南海等密度面环流季节变化的数值模拟研究

In this study, we develop a variable-grid global ocean general circulation model(OGCM) with a fine grid(1/6)°covering the area from 20°S–50°N and from 99°–150°E, and use the model to investigate the isopycnal surface circulation in the South China Sea(SCS). The simulated results show four layer structures in vertical: the surface and subsurface circulation of the SCS are characterized by the monsoon driven circulation, with basin-scaled cyclonic gyre in winter and anti-cyclonic gyre in summer. The intermediate layer circulation is opposite to the upper layer, showing anti-cyclonic gyre in winter but cyclonic gyre in summer. The circulation in the deep layer is much weaker in spring and summer, with the maximum velocity speed below 0.6 cm/s. In fall and winter, the SCS deep layer circulation shows strong east boundary current along the west coast of Philippine with the velocity speed at 1.5 m/s, which flows southward in fall and northward in winter. The results have also revealed a fourlayer vertical structure of water exchange through the Luzon Strait. The dynamics of the intermediate and deep circulation are attributed to the monsoon driving and the Luzon Strait transport forcing.     

Numerical investigation of the South China Sea deep circulation

Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea（SCS）, the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer（July）, a northeastsouthwest through-flow pattern across the deep basin without a western boundary current in winter（January）,and a transitional pattern in spring and autumn. The sensitivity ...     

The bimodality of the Luzon Strait deep water

Combined conductivity-temperature-depth(CTD) casts and Argo profiles, 3 086 historical hydrocasts were used to quantify the water column characteristics in the northern South China Sea(SCS) and its adjacent waters. Based on a two-dimensional "gravest empirical mode"(GEM), a gravitational potential(, a vertically integrated variable) was used as proxy for the vertical temperature profiles TG(p,). integrated from 8 MPa to the surface shows a close relationship with the temperature, except in the deep layer greater than 15 MPa, which was caused by the bimodal deep water in the region. The GEM temperature profiles successfully revealed the bimodality of the Luzon Strait deep water, that disparate hydrophic vertical profiles can produce distinct specific volume anomaly() in the SCS and the western Philippine Sea(WPS), but failed in the Luzon Strait, where different temperature profiles may produce a same. A significant temperature divergence between the SCS water and the WPS water confirmed that the bimodal structure is strong. The deepwater bifurcation starts at about 15 MPa, and gets stronger with increasing depth. As the only deep channel connecting the bimodal-structure waters, water column characteristics in the Luzon Strait is in between, but much closer to the SCS water because of its better connectivity with the SCS. A bimodal temperature structure below 15 MPa reveals that there was a persistent baroclinic pressure gradient driving flow through the Luzon Strait. A volume flux predicted through the Bashi Channel with the hydraulic theory yields a value of 5.62×106 m3/s using all available profiles upstream and downstream of the overflow region, and 4.03×106and 2.70×106 m3/s by exclusively using the profiles collected during spring and summer, respectively. No volume flux was calculated during autumn and winter because profiles are only available for the upstream of the Bashi Channel during the corresponding period.     

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.     

An anticyclonic eddy in the intermediate layer of the Luzon Strait in Autumn 2005

The complicated flow pattern in the intermediate layer of the Luzon Strait could directly affect the efficiency of the water and energy exchange between the South China Sea (SCS) and the North Pacific. Here we present a subsurface anticyclonic eddy in the Luzon Strait deduced using observations conducted in October 2005. On the basis of the hydrographic and current measurements, an anticyclonic eddy was found in the intermediate layer, i.e., about 26.8–27.3σ_θ, 500–900 m. It captures part of the SCS Intermediate Water outflow in the northern Luzon Strait, and carries it to flow southward and then westward back into the SCS in the southern Luzon Strait, with volume transport of about 1.9 × 10⁶ m³ s⁻¹. The simulated results from Hybrid Coordinate Ocean Model also suggest the existence of this anticyclonic eddy that develops and lingers for a month long.     

The destiny of the North Pacific Intermediate Water in the South China Sea

The previous studies show that the spreading path of the subtropical salinity minimum of the North Pacific Intermediate Water (NPIW) is southwestward pointing to the Luzon Strait. Based on the P -vector method and generalized digital environmental model (GDEM) data, the volume transport of NPIW through Luzon Strait and the upward transport on the NPIW lower and upper boundaries are calculated to examine the destiny of NPIW in the South China Sea (SCS). On the annual mean, the estimation of NPIW transport into the SCS through the Luzon Strait is 1.72 Sv (1Sv=10 6 m 3 /s). The upward transport over the SCS is 0.31 Sv on the NPIW upper boundary and 1.31 Sv on the NPIW lower boundary. There is no strait or passage deeper than the surface for the NPIW to extend, except for the Luzon Strait. For the volume balance in the SCS NPIW, the volume transport of 2.72 Sv has to flow out of the SCS NPIW layer through the Luzon Strait.     

1998年春夏南海温盐结构及其变化特征

利用1998年5～8月“南海季风试验”期间“科学1”号和“实验3”号科学考察船两个航次CTD资料,分析了1998年南海夏季风暴发前后南海主要断面的温盐结构及其变化特征.观测发现,南海腹地基本被典型的南海水团所控制,但在南海东北部尤其是吕宋海峡附近,表层和次表层水明显受到西太平洋水的影响.季风暴发以后,南海北部表面温度有显著升高,升幅由西向东递减,而南海中部和南部表面温度基本没变,这使得南海北部东西向温度梯度和整个海盆南北向温度梯度均减小.北部断面表层盐度普遍由34以上降低到34以下,混合层均有所发展,是季风暴发后降水和风力加剧的结果.观测期间黑潮水跨越吕宋海峡的迹象明显但变化剧烈.4～5月,黑潮次表层水除在吕宋海峡中北部出现外,在吕宋岛以西亦有发现,表明有部分黑潮水从吕宋海峡南端沿岸向西进而向南进入南海.6～7月,次表层高盐核在吕宋海峡中北部有极大发展,但在吕宋岛以西却明显萎缩;虽然看上去黑潮水以更强的流速进、出南海,但对南海腹地动力热力结构的影响未必更大.一个超过34.55的表层高盐水体于巴拉望附近被发现,似与通过巴拉望两侧水道入侵南海的西太平洋水有关.     

苏禄海的“通风”过程：历史数据分析

Based on historical observations, ventilation of the Sulu Sea （SS） is investigated and, its interbasin exchange is also partly discussed. The results suggest that near the surface the water renewal process not only occurs through the Mindoro Strait （MS） and the Sibutu Passage, but also depends on the inflows through the Surigao Strait and the Bohol Sea from the Pacific and through the Balabac Strait from the South China Sea （SCS）. Both inflows are likely persistent year round and their transports might not be negligible. Below the surface, the core layer of the Subtropical LowerWater （SLW） lies at about 200 m, which enters the SS through the Mindoro Strait not hampered by topography. Moreover, there is no indication of SLW inflow through the Sibutu Passage even though the channel is deep enough to allow its passage. The most significant ventilation process of the SS takes place in depths from 20a m to about 1200 m where intermediate convection driven by quasi-steady inflows through the Mindoro and Panay straits （MS-PS） dominates. Since the invaded water is drawn from the upper part of the North Pacific Intermediate Water （NPIW） of the SCS, it is normally not dense enough to sink to the bottom. Hence, the convective process generally can only reach some intermediate depths resulting in a layer of weak salinity minimum （about 34.45）. Below that layer, there is the Sulu Sea Deep Water （SSDW） homogeneously distributed from 1200 m down to the sea floor, of which the salinity is only a bit higher （about 34.46） above the minimum. Observational evidence shows that hydrographic conditions near the entrance of the MS in the SCS vary significantly from season to season, which make it possi- ble to provide the MS-PS overflowwith denser water of higher salinity sporadically. It is hence proposed that the SSDW is derived from intermittent deed convection resulted from DroDertv changes of the MS-PS inflow.     

Sub-seasonal variability of Luzon Strait Transport in a high resolution global model

The Luzon Strait is the main impact pathway of the Kuroshio on the circulation in South China Sea (SCS). Based on the analysis of the 1997–2007 altimeter data and 2005–2006 output data from a high resolution global HYCOM model, the total Luzon Strait Transport (LST) has remarkable subseasonal oscillations with a typical period of 90 to 120 days, and an average value of 1.9 Sv into SCS. Further spectrum analysis shows that the temporal variability of the LST at different depth is remarkable different. In the upper layer (0–300 m), westward inflow has significant seasonal and subseasonal variability. In the bottom layer (below 1 200 m), eastward outflow exhibits remarkable seasonal variability, while subseasonal variability is also clear. In the intermediate layer, the westward inflow is slightly bigger than the eastward outflow, and both of them have obvious seasonal and subseasonal variability. Because the seasonal variation of westward inflow and eastward outflow is opposite, the total transport of intermediate layer exhibits significant 50–150 days variation, without obvious seasonal signals. The westward Rossby waves with a period of 90 to 120 days in the Western Pacific have very clear correlationship with the Luzon Strait Transport, this indicates that the interaction between these westward Rossby waves and Kuroshio might be the possible mechanism of the subseasonal variation of the LST.     

2009-2010年冬季南海东北部中尺度过程观测

根据南海北部陆架陆坡海域2009-2010年冬季航次的CTD调查资料,发现西北太平洋水在上层通过吕宋海峡入侵南海,其对南海东北部上层水体温盐性质的影响自东向西呈减弱趋势,影响范围可达114°E附近。入侵过程中受东北部海域反 气旋式涡旋(观测期间,其中心位于20.75°N,118°E附近) 的影响,海水的垂向和水平结构发生了很大变化,特别是涡旋中心区域,上层暖水深厚,混合层和盐度极大值层显著深于周边海域。该暖涡在地转流场、航载ADCP观测海流及卫星高度计资料中均得到了证实。暖涡的存在还显著影响了海水化学要素的空间分布,暖涡引起的海水辐聚将上层溶解氧含量较高的水体向下输运,使次表层的暖涡中心呈现高溶解氧的分布特征。     

A Lagrangian study of the near-surface intrusion of Pacific water into the South China Sea

Satellite-tracked Lagrangian drifters are used to investigate the transport pathways of near-surface water around the Luzon Strait. Particular attention is paid to the intrusion of Pacific water into the South China Sea(SCS).Results from drifter observations suggest that except for the Kuroshio water, other Pacific water that carried by zonal jets, Ekman currents or eddies, can also intrude into the SCS. Motivated by this origin problem of the intrusion water, numerous simulated trajectories are constructed by altimeter-based velocities. Quantitative estimates from simulated trajectories suggest that the contribution of other Pacific water to the total intrusion flux in the Luzon Strait is approximately 13% on average, much smaller than that of Kuroshio water. Even so, over multiple years and many individual intrusion events, the contribution from other Pacific water is quite considerable. The interannual signal in the intrusion flux of these Pacific water might be closely related to variations in a wintertime westward current and eddy activities east of the Luzon Strait. We also found that Ekman drift could significantly contribute to the intrusion of Pacific water and could affect the spreading of intrusion water in the SCS. A case study of an eddy-related intrusion is presented to show the detailed processes of the intrusion of Pacific water and the eddy-Kuroshio interaction.     

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     

南海与西太平洋海水的交换:氧、氦同位素证据

研究了西太平洋海域(7°～26°N,122°～130°E)不同深度海水的氧、氦同位素组成和分布特征.结果表明,巴士海峡附近海域几个深度上δ18O等值线均向东弯曲,δ3He等值线也出现了类似的分布特征,可能反映了南海海水与黑潮水的混合作用.氧、氦同位素的研究结果为南海海水通过巴士海峡侵入了西太平洋提供了地球化学证据.     

关于南海北部上层水团的分类及三维分布的研究

Using the fuzzy cluster analysis and the temperature-salinity(T-S) similarity number analysis of cruise conductivity-temperature-depth(CTD) data in the upper layer(0–300 m) of the northern South China Sea(NSCS), we classify the upper layer water of the NSCS into six water masses: diluted water(D), surface water(SS),the SCS subsurface water mass(U_S), the Pacific Ocean subsurface water mass(U_P), surface-subsurface mixed water(SU) and subsurface-intermediate mixed water(UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of U_P, combining with the sea surface height data and geostrophic current field. The results show that water mass U_P exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the U_P intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the U_P is sporadically distributed in the study area. In autumn, the U_P is located in the upper 200 m layer east of Hainan Island.     

基于拉格朗日拟序结构的吕宋海峡表层水交换研究

黑潮通过吕宋海峡入侵南海呈现明显的瞬态特征。以往的研究通常将黑潮在吕宋海峡附近的流态分为几种不同类型。本文基于表层地转流计算得到的有限时间李雅普诺夫指数场（FTLE）,展示了拉格朗日视角下的吕宋海峡上层水交换特征。从FTLE场提取的拉格朗日拟序结构（LCSs）很好地识别了吕宋海峡附近的典型流态和旋涡活动。此外,这些LCSs还揭示了吕宋海峡周围复杂的输运路径和流体域,这些特征得到了卫星跟踪浮标轨迹的验证,且从流速场中是无法直接识别的。FTLE场显示,吕宋海峡附近表层水体的输运形态主要可分为四类。其中,黑潮直接向北流动的“跨越”形态和顺时针旋转的“流套”形态的发生频次明显高于直接进入南海的黑潮分支“渗入”形态和南海水流出至太平洋的“外流”形态。本文还进一步分析了黑潮在吕宋海峡处的涡旋脱落事件,突出强调了LCSs在评估涡旋输运方面的重要性。反气旋涡旋的脱落个例表明,这些涡旋主要源自黑潮“流套”,涡旋脱落之前可有效地俘获黑潮水。LCS所指示的输运通道信息有助于预测最终被反气旋涡所挟卷水体在上游的位置。而在气旋涡的形成过程中,LCS的分布特征表明,大部分气旋涡并未与黑潮水的输运路径相连通。因此,气旋涡对从太平洋到南海的上层水交换的贡献较小。     

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。     

2001年冬春转换期间南海东北部水文和环流特征

根据2001年3月份南海东北部航次调查温、盐资料,分析了2001年冬末春初南海东北部温、盐结构和环流的特征.分析结果表明:观测期间南海东北部环流主要受一次海盆尺度气旋型冷环流支配,冷环流呈现双核结构,垂向尺度接近1000 m.吕宋海峡内侧断面的水交换在600 m以浅海水流入南海,在断面南部(20°N以南)中层和深层有流出,断面法向地转流向西净输运量为6.9×106m3/s;直接的黑潮入侵不超过120.5°E,但有部分的黑潮水沿陆坡达到台湾岛西南部海域,并更有一部分逸入东沙岛以西海域,与南海水混合变性.     

LICOM模拟的南海贯穿流及其对南海上层热含量的影响

利用SODA(Simple Ocean Data Assimilation)数据、XBT(Expendable Bathythermograph)观测数据和绕岛环流理论(island rule)诊断计算结果评估了一个涡相容(eddy-permitting)全球海洋环流模式——LICOM对南海贯穿流及南海上层热含量的模拟能力,同时利用模式输出探讨了南海贯穿流对南海上层热含量的影响。NEC(North Equatorial Current)分叉的垂向结构、南海内区环流的季节和吕宋海峡体积输送的年际变化等分析结果都表明,LICOM能获取西北太平洋-印尼海域环流和南海贯穿流的合理模拟结果。模式模拟的南海上层热含量季节变化与观测及同化数据都表现出良好的一致性,尤其在南海内区。相关分析表明,吕宋海峡热输送主要控制着南海内区上层的热含量变化,两者呈显著负相关,这进一步证实了南海贯穿流作为一支冷平流调制着南海上层热含量变化的重要事实。     

Optimal estimation of absolute geostrophic velocity field in vicinity of the Luzon Strait using variational data assimilation technique

A P - vector method is optimized using the variational data assimilation technique(VDAT). The absolute geostrophic velocity fields in the vicinity of the Luzon Strait (LS) are calculated, the spatial structures and seasonal variations of the absolute geostrophic velocity field are investigated. Our results show that the Kuroshio enters the South China Sea (SCS) in the south and middle of the Luzon Strait and flows out in the north, so the Kuroshio makes a slight clockwise curve in the Luzon Strait, and the curve is strong in winter and weak in summer. During the winter, a westward current appears in the surface, and locates at the west of the Luzon Strait. It is the north part of a cyclonic gyre which exits in the northeast of the SCS; an anti-cyclonic gyre occurs on the intermediate level, and it exits in the northeast of the SCS, and an eastward current exits in the southeast of the anti-cyclonic gyre.     

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？