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.

Numerical simulations of Atlantic meridional overturning circulation(AMOC)from OMIP experiments and its sensitivity to surface forcing

Atlantic meridional overturning circulation(AMOC)plays an important role in transporting heat meridionally in the Earth’s climate system and is also a key metrical tool to verify oceanic general circulation models.Two OMIP(Ocean Model Intercomparison Project phase 1 and 2)simulations with LICOM3(version 3 of the LASG/IAP Climate System Ocean Model)developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics(LASG),Institute of Atmospheric Physics(IAP),are compared in this study.Both simulations well reproduce the fundamental characteristics of the AMOC,but the OMIP1 simulation shows a significantly stronger AMOC than the OMIP2 simulation.Because the LICOM3 configurations are identical between these two experiments,any differences between them must be attributed to the surface forcing data.Further analysis suggests that sea surface salinity(SSS)differences should be mainly responsible for the enhanced AMOC in the OMIP1 simulation,but sea surface temperature(SST)also play an unignorable role in modulating AMOC.In the North Atlantic,where deep convection occurs,the SSS in OMIP1 is more saline than that in OMIP1.We find that in the major region of deep convection,the change of SSS has more significant effect on density than the change of SST.As a result,the SSS was more saline than that in OMIP2,leading to stronger deep convection and subsequently intensify the AMOC.We conduct a series of numerical experiments with LICOM3,and the results confirmed that the changes in SSS have more significant effect on the strength of AMOC than the changes in SST.

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 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.

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.

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.

Tide simulation in a global eddy-resolving ocean model

The tide plays a pivotal role in the ocean,affecting the global ocean circulation and supplying the bulk of the energy for the global meridional overturning circulation.To further investigate internal tides and their impacts on circulation,it is imperative to incorporate tidal forcing into the eddy-resolving global ocean circulation model.In this study,we successfully incorporated explicit tides(eight major constituents)into a global eddy-resolving general ocean circulation model and evaluated its tidal simulation ability.We obtained harmonic constants by analyzing sea surface height through tidal harmonic analysis and compared them with the analysis data Topex Poseidon Cross-Overs v9(TPXO9),the open ocean tide dataset from 102 open-ocean tide observations,and tide gauge stations from World Ocean Circulation Experiment.The results demonstrated that the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics(LASG/IAP)Climate System Ocean Model 3.0(LICOM3.0)effectively simulated tides,with errors predominantly occurring in nearshore regions.The tidal amplitude simulated in LICOM3.0 was greater than that of TPXO9,and these high-amplitude areas exhibited greater errors.The amplitude error of the M_(2) constituent was larger,while the phase error of the K_(1) constituent was more significant.Furthermore,we further compared our results with those from other models.

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.

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.

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.

Meso-scale eddy in the South China Sea simulated by an eddy-resolving ocean model

Mesoscale eddies （MEs） in the South China Sea （SCS） simulated by a quasi-global eddy-resolving ocean general circulation model are evaluated against satellite data during 1993-2007. The modeled ocean data show more activity than shown by the satellite data and reproduces more eddies in the SCS. A total of 345 （428） cyclonic eddies （CEs） and 330 （371） anti-cyclonic eddies （AEs） generated for satellite （model） data are identified during the study period, showing increase of -24% and -12% for the model data, respectively. Compared with eddies in satellite, the simulated eddies tend to have smaller radii, larger amplitudes, a slightly longer lifetime, faster movement and rotation speed, a slightly larger nonlinear properties （U/c） in the model. However, the spatial distribution of generated eddies appears to be inhomogeneous, with more CEs in the northern part of SCS and fewer AEs in the southern part. This is attributed to the exaggerated Kuroshio intrusion in the model because the small islands in the Luzon Strait are still not well resolved although the horizontal resolution reaches （1/10）°. The seasonal variability in the number and the amplitude of eddies generated is also investigated.

Estimates of Anthropogenic CO_2 Uptake in a Global Ocean Model

A global ocean general circulation model （L30T63） is employed to study the uptake and distribution of anthropogenic CO2 in the ocean. A subgrid-scale mixing scheme called GM90 is used in the model. There are two main GM90 parameters including isopycnal diffusivity and skew （thickness） diffusivity. Sensitivities of the ocean circulation and the redistribution of dissolved anthropogenic CO2 to these two parameters are examined. Two runs estimate the global oceanic anthropogenic CO2 uptake to be 1.64 and 1.73 Pg C yr^-1 for the 1990s, and that the global ocean contained 86.8 and 92.7 Pg C of anthropogenic CO2 at the end of 1994, respectively. Both the total inventory and uptake from our model are smaller than the data-based estimates. In this presentation, the vertical distributions of anthropogenic CO2 at three meridional sections are discussed and compared with the available data-based estimates. The inventory in the individual basins is also calculated. Use of large isopycnal diffusivity can generally improve the simulated results, including the exchange flux, the vertical distribution patterns, inventory, storage, etc. In terms of comparison of the vertical distributions and column inventory, we find that the total inventory in the Pacific Ocean obtained from our model is in good agreement with the data-based estimate, but a large difference exists in the Atlantic Ocean, particularly in the South Atlantic. The main reasons are weak vertical mixing and that our model generates small exchange fluxes of anthropogenic CO2 in the Southern Ocean. Improvement in the simulation of the vertical transport and sea ice in the Southern Ocean is important in future work.

Ocean Data Assimilation Using Intermittent Analyses and Continuous Model Error Correction

A new data insertion approach is applied to the Derber and Rosati ocean data assimilation (ODA) system, a system that uses a variational scheme to analyze ocean temperature and provide ocean model corrections continuously. Utilizing the same analysis component as the original system, the new approach conducts analyses to derive model corrections intermittently at once-daily intervals. A technique similar to the Incremental Analysis Update (IAU) method of Bloom et al. is applied to incorporate the corrections into the model gradually and continuously. This approach is computationally more economical than the original.A 13-year global ocean analysis from 1986 to 1998 is produced using this new approach and compared with an analysis based on the original one. An examination of both analyses in the tropical Pacific Ocean shows that they have qualitatively similar annual and interannual temperature variability. However, the new approach produces smoother monthly analyses. Moreover, compared to the independent

Ocean Response to a Climate Change Heat-Flux Perturbation in an Ocean Model and Its Corresponding Coupled Model

State-of-the-art coupled general circulation models(CGCMs)are used to predict ocean heat uptake(OHU)and sealevel change under global warming.However,the projections of different models vary,resulting in high uncertainty.Much of the inter-model spread is driven by responses to surface heat perturbations.This study mainly focuses on the response of the ocean to a surface heat flux perturbation F,as prescribed by the Flux-Anomaly-Forced Model Intercomparison Project(FAFMIP).The results of ocean model were compared with those of a CGCM with the same ocean component.On the global scale,the changes in global mean temperature,ocean heat content(OHC),and steric sea level(SSL)simulated in the OGCM are generally consistent with CGCM simulations.Differences in changes in ocean temperature,OHC,and SSL between the two models primarily occur in the Arctic and Atlantic Oceans(AA)and the Southern Ocean(SO)basins.In addition to the differences in surface heat flux anomalies between the two models,differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins.These discrepancies are largely due to both the larger initial value and the greater weakening change of the Atlantic meridional overturning circulation(AMOC)in CGCM.The greater weakening of the AMOC in the CGCM is associated with the atmosphere–ocean feedback and the lack of a restoring salinity boundary condition.Furthermore,differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes.

Effect of opening the Drake Passage on the oceanic general circulation:A box model study

The impacts of opening the Drake Passage(DP) on the oceanic general circulation are examined.When the DP is open,wind stress at mid-and high latitudes gives rise to a wind-driven gyre,which induces a meridional heat exchange between mid-and high latitudes in the Southern Ocean.After the opening of the DP,the Antarctic Circumpolar Current(ACC) forms and its associated strong temperature front blocks the heat transport from mid-latitudes to high latitudes.A simple box model is formulated,in which the effects of the wind stress(for the case of DP closed) and the thermal front(for the case of DP open) on the variability of Antarctic Bottom Water(AABW) and North Atlantic Deep Water(NADW) are explored.The sensitivity experiments demonstrate that:(1) When the DP is closed,the enhancement of the wind-driven gyre leads to the decline of AABW formation in the Southern Ocean and the increase of NADW formation in the North Atlantic.As a result,water in high latitudes of the Southern Ocean becomes warmer,so does the bottom water of global ocean.(2) When the DP is open,there is no formation of AABW until the intensity of thermal front along ACC exceeds a threshold value(it is 4.03℃ in our model).Before the formation of AABW,temperature in most of the oceans is higher than that after the formation of AABW,which usually leads to the cooling of high latitudes of the Southern Hemisphere and the bottom water in global ocean.When the strength of the thermal front is lower than the critical value,there is no AABW formation,and temperature in most of the oceans is slightly higher.These results demonstrate that during the opening of the DP,changes in wind stress and the formation of the thermal front in the Southern Ocean can substantially affect the formation of AABW and NADW,thus changing the state of meridional overturning circulation in the global ocean.

Comparative simulation study of effects of eddy-topography interaction in the East/Japan Sea deep circulation

In this study the structure and seasonal variations of deep mean circulation in the East/lapan Sea （E/S） were numerically simulated using a mid-resolution ocean general circulation model with two different parameterizations for the eddy-topography interaction （ETI）. The strong deep mean circulations observed in the EIS are well reproduced when using the ETI parameterizations. The seasonal variability in the EIS deep layer is shown by using ETI parameterization based on the potential vorticity approach, while it is not shown in the statistical dynamical parameterization. The driving mechanism of the strong deep mean currents in the E/S are discussed by investigating the effects of model grids and parameterizations. The deep mean circulation is more closely related to the baroclinic process and potential vorticity than it is to the wind driven circulation.

Effects of adjusting vertical resolution on the eastern equatorial Pacific cold tongue

The vertical resolution of LICOM1.0 (LASG/IAP Climate System Ocean Model, version 1.0) is adjusted by increasing the level amount within the upper 150 m while keeping the total of levels. It is found that the eastern equatorial Pacific cold tongue is sensitive to the adjustment. Compared with the simulation of the original level scheme, the adjusting yields a more realistic structure of cold tongue extending from the coast of Peru to the equator, as well as a temperature minimum at Costa Rica coast, north of the cold tongue. In the original scheme experiment, the sharp heating by net surface heat flux at the beginning of spin-up leads to a great warm- ing in the eastern equatorial Pacific Ocean. The weak vertical advection due to a too thick mixed layer in the coarse vertical structure also accounts for the warm bias. The fact that most significant improvements of the upper 50 m temperature appear at the region of the thinnest mixed layer indicates the necessity of fine vertical resolution for the eastern equatorial Pacific Ocean. However, the westward extension of equatorial cold tongue, a defect in the original scheme, gets even more serious in the adjusting scheme due to the intensi- fied vertical velocity and hence vertical advection in the central-eastern equatorial Pacific Ocean.

The Tropical Pacific–Indian Ocean Associated Mode Simulated by LICOM2.0

Oceanic general circulation models have become an important tool for the study of marine status and change. This paper reports a numerical simulation carried out using LICOM2.0 and the forcing field from CORE. When compared with SODA reanalysis data and ERSST.v3 b data, the patterns and variability of the tropical Pacific–Indian Ocean associated mode(PIOAM) are reproduced very well in this experiment. This indicates that, when the tropical central–western Indian Ocean and central–eastern Pacific are abnormally warmer/colder, the tropical eastern Indian Ocean and western Pacific are correspondingly colder/warmer. This further confirms that the tropical PIOAM is an important mode that is not only significant in the SST anomaly field, but also more obviously in the subsurface ocean temperature anomaly field. The surface associated mode index(SAMI) and the thermocline(i.e., subsurface) associated mode index(TAMI) calculated using the model output data are both consistent with the values of these indices derived from observation and reanalysis data. However, the model SAMI and TAMI are more closely and synchronously related to each other.

Study on the mesoscale eddies around the Ryukyu Islands

Results of the Ocean General Circulation Model for the Earth Simulator（OFES） from January 1977 to December2006 are used to investigate mesoscale eddies near the Ryukyu Islands. The results show that：（1） Larger eddies are mainly east of Taiwan, above the Ryukyu Trench and south of the Shikoku Island. These three sea areas are all in the vicinity of the Ryukyu Current.（2） Eddies in the area of the Ryukyu Current are mainly anticyclonic, and conducive to that current. The transport of water east of the Ryukyu Islands is mainly toward the northeast.（3）The Ryukyu Current is significantly affected by the eddies. The lower the latitude, the greater these effects.However, the Kuroshio is relatively stable, and the effect of mesoscale eddies is not significant.（4） A warm eddy south of the Shikoku Island break away from the Kuroshio and move southwest, and is clearly affected by the Ryukyu Current and Kuroshio. Relationships between the mesoscale eddies, Kuroshio meanders, and Ryukyu Current are discussed.