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

A fine-resolution MOM code is used to study the South China Sea basin-scale circulationand its relation to the mass transport through the Luzon Strait. The model domain includes the South China Sea, part of the East China Sea, and part of the Philippine Sea so that the currents in the vicinity of the Luzon Strait are free to evolve. In addition, all channels between the South China Sea and the Indonesian seas are closed so that the focus is on the Luzon Strait transport. The model is driven by specified Philippine Sea currents and by surface heat and salt flux conditions. For simplicity, no wind-stress is applied at the surface.The simulated Luzon Strait transport and the South China Sea circulation feature a sandwich vertical structure from the surface to the bottom. The Philippine Sea water is simulated to enter the South China Sea at the surface and in the deep ocean and is carried to the southern basin by western boundary currents. At the intermediate depth, the net Luzon Strait transport is out of t     

对吕宋海峡深水瀑布的重新认识

On the basis of the latest version of a U.S. Navy generalized digital environment model(GDEM-V3.0) and World Ocean Atlas(WOA13), the hydraulic theory is revisited and applied to the Luzon Strait, providing a fresh look at the deepwater overflow there. The result reveals that:(1) the persistent density difference between two sides of the Luzon Strait sustains an all year round deepwater overflow from the western Pacific to the South China Sea(SCS);(2) the seasonal variability of the deepwater overflow is influenced not only by changes in the density difference between two sides of the Luzon Strait, but also by changes in its upstream layer thickness;(3) the deepwater overflow in the Luzon Strait shows a weak semiannual variability;(4) the seasonal mean circulation pattern in the SCS deep basin does not synchronously respond to the seasonality of the deepwater overflow in the Luzon Strait.Moreover, the deepwater overflow reaches its seasonal maximum in December(based on GDEM-V3.0) or in fall(October–December, based on the WOA13), accompanied by the lowest temperature of the year on the Pacific side of the Luzon Strait. The seasonal variability of the deepwater overflow is consistent with the existing longest(3.5 a) continuous observation along the major deepwater passage of the Luzon Strait.     

吕宋海峡及南海北部海域的水团分析

根据1992年3月和1994年9月台湾海峡两岸科学家对南海北部两次协同调查的CTD资料以及由此计算的重力势资料,对吕宋海峡及南海北部400m以上海水的温盐性质进行分析。结果发现,调查海区基本可划分为两种水团,即黑潮水和南海水。黑潮水主要从吕宋海峡中部和北部进入南海,侵入的黑潮水向西北方向扩展,受到台湾海峡海底地形的阻挡而大部分集中于台湾西南海域,向西的范围基本不超过119°E。虽然两次观测所处的季节不同（分别为春初和夏末）,但黑潮入侵南海的差异并不明显。另外,在二次调查的部分层次上,南海北部陆坡边缘都发现有一团水平尺度约百公里的黑潮性质水。配合重力势的水平分布形式,可以用地转流场的结构解释水团分析的结果。     

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

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

Assessing the west ridge of Luzon Strait as an internal wave mediator

The Luzon Strait is blocked by two meridional ridges at depths, with the east ridge somewhat higher than the west ridge in the middle reaches of the Strait. Previous numerical models identified the Luzon Strait as the primary generation site of internal M₂ tides entering the northern South China Sea (Niwa and Hibiya, 2004), but the role of the west-versus-east ridge was uncertain. We used a hydrostatic model for the northern South China Sea and a nonhydrostatic, process-oriented model to evaluate how the west ridge of Luzon Strait modifies westward propagation of internal tides, internal bores and internal solitary waves. The dynamic role of the west ridge depends strongly on the characteristics of internal waves and is spatially inhomogeneous. For M₂ tides, both models identify the west ridge in the middle reaches of Luzon Strait as a dampener of incoming internal waves from the east ridge. In the northern Luzon Strait, the west ridge is quite imposing in height and becomes a secondary generation site for M₂ internal tides. If the incoming wave is an internal tide, previous models suggested that wave attenuation depends crucially on how supercritical the west ridge slope is. If the incoming wave is an internal bore or internal solitary wave, our investigation suggests a loss of sensitivity to the supercritical slope for internal tides, leaving ridge height as the dominant factor regulating the wave attenuation. Mechanisms responsible for the ridge-induced attenuation are discussed.     

吕宋海峡两侧中尺度涡统计

利用1993-2000年间的T/P卫星高度计轨道资料的时间序列和MODAS同化产品中的卫星高度计最优插值资料对南海东北部海区中尺度涡旋进行动态追踪。按照给定的标准从2种资料中提取了涡旋信息并对其特征量进行统计分析。结果表明,南海东北部海区中尺度涡旋十分活跃,平均每年6个,其中暖涡4个,尺度一般为200~250 km,平均地转流速为44 cm/s;冷涡每年平均2个,尺度一般为150~200 km,平均地转流速为-37 cm/s。吕宋海峡两侧涡旋的比较分析表明,南海东北部海区仍属于西北太平洋副热带海区的涡旋带,冷、暖涡旋处于不断的形成—西移—消散过程中。南海东北部中尺度冷涡大多是南海内部产生的,而暖涡与吕宋海峡外侧暖涡有一定的联系又具有相对的独立性。分析认为西北太平洋的西行暖涡在到达吕宋海峡时,受到黑潮东翼东向下倾的等密度面的抑制和岛链的阻碍,涡旋停滞于吕宋海峡外侧并逐渐消弱,被阻挡于吕宋海峡东侧涡旋释放的能量,形成一支横穿吕宋海峡(同时横穿过黑潮)的高速急流,把能量传递给吕宋海峡西侧的涡旋,使其得到强化,这是吕宋海峡两侧涡旋联系的一种重要机制。     

吕宋海峡西侧海域混合层盐度低频变化特征分析

利用WOD09和Argo剖面资料,结合SODA、OFES、GPCP和WHOI等资料分析了吕宋海峡西侧海域混合层盐度1984—2010年间的长期变化趋势和年代际变化特征,并利用混合层盐度平衡方程探讨了其变化机理。研究结果表明:(1)吕宋海峡西侧海域的盐度整体上呈现下降趋势,线性趋势为-0.020/a;(2)以1999年为界,吕宋海峡西侧海域的盐度在1999年之前为正距平,最大正距平值可达0.14,平均值为0.05,在1999年之后为负距平,最大负距平值为-0.22,平均值为-0.06;(3)影响该海域混合层盐度的长期变化趋势和年代际变化的主要因素为平流作用,特别是黑潮入侵的变化。     

吕宋海峡附近海域水团分布及季节变化特征

利用2003年2月至2009年4月期间在吕宋海峡附近海域由Argo剖面浮标观测的温、盐度资料,对该海域的水团分布及季节变化特征进行了探讨.结果表明,在120.5°-122.75°E、19°-23°N范围内,水团特征介于南海水和北太平洋水之间,而19°N以南区域的水交换并不显著.北太平洋热带水(NPTW)和北太平洋中层水(NPIW)通过吕宋海峡入侵南海的趋势在夏季较弱.秋季,NPTW入侵南海的趋势增强,而到了冬季,受东北季风控制,北太平洋水的入侵程度最强,然而并无NPIW进入南海的迹象.值得指出的是,整年没有发现明显的NPIW进入南海,而南海中层水可以通过海峡流入太平洋,其强度在秋、冬季节达到最大.     

吕宋海峡Ekman输运和南海SST的相关性分析

在利用1950—2009年NCEP(National Center for Environmental Prediction)资料分析风场数据的基础上,计算吕宋海峡的Ekman输运,研究表明其存在显著的季节变化,除了夏季外,其它季节均为由太平洋向南海输运。分析吕宋海峡Ekman输运和南海海盆表征上层热力状况的海表面温度SST(Sea Surface Temperature)之间的关系发现:在年内时间尺度上,两者不存在显著的同期相关,Ekman输运对SST的影响开始于一个月之后,从北部向南扩展,第二个月最为明显,并扩展至整个海盆,第三个月开始衰减,第四个月影响消失,且相关性为正;在年际尺度上,吕宋海峡Ek-man输运的异常同南海SSTA(Sea Surface Temperature Abnormal)的第二模态存在显著的相关联系,并且吕宋海峡Ekman输运和南海SSTA的相关关系在北部为正,南部为负。吕宋海峡Ekman输运调制南海大尺度环流,通过暖、冷平流的作用影响南海SST的变化。     

A numerical study of the summertime flow around the Luzon Strait

Luzon Strait, a wide channel between Taiwan and Luzon islands, connects the northern South China Sea and the Philippine Sea. The Kuroshio, South China Sea gyre, monsoon and local topography influence circulation in the Luzon Strait area. In addition, the fact that the South China Sea is a fairly isolated basin accounts for why its water property differs markedly from the Kuroshio water east of Luzon. This work applies a numerical model to examine the influence of the difference in the vertical stratification between the South China Sea and Kuroshio waters on the loop current of Kuroshio in the Luzon Strait during summer. According to model results, the loop current’s strength in the strait reduces as the strongly stratified South China Sea water is driven northward by the southwest winds. Numerical results also indicate that Kuroshio is separated by a nearly meridional ridge east of Luzon Strait. The two velocity core structures of Kuroshio can also be observed in eastern Taiwan. Moreover, the water flowing from the South China Sea contributes primarily to the near shore core of Kuroshio.     

ADCP观测得到的2008年4月吕宋海峡流速剖面结构

基于2008年4月22—26日吕宋海峡调查航次的下放式声学多普勒流速剖面仪(LADCP)和船载ADCP(SADCP)等观测资料,并采用潮波模式模拟结果去除潮流对观测资料的影响,观测结果表明:调查期间黑潮入侵南海的位置与1992年春季比较接近,其分支位于调查海区中部C2、C7、C8和C9站,表层黑潮在C8站分离为两支,分别流向C9和C2站,C9站北向流明显比C8站减弱。在C2站,黑潮分支位于400m层以浅,其最大西向流速为77cm/s,而在C7、C8和C9站黑潮分支位于500m层以浅,黑潮在入侵南海的过程中其核心深度逐渐变浅。上层黑潮明显作反气旋弯曲。本调查航次的观测结果在定性上支持吕宋海峡水交换有"三明治"垂直结构的特性。     

吕宋海峡附近中尺度涡特征的统计分析

采用1993年1月到2008年12月16a融合海面高度距平数据,追踪吕宋海峡附近海域(18°～23°N,116°～126°E)中尺度涡的移动轨迹,结果表明:时间分辨率为7d的卫星高度计资料难以观测到中尺度涡从西北太平洋通过吕宋海峡传进南海的过程,但对1994年吕宋海峡中部观测到的一个气旋涡及其附近中尺度涡的运动轨迹进行分析可见,西北太平洋海面高度变化会与吕宋海峡内部海面高度耦合后向南海传播。海面高度距平数据的时间-经度图表明,西北太平洋海面高度变化信号在西传至吕宋海峡附近(121°～122°E)时出现信号不连续。对21°N,116°～140°E断面的海面高度距平数据按周期分别为1～3月、3～6月、330～390d(年信号)进行分段带通滤波,发现不同周期的西北太平洋信号穿过吕宋海峡传入南海受到的阻隔作用、向西传播的速度以及它们所受的强迫机制均不同。     

四维变分资料同化在风暴潮模拟中的平流作用分析

利用基于三维斜压非线性的普林斯顿海洋模型(POM)及其切向线性伴随模式的四维变分数据同化(4DVAR)系统对一个近岸风暴潮个例进行理想同化试验和模拟,着重分析了四维变分数据同化增水改善中的二维平流作用。试验结果表明,同化后的水位结果明显优于同化前的结果,而且同化对预报的影响主要在前5h,尤其是积分到3—4h时同化和无同化的结果差异最大,在5h后同化的影响变得非常小。从平流作用中分析得知,无论是只同化水位还是同化水位和海表流速,同化的结果都增大了沿岸和向岸的平流。与无同化结果相比,同化试验的增水改善主要是来自向岸平流的增大,但只同化水位时由于水位增大使向岸压力梯度减小从而阻碍了向岸流的进一步增大;而同时同化水位和海表流速时,由于流速也被改善,故增水改善更明显和合理。     

变分资料同化中不同的变分求解方法

在应用变分资料同化方法时面临着两方面的难题:一是背景场误差协方差矩阵的求逆问题;二是与背景场误差协方差矩阵相关的计算与存储问题。为了解决这两方面的问题,不同的求解方法便被提出来了。对主要的变分求解方法,包括增量法、运用空间滤波算子的变分分析法、预处理化法、物理空间统计分析法、谱统计插值法等进行了系统的回顾,对它们的优缺点进行了分析与讨论,并指出了变分资料同化中各种求解方法的适用条件。     

Observations of near surface current at the Luzon Strait in winter

One hundred and ninety-one Argos satellite-tracked drifters deployed at the Luzon Strait in winter during 1991 to 2004 were ana- lyzed to understand the near surface current in northern South China Sea (SCS). Several major track patterns of these drifters have been found. There are: (1)shelf slope landing way (SLW) ; (2)deep basin way (DBW) ;(3) weak loop current pattern; (4) northward movement directly driven by the Kuroshio. These observations show the effects of the basin scale gyre circulation, mesoscale eddies and the Kuroshio on the drifters' ovement.     

Several characteristics of water exchange in the Luzon Strait

1IntroductionTheLuzonChannelissituatedonthewest-ernsideofthenorthernPacificandbetweenTaiwanandLuzonIslands.ItisthemainpassageofthePacificwaterenteringtheSCS.Therearenumerousdifferent-sizedislandsformingmanynarrowwaterpassagesinthischannel,sotheLuzonChannelisthegeneralnameofthesepas-sages(includingBabuyan,BalintangandBashiChannels,etc.).Customarily,theLuzonStraitiscalledtheBashiChannel.Itswidthis386kmandhasameandepthof1400m. Toagreatextent,thehydro-meteorologi-calconditionso…     

Vertical velocity and transport in the South China Sea

Deep water in the South China Sea is renewed by the cold and dense Luzon Strait overflow. However, from where and how the deep water upwells is poorly understood yet. Based on the Hybrid Coordinate Ocean Model reanalysis data, vertical velocity is derived to answer these questions. Domain-integrated vertical velocity is of two maxima, one in the shallow water and the other at depth, and separated by a layer of minimum at the bottom of the thermocline. Further analysis shows that this two-segmented vertical transport is attributed to the vertical compensation of subsurface water to the excessive outflow of shallow water and upward push of the dense Luzon Strait overflow, respectively. In the abyssal basin, the vertical transport increases upward from zero at the depth of 3 500–4 000 m and reaches a maximum of 1.5×106 m3/s at about 1 500 m. Deep water upwells mainly from the northeastern and southwestern ends of the abyssal basin and off the continental slopes. To explain the upward velocity arising from slope breaks, a possible mechanism is proposed that an onshore velocity component can be derived from the deep western boundary current above steep slopes under bottom friction.     

基于ROMS模式的南海SST与SSH四维变分同化研究

卫星遥感观测获得了大量高分辨率的海面实时信息,包括海面温度（SST）和海面高度（SSH）等,同化进入数值模式可有效提升模拟精度。本文基于ROMS模式与四维变分同化方法（4DVAR）,使用AVHRR SST和AVISO SSH数据,开展了南海区域同化实验。为检验同化的效果,分别利用HYCOM再分析资料和Argo温盐实测数据分析了同化结果的海面高度、流场及温盐剖面的精度。对比结果表明,SST和SSH的同化能够改善ROMS的模拟结果:同化后海面高度场能够更为准确地捕捉海洋的中尺度特征,与HYCOM海面高度再分析资料相比,平均绝对偏差和均方根误差分别为0.054 m和0.066 m;与HYCOM 10 m层流场相比,东向与北向流速平均绝对偏差分别为0.12 m/s和0.11 m/s,相比未同化均提升约0.01 m/s;温盐同化结果与Argo温盐实测具有较高的一致性,温度和盐度平均绝对偏差为0.45℃、0.077,均方根误差为0.91℃、0.11,单个的温盐廓线对比说明,同化结果与HYCOM再分析资料精度相当。     

A theoretical study of the multigrid three-dimensional variational data assimilation scheme using a simple bilinear interpolation algorithm

In order to solve the so-called "bull-eye" problem caused by using a simple bilinear interpolation as an observational mapping operator in the cost function in the multigrid three-dimensional variational(3DVAR) data assimilation scheme,a smoothing term,equivalent to a penalty term,is introduced into the cost function to serve as a means of troubleshooting.A theoretical analysis is first performed to figure out what on earth results in the issue of "bull-eye",and then the meaning of such smoothing term is elucidated and the uniqueness of solution of the multigrid 3DVAR with the smoothing term added is discussed through the theoretical deduction for one-dimensional(1D) case,and two idealized data assimilation experiments(one-and two-dimensional(2D) cases).By exploring the relationship between the smoothing term and the recursive filter theoretically and practically,it is revealed why satisfied analysis results can be achieved by using such proposed solution for the issue of the multigrid 3DVAR.     

吕宋海峡纬向海流及质量输送

分析和计算了吕宋海峡PR21断面最近海洋调查的部分CTD资料和ADCP资料,再一次证明吕宋海峡常年存在纬向流。但对于天气尺度而言,该流型是多变的。根据高分辨率的海洋环流数值模式4a(1992～1996年)海平面高度(SSH)的输出值,运用地转关系估计了吕宋海峡纬向流的月平均值。研究表明;通过海峡流入、流出南海纬向流的深度一般达到500m左右,200m以上流速较大,平均流速为50cm/s,最大时达80cm/s以上。500m以下的纬向地转流流速较小,通常小于10cm/s.由大洋进入海峡的入流位置位于海峡的中部和南部,月平均入流最大值出现在11月,为50cm/s.位于海峡的北部和南部上层海洋的月平均出流,最大流速亦出现在11月,也为50cm/s,这与秋季北赤道流分叉位置最北(15°N),春季分叉位置最南(14°N)有关。上层流入、流出海峡的流量的月平均值分别约为10×106m3/s和5×106m3/s.当东北季风盛行时(从10月到翌年2月),流入海峡的流量远大于流出海峡的流量,两者的差可达8×106m3/s,而在其他季节两者的差仅为3×106m3/s.这说明东北季风盛行时,会有较多的水从南海南？