Fundamental Framework and Experiments of the ThirdGeneration of IAP/LASG World OceanGeneral Circulation Model

A new generation of the IAP / LASG world ocean general circulation model is designed and presented based on the previous 20-layer model, with enhanced spatial resolutions and improved parameterizations. The model uses a triangular-truncated spectral horizontal grid system with its zonal wave number of 63 (T63) to match its atmospheric counterpart of a T63 spectral atmosphere general circulation model in a planned coupled ocean-atmosphere system. There are 30 layers in vertical direction, of which 20 layers are located above 1000 m for better depicting the permanent thermocline. As previous ocean models developed in IAP / LASG, a free surface (rather than “rigid-lid” approximation) is included in this model. Compared with the 20-layer model, some more detailed physical parameterizations are considered, including the along / cross isopycnal mixing scheme adapted from the Gent-MacWilliams scheme. The model is spun up from a motionless state. Initial conditions for temperature and salinity are taken from the three-dimensional distributions of Levitus’ annual mean observation. A preliminary analysis of the first 1000-year integration of a control experiment shows some encouraging improvements compared with the twenty-layer model, particularly in the simulations of permanent thermocline, thermohaline circulation, meridional heat transport, etc. resulted mainly from using the isopycnal mixing scheme. However, the use of isopycnal mixing scheme does not significantly improve the simulated equatorial thermocline. A series of numerical experiments show that the most important contribution to the improvement of equatorial thermocline and the associated equatorial under current comes from reducing horizontal viscosity in the equatorial regions. It is found that reducing the horizontal viscosity in the equatorial Atlantic Ocean may slightly weaken the overturning rate of North Atlantic Deep Water.

A Nine-layer Atmospheric General Circulation Model and Its Performance

Different versions of a new nine-layer general circulation model which is rhomboidally truncated at zonal wavenumber 15(L9R15)are introduced in this paper.On using the observed global monthly sea surfaCe temperature(SST)and sea ice(SI)data from 1979 to 1988 offered by the internahonal Atmospheric Model Iute-comparison Program(AMIP),these different model versions were integrated for the ten-year AMIP period. Results show that the model iscapable of simulating the basic states of the atmosphere and its interannual variability,and in performing reasonablesensitivity experiments.

An Eddy-Permitting Oceanic General Circulation Model and Its Preliminary Evaluation

An eddy-permitting, quasi-global oceanic general circulation model, LICOM (LASG/IAP (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics) Climate System Ocean Model), with a uniform grid of 0.5? × 0.5? is established. Forced by wind stresses from Hellerman and Rosenstain (1983), a 40-yr integration is conducted with sea surface temperature and salinity being restored to the Levitus 94 datasets. The evaluation of the annual mean climatology of the LICOM control run shows that the large-scale circulation can be well reproduced. A comparison between the LICOM control run and a parallel integration of L30T63, which has the same framework but a coarse resolution, is also made to con?rm the impact of resolution on the model performance. On account of the reduction of horizontal viscosity with the enhancement of the horizontal resolution, LICOM improves the simulation with respect to not only the intensity of the large scale circulations, but also the magnitude and structure of the Equatorial Undercurrent and South Equatorial Current. Taking advantage of the ?ne grid size, the pathway of the Indonesian Through?ow (ITF) is better represented in LICOM than in L30T63. The transport of ITF in LICOM is more convergent in the upper layer. As a consequence, the Indian Ocean tends to get warmer in LICOM. The poleward heat transports for both the global and individual basins are also signi?cantly improved in LICOM. A decomposed analysis indicates that the transport due to the barotropic gyre, which primarily stands for the barotropic e?ect of the western boundary currents, plays a crucial role in making the di?erence.

Dynamical Framework of IAP Nine-Level Atmospheric General Circulation Model

The dynamical framework of the nine-level version of the IAP AGCM is presented in this paper. The emphasis of the model's description is put on the following two aspects:(1) A model's standard atmosphere, which is a satisfactory approximation to the observed troposphere and lower stratosphere standard atmosphere, is introduced into the equations of the model to permit a more accurate calculation of the vertical transport terms, especially near the tropopause; (2) The vertical levels of the model are carefully selected to guarantee a smooth dependence of layer thickness upon pressure in order to reduce the truncation error involved in the unequal interval vertical finite-differencing. For testing the model, two kinds of linear baroclinic Rossby-Haurwitz waves, one of which has a dynamically stable vertical structure and the other has a relatively unstable one, are constructed to provide initial conditions for numerical experiments. The two waves have been integrated for more than 300 days and 100 days respectively by using the model and both of them are propagating westward with almost identical phase-speed during the time period of the integrations. No obvious change of the wave patterns is found at the levels in the model's troposphere. The amplitudes of both two waves at the uppermost level, however, exhibit rather significant oscillation with time, of which the periods are exactly 20 days and 25 days espectively.The explanation of this interesting phenomena is still under investigation.

A Numerical World Ocean General Circulation Model

This paper describes a numerical model of the world ocean based on the fully primitive equations. A 'Standard' ocean state is introduced into the equations of the model and the perturbed thermodynamic variables are used in the modlc's calculations. Both a free upper surface and a bottom topography are included in the model and a sigma coordinate is used to normalize the model's vertical component. The model has four unevenly-spaced layers and 4 × 5 horizontal resolution based on C-grid system. The finite-difference scheme of the model is designed to conserve the gross available energy in order to avoid fictitious energy generation or decay.The model has been tested in response to the annual mean surface wind stress, sea level air pressure and sea level air temperature as a preliminary step to its further improvement and its coupling with a global atmospheric general circulation model. Some of results, including currents, temperature and sea surface elevation simulated by the mode! arc presented.

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 Sulfate Optical Properties and Direct Radiative Forcing Using LASG-IAP General Circulation Model

The direct radiative forcing （DRF） of sulfate aerosols depends highly on the atmospheric sulfate loading and the meteorology, both of which undergo strong regional and seasonal variations. Because the optical properties of sulfate aerosols are also sensitive to atmospheric relative humidity, in this study we first examine the scheme for optical properties that considers hydroscopic growth. Next, we investigate the seasonal and regional distributions of sulfate DRF using the sulfate loading simulated from NCAR CAM-Chem together with the meteorology modeled from a spectral atmospheric general circulation model （AGCM） developed by LASG-IAP. The global annual-mean sulfate loading of 3.44 mg m-2 is calculated to yield the DRF of -1.03 and -0.57 W m-2 for clear-sky and all-sky conditions, respectively. However, much larger values occur on regional bases. For example, the maximum all-sky sulfate DRF over Europe, East Asia, and North America can be up to -4.0 W m-2. The strongest all-sky sulfate DRF occurs in the Northern Hemispheric July, with a hemispheric average of -1.26 W m-2. The study results also indicate that the regional DRF are strongly affected by cloud and relative humidity, which vary considerably among the regions during different seasons. This certainly raises the issue that the biases in model-sinmlated regional meteorology can introduce biases into the sulfate DRF. Hence, the model processes associated with atmospheric humidity and cloud physics should be modified in great depth to improve the simulations of the LASG-IAP AGCM and to reduce the uncertainty of sulfate direct effects on global and regional climate in these simulations.

Primary Reasoning behind the Double ITCZ Phenomenon in a Coupled Ocean-Atmosphere General Circulation Model

This paper investigates the processes behind the double ITCZ phenomenon, a common problem in Coupled ocean-atmosphere General Circulation Models (CGCMs), using a CGCM—FGCM-0 (Flexible General Circulation Model, version 0). The double ITCZ mode develops rapidly during the ?rst two years of the integration and becomes a perennial phenomenon afterwards in the model. By way of Singular Value Decomposition (SVD) for SST, sea surface pressure, and sea surface wind, some air-sea interactions are analyzed. These interactions prompt the anomalous signals that appear at the beginning of the coupling to develop rapidly. There are two possible reasons, proved by sensitivity experiments: (1) the overestimated east-west gradient of SST in the equatorial Paci?c in the ocean spin-up process, and (2) the underestimated amount of low-level stratus over the Peruvian coast in CCM3 (the Community Climate Model, Version Three). The overestimated east-west gradient of SST brings the anomalous equatorial easterly. The anomalous easterly, a?ected by the Coriolis force in the Southern Hemisphere, turns into an anomalous westerly in a broad area south of the equator and is enhanced by atmospheric anomalous circulation due to the underestimated amount of low-level stratus over the Peruvian coast simulated by CCM3. The anomalous westerly leads to anomalous warm advection that makes the SST warm in the southeast Paci?c. The double ITCZ phenomenon in the CGCM is a result of a series of nonlocal and nonlinear adjustment processes in the coupled system, which can be traced to the uncoupled models, oceanic component, and atmospheric component. The zonal gradient of the equatorial SST is too large in the ocean component and the amount of low-level stratus over the Peruvian coast is too low in the atmosphere component.

The influence of explicit tidal forcing in a climate ocean circulation model

The eight main tidal constituents have been implemented in the global ocean general circulation model with approximate 1° horizontal resolution.Compared with the observation data,the patterns of the tidal amplitudes and phases had been simulated fairly well.The responses of mean circulation,temperature and salinity are further investigated in the global sense.When implementing the tidal forcing,wind-driven circulations are reduced,especially those in coastal regions.It is also found that the upper cell transport of the Atlantic meridional overturning circulation（AMOC） reduces significantly,while its deep cell transport is slightly enhanced from 9×106m3/s to 10×106 m3/s.The changes of circulations are all related to the increase of a bottom friction and a vertical viscosity due to the tidal forcing.The temperature and salinity of the model are also significantly affected by the tidal forcing through the enhanced bottom friction,mixing and the changes in mean circulation.The largest changes occur in the coastal regions,where the water is cooled and freshened.In the open ocean,the changes are divided into three layers：cooled and freshened on the surface and below 3 000 m,and warmed and salted in the middle in the open ocean.In the upper two layers,the changes are mainly caused by the enhanced mixing,as warm and salty water sinks and cold and fresh water rises;whereas in the deep layer,the enhancement of the deep overturning circulation accounts for the cold and fresh changes in the deep ocean.

Assimilation of High Frequency Radar Data into a Shelf Sea Circulation Model

High Frequency(HF) radar current data is assimilated into a shelf sea circulation model based on optimal interpolation(OI) method. The purpose of this work is to develop a real-time computationally highly efficient assimilation method to improve the forecast of shelf current. Since the true state of the ocean is not known, the specification of background error covariance is arduous. Usually, it is assumed or calculated from an ensemble of model states and is kept in constant. In our method, the spatial covariances of model forecast errors are derived from differences between the adjacent model forecast fields, which serve as the forecast tendencies. The assumption behind this is that forecast errors can resemble forecast tendencies, since variances are large when fields change quickly and small when fields change slowly. The implementation of HF radar data assimilation is found to yield good information for analyses. After assimilation, the root-mean-square error of model decreases significantly. Besides, three assimilation runs with variational observation density are implemented. The comparison of them indicates that the pattern described by observations is much more important than the amount of observations. It is more useful to expand the scope of observations than to increase the spatial interval. From our tests, the spatial interval of observation can be 5 times bigger than that of model grid.

A two-time-level split-explicit ocean circulation model (MASNUM) and its validation

A two-time-level, three-dimensional numerical ocean circulation model（named MASNUM） was established with a two-level, single-step Eulerian forward-backward time-differencing scheme. A mathematical model of large-scale oceanic motions was based on the terrain-following coordinated, Boussinesq, Reynolds-averaged primitive equations of ocean dynamics. A simple but very practical Eulerian forward-backward method was adopted to replace the most preferred leapfrog scheme as the time-differencing method for both barotropic and baroclinic modes. The forward-backward method is of second-order of accuracy, computationally efficient by requiring only one function evaluation per time step, and free of the computational mode inherent in the three-level schemes. This method is superior to the leapfrog scheme in that the maximum time step of stability is twice as large as that of the leapfrog scheme in staggered meshes thus the computational efficiency could be doubled. A spatial smoothing method was introduced to control the nonlinear instability in the numerical integration. An ideal numerical experiment simulating the propagation of the equatorial Rossby soliton was performed to test the amplitude and phase error of this new model. The performance of this circulation model was further verified with a regional（northwest Pacific） and a quasi-global（global ocean simulation with the Arctic Ocean excluded） simulation experiments. These two numerical experiments show fairly good agreement with the observations. The maximum time step of stability in these two experiments were also investigated and compared between this model and that model which adopts the leapfrog scheme.

A parameterization scheme of vertical mixing due to inertial internal wave breaking in the ocean general circulation model

Based on the theoretical spectral model of inertial internal wave breaking （fine structure） proposed previ- ously, in which the effects of the horizontal Coriolis frequency component f-tilde on a potential isopycnal are taken into account, a parameterization scheme of vertical mixing in the stably stratified interior be- low the surface mixed layer in the ocean general circulation model （OGCM） is put forward preliminarily in this paper. Besides turbulence, the impact of sub-mesoscale oceanic processes （including inertial internal wave breaking product） on oceanic interior mixing is emphasized. We suggest that adding the inertial inter- hal wave breaking mixing scheme （F-scheme for short） put forward in this paper to the turbulence mixing scheme of Canuto et al. （T-scheme for short） in the OGCM, except the region from 15°S to 15°N. The numeri- cal results ofF-scheme by usingWOA09 data and an OGCM （LICOM, LASG/IAP climate system ocean model） over the global ocean are given. A notable improvement in the simulation of salinity and temperature over the global ocean is attained by using T-scheme adding F-scheme, especially in the mid- and high-latitude regions in the simulation of the intermediate water and deep water. We conjecture that the inertial internal wave breaking mixing and inertial forcing of wind might be one of important mechanisms maintaining the ventilation process. The modeling strength of the Atlantic meridional overturning circulation （AMOC） by using T-scheme adding F-scheme may be more reasonable than that by using T-scheme alone, though the physical processes need to be further studied, and the overflow parameterization needs to be incorporated. A shortcoming in F-scheme is that in this paper the error of simulated salinity and temperature by using T-scheme adding F-scheme is larger than that by using T-scheme alone in the subsurface layer.

Seasonal variability of the isopycnal surface circulation in the South China Sea derived from a variable-grid global ocean circulation model

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.

The African Climate as Predicted by the IAP Grid-Point Nine-Layer Atmospheric General Circulation Model (IAP-9L-AGCM)

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EXPERIMENTS OF A REDUCED GRID IN LASG/IAP WORLD OCEAN GENERAL CIRCULATION MODELS (OGCMs)

Due to the decrease in grid size associated with the convergence of meridians toward the poles inspherical coordinates, the time steps in many global climate models with finite-difference method are restrictedto be unpleasantly small. To overcome the problem, a reduced grid is introduced to LASG/IAP world oceangeneral circulation models. The reduced grid is implemented successfully in the coarser resolutions versionmodel L30T63 at first. Then, it is carried out in the improved version model LICOM with finer resolutions. Inthe experiment with model L30T63, under time step unchanged though, execution time per single model run isshortened significantly owing to the decrease of grid number and filtering execution in high latitudes. Resultsfrom additional experiments with L30T63 show that the time step of integration can be quadrupled at most inreduced grid with refinement ratio 3. In the experiment with model LICOM and with the model’s original timestep unchanged, the model covered area is extended to the whole globe from its original case with the grid pointof North Pole considered as an isolated island and the results of experiment are shown to be acceptable.

Development of a High-Resolution Coastal Circulation Model for the Ocean Observatory in Lunenburg Bay

An advanced ocean observatory has been established in Lunenburg Bay of Nova Scotia, Canada as part of an interdisciplinary research project of marine environmental prediction. The development of a high-resolution coastal circulation model is one of important components of the observatory. The model horizontal resolution is 60 m and the vertical resolution is about 1 m. The coastal circulation model is used to simulate the semi-diurnal tidal circulation and associated nonlinear dynamics with the M2 forcing specified at the model open boundaries. The model is also used to simulate the storm-induced circulation in the bay during Hurricane Juan in September 2003, with the model forcing to be the combination of tides and remotely generated waves specified at the model open boundaries and wind stress applied at the sea surface. The model results demonstrate strong interactions between the local wind stress, tidal forcing, and remotely generated waves during this period. Comparison of model results with the surface elevation and current observations demonstrates that the coastal circulation model has reasonable skills in simulating the tidal and storm-induced circulation in the bay.

Simulating Tropical Instability Waves in the Equatorial Eastern Pacific with a Coupled General Circulation Model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20-40-day periodicity with westward phase speeds of 0.4-0.6 m s^-1 and wavelengths of 1000-2000 km during boreal summer and fall. They are generally called tropical instability waves （TIWs）. This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model （AOGCM）. The horizontal resolution of the model is 120 km in the atmosphere, and 30 km longitude by 20 km latitude in the ocean. Model simulations show good agreement with the observed main features associated with TIWs. The results of energetics analysis reveal that barotropic energy conversion is responsible for providing the main energy source for TIWs by extracting energy from the meridional shear of the climatological-mean equatorial currents in the mixed layer. This deeper and northward-extended wave activity appears to gain its energy through baroclinic conversion via buoyancy work, which further contributes to the asymmetric distribution of TIWs. It is estimated that the strong cooling effect induced by equatorial upwelling is partially （-30%-40%） offset by the equatorward heat flux due to TIWs in the eastern tropical Pacific during the seasons when TIWs are active. The atmospheric mixed layer just above the sea surface responds to the waves with enhanced or reduced vertical mixing. Furthermore, the changes in turbulent mixing feed back to sea surface evaporation, favoring the westward propagation of TIWs. The atmosphere to the south of the Equator also responds to TIWs in a similar way, although TIWs are much weaker south of the Equator.

Symmetry Reductions of a 1.5-Layer Ocean Circulation Model

The （2＋1）-dimensional nonlinear 1.5-layer ocean circulation model without external wind stress forcing is analyzed by using the classical Lie group approach. Some Lie point symmetries and their corresponding two-dimensional reduction equations are obtained.

A 3-dimensional baroclinic circulation model of the South China Sea

The summer and winter circulations in the South China Sea (SCS) including the surface elevation and water temperature are simulated using the model described by Cai and Li (1996) with the monthly mean wind stress and air temperature field at the 1000 mb level from the European Centre for Medium-Range Weather Forecasts as inputs. The boundary conditions at Bashi Channel and Taiwan Strait are taken from the simulation results of the Kuroshio using the same numerical model with a grid size of 0.5°×0.5° and the results of Cai and Li (1996) as boundary conditions. The computational domain for the present paper is between 100°E and 123°E and between 4.5°N and 27°N. The horizontal resolution is 0.25°×0.25° and the vertical variations of the velocity components are resolved by 6 layers The computed steady flow, temperature and elevation fields are consistent with the corresponding fields observed. In particular, the temperature and elevation fields of the South China Sea Warm Current (SCSWC) have been successfully simulated. The paths of the branch of the Kuroshio entering the South China Sea (SCSBK) through Bashi Channel in winter and summer are discussed It is found that the SCSBK flows southward to the southern SCS from the coast of the Guangdong Province. A portion of the SCSBK returns to the Bashi Channel and subdivides again in deep waters in winter with a branch flows to the south along the coast of the Philippines instead of flowing back to the Pacific In addition, our results confirm the existence of a eastward current to the northeast of Dongsha in summer with the Kuroshio as its source as suggested by Huang et al. Since the value of the eddy viscosity adopted for the simulation of the Kuroshio is on the high side, resulting in a weaker west boundary current in the western Pacific as the boundary conditions for the present simulations, some deviations from the actual situations are expected although the results are in general consistent with observations.

A three-dimensional ocean general circulation model for mesoscale eddies——Ⅱ Diagnostic analysis

A three-dimensional density field associated with mesoscaie unstable waves generated by the 3-D, primitive-equation model (Wang and Ikeda, 1996) is provided to the quasi-geostrophic pressure tendency and ω-equations, and to the (ageostrophic) Q-vector equation. Diagnostic analyses, analogous to the approaches in meteorology: ω-equation and Q-vector method, are for the first time developed to examine the mesoscaie dynamical processes and mechanisms of the unstable waves propagating in the mid-latitude ocean. The weaknesses and strengths of these two diagnostic approaches are evaluated and compared to the model results. The Q-vector method is then recommended to diagnose the vertical motion associated with the mesoscaie dynamics from a hydrographic CTD (conductivity-temperature-depth) array, while the quasi-geostrophic equations produce some small-scale features (errors) in the diagnosed fields.