanisms have been described in recent studies. The water exchange through the Luzon Strait and other straits could give rise to the relation between the Pacific and the SCS. This paper reviews the research results mentioned above.     

南海冬、夏季环流的三维数值模拟

本文利用一个斜压三维陆架海模式——HAMSOM模式对12月份和8月份的南海环流进行数值模拟,结果为:对上层流场,在12月份,在西沙群岛－中沙群岛海区间呈现一个气旋式环流,在越南中部东岸存在一支南向西边界流,在金兰湾的远海为一局地反气旋涡,在南海南部,主要表现为万安滩的气旋式大弯曲(气旋涡)及在北康暗沙北侧的反气旋涡;在8月份,在东沙群岛－中沙群岛－吕宋岛西侧海域间存在一大尺度的气旋涡,在南海西部主要表现为以西沙群岛南部的气旋涡与金兰湾－礼乐滩间的反气旋式大环流相对峙的局面,同时在万安滩东侧有－气旋涡.由于斜压效应、底形效应的作用,使冬、夏季的南海南部中层流场几乎与上层流场相反.     

孟加拉湾上层环流研究综述

综述了孟加拉湾上层环流研究的主要成果并指出,研究海区环流与季风转换不完全同步。在西南季风期间,南、北海区各有一气旋式环流;在秋季季风过渡期间,出现海湾尺度的气旋式环流;在东北季风期间,气旋式环流减弱北移,南部则为一反气旋式环流控制;春季与秋季的情形相反,整个湾出现一海湾尺度的反气旋式环流。研究海区环流的变异主要受季风、赤道远地作用和浮力通量等复杂外源作用的影响。东印度沿岸流的季节变化与季风转换也不同步,局地风、内部Ekman抽吸、远地沿岸风及赤道远地作用的影响对沿岸流周年变化有重要作用。孟加拉湾上层环流年际变化显著,此年际变化主要受赤道风场的影响。     

Interannual variability of the Kuroshio intrusion in the South China Sea

The interannual variability of intrusions of the Kuroshio into the South China Sea (SCS) is investigated using satellite remote sensing data supported by in-situ measurements. The mesoscale circulation of the SCS is predominantly wind-forced by the northeast winter and southwest summer monsoons. Although the region has been studied extensively, considerable uncertainty remains about the annual and interannual mesoscale nature of the circulation. The frequency and characteristics of Kuroshio intrusions and their effect on circulation patterns in the northeast SCS are also not well understood. Satellite observations of Sea Surface Temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM) and the Advanced Very High Resolution Radiometer (AVHRR) and Sea Surface Height Anomalies (SSHA) from TOPEX/ Poseidon for the period 1997–2005 are used here to analyze the annual and interannual variability in Kuroshio intrusions and their effects on the region. Analysis of SST and SSHA shows the formation and characteristics of intrusions vary considerably each year. Typically, the intrusion occurs in the central region of Luzon Strait and results in an anticyclonic circulation in the northeastern SCS. However, in some years, the intrusion is located in the northern portion of Luzon Strait and a cyclonic intrusion results. Wind stress and wind stress curl derived from the National Aeronautics and Space Administration (NASA) QuikSCAT satellite scatterometer are used to evaluate the relationship between wind stress or wind stress curl and the presence of winter Kuroshio intrusions into the SCS.     

1998年夏季季风爆发前后南海环流的多涡特征

利用南海季风实验（SCSMEX－IOP1、IOP2）期间（1998年4月底－7月初）所获得的温盐深（CTD）、声学多普勒流速剖面仪（ADCP）资料及TOPEX／POSEIDON卫星高度计遥感资料，分析了南海表层、1．0MPa层和3．0MPa层得力势异常场的分布格局，探讨了夏季季风爆发前后南海的环流特征。结果表明：在夏季季风爆发前（IOP1期间）南海北部以气旋试流动为主，并在此气旋式环流的东部镶嵌着一个较小的反气旋型涡；南海中部和南部以反气旋式流动为主，其中越南以东海域存在着两个南北对峙分布的反气旋型涡，在它们的东侧伴随一气旋型涡。季风爆发后（IPO2期间），南海北部仍然以气旋式流动为主，黑潮水越过巴士海峡南北中线，一部分可能入侵南海北部，另一部分向东北折回黑潮主干；南海中部和南部仍以反气旋式流动为主，越南以东海域北部的反气旋型涡消失，但南西的反气旋型涡加强，与IOP1类似，仍伴随着一个气旋型涡。总体而方，强流区出现在巴士海峡西北侧和南海西部（尤其是越东南东沿岸），南海东部和东南部为弱流区。     

南黄海及东海北部夏季若干水文特征和环流的分析

利用“中韩黄海水循环动力学合作研究”1 997年 7月航次和“黄海综合环境调查”1 998年 8月航次观测所得的CTD资料 ,对南黄海及东海北部夏季的水文特征和特大洪水年长江冲淡水扩展特征进行探讨。同时还根据夏季所施放的卫星跟踪漂流浮标的轨迹、底层人工水母的漂移路径及等密面深度的分布对夏季环流作了阐述 ,提出夏季南黄海环流并非单一的气旋式系统 ,其内部还存在着气旋、反气旋的多个较小的环流 ;东海北部交替出现气旋、反气旋涡旋。     

南海的季节环流─TOPEX/POSEIDON卫星测高应用研究

应用1992～1996年的TOPEX/POSEIDON卫星高度计遥感资料,研究了冬、夏季风强盛期多年平均的南海上层环流结构。研究结果表明,南海上层流结构呈明显的季节变化,在很大程度上受该海区冬、夏交替的季风支配。冬季总环流呈气旋型,并发育有两个次海盆尺度气旋型环流;夏季总环流大致呈反气旋型、但在南海东部18°N以南海域未见明显流系发育。研究还表明,南海环流的西向强化趋势明显,无论冬、夏在中南半岛沿岸和巽他陆架外缘均存在急流,其流向冬、夏相反,是南海上层环流中最强劲的一支。鉴于该海流的动力特征与海洋动力学中定义的漂流不同,有相当大的地转成分,建议称为“南海季风急流(South China Sea MonsoonJet)”.冬季南下的季风急流在南海南部受巽他陆架阻挡折向东北,沿加里曼丹岛和巴拉望岛外海有较强东北向流发育。夏季北上的季风急流在海南岛东南分为两支:北支沿陆架北上,似为传统意义上的南海暖流;南支沿18°N向东横穿南海后折向东北;二者之间(陆架坡折附近)为弱流区。两分支在汕头外海汇合后,南海暖流流速增强。就多年平均而言,黑潮只在冬季侵入南海东北部,并在南海北部诱生一个次海盆尺度的气旋型环流,这时南海暖流只出现在汕头以东海域.夏季南海北部完全受东北向流控制,未见黑潮入侵迹象.用卫星跟踪海面漂流浮标观测进行的对比验证表明,以上遥感分析结果与海上观测一致。     

The role of atmospheric circulation in spatial and temporal variations in the structure of currents in the western South China Sea

Based on numerical simulations, we calculate the integral water circulation of the South China Sea on the eastern Vietnam shelf in the Vietnam coastal current area. The main objective of simulations was to study the hydrodynamic structures of this current in the winter–summer interseasonal period. The calculations were performed for the period from April to June 1999, which had the necessary primary field data. Two types of atmospheric processes were considered: the first is characterized by a small pressure gradient over the South China Sea and the second includes tropical cyclones in the southern part of the sea. The simulation results showed that there are three hydrodynamic gyres in the study area during the given time period: two anticyclonic gyres and a cyclonic gyre that separates them, which together form a complex pattern of the Vietnam current. These gyres persist for the given types of atmospheric processes and are quasi-stationary structures. The Vietnam current carries coastal water masses from south to north within the anticyclonic gyres in summer and from north to south within the cyclonic gyres in winter.     

Seismic ambient noise around the South China Sea: seasonal and spatial variations, and implications for its climate and surface circulation

With its strong seasonal variation in wave climate and various bathymetric features due to the complex tectonics, the South China Sea (SCS) provides a natural laboratory to study the microseism. We collected data from seismic stations around the SCS and calculated their noise spectra, through which seasonal and spatial variations of microseism, as well as the general feature of seismic ambient noise in this marginal sea were revealed. Microseism seasonal variations in general reflect influences of the East Asian monsoon in winter and the Indian monsoon in summer, respectively. The two microseism components, the single frequency microseism (SFM) and the double frequency microseism (DFM), show striking alternating variation patterns both seasonally and spatially. These variation patterns, along with the bathymetric feature near the stations, indicate SFM and DFM are generated through different physical mechanisms. More interestingly, seasonal and spatial variations of DFM appear to be consistent with the basin-scale surface circulation model of the SCS, in which the upper SCS experiences cyclonic in winter and anti-cyclonic in summer. These consistencies provide observational evidence for the hypothesis that the cyclonic depression is a favorable condition to generate DFM.     

南海环流动力机制研究综述

南海的环流复杂,但通过近20 a来的研究工作,国内外学者对此已取得了不少的成果.本文就南海环流框架性的问题,综述了有关的文献,认为对南海上层海洋三方面的环流分量的驱动机制已有了初步的认识.这三方面分别是:(1)准季节性风场;(2)黑潮向南海的净输运;(3)黑潮向南海的涡度平流输送.但是对这些驱动的时空变化仍相当不清楚.三者皆增强了南海北部的海盆尺度气旋式环流,其强化的西南向西边界流靠近东沙群岛,建议称为“东沙海流”.没有水文证据显示黑潮水是以分支形式进入南海,其向南海的输运也不可能主要通过中尺度涡过程,具体机制有待研究.每年在南海生成的中尺度涡平均约有10个,风场与沿岸地形所生成的强风应力旋度可能是其主要的驱动机制.作为框架性的认识,也有三方面的工作进行得较少,即:(1)吕宋海峡的上层水交换;(2)南海的中尺度涡生成机制,虽然强风应力旋度及前述的第三种环流驱动机制也有中尺度涡伴生;(3)自吕宋海峡进入的深层水对南海上层海洋环流的影响.     

1998年冬季南海环流的三维结构

利用1998年11月28日至12月27日南海的调查资料,采用三维海流诊断模式,计算了冬季南海三维海流,所得结果如下:(1)冬季南海环流系统方面:1)南海北部,在吕宋西北海域分别存在一个气旋式、反气旋式涡.2)南海中部,在越南近岸存在较强的、南向的西边界射流.其以东海域出现较强的气旋式环流.南海中部东侧海域存在一个较弱的反气旋式环流.3)南海南部,一般流速较弱.在112°E以西受反气旋式环流所控制,加里曼丹岛西北海域存在气旋性环流.由于受调查海域所限,这两个环流只部分出现.(2)上述环流系统与200 m层水平温度、密度分布对应较好.(3)南海冬季环流垂向速度分布方面:1)表层,南海北部,在吕宋西北为范围较大的上升流海区.而在东沙群岛附近海域出现了下降流.海南岛以南及东南海域也存在下降流.南海中部,越南以东海域出现范围较大的下降流,其以东为上升流海域,而在巴拉望岛西北海域又出现下降流.南海南部,基本上被上升流海域所控制.2)次表层与表层不同,例如在次表层,海南岛东南部海域出现上升流.中层和深层垂向速度分布与次表层相似.(4)关于南海垂向速度分量分布的动力原因:在表层,风应力旋度场起着主要作用;在次表层,β效应与斜压场相互作用是重要的动力因子,而风应力旋度场和β效应与正压场相互作用也有一定影响;在南海中部等区域的中层以及在南海的深层,主要受B效应与斜压场相互作用和B效应与正压场相互作用的共同作用.     

1998年夏季季风爆发前后南海上层环流的诊断分析

基于1998年南海季风试验（SCSMEX，South China Sea Monsoon Experiment)期间2个强化观测航次（4－5月及6－7）所获温盐深（CTD）资料，利用一个改进逆模式研究了夏季季风爆发前后南海环流的演变特征。诊断计算表明，在此期间南海环流主要表现为两脊两槽型，即越南以东和菲律宾以西呈反气旋式环流，南海北部和南海中部呈气旋式环流。但对局部区域而言，可以发现在季风爆发前后其环流结构有明显的差异。上述计算结果亦与等压面上海水密度分布的定性分析结果及同期观测的ADCP资料进行了比较，结果表明模式计算所得到的南海上层环流主要特征与定性分析结果及实测资料大体一致，诊断结果可作为南海上层季风环流演变机制研究的依据。     

A Review on the Currents in the South China Sea: Seasonal Circulation, South China Sea Warm Current and Kuroshio Intrusion

Researches on the currents in the South China Sea (SCS) and the interaction between the SCS and its adjacent seas are reviewed. Overall seasonal circulation in the SCS is cyclonic in winter and anticyclonic in summer with a few stable eddies. The seasonal circulation is mostly driven by monsoon winds, and is related to water exchange between the SCS and the East China Sea through the Taiwan Strait, and between the SCS and the Kuroshio through the Luzon Strait. Seasonal characteristics of the South China Sea Warm Current in the northern SCS and the Kuroshio intrusion to the SCS are summarized in terms of the interaction between the SCS and its adjacent seas.