An extended variable-grid global ocean circulation model and its preliminary results of the equatorial Pacific circulation

To investigate the interaction between the tropical Pacific and China seas a variable-grid global ocean circulation model with fine grid covering the area from 20°S to 50°N and from 99° to 150°E is developed. Numerical computation of the annually cyclic circulation fields is performed. The results of the annual mean zonal currents and deep to abyssal western boundary currents in the equatorial Pacific Ocean are reported. The North Equatorial Current,the North Equatorial Countercurrent, the South Equatorial Current and the Equatorial Undercurrent are fairly well simulated. The model well reproduces the northward flowing abyssal western boundary current.From the model results a lower deep western boundary current east of the Bismarck-Solomon-New Hebrides Island chain at depths around 2 000 m has been found. The model results also show that the currents in the equatorial Pacific Ocean have multi-layer structures both in zonal currents and western boundary currents, indicating that the global ocean overturning thermohaline circulation appears of multi-layer pattern.

A STUDY ON THE BAROCLINIC STRUCTURE OF THE ABYSSAL CIRCULATION

In this paper, the linear continuously stratified model of the abyssal circulation proposed by Pedlosky (1992) was extended to include the second order term -(γθ zzz ) in the vertical turbulent mixing parameterization of - (w′θ′) z=k υθ zz -γθ zzz , in which k υ is a vertical diffusion coefficient, and γ is the second order coefficient of turbulent mixing (or simply called γ term and γ<0 is only allowed). The influence of the γ term on the baroclinic structure of the abyssal circulation driven by upwelling out of the abyss was investigated. It was found that the γ term has a noticeable influence on the baroclinic structure of the upwelling driven abyssal circulation. For uniform upwelling, it favors the baroclinic layering of the abyssal circulation in the eastern part of the basin, but prevents the layering in the west. In addition, this parameter was found to decrease the vertically averaging meridional velocity of the abyssal circulation from the west to the east on the southern boundary. For upwelling localized near the eastern boundary, the γ-term favors baroclinic layering of the abyssal circulation in the whole basin. Especially, on the southern boundary the γ-term could strengthen the vertically averaging meridional velocity in the west, but greatly weaken it in the east. The model presented here might be considered as an extension of the Pedlosky baroclinic model of the abyssal circulation.

NUMERICAL MODELLING OF THE QUASI-GLOBAL OCEAN CIRCULATION BASED ON POM

A free surface quasi-global ocean circulation model, Princeton Ocean Model(POM), was adopted to simulate the climatological circulation. The horizontal resolution of themodel was 1/2° x 1/2° with 16 vertical sigma layers. The initial temperature and salinity fieldsof the model were interpolated from the Levitus data, and the COADS (Comprehensive Ocean-AtmosphereData Set) monthly mean SST and wind fields were used as the surface forcing. The integral timelength is 6a. The main general circulation components such as the equatorial current, the equatorialundercurrent, the south and north equatorial currents, the Antarctic Circumpolar Current (ACC), theKuroshio and the Gulf Stream were well reconstructed. The volume transports of PN section and ACCa-gree well with the estimations on field survey. Up to now there is no global or quasi-globalcirculation model results u-sing POM in literature. Our results demonstrate that POM has soundability to simulate the coastal circulation as well as the general ocean circulation. And thisresult can provide open boundary conditions for fine resolution regional ocean circulation models.

Analysis of Yellow Sea circulation

Some features of the circulation in the YellowSea (hereafter referred to as YS) in the cold season (mid-November through mid-April) are analyzed emphatically by using the observations of several large-scale surveys in recent years together with historical data. Compared with the previous studies, a wealth of observed data have firstly been used in this study to further discuss the origin and path of the Yellow Sea Warm Current (YSWC). The analyses show that the YSWC is not a direct branch of the Tsushima Warm Current (TSWC), but derives from the mixed water region formed by TSWC water and the continental shelf water of the East China Sea (ECS). The path of the YSWC has certain seasonal and interannual variations. Then, the basic features of the Cheju Warm Current (CWC) discovered in recent years and its relation to the YSWC are expounded.The studies indicate that the CWC and the YSWC come basically from the same origin, and the main path of CWC is located roughly in water depths of greater than 80 m northwest of Chejudo. At last, the distribution features of the Korea Coastal Current (KCC) are delineated according to the new observations. The results show that the KCC moves toward south along the 40-50 m isobaths west of the Korean Peninsula, and its velocity increases from north to south.

LATITUDINAL VARIATION OF DEEP SCATTERING LAYER IN THE WESTERN PACIFIC

The acoustic Echo Intensity (EI) was recorded with 38k shipborne AcousticDoppler Current Profiler (AD-CP) in the Western Pacific in four cruises between Sept. 2001 and Oct.2002. The main Deep Scattering Layer (DSL) was observed at 400m-600 m depth in the four cruises. Thelatitudinal variation of the main DSL, which has high level of back-scatter strength (BS) at highlatitude, is prominent during both nighttime and daytime. The influences of environmental conditionson the DSL are discussed. Since high-oxygen water in the north is a friendly environment of marineanimals which form the main DSL, more animals are expected to aggregate in the 400dbars-600dbarslayer in the north. Dissolved Oxygen (DO) is the principal factor that causes the main DSL to varywith latitude, and its spatial distributions result from formation and transport of North PacificIntermediate Water (NPIW).

EI NIO PHENOMENON AND EXTENDED ASSOCIATE PATTERN ANALYSIS

To study the causes of El Nino Phenomenon the Extended Associate PatternAnalysis (EAPA) is set up with monthly sea surface wind stress and precipitation besides Sea LevelPressure (SLP) and Sea Surface Temperature (SST) in or over the Pacific and related seas since 1979in present paper. The results show that El Nino Phenomenon means a SST redistribution, coveringalmost the whole Pacific that needs a huge amount of energy provided by air-sea interaction in themost Pacific regions; It is the wester burst and wind convergence, coming from meddle latitudesdirectly, instead of Kelvin waves that produce the strong SST warm signal in Nino regions; Twonegative SLP anomalies in middle of the North and South Pacific, most likely produced by the averageair pressure decreasing effect of explosive cyclones through precipitation, and the positive SLPanomaly o-ver the Asia-Australia land bridge, coming mainly from the south hemisphere with a littleinfluence from the Asian continent, are the three major causes for El Nino onset, they work togetherto produce anomalous wind convergence in Nino regions and wester burst in middle of the North andSouth Pacific that result in a warm tongue and cold pincers gearing together; Basically, during itsearly stage the ocean is forced by the atmosphere, while during its late stage the atmosphere isforced by the ocean.

SEASONAL VARIABILITY OF THE INDONESIAN THROUGHFLOW FROM A VARIABLE-GRID GLOBAL OCEAN MODEL

The objective of this study is to model the mean and seasonal mass transportof the Pacific to Indian O-cean throughflow using variable-grid global Ocean General CirculationModel (OGCM) with fine grid (1°/6) covering the area from 20°S to 60°N and from 98°E to 156°E.The computations show that Indonesian Throughflow (ITF) mass transport, computed as a sum ofthrough-strait transport, has maximum transport in Sept. (17. 5Sv) and minimum transport in Jan. (9.5Sv). The annual mean ITF transport amounts to 14. 5Sv. Twenty-two percent of this transport passesthrough Lombok Strait. Sixty-five percent of this transport passes through Timor Passage.Semi-annual variability is apparent in Lombok and Ombai Straits while annual variability is apparentin Timor Passage.

NUMERICAL SIMULATION OF THE TIDAL MOTION AND SUSPENDED SEDIMENT TRANSPORT IN THE RADIAL SANDBANK AREA OF THE SOUTHERN YELLOW SEA

A coupled 2-D numerical model for hydrody-namic-sediment transport wasestablished and applied to simulate the tides, tidal currents and sediment movement in the submarineRadial Sandbank area of the southern Yellow Sea. With a high-resolution topography dataset used inthe model, the simulation reproduced a fine-structured current field and e-rosion-siltationdistribution. The modeled results show that, in the area of Radial Sandbanks, reversing tidalcurrent and seabed erosion occurs within troughs while tidal current with more rotary feature anddeposition occurs above sandbanks, which indicates the tidal-induced formation of the RadialSandbanks. During a tidal period, associating with the variation of current speed, erosionalternates with siltation. The seabed deformation depends on the relative strength of erosion andsiltation in a tidal period.

FISSION LAWS OF INITIALLY INTERFACE SOLITARY WAVES IN TWO-LAYER OCEAN

A 2-D KdV equation of two-layer stratified o-cean with 2-D topography isrecapitulated in the present paper. Based on a reduced version of this 2-D KdV equation, the fissionlaws of the initially internal solitary waves are studied by means of numerical calculation. Fromthe numerical results , it is shown that the initially interface solitary waves can fission on thecontinental slope like the initially surface solitary waves and the fission process is a significantgenerating mechanism of internal interface soliton packet on the continental shelf.

UNIFICATION AND APPLICATIONS OF MODERN OCEANIC/ATMOSPHERIC DATA ASSIMILATION ALGORITHMS

The key mathematics and applications of various modern atmospheric/oceanicdata assimilation methods including Optimal Interpolation (OI), 4-dimensional variational approach(4D-Var) and filters were systematically reviewed and classified. Based on the data assimilationphilosophy, i. e. , using model dynamics to extract the observational information, the commoncharacter of the problem, such as the probabilistic nature of the evolution of theatmospheric/oceanic system, noisy and irregularly spaced observations, and the advantages anddisadvantages of these data assimilation algorithms, were discussed. In the filtering framework, allmodern data assimilation algorithms were unified: OI/3D-Var is a stationary filter, 4D-Var is alinear (Kalman) filter and an ensemble of Kalman filters is able to construct a nonlinear filter.The nonlinear filter such as the Ensemble Kalman Filter (EN-KF), Ensemble Adjustment Kalman Filter(EAKF) and Ensemble Transformation Kalman Filter (ETKF) can, to some extent, account for thenon-Gaussian information of the prior distribution from the model. The flow-dependent covarianceestimated by an ensemble filter may be introduced to OI and 4D-Var to improve these traditionalalgorithms. In practice, the performance of algorithms may depend on the specific numerical modeland the choice of algorithm may depend on the specific problem. However, the unification ofalgorithms allows us to establish a unified test system to evaluate these algorithms, which providesmore insights into data assimilation philosophies and helps improve data assimilation techniques.

ANALYTIC METHOD FOR INVERSING THE OPEN BOUNDARY CONDITIONS IN A SEMI-CLOSED SEA AREA—CASE I FOR INVERSING THE TIDAL LEVELS AT OPEN BOUNDARY

In this paper, a computational case was employed to describe thecomputational procedure for inversing the tidal level open boundary conditions using an analyticmethod. The area for finding the solution is a circular area with a circular arc with the openingangle 60°-being the open boundary and the other circular arc being the solid wall boundary.Proceeding from the reestablished elliptic partial differential equation satisfied by the tidallevel function, the extended spectrum method was used to derive the general solution of the equationfor the sea of constant depth, and the impermeable solid wall condition (the second class boundarycondition) and the adequately specified open boundary conditions were then applied to determine theundetermined coefficients of the general solution, thus obtaining the tidal level distributionfunction. In this way, both the first and second class boundary values at the solid wall boundarywere obtained. With the above boundary values as the boundary conditions, the tidal level values atthe open boundary were then inversed by means of the general solution of tidal wave equation. Thevalidity of inversion method could be verified by comparing the inversed tidal level distributionswith the originally specified open boundary values.

A STUDY ON VARIABILITY OF SEA SURFACE TEMPERATURE IN TROPICAL PACIFIC, INDIAN OCEAN AND RELATED AIR CIRCULATION

Canonical Correlation Analysis (CCA) was a-dopted in the present paper tostudy the of Sea Surface Temperature (SST) in the tropical Pacific, Indian Ocean and related aircirculation. The results show that on the seasonal time scale, El Nino events can be divided intotwo types: the east one and the middle one. For the middle type the SST variations appear contrarilyin the tropical Pacific and Indian O-cean, and the anomalous SST decreases in the east butincreases in the northwest and south-middle of the tropical Indian O-cean, specially in the east ofMadagascar Island. And vice versa. On annual time scale, when the Asian continent high gets strongerand the deepened Aleutian low shifts southeastward, both of them trigger an onset of the El Ninoevents. Contrarily, the La Nina events take place. On decadal time scale, there are two basic modesof air-sea system over the tropical Pacific and Indian Ocean. Firstly, when the Asian continent highgets stronger and deepened Aleutian low shifts southeastward, the anomalous SST increases in themiddle and east of the proical Pacific, extending to the subtropical regions, and so in most of thetropical Indian Ocean, specially in the northeast of Madagascar Island and nearby. And vice versa.Secondly, when the Asian continent high gets stronger in the north and the Aleutian low decreasesfixedly or even disappears, the anomalous SST decreases slightly in middle of the tropical Pacificand the temperate northern Pacific but increases weakly in other regions, the anomalous SSTincreases in the south but decreases in the north of the tropical Indian O-cean, and the SSTincreases more obviously in southeast of Madagascar Island. And vice versa. The linear trends ofglob- al warming seems to play a certain role for the El Nino onsets.

DYNAMICS OF THE ZONAL ASYMMETRIC STRUCTURE OF ARCTIC OSCILLATION

The Arctic Oscillation (AO), though basically is a zonal symmetricphenomenon, also shows zonal asymmetric variation. The dynamics of this zonal variation isconsidered here from the point of view of the planetary propagation on the earth sphere. Based onthe linear barotropic vorticity e-quation, the planetary wave ray path is calculated in the winterJanuary. It shows that NAO( North Atlantic Oscillation), AO and North Pacific could be linkedthrough the planetary wave propagation, which acts as the atmospheric bridge. The zonal symmetricand asymmetric structures both have association with these Rossby wave activities.

CORRELATION BETWEEN ASO AND AAO

Using a 34 years database consisting of sea level pressure and sea iceconcentration, the relationship between Antarctic Oscillation (AAO) and Antarctic Sea-iceOscillation (ASO) was investigated. Firstly, Empirical Orthogonal Function (EOF) analysis depictsthe spatial variability of AAO and ASO and Antarctic Sea-ice Oscillation Index (ASOI) was definedfor the first time. Secondly, power spectrum and head-lag correlation analysis show that ASO and AAOhave the same periods of quasi-three years, quasi-one year and quasi-half a year. Corresponding toAAO, ASO has an evident anti-oscillation after one and half a year.

STUDY OF NON-BOUSSINESQ EFFCET ON SEA SURFACE HEIGHT

A set of equations was derived for a non-Boussinesq ocean model in thispaper. A new time-splitting scheme was introduced which incorporates the 4th-order Runge-Kuttaexplicit scheme of low-frequency mode and an implicit scheme of high-frequency mode. With thismodel, potential temperature, salinity fields and sea surface height were calculated simultaneouslysuch that the numerical error of extrapolation of density field from the current time level to thenext one could be reduced while using the equation of mass conservation to determine sea surfaceheight. The non-Bouss-inesq effect on the density field and sea surface height was estimated bynumerical experiments in the final part of this paper.

A UNIVERSAL IN TAILING WAVETRAIN GENERATION

A universal in tailing wave-train generation of forced soliton generationover topography is found theoretically as the flows are at the resonant points and it is examinedwith the numerical calculation of the corresponding fKdV e-quatioa From the comparisons, it is shownthat theoretical and numerical results on the invariance is in good agreement and the theory givenin this paper does not include the modulus truncation, any free constant and unknown function.

SEASONAL REVERSE OF SEA SURFACE SLOPE IN THE NORTHERN YELLOW SEA AND ITS DYNAMIC RELATION WITH MONSOON EFFECTS

Based on the monthly average sea level data from the tide gauge measurement(1999-2001), the seasonal variability of the sea level in the Northern and Middle Yellow Sea isstudied to reveal that the sea surface height at all the tide gauges becomes higher in summer thanthat in winter. In addition, the sea surface height of the Northern Yellow Sea is higher than theone of the Middle Yellow Sea with a slope downward from the north to the south in summer, while itis lower with a reversed slope in winter. The seasonal reverse of the sea surface slope can beattributed to the monsoon effects i. e. the annual reverse of the monsoon direction and the annualvariation of the monsoon rainfall. A set of equations are established in light of the dynamicprinciples to expound how the monsoon forcing and the sea surface slope generate a summer outflowand a winter inflow in the Yellow Sea.