ON THE SOURCES OF THE TAIWAN WARM CURRENT FROM THE SOUTH CHINA SEA

Historical hydrographic data across several sections in the South China Sea (SCS) and Taiwan Strait have been reconsidered. The year-around existence of the South China Sea Warm Current (SCSWC) along the shelf break off the Guangdong coast and the seemingly year-around southwestward current to the east of SCSWC are both evident in the data. The data also showed that the northward current in the Taiwan Strait seemed to be the extension of SCSWC.A barotropic numerical model is employed to explain some of the observed features. Reasonable simulation of the circulation in the northeast part of SCS has been found.     

NUMERICAL STUDY ON THE FORMATION OF THE SOUTH CHINA SEA WARM CURRENT Ⅰ.BAROTROPIC CASE

In this work, Princeton Ocean Model (POM) was used to study the formation of the South China Sea Warm Current (SCSWC) in the barotropic case. Monthly averaged wind stress and the inflow/outflow transports in January were used in the numerical simulation which reproduced the SCSWC. The effects of wind stress and inflow/outflow were studied separately. Numerical experiments showed thatthe Kuroshio intrusion through the Luzon Strait and the slope shelf in the northern SCS are necessary conditions for the founation of the SCSWC. In a flat bottom topography experiment, the wind stress drivennortheast current in the northern SCS is a compensatory current.     

NUMERICAL MODELLING OF THREE-DIMENSION CHARACTERISTICS OF WIND-DRIVEN CURRENT IN THE BOHAI SEA

Three- dimension (3-D) wind-driven currents in the Bohai Sea in both winter and summer are calculated by using a 3- D barotropic steady model, and the results are consistent with observed flow char -acteristics. Based on the results, 3- D characteristics of flow, currents at different depths, compensated flow in the lower layer , long and narrow alongshore current, the area of upwelling and downwelling, main circulation in vertical profile, and the current in Bohai Strait are discussed.     

A TWO-DIMENSIONAL VORTICITY MODEL OF INTERNAL TIDES GENERATED ON THE CONTINENTAL SHELF/SLOPE

A two-dimensional spectral-difference mode (with vorticity and density equations) of internal tides isdeveloped for studying the genration and propagration of internal tides generated at the continentalshelf/slope. In general, internal tides propagate seaward in deep sea regions and shoreward on the shelf,and are dissipated rapidly. When the Vaisala frequency decreases vertically, waves may be mostly limited to thecontinental slope region. in deep sea region, motions may have either boam-like structure or modal structure,depending on the stratification strerigth and structure, whereas a modal structure may always exist onthe shelf. Waves show strong bottom intensification on the slope when strong stratification exists on thebottom. The barotropic tidal advection may affed the temporal character of internal tides at thecontinental slope, shelf break and shelf regions. but may have little influence on the energy density and energy flux of internal tides. ln the case of strong stratification, waverforms of     

Effects of tidal currents on nonlinear internal solitary waves in the South China Sea

The propagation and fission process of internal solitary waves (ISWs) with amplitudes of about 170 m are simulated in the northeast of the South China Sea (NSCS) by using the generalized Korteweg-de Vries (KdV) equation under continuous stratification. More attention is paid to the effects of the ebb and flood background currents on the fission process of ISWs. This kind of background current is provided by the composed results simulated in terms of monthly mean baroclinic circulation and barotropic tidal current. It is found that the obtained relation of the number of fission solitons to the water depth and stratification is roughly in accordance with the fission law derived by Djordjevic and Redekopp in 1978; however, there exists obvious difference between the effects of the ebb and flood background currents on the wave-lengths of fission solitons (defined as the distance between two neighboring peaks of ISWs). The difference in nonlinearity coefficient α between the ebb and flood background currents is a main cause for the different wave-lengths of fission solitons.     

THE DIVERGENCE-CONVERGENCE EFFECT OF SURFACE CURRENT AND ITS ACTION ON THE BOHAI SEA

The action of the wind field and the influence of topography can cause divergence or convergence of surface current. The existence of the divergence-convergence effect is proved and the dynamical significance of the divergent or convergent state and its link with many marine phenomena are pointed out. Divergence fields of surface current in the Bohai Sea in winter and summer are obtained by numerical modelling describing the divergence-convergence character of seasonally wind-driven current. The relation between the effect and seasonal marine phenomena is discussed. Study on the divergence-convergence effect of surface current (DCESC)can be an indirect method for testing the calculated results.     

NUMERICAL STUDY ON THE INTERANNUAL OSCILLATION OF SEA SURFACE TEMPERATURE IN THE SOUTH CHINA SEA

A two and a half layer oceanic model of wind-driven, thermodynamical general circulation is appliedto study the interannual oscillation of sea surface temperature (SST) in the South China Sea (SCS). Themodel consists of two active layers: the upper mixed layer (UML) and the seasonal thermocline, with themotionless abyss beneath them. The governing equations which include momentum, continuity and sea.temperature for each active layer, can describe the physics of Boussinseq approximation, reduced gravityand equatorial β-plane. The formulas for the heat flux at the surface and at the interface between twoactive layers are designed on the Haney scheme. The entrainment and detrainment at the bottom of theUML induces vertical transport of mass,momentum and heat, and couples of dynamic andthermodynamic effect.Using leap-frog integrating scheme and the Arakawa-C grid the model is forced bya time-dependent wind anomaly stress pattern obtained from category analysis of COADS. The numerical results indicate that t     

THE BAROCLINIC RESIDUAL CIRCULATION IN SHALLOW SEAS I. THE HYDRODYNAMIC MODELS

There are two different modeling approaches which have been proposed and utilized to derive residual currents: the first approach is equivalent to deducing residual currents from current-meter records using filtering techniques or time averages of time-series records to remove tidal variations; in the second approach, filters or time averages over several tidal cycles are applied to the hydrodynamic equations to generate the governing equations for residual circulation. Based on the latter, both the baroclinic dynamic models of residual circulation are proposed, of which one is a two-dimensional transport model and the other is a three-dimensional model with variable eddy viscosity. The two-dimensional transport model is a direct generalization from the barotropic model of residual circulation presented by Nihoul and Randy (1975) and Heaps (1978) to the baroclinic model. In the three-dimensional model with variable eddy viscosity, using a Sturm-Liouville system adopted in the reference [5], the nondimen     

UPWELLING AND SEDIMENTATION DYNAMICS I. THE ROLE OF UPWELLING IN SEDIMENTATION IN THE HUANGHAI SEA AND EAST CHINA SEA-A DESCRIPTION OF GENERAL FEATURES

It is found that the mud distributions in the Huanghai Sea and East China Sea are extremely identical with the upwelling's in terms of locality and extent. The process of the mud formation is examined. It is concluded that vertical circulation i.e. upwelling plays a determinant role in the: process and furthermore, it is inferred that wherever upwelling occurs on continental shelf, mud should be formed and, conversely, wherever downwelling takes place, mud should not exist, for which coarse: sediment should substitute.     

A THEORETICAL SOLUTION FOR THE THERMOHALINE CIRCULATION IN THE SOUTHERN YELLOW SEA

Application of the thermocline equations in the thermocline areas and the boundary layer and the asymptotic matching techniques in each boundary in order to satisfy the surface and bottom conditions yielded a theoretical 2- D solution of the vertical thermohaline circulation of the Southern Yellow Sea in summer when the quasi-statically varying seasonal thermocline (density layer) is the background density structure , the deviations from which cause the secondary vertical circulation . The results show that the thermocline can be considered as an internal boundary or a barrier to the vertical heat advection so that in the central areas of the Southern Yellow Sea or the center of the Yellow Sea Cold Water Mass(YCWM)> the downwelling in the upper layer and upwelling in the lower or bottom layer form a double cell vertical circulation . The solution is similar to Hu's conceptual model ( 1986) in the central areas of the YCWM and is consistent with observed temperature . salinity and dissolved oxygen distri     

A numerical study of the South China Sea Warm Current during winter monsoon relaxation

Using a Finite-Volume Community Ocean Model, we investigated the dynamic mechanism of the South China Sea Warm Current(SCSWC) in the northern South China Sea(NSCS) during winter monsoon relaxation. The model reproduces the mean surface circulation of the NSCS during winter, while model-simulated subtidal currents generally capture its current pattern. The model shows that the current over the continental shelf is generally southwestward, under a strong winter monsoon condition, but a northeastward counter-wind current usually develops between 50-and 100-m isobaths, when the monsoon relaxes. Model experiments, focusing on the wind relaxation process, show that sea level is elevated in the northwestern South China Sea(SCS), related to the persistent northeasterly monsoon. Following wind relaxation, a high sea level band builds up along the mid-shelf, and a northeastward current develops, having an obvious vertical barotropic structure. Momentum balance analysis indicates that an along-shelf pressure gradient provides the initial driving force for the SCSWC during the first few days following wind relaxation. The SCSWC subsequently reaches a steady quasi-geostrophic balance in the cross-shelf direction, mainly linked to sea level adjustment over the shelf. Lagrangian particle tracking experiments show that both the southwestward coastal current and slope current contribute to the northeastward movement of the SCSWC during winter monsoon relaxation.     

On the sources of the Taiwan Warm Current from the South China Sea

Historical hydrographic data across several sections in the South China Sea (SCS) and Taiwan Strait have been reconsidered. The year-around existence of the South China Sea Warm Current (SCSWC) along the shelf break off the Guangdong coast and the seemingly year-around southwestward current to the east of SCSWC are both evident in the data. The data also showed that the northward current in the Taiwan Strait seemed to be the extension of SCSWC. A barotropic numerical model is employed to explain some of the observed features. Reasonable simulation of the circulation in the northeast part of SCS has been found.     

Effect of low-frequency Rossby wave on thermal structure of the upper southwestern tropical Indian Ocean

We investigate the influence of low-frequency Rossby waves on the thermal structure of the upper southwestern tropical Indian Ocean (SWTIO) using Argo profiles, satellite altimetric data, sea surface temperature, wind field data and the theory of linear vertical normal mode decomposition. Our results show that the SWTIO is generally dominated by the first baroclinic mode motion. As strong downwelling Rossby waves reach the SWTIO, the contribution of the second baroclinic mode motion in this region can be increased mainly because of the reduction in the vertical stratification of the upper layer above thermocline, and the enhancement in the vertical stratification of the lower layer under thermocline also contributes to it. The vertical displacement of each isothermal is enlarged and the thermal structure of the upper level is modulated, which is indicative of strong vertical mixing. However, the cold Rossby waves increase the vertical stratification of the upper level, restricting the variability related to the second baroclinic mode. On the other hand, during decaying phase of warm Rossby waves, Ekman upwelling and advection processes associated with the surface cyclonic wind circulation can restrain the downwelling processes, carrying the relatively colder water to the near-surface, which results in an out-of-phase phenomenon between sea surface temperature anomaly (SSTA) and sea surface height anomaly (SSHA) in the SWTIO.     

The baroclinic residual circulation in shallow seas

There are two different modeling approaches which have been proposed and utilized to derive residual currents: the first approach is equivalent to deducing residual currents from current-meter records using filtering techniques or time averages of time-series records to remove tidal variations; in the second approach, filters or time averages over several tidal cycles are applied to the hydrodynamic equations to generate the governing equations for residual circulation. Based on the latter, both the baroclinic dynamic models of residual circulation are proposed, of which one is a two-dimensional transport model and the other is a three-dimensional model with variable eddy viscosity. The two-dimensional transport model is a direct generalization from the barotropic model of residual circulation presented by Nihoul and Randy (1975) and Heaps (1978) to the baroclinic model. In the three-dimensional model with variable eddy viscosity, using a Sturm-Liouville system adopted in the reference [5], the nondimensional problem for residual circulation reduces to the nondimensional problem of the elevation and the expression of residual currents. It should be pointed out that both the baroclinic models developed in the present paper are confined to describe the Eulerian residual circulation only.     

Rotary spectrum analysis of tidal current in the southern Yellow Sea

A complete set of one-month Acoustic Doppler Profiler (ADP) current data at a station in the southern Yellow Sea (SYS) is analyzed using the rotary spectrum method. The results revealed different rotary properties between barotropic and baroclinic tidal currents. The barotropic and baroclinic tidal currents rotate elliptically counter-clockwise and clockwise, respectively. Meanwhile, baroclinic bottom tidal currents are almost along-isobath. The baroclinic cross-isobath velocities attenuate quickly at the bottom, implying important effects of bottom topography on the cross-isobath motions.     

A time-dependent baroclinic model on NEC bifurcation

As it is well-known, the North Equatorial Current (NEC) bifurcates into the Kuroshio flowing northward and the equatorward Mindanao Current, which is well depicted by Munk’s theory in 1950 in terms of its climatology. However, Munk’s theory is unable to tell the NEC bifurcation variability with time. In the present paper, a time-dependent baroclinic model forced by wind, in which temporal and baroclinic terms are added to Munk’s equation, is proposed to examine the seasonal variability of the NEC bifurcation latitude. An analytical solution is obtained, with which the seasonal variability can be well described: NEC bifurcation reaches its northernmost position in December and its southernmost position in June with a range of about 1° in latitude, consistent with previous results with observations. The present solution will degenerate to Munk’s one in the case of steady and barotropic state.     

Study on numerical simulation and dynamic mechanism of winter-time circulation in the eastern China seas

An MOM2 based 3-dimentional prognostic baroclinic Z-ordinate model was established to study the circulation in eastern China seas, considering the topography, inflow and outflow on the open boundary, wind stress, temperature and salinity exchange on the sea surface. The results were consistent with observation and showed that the Kuroshio intrudes in large scale into the East China Sea continental shelf East China, during which its water is exchanged ceaselessly with outer sea water along Ryukyu Island. The Tsushima Warm Current is derived from several sources, a branch of the Kuroshio, part of the Taiwan Warm Current, and Yellow Sea mixed water coming from the west of Cheju Island. The water from the west of Cheju Island contributes approximately 13% of the Isushima Warm Current total transport through the Korea Strait. The circulation in the Bohai Sea and Yellow Sea is basically cyclonic circulation, and is comprised of coastal currents and the Yellow Sea Warm Current. Besides simulation of the real circulation, numerical experiments were conducted to study the dynamic mechanism. The numerical experiments indicated that wind directly drives the East China Sea and Yellow Sea Coastal Currents, and strengthens the Korea Coastal Current and Yellow Sea Warm Current. In the no wind case, the kinetic energy of the coastal current area and main YSWC area is only 1% of that of the wind case.Numerical experiments also showed that the Tsushima Warm Current is of great importance to the formation of the Korea Coastal Current and Yellow Sea Warm Current.     

Numerical Simulation of Wintertime Mesoscale Eddies in the East China Sea

INTRODUCTIONAnimportantachievementofoceanographysincethe 1960swasthediscoveryofmesoscaleed dieswithspatialscaleofhundredsofmeters,andtimescaleofhours;andaverageflowvelocityofabout 10cm s.Theenormousenergyofthemesoscaleeddyiscomparabletothatofacycloneoran ticycloneintheatmosphere .Themesoscaleeddyisoneoftheimportantfactorsthatdecidethechangeoftheocean .Intherecentdecades,ChineseandforeignscientistshavedonelotsofworkontheEastChinaSeasmesoscaleeddies,theformationmechanismofwhicharethefocuso…