An eddy-permitting oceanic general circulation model and its preliminary evaluation

1. Introduction The current oceanic general circulation models(GCMs) used in climate studies, especially those serv-ing as component models in coupled GCMs, havean average resolution around 2? (IPCC, 2001). Theoceanic GCMs with relatively coarse resolutions canreproduce the major observed features of the large-scale circulations, but there are some insurmount-able di?culties. It was documented by Gates (1992)that the main defects of the coarse resolution oceanicGCMs include the repres…     

Impact of the moisture transport formulation on the simulated tropical rainfall in a general circulation model

The impact of numerical modeling of moisture transport on the simulation of the seasonal mean pattern of precipitation in the tropics is studied. The NCAR CCM2 with spectral and semi-Lagrangian moisture transport has been used for this purpose. The differences in the numerical modeling of moisture transport are found to have a significant impact on the simulation of the seasonal mean patterns. The major differences while using the spectral method (vis-a-vis the semi-Lagrangian method) are (1) a decrease in rainfall over the Indian monsoon region, (2) a decrease in rainfall over the west Pacific region and (3) an increase in rainfall over the central and east Pacific regions. There are substantial differences in the amount of precipitable water vapor simulated by the two moisture transport techniques. It is shown that the difference in precipitable water vapor between the two simulations is associated with changes in the vertical moist static stability (VMS) of the atmosphere, and differences in the simulated precipitation patterns. Received: 7 August 1998 / Accepted: 15 October 1999     

A low latitude general circulation model

Summary The primitive equations of motion are solved to find peak latitudes of families of trajectories of air moving from low latitudes towards the pole from a state of rest under the action of a constant meridional pressure gradient. The general case is discussed first and then calculations are applied to the 20° West meridian in January and July for the Northern Hemisphere.The resulting mass transposition indicates that a belt of convergence should occur at about 32° N in January; this agrees well with the latitude of the axis of the mean sub-tropical high pressure belt at that time of year.The nature of the climatological model in low latitudes suggested by the calculation is then discussed.

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Zusammenfassung Die vereinfachten Bewegungsgleichungen werden gelöst, um Scheitelbreiten für Trajektorienscharen der Luftbewegungen zu finden, die ausgehend von dem Ruhezustand unter Einwirkung eines konstanten meridionalen Druckgradienten polwärts gerichtet sind. Der allgemeine Fall wird diskutiert und es werden Berechnungen für den 20. westlichen Längengrad der Nordhemisphäre für Januar und Juli durchgeführt.Der resultierende Massentransport zeigt, daß im Januar in 32° N ein Konvergenzgürtel auftreten sollte. Dies stimmt gut überein mit der geographischen Breite der Achse des mittleren subtropischen Hochdruckgürtels zu dieser Jahreszeit.Schließlich wird die Form des klimatologischen Modells für niedere Breiten, das durch diese Berechnungen nahegelegt wird, diskutiert.

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Résumé Résolution des équations simpflifiées du mouvement pour déterminer l'origine des faisceaux de trajectoires des courants aériens qui, partant du repos, sont dirigés vers le pôle sous l'effet d'un gradient constant de pression méridien. Discussion du cas général; calcul pour le 20^e méridien Ouest de l'hémisphère Nord en janvier et en juillet.Le transport de masse résultant montre qu'il doit y avoir en janvier, a 32° de latitude Nord, une ceinture de convergence ce qui concorde bien avec la latitude de l'axe de la zone moyenne des hautes pressions subtropicales en cette saison.Forme du modèle climatologique aux basses latitudes suggéré par ces calculs.

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With 1 Figure     

Fluctuation dissipation in a general circulation model

This paper considers the climate response to step function changes in the solar constant in two versions of a general circulation model with simplified geography. The NCAR CCM0 model is applied to an all-land planet with no topography (Terra Blanda). In one version there is moisture in the air (as well as self-generated clouds) as evaporated from an ideal surface at a fixed 80% of saturation. In the other version there is no moisture in the atmosphere. We examine the decay of natural anomalies in the large-scale temperature field in each model and compare the time dependence of the ensemble average with the average temporal behavior of the response to step function changes in the solar constant. The fluctuation-dissipation theorem of statistical mechanics makes specific predictions about the relationship between the two curves. We conduct the experiments for both versions of the model since the sensitivity is quite different for each. The theorem is found to hold reasonably well in each case.     

A new framework for isolating individual feedback processes in coupled general circulation climate models. Part I: formulation

This paper proposes a coupled atmosphere–surface climate feedback–response analysis method (CFRAM) as a new framework for estimating climate feedbacks in coupled general circulation models with a full set of physical parameterization packages. The formulation of the CFRAM is based on the energy balance in an atmosphere–surface column. In the CFRAM, the isolation of partial temperature changes due to an external forcing or an individual feedback is achieved by solving the linearized infrared radiation transfer model subject to individual energy flux perturbations (external or due to feedbacks). The partial temperature changes are addable and their sum is equal to the (total) temperature change (in the linear sense). The decomposition of feedbacks is based on the thermodynamic and dynamical processes that directly affect individual energy flux terms. Therefore, not only those feedbacks that directly affect the TOA radiative fluxes, such as water vapor, clouds, and ice-albedo feedbacks, but also those feedbacks that do not directly affect the TOA radiation, such as evaporation, convections, and convergence of horizontal sensible and latent heat fluxes, are explicitly included in the CFRAM. In the CFRAM, the feedback gain matrices measure the strength of individual feedbacks. The feedback gain matrices can be estimated from the energy flux perturbations inferred from individual parameterization packages and dynamical modules. The inter-model spread of a feedback gain matrix would help us to detect the origins of the uncertainty of future climate projections in climate model simulations.     

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 modle’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 model are presented.     

一个灵活的海洋——大气耦合环流模式

Based on the National Center for Atmospheric Research (NCAR) Climate System Model version 1(CSM-1), a Flexible coupled General Circulation Model version 0 (FGCM-0) is developed in this study through replacing CSM-1＇s oceanic component model with IAP L30T63 global oceanic general circulation model and some necessary modifications of the other component models. After the coupled model FGCM--0 is spun up for dozens of years, it has been run for 60 years without flux correction. The model does not only show the reasonable long-term mean climatology, but also reproduce a lot of features of the interannual variability of climate, e.g. the ENSO-like events in the tropical Pacific Ocean and the dipole mode pattern in the tropical Indian Ocean. Comparing FGCM-0 with the NCAR CSM-1, some common features are found, e.g. the overestimation of sea ice in the North Pacific and the simulated double ITCZ etc.The further analyses suggest that they may be attributed to errors in the atmospheric model.     

Multi-year predictability in a coupled general circulation model

Multi-year to decadal variability in a 100-year integration of a BMRC coupled atmosphere-ocean general circulation model (CGCM) is examined. The fractional contribution made by the decadal component generally increases with depth and latitude away from surface waters in the equatorial Indo-Pacific Ocean. The relative importance of decadal variability is enhanced in off-equatorial “wings” in the subtropical eastern Pacific. The model and observations exhibit “ENSO-like” decadal patterns. Analytic results are derived, which show that the patterns can, in theory, occur in the absence of any predictability beyond ENSO time-scales. In practice, however, modification to this stochastic view is needed to account for robust differences between ENSO-like decadal patterns and their interannual counterparts. An analysis of variability in the CGCM, a wind-forced shallow water model, and a simple mixed layer model together with existing and new theoretical results are used to improve upon this stochastic paradigm and to provide a new theory for the origin of decadal ENSO-like patterns like the Interdecadal Pacific Oscillation and Pacific Decadal Oscillation. In this theory, ENSO-driven wind-stress variability forces internal equatorially-trapped Kelvin waves that propagate towards the eastern boundary. Kelvin waves can excite reflected internal westward propagating equatorially-trapped Rossby waves (RWs) and coastally-trapped waves (CTWs). CTWs have no impact on the off-equatorial sub-surface ocean outside the coastal wave guide, whereas the RWs do. If the frequency of the incident wave is too high, then only CTWs are excited. At lower frequencies, both CTWs and RWs can be excited. The lower the frequency, the greater the fraction of energy transmitted to RWs. This lowers the characteristic frequency (reddens the spectrum) of variability off the equator relative to its equatorial counterpart. At low frequencies, dissipation acts as an additional low pass filter that becomes more effective, as latitude increases. At the same time, ENSO-driven off-equatorial surface heating anomalies drive mixed layer temperature responses in both hemispheres. Both the eastern boundary interactions and the accumulation of surface heat fluxes by the surface mixed layer act to low pass filter the ENSO-forcing. The resulting off-equatorial variability is therefore more coherent with low pass filtered (decadal) ENSO indices [e.g. NINO3 sea-surface temperature (SST)] than with unfiltered ENSO indices. Consequently large correlations between variability and NINO3 extend further poleward on decadal time-scales than they do on interannual time-scales. This explains why decadal ENSO-like patterns have a broader meridional structure than their interannual counterparts. This difference in appearance can occur even if ENSO indices do not have any predictability beyond interannual time-scales. The wings around 15–20°S, and sub-surface variability at many other locations are predictable on interannual and multi-year time-scales. This includes westward propagating internal RWs within about 25° of the equator. The slowest of these take up to 4 years to reach the western boundary. This sub-surface predictability has significant oceanographic interest. However, it is linked to only low levels of SST variability. Consequently, extrapolation of delayed action oscillator theory to decadal time-scales might not be justified.     

The generation of available potential energy: a comparison of results from a general circulation model with observations

The generation of available potential energy (APE) in the space-time domain in January and July in a present-day climate simulation with a general circulation model (GCM) is compared with observations. An attempt is made to explain the differences. The generation is computed from the fields of diabatic heating and temperature. The heating is computed with the residual method, using UKMO (United Kingdom Meteorological Office) Unified Model GCM circulation data and ECMWF (European Centre for Medium-Range Weather Forecasts) initialized analyses for the period 1989–1992. The global value of the generation of APE is about 35% larger in the GCM climate than the value computed from the analyses. This is mainly because the generation of transient eddy APE in the GCM is too large, due to the more than 40% too large generation of transient eddy kinetic energy by baro-clinic processes. In most of the extratropics the local contribution to the generation of transient eddy APE in the GCM climate is more than twice as large as the contribution computed from the analyses. A possible qualitative explanation for this difference is presented.     

A Nine-layer Atmospheric General Circulation Model and Its Performance

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Simulation of thermohaline circulation with a twenty-layer oceanic general circulation model

Summary This paper presents the basic configuration and preliminary performance of a twenty-layer oceanic general circulation model which represents a portion of the recent progress in developing coupled ocean-atmosphere general circulation models made by the authors. The model uses latitude/depthdependent thermohaline-stratification subduction, -coordinate, three-dimensional implicit diffusion, complete convective adjustment, separating and coupling of external and internal modes and Asselin temporal filter, and thermodynamic sea-ice calculation. With seasonally varying climatological forcing at the surface and enhanced surface salinities in the region adjacent Antarctica, the model has been integrated for one thousand years to reach a quasiequilibrium state. Preliminary verification shows that the model is capable of simulating successfully not only many aspects of the upper ocean circulation but also an acceptable thermohaline circulation. The modelled overturning rate of the North Atlantic Deep Water (NADW) is greater than 15Sv. The simulated overturning rate of the Antarctic Bottom Water (AABW) is about 20Sv. The southward outflow of NADW can be identified from not only the meridional overturning streamfunction but also the current fields at four deeper levels from 1455m to 2475m. The AABW northward outflow exists at some bottom levels below 2600m, and mainly flows towards the Pacific basin.Major problems in the present simulation include the underestimate of the NADW outflow, the failure to simulate the Antarctic Intermediate Water (AAIW), the too fresh bottom water and the too diffuse thermocline of the model. A sensitivity experiment has revealed that the model diffusion process has an important impact on the simulation of both the thermocline and the NADW outflow.With 16 Figures     

Climate predictability experiments with a general circulation model

The atmospheric response to the evolution of the global sea surface temperatures from 1979 to 1992 is studied using the Max-Planck-Institut 19 level atmospheric general circulation model, ECHAM3 at T 42 resolution. Five separate 14-year integrations are performed and results are presented for each individual realization and for the ensemble-averaged response. The results are compared to a 30-year control integration using a climate monthly mean state of the sea surface temperatures and to analysis data. It is found that the ECHAM3 model, by and large, does reproduce the observed response pattern to El Nino and La Niña. During the El Nino events, the subtropical jet streams in both hemispheres are intensified and displaced equatorward, and there is a tendency towards weak upper easterlies over the equator. The Southern Oscillation is a very stable feature of the integrations and is accurately reproduced in all experiments. The inter-annual variability at middle- and high-latitudes, on the other hand, is strongly dominated by chaotic dynamics, and the tropical SST forcing only modulates the atmospheric circulation. The potential predictability of the model is investigated for six different regions. Signal to noise ratio is large in most parts of the tropical belt, of medium strength in the western hemisphere and generally small over the European area. The ENSO signal is most pronounced during the boreal spring. A particularly strong signal in the precipitation field in the extratropics during spring can be found over the southern United States. Western Canada is normally warmer during the warm ENSO phase, while northern Europe is warmer than normal during the ENSO cold phase. The reason is advection of warm air due to a more intense Pacific low than normal during the warm ENSO phase and a more intense Icelandic low than normal during the cold ENSO phase, respectively.     

Snowline instability in a general circulation model: application to Carboniferous glaciation

For over twenty years it has been known that energy balance models (EBMs) with snow-albedo feedback are characterized by unstable behavior in some areas of parameter space. This behavior leads to rapid changes in snow area due to small changes in forcing, and has been termed the small ice cap instability (SICI). It has never been clarified whether this behaviour reflects a real feature of the climate system or a limitation in EBMs. In this study we demonstrate that evidence for similar unstable behavior can also be found in an atmospheric general circulation model (GCM), using a realistic set of boundary conditions for the Carboniferous (300 Ma), one of the most extensive periods of glaciation in Earth history. When solar luminosity is sequentially lowered to near values appropriate for the Carboniferous, there is a discontinuous increase in summer snow area. The instability occurs in approximately the same area of parameter space as one previously found in an EBM. Analysis of selected fields indicates that the circulation is primarily affected in the area of snow increase; far-field effects are minimal. There is good agreement between model-generated summer snowcover and one reconstruction of Carboniferous ice cover. Although more work is required on this topic, our results provide increased support for the possibility that the snowline instability represents a real feature of the climate system, and that it may help explain some cases of glacial inception and abrupt transitions in Earth history.     

A global ocean biogeochemistry general circulation model and its simulations

An ocean biogeochemistry model was developed and incorporated into a global ocean general circulation model (LICOM) to form an ocean biogeochemistry general circulation model (OBGCM). The model was used to study the natural carbon cycle and the uptake and storage of anthropogenic CO2 in the ocean. A global export production of 12.5 Pg C yr-1 was obtained. The model estimated that in the pre-industrial era the global equatorial region within 15o of the equator released 0.97 Pg C yr-1 to the atmosphere, which was balanced by the gain of CO2 in other regions. The post-industrial air-sea CO2 flux indicated the oceanic uptake of CO2 emitted by human activities. An increase of 20－50 mol kg-1 for surface dissolved inorganic carbon (DIC) concentrations in the 1990s relative to pre-industrial times was obtained in the simulation, which was consistent with data-based estimates. The model generated a total anthropogenic carbon inventory of 105 Pg C as of 1994, which was within the range of estimates by other researchers. Various transports of both natural and anthropogenic DIC as well as labile dissolved organic carbon (LDOC) were estimated from the simulation. It was realized that the Southern Ocean and the high-latitude region of the North Pacific are important export regions where accumulative air-sea CO2 fluxes are larger than the DIC inventory, whereas the subtropical regions are acceptance regions. The interhemispheric transport of total natural carbon (DIC+LDOC) was found to be northward (0.11 Pg C yr-1), which was just balanced by the gain of carbon from the atmosphere in the Southern Hemisphere.     

Ocean-atmosphere interactions and climate drift in a coupled general circulation model

 We have analysed numerical simulations performed with a global 3D coupled atmosphere-ocean model to focus on the role of atmospheric processes leading to sea surface temperature (SST) drift in the tropics. Negative SST errors occur coherently in space and time with large positive errors in latent heat and momentum fluxes at the tropical air-sea interface, as diagnosed from forced SST simulations. The warm pool in the western Pacific disappears after a few years of simulation. Strong SST gradients enforce regions of high precipitation that are thin and stationary north of the equator. We detail the implications for the ocean-atmosphere system of such upheaval in the deep convection location. A sensitivity experiment to empirically formulate air-sea drag coefficient shows that the rapid warm pool erosion is not sensitive to changes in the formulation of the surface drag coefficient over the oceans because the corresponding changes in turbulent heat fluxes and LW cooling approximately cancel one another. In the eastern Pacific, the improvement in SST is striking and caused by feedbacks between SST, surface turbulent fluxes and boundary layer cloud fraction, which decreases as SST warms. Received: 8 December 1998 / Accepted: 6 January 2000     

Decadal climate variability simulated in a coupled general circulation model

A 1000 year integration of the CSIRO coupled ocean-atmosphere general circulation model is used to study low frequency (decadal to centennial) climate variability in precipitation and temperature. The model is shown to exhibit sizeable decadal variability for these fields, generally accounting for approximately 20 to 40% of the variability (greater than one year) in precipitation and up to 80% for temperature. An empirical orthogonal function (EOF) analysis is applied to the model output to show some of the major statistical modes of low frequency variability. The first EOF spatial pattern looks very much like that of the interannual ENSO pattern. It bears considerable resemblance to observational estimates and is centred in the Pacific extending into both hemispheres. It modulates both precipitation and temperature globally. The EOF has a time evolution that appears to be more than just red noise. Finally, the link between SST in the Pacific with Australian rainfall variability seen in observations is also evident in the model. Received: 29 August 1998 / Accepted: 31 July 1999     

Design and performance of a new cloud microphysics scheme developed for the ECHAM general circulation model

A new cloud microphysics scheme including a prognostic treatment of cloud ice (PCI) is developed to yield a more physically based representation of the components of the atmospheric moisture budget in the general circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. The precipitation formation scheme for warm clouds distinguishes between maritime and continental clouds by considering the cloud droplet number concentration, in addition to the liquid water content. Based on several observational data sets, the cloud droplet number concentration is derived from the sulfate aerosol mass concentration as given from the sulfur cycle simulated by ECHAM. Results obtained with the new scheme are compared to satellite observations and in situ measurements of cloud physical and radiative properties. In general, the standard model ECHAM4 and also PCI capture the overall features, and the simulated results usually lie within the range of observed uncertainty. As compared to ECHAM4, only slight improvements are achieved with the new scheme. For example, the overestimated liquid water path and total cloud cover over convectively active regions are reduced in PCI. On the other hand, some shortcomings of the standard model such as underestimated shortwave cloud forcing over the extratropical oceans of the respective summer hemisphere are more pronounced in PCI.This paper was presented at the Third International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 4–9 Sept. 1995 under the auspices of the Max Planck Institute for Meteorology, Hamburg. Editor for these papers is L. Dümenil.     

Climate feedbacks in a general circulation model incorporating prognostic clouds

 This study performs a comprehensive feedback analysis on the Bureau of Meteorology Research Centre General Circulation Model, quantifying all important feedbacks operating under an increase in atmospheric CO₂. The individual feedbacks are analysed in detail, using an offline radiation perturbation method, looking at long- and shortwave components, latitudinal distributions, cloud impacts, non-linearities under 2xCO₂ and 4xCO₂ warmings and at interannual variability. The water vapour feedback is divided into terms due to moisture height and amount changes. The net cloud feedback is separated into terms due to cloud amount, height, water content, water phase, physical thickness and convective cloud fraction. Globally the most important feedbacks were found to be (from strongest positive to strongest negative) those due to water vapour, clouds, surface albedo, lapse rate and surface temperature. For the longwave (LW) response the most important term of the cloud ‘optical property’ feedbacks is due to the water content. In the shortwave (SW), both water content and water phase changes are important. Cloud amount and height terms are also important for both LW and SW. Feedbacks due to physical cloud thickness and convective cloud fraction are found to be relatively small. All cloud component feedbacks (other than height) produce conflicting LW/SW feedbacks in the model. Furthermore, the optical property and cloud fraction feedbacks are also of opposite sign. The result is that the net cloud feedback is the (relatively small) product of conflicting physical processes. Non-linearities in the feedbacks are found to be relatively small for all but the surface albedo response and some cloud component contributions. The cloud impact on non-cloud feedbacks is also discussed: greatest impact is on the surface albedo, but impact on water vapour feedback is also significant. The analysis method here proves to be a␣powerful tool for detailing the contributions from different model processes (and particularly those of the clouds) to the final climate model sensitivity. Received: 15 June 2000 / Accepted: 10 January 2001     

On the climatology of persistent circulation anomalies in the atmosphere and in a general circulation model

Abstract

The climatologies of upper‐air persistent circulation anomalies found in observations of the Northern Hemisphere and in a General Circulation Model (GCM) integration are compared with each other and with those found in previous studies. The model simulation is that of the Canadian Climate Centre GCM run at resolution T20. The objective criteria that define the persistent events differ from those of some earlier investigations in that the anomalies are not required to be as nearly stationary. It is found that the GCM generates persistent circulation anomalies downstream of the synoptic‐scale storm tracks, in very nearly the correct geographical locations, but that the frequency of occurrence is too low in the model. A kinetic energy and streamfunction variance analysis is presented for both dataseis to clarify the differences between the observed and simulated distributions of circulation anomalies. It is evident that, apart from the mean annual cycle, the middle‐latitude transient eddies of the model are too weak.     

Effect of seasonal forcing on global circulation in a world ocean general circulation model

 Effects of the seasonal variation in thermohaline and wind forcing on the abyssal circulation are investigated by using an ocean general circulation model. To isolate effects of the seasonality in the thermohaline forcing from those in the wind forcing, we carry out three experiments with (1) annual-mean wind forcing and perpetual-winter thermohaline forcing, (2) annual-mean wind forcing and seasonal thermohaline forcing, and (3) seasonal wind forcing and seasonal thermohaline forcing. The deep water under the seasonal thermohaline forcing becomes warmer than under the perpetual-winter thermohaline forcing. Although the perpetual-winter thermohaline forcing is widely used and believed to reproduce the deep water better than the annual-mean forcing, the difference between the results of the perpetual-winter and the seasonal thermohaline forcing is significant. The seasonal variation of the Ekman convergence and divergence produces meridional overturning cells extending to the bottom because the period of seasonal cycle is shorter than the adjustment timescale by baroclinic Rossby waves. The heat transport owing to those Ekman flows and temperature anomalies makes the upper water (0–200 m) colder at low to mid-latitudes (40S–40N) and warmer at high latitudes. Also the deep water becomes warmer owing to the warming of the northern North Atlantic, the main source region of North Atlantic Deep Water. The model is also synchronously (i.e., without acceleration) integrated with seasonal forcing for 5400 y. A past study suggested that under seasonal forcing, a sufficient equilibrium state can be achieved after only decades of synchronous integration following more than 10 000 y of accelerated integration. Here, the result so obtained is compared with that of the 5400-y synchronous integration. The difference in the global average temperature is as small as 0.12 °C, and most of the difference is confined to the Southern Ocean. Received: 1 May 1998 / Accepted: 5 January 1999