JPM9b
Simulation with an O-AGCM of the influence of variations of the solar constant on the global climate
Ulrich Cubasch, Gabriele Hegerl, Reinhard Voss, J¸rgen Waszkewitz and Thomas Crowley
Deutsches Klimarechenzentrum GmbH, Hamburg,
Germany
Max-Planck-Institut f¸r Meteorologie, Hamburg, Germany
Dept. of Oceanography, Texas A&M University, College Station,
USA
A globally coupled ocean-atmosphere model has been forced with an estimate of solar variability since 1700 in order to assess its potential impact on climate.
The coupled model responds to the variations of the solar constant in the near-surface temperature with a time lag of about 25 to 30 years. The dominant response in the model is at the centennial-scale Gleissberg cycle, with peak-to-trough changes in global temperature on the order of 0.5 K with a stronger response to a decrease of the solar intensity than to an increase The prime response to an increase of solar forcing is an increase in the land-sea contrast, similar to the effect of an enhancement of the greenhouse gases. The vertical structure of the warming is similar for the greenhouse gas and the solar warming in the troposphere, but in the stratosphere considerable differences appear. The greenhouse gas response pattern generally shows a cooling in the stratosphere, while an increase in solar radiation tends to increase temperatures there. The observed stratospheric cooling cannot be explained by solar variability. However, other effects, like volcanic aerosols and ozone variations, also have to be considered at this layer of the atmosphere.
During recent years, the modeled global warming due to the solar constant increase explains only a fraction of the observed global warming. The global mean temperature trends between the model simulation and the observations disagree at a 95% confidence level. Also, the warming pattern of the recent observed 30-year trends disagree from a response to solar variability alone and point clearly towards the CO2 pattern.
JPM9c
Global warming, ENSO and Australian climate in a coupled GCM
S. Power, F. Tseitkin, R. Colman, B. McAvaney, J. Fraser, R. Kleeman and A. Sulaiman
Bureau of Meteorology Research Centre, Melbourne, AUSTRALIA
Anthropogenically-induced climate change has the potential to alter not only the average climate state but also the nature of the variability about this average. A large part of the variability evident from year-to-year can be attributed to the ENSO phenomenon. This phenomenon, while centred in the equatorial Pacific, has a profound influence on the climate in many countries around the world. It is therefore important that research continues to focus on how ENSO and climate variability linked with ENSO may be altered by global warming.
The original BMRC coupled general circulation model (CGCM, Power et al. 1993) incorporated a global version of the GFDL modular ocean model (Pacanowski et al. 1991) which had a rather coarse horizontal and vertical resolution. So while this model is useful for climate change research (eg. Colman et al., 1995), it is limited in its ability to simulate the climate and variability of the tropical ocean. Given the importance of the tropical Pacific Ocean in the dynamics of ENSO, we have improved the CGCM (Power et al. 1996) by substantially upgrading its oceanic component (Power et al. 1995). The resolution is now increased in both the vertical (to 25 levels) and horizontal (to 0.5 x 2 degrees in the tropics), and a much improved surface and interior mixing scheme has been incorporated (Chen et al. 1994). Allowance has also been made for the partial penetration of short wave radiation into the upper levels of the model. Accommodating this new OGCM has entailed significant change to the coupling strategy due to a number of factors which will be described.
In this (preliminary) study, we will focus on the last 20 years of two multidecadal coupled integrations. The climate and interannual variability during this period has been examined in the first of these integrations. It exhibits significant interannual variability in the tropical Pacific which appears to modulate Australian rainfall, as it does in the actual climate system. The concentration of atmospheric carbon-dioxide has been increased in the second integration which is currently (October, 1996) underway. The variability in the two integrations will be compared and contrasted to determine if there is any significant difference.
Chen, D, L.M. Rothstein, and A.J. Busalacchi (1994) A hybrid vertical mixing scheme and its application to tropical ocean models, J. Phys. Oceanogr., 24, 2156-79.
Colman, R.A., S.B. Power, B.J. McAvaney and R.R. Dahni (1995) A non-flux corrected transient CO2 experiment using the BMRC coupled atmosphere/ocean GCM. Geophys. Res. Lett, 22, 3047-3050.
Pacanowski, R.C., K. Dixon and A. Rosati (1991) The GFDL Modular Ocean Model Users Guide, version 1.0. GFDL Ocean Group Tech. Rep. No. 2, 376pp.
Power S.B., R.A. Colman, B.J. McAvaney, R.R. Dahni A.M. Moore and N.R. Smith (1993) The BMRC coupled atmosphere/ocean/sea-ice model. BMRC Res. Rep. No. 37, 58pp.
Power, S.B., R. Kleeman, F. Tseitkin and N.R. Smith (1995) BMRC Technical Report on a global version of the GFDL Modular Ocean Model for ENSO studies. BMRC, 18pp.
Power S.B., F. Tseitkin, R. Colman, B.J. McAvaney, J.R. Fraser, R. Kleeman and A. Sulaiman (1996) A CGCM for seasonal prediction and climate change research. In: BMRC Workshop on Climate Prediction, Nov., 1996.
JPm9e
Features of Individual Atmospheric Aerosol Particles in Coastal Areas of China: Case Studies in Qingdao
Daizhou Zhang and Min Hu
Center of Environmental Sciences of Peking University, Beijing 100871, CHINA
Thin film method and X-ray dispersive analysis were applied to examine the individual atmospheric aerosol particles collected in the coastal areas of Qingdao city, China. Sampling was carried out in urban areas, on an island over sea near the coastal line of a ship on October 9,10 and 11. Nitrate and sulfate in particles were identified by nitron and barium chloride multiple thin-film. Elemental composition of individual particles was obtained through their X-ray spectra. Nitrate and sulfate-containing particles were found in samples taken at all three observational sites, but their distributions were of submicron size. In the samples in urban areas and the island, the frequencies of nitrate-containing particles in coarse particles were much larger than over the sea. On the contrary, sulfate mainly existed in submicron range. Morphological analysis revealed there were no sulfuric acid particles in the samples on carbon films but there were a huge number of submicron sulfate-containing particles identified by the multiple thin-film. This result suggests that sulfuric acid formed in the marine atmosphere were totally neutralised by ammonia emitted from Qingdao city in these areas.
Internal mixed particles, which contained simultaneously sulfate and nitrate, were also found.. Their X-ray spectra showed most of them contained crustal elements, suggesting these particles initially originated from land surface and then sulfate and nitrate were formed on their surfaces. Some statistical results and electron microscope pictures of particle, and their X-ray spectra will be presented.
JPM9f
Earth-Ocean-Atmosphere Coupled Model Based on Gravitational Teleconnection
B. A. Leybourne
Geophysics Division, Naval Oceanographic Office, Stennis Space Center, USA
Cross-correlations over great distances between time series of meteorological variables, such as sea level pressure (SLP), were realized in the early 1900's (Walker, 1924; Walker and Bliss, 1932; Wise, 1927). These linkages between weather anomalies were later called teleconnections. Extensive analysis of SLP and surface air temperature (SAT) has resulted in the discovery of three major oscillation systems or teleconnections around the globe. The most familiar are the Southern Oscillation (SO) associated with El Nino (ENSO), the North Pacific Oscillation (NPO) controlling fronts moving toward North America, and the North Atlantic Oscillation (NAO) exerting control over European weather patterns. Solar modulated ocean/atmosphere coupled models predict short-range weather phenomena adequately, but longer range climatic variability such as El Nino and global warming trends prove difficult to forecast accurately because they tend to be tectonically modulated.
By coupling geodynamic tectonic flow to ocean/atmosphere models based on principles of surge tectonics (Meyerhoff et al., 1992), we can incorporate the influences of gravitational teleconnection on atmospheric pressure into current models. In order to do this we must realize that the global teleconnected systems are influenced by major tectonic vortex structures as illustrated by surge theory. The SO is controlled by the largest upwelling tectonic vortex structure on earth, the Indonesian Island Arc. Across the Pacific Basin the SO is controlled by strong downwelling vortices along offsets on the East Pacific Rise near Easter Island. The NPO is considered a seesaw of SLP between a belt at high latitudes extending from eastern Siberia to western Canada and a broad region at lower latitudes including the subtropics. The NPO is controlled by island arcs and deep trench systems in the north and northwest Pacific, which includes the Japan, Kuril and Aleutian island arcs and trench systems. To the south the NPO pressure is controlled by the Mid-Pacific and Hawaiian volcanic systems. The NAO is controlled by an upwelling tectonic vortex beneath Iceland and a downwelling tectonic vortex along an offset of the Mid-Atlantic Ridge near the Azores.
We know how these teleconnected pressure systems affect weather patterns around the globe, but what creates the large-scale changes in pressure that cause a vacillation between zonal and meridional flow? By understanding geodynamic tectonic flow as analogous to ocean and atmosphere flow, which is inferred in surge tectonic theory, we can make the following comparisons. Atmospheric jetstream flow is analogous to the Gulf Stream or Kuroshio Current flow structure in oceans, in another word, aquastreams. These are in turn analogous to horizontal upper asthenosphere tectonic flow or geostreams, which create surface trends in the crust. The high/low pressure cells in the atmosphere are analogous to cold/warm core eddies in the oceans and downwelling/upwelling vortex structures in the earth’s crust and mantle. Weather fronts are analogous to Kelvin/Rossby waves in the oceans, or oceanic fronts. These fronts are simply pressure/temperature waves moving through their corresponding medium and in the earth are called surges, gravity waves, or tectonic fronts. Earth oscillations of various periods generate these surges, and affect the vortex structures and geostreams in predictable ways.
The problem lies in modeling these undulations of the geoid over time frames corresponding to periods of climatic changes such as El Ninos and global warming/cooling trends. Time series gravitational data associated with the teleconnected earth oscillation system are needed to monitor these long-range climatic trends and create a surge index to factor into the solar modulated simulations of ocean/atmospheric coupled models.
B. A. Leybourne is an employee of the Naval Oceanographic Office. However, this paper was prepared in his personal time. As such, the opinions and assertions contained herein are those of the Authors, and are not to be considered as official statements of the US Department of the Navy.
jpm9g
SEA LEVEL CHANGES DUE TO THE GRADUAL INCREASE IN ATMOSPHERIC CO2 USING A COUPLED OCEAN-ATMOSPHERE MODEL
T. Motoi, A. Noda, S. Nakagawa, S. Yukimoto, H. Ishizaki and M. Endoh
Meteorological Research Institute, Nagamine, Tsukuba, Ibaraki, Japan
Sea level changes associated with enhanced greenhouse warming are investigated with an MRI global coupled ocean-atmosphere general circulation model. A transient response experiment on the gradual increase in atmospheric carbon dioxide was performed after spin up of the atmosphere (3 years), ocean (1500 years) and coupled system (30 years). The experiment consists of a control run with a fixed atmospheric carbon dioxide concentration (345 ppmv) and a transient run with a gradual increase in atmospheric carbon dioxide at a compound rate of 1%yr-1. Both control and transient run were carried out for 140 years, using a flux adjustment for heat and water fluxes.
The model predicts that the globally averaged rise in sea level caused by sea-water expansion is 12 cm by the time atmospheric carbon dioxide doubles (year 70). A significant contribution to the rise in sea level is expected from changes in the mountain glaciers and ice sheets over land on this time scale, but the model does not represent these factors. The spatial distribution of the sea level change is also predicted by the model, neglecting the background rise due to land ice changes. The response from the climate system to increasing atmospheric carbon dioxide is not uniform in space and has some interesting structures. A rather small rise is estimated around Antarctica. A minimum rise in the eastern tropical Pacific and a maximum rise in the North and South Pacific (wedge-like pattern) are predicted for sea level changes.
JPM9h
influences of sea surface temperature and ground wetness variations on the asian summer monsoon
Song Yang and K.-M. Lau
Climate and Radiation Branch, Laboratory for Atmospheres, NASA-Goddard Space Flight Center, Greenbelt, Maryland, US
Using the Goddard Laboratory for Atmospheres general circulation model, we have carried out a number of experiments to understand the influences of variations of sea surface temperature (SST) and ground wetness (GW; snow and soil moisture content) on the Asian summer monsoon. We attempt to identify the robust signals associated with these influences and to isolate their relative importance for the monsoon by specifying the boundary forcing functions in different ways.
Results indicate that, compared to GW, SST anomalies cause a more significant change in the variability of the Asian summer monsoon. The impact of SST anomalies appears asymmetric with respect to warm and cold events. This is linked to the nonlinear response of atmospheric diabatic heating to SST anomalies. The monsoon becomes significantly weaker during the warm events and changes little during the cold events. Associated with the warm SST anomalies, both the Walker circulation and local Hadley circulation diminish substantially. Consistently, atmospheric water vapor transported into tropical Asia reduces to a remarkable extent.
The Asian summer monsoon changes little following cold seasons of small GW (reduced snow and soil moisture). However, a wet Asian continent, which is accompanied by a low surface temperature, is followed by a moderately weak summer monsoon. During the warm events, large GW appears in the Asian continent. It reinforces the anomalies of the Asian monsoon produced by warm SST forcing. Therefore, the influences of SST and GW on the Asian summer monsoon can be viewed clearly through mutually interactive processes.
JPM9i
A coupled atmosphere-ocean general circulation model for climate studies
C.H. Poncin, T. Chefet and H. Enier
Institut d'Astronomie et de Geophysique G. Lemaitre, UCL, BELGIUM.
A new coupled atmosphere-ocean model has been developed for climate predictions at the decade to century time scales. The atmospheric model is an improved version of the atmospheric general circulation model built at the "Laboratoire de Meteorologie Dynamique" of the CNRS (Paris). Its horizontal resolution is 64 regularly spaced points in longitude and 50 points spaced in a sinusoidal regular distribution from pole to pole. Vertically there are eleven levels. The oceanic model is a free-surface, primitive-equation model that includes a detailed treatment of the vertical mixing and of the thermodynamic and dynamic sea-ice processes. The horizontal resolution of this global ocean model is 3 degrees by 3 degrees, and there are 20 levels along the vertical. The normal latitude-longitude singularity at the North Pole is overcome by using two grids, the second rotated grid covering the Atlantic from the equator to the Bering Strait. At this point, the throughflow is parameterized according to the geostrophic control theory. The coupling of the two models is achieved by means of a coupler. Here we discuss results of a 30-yr control run (without flux adjustment) conducted with the coupled model. A particular attention is paid to the model behaviour at high latitudes. Despite some systematic biases, the model robustness and stability are quite promising for future studies.
JPM9J
Decadal Climate Variability and Predictability
Stephen Jewson and Scott Power
CRCSHM, Monash University, Melbourne,
Australia
Bureau of Meteorology Research Centre, Melbourne, Australia
The slow timescales of ocean and ice dynamics lead to the possibility of predictable climate variability on decadal time scales. Research in this area is hampered by the fact that the most complex coupled models run too slowly for multiple integrations to be performed. In this study, a global coupled model is designed and built using an intermediate complexity dynamical primitive equation atmosphere model and a course grid global ocean GCM.
The atmosphere model is complex enough to simulate the response of the atmosphere to changing SSTs in terms of changing heat, freshwater and momentum fluxes, but simple enough to integrate as fast as the ocean model.
A description of the model components, the coupling strategy and the decadal variability evident will be provided. The variability will be compared to that evident in observational records and more sophisticated coupled models. Special attention will be given to variability evident in rainfall and surface temperature over Australia, for which high quality observational records have recently become available.
Additional experiments aimed at further elucidation of the nature of the variability are expected to be well underway, and it is hoped that these will also be discussed.
JPM9L
On the relative contribution of heat, freshwater und momentum fluxes for changes in ocean circulation in case of CO2 induced warming
Uwe Mikolajewicz and Reinhard Voss
Max-Planck-Institut fuer Meteorologie,
Hamburg, GERMANY
Deutsches Klimarechenzentrum, Hamburg, GERMANY
Many simulations with various coupled ocean-atmosphere general circulation models (COAGCMs) have shown that the formation rate of North Atlantic deep water will be reduced in case of increasing atmospheric concentrations of greenhouse gases. The northward heat transport and the overturning circulation in the Atlantic will be reduced as well. From analysis of the model data it can be shown that both temperature and salinity changes are responsible for these changes in ocean circulation. The direct analysis of the relative importance of the individual flux components, however, is complicated due to feedback processes in the ocean. To overcome this difficulty, we performed a set of greenhouse experiments with the ECHAM3/LSG COAGCM where we suppressed the coupling in individual components of the flux between atmosphere and ocean. Thus we are able to quantify the relative role of the individual atmosphere-ocean flux components in the circulation changes. It turns out that the effect of the heat flux dominates. Changes in salinity stem in almost equal parts from changes in atmospheric water vapour transport and heat flux induced changes in ocean circulation.
JPM9m
Sensitivity of global warming patterns to oceanic eddy physics
Anthony C. Hirst
CSIRO Division of Atmospheric Research, Aspendale, Vic., AUSTRALIA
The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the ocean component of a global coupled general circulation model. Parallel simulation with and without the GM scheme are performed to examine the effect of this parameterization on model behavior under constant atmospheric CO2 and on the model response to increasing CO2. The control (constant CO2) runs show substantial differences in the oceanic stratification and extent of convection, similar to differences found previously using uncoupled ocean models. The transient (increasing CO2) runs show moderate differences in the rate of oceanic heat sequestration (less in the GM case), as expected based on passive tracer uptake studies. However, the surface warming is weaker in the GM case, especially over the Southern Ocean, which is contrary to some recent supposition. Reasons for the reduced warming in the GM case will be discussed.
JPM9n
Review of global coupled modelling in the french climate community
Laurent Terray, Pierre Barthelet, Eric Guilyardi, Olivier Thual, Michel Deque, Gurvan Madec, Herve le Treut and Olivier Marti
CERFACS, Toulouse, FRANCE
IMFT/CERFACS, Toulouse, FRANCE
CNRM, Toulouse, FRANCE
LODYC, Paris, FRANCE
LMD, Paris, FRANCE
LMCE, Paris, FRANCE
The natural variability of the climate system at interannual, decadal and multidecadal timescales is receiving increased attention, particularly under international research programs such as CLIVAR. It is important to study this variability both for a better understanding of the underlying physical processes and its implications for the detection of anthropogenic climate change. Several coupled atmosphere-ocean general circulation models have been developed in the past years in the French modelling climate community.
Results from both control and transient climate change experiments performed with these AOGCM’s will be presented. Numerous regional diagnostic studies have been performed to study the model’s drift as well as the variability at different timescales. In particular, the sensitivity of the water mass formation to the ocean physics has been investigated. Also, coupled processes at play in the western Pacific warm pool have been looked at in connection with the proposed thermostat hypothesis and its possible role in climate change of the Pacific region.
jpm9p
a conceptual model for pacific cold tongue and enso
Fei-Fei Jin
Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, USA
A conceptual model is constructed based upon the Bjerknes hypothesis of tropical atmosphere-ocean interaction. It is shown that strong feedbacks among the trade winds, equatorial zonal sea surface temperature contrast, and upper ocean heat content occur in the tropical basin. Coupled atmosphere-ocean dynamics produce both the strong Pacific cold-tongue and the El NiÒo-Southern Oscillation (ENSO) phenomenon. The cold tongue climate state is unstable and gives rise to the self-sustained ENSO which can be understood as an equatorial ocean recharge oscillator. The small basin size and the influence of a wind system resulting from heating sources of its adjacent land masses are responsible for a weak and stable Atlantic cold-tongue state which cannot support ENSO-like interannual variability. The presence of westerly wind associated with the Walker circulation ascending at the western Pacific warm pool disables the dynamical coupling processes in the equatorial Indian ocean. As a result, the equatorial Indian ocean maintains a stable warm climate state. The conceptual coupled model reproduces the basic features of the climate states of the tropical Pacific, Atlantic and Indian ocean basins and the dominant interannual climate variability of the tropical climate system.
JPM9q
Linear and non-linear feedbacks in a GCM
R.A. Colman, S.B. Power and B.J. McAvaney
Bureau of Meteorology Research Centre, Melbourne, AUSTRALIA
The study of climate feedbacks in General Circulation Models (GCMs) is critical to an understanding of the spread in climate change sensitivity which is currently found between models. Traditionally, in the evaluation of feedbacks, a simple linear assumption is made about the manner in which the model responds to changes in a given physical process. However, many of the processes determining model feedbacks are known to be inherently non-linear in nature (e.g. changes to water vapour and clouds), and it is unclear whether this linear assumption is justified over the range of climate change typically considered. Indeed it is of considerable interest to explore these non-linear processes for a deeper understanding of the mechanisms underlying climate change in models.
This paper outlines a technique for evaluating both linear and non-linear contributions to global climate feedback in a GCM. The method does this by expressing the global radiative change as a sum of linear and non-linear terms due to changes in globally averaged parameters, with these parameters in turn expressed as quadratic functions of global mean SST. The climate perturbation used entailed five uniform SST change experiments: ±2oC, ±1oC and 0oC (run in perpetual season mode).
A theoretical framework is described which shows that with these five climate snapshots, both linear and non-linear terms in the global climate sensitivity may be estimated. The evaluation of the radiative change resulting from each physical process is calculated by repeated offline running of the GCM radiation code, substituting one field at time from the ‘perturbed’ climate into the ‘control’ following the method of Wetherald and Manabe (1980). The contributions to climate sensitivity from individual physical processes are evaluated, and thus may be ‘ranked’ in terms of those with the greatest linear and non-linear impacts over the full 4oC of temperature change.
jpm9r
An investigation of the impact of tundra ecosystems on surface energy budgets in a regional climate system model
A.H. Lynch and F.S. Chapin III
CIRES, Colorado, USA
The surface energy and moisture budgets are crucial elements in the simulation of regional land-atmosphere interactions. Regional coupled model experiments investigating the performance of different land surface exchange models have shown a strong impact on climate simulations. Summer surface energy budgets measured by eddy correlation in arctic Alaska showed large ecosystem differences in both energy absorption and dissipation. The major tundra ecosystems show differences large enough to have the potential to impact regional climate simulation. In addition, observations indicate that changes in climatic regimes lead to shifts in vegetation distributions favouring boreal forest over tundra, and to shifts in the permafrost line.
The Arctic Region Climate System Model (ARCSyM) is used to investigate the atmospheric response to the inclusion of elements of these differing ecosystems in a regional climate model. ARCSyM is a fully coupled atmosphere-land-sea ice-ocean model designed for simulations of the high latitude climate system. In these experiments, the impacts of sub-grid scale variability of tundra plant types and of changing vegetation distributions are addressed.
JPM9s
Estimation of the air-sea fluxes and the subsurface contents of heat, salt and CO2 using assimilation of surface data into a mixed layer model
M. Ikeda
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan
Oceanographic data are collected much more extensively from the surface than subsurface using satellite remote-sensing and ship of opportunity. A data assimilation method is proposed for determining the heat, salt and CO2 contents in the subsurface as well as the air-sea fluxes. The method is tested with a much simpler model, which is chosen to be a vertical 1-D model here. The period from fall to winter and the subpolar ocean are chosen, in which the ocean interacts most extensively with the atmosphere via mixed layer development.
During mixed layer development, intense vertical mixing occurs due to static instability. Since the vertical diffusion coefficient becomes extremely large in a numerical model, it is difficult to solve the equations that are adjoint to the governing equations for a continuously stratified (z-grid) model. A bulk (1-layer) mixed-layer model is instead used in this work. The primary properties of the mixed layer are temperature, salinity and layer thickness. The mixed layer receives heat flux through the sea surface. This negative buoyancy flux works on mixing at the bottom of the mixed layer. In addition to these physical properties, CO2 is also considered as a model variable. Total carbonate and pCO2 increase in fall through winter, as the mixed layer develops and entrain carbonate-rich subsurface water.
In the variational method to be solved by an iterative procedure, a cost function is used which is based upon squared differences between data and a model solution. Initial values, air-sea fluxes and subsurface properties are chosen as control variables, which are varied to fit a solution to data. Without the CO2 information, reconstruction of a solution and the control variables highly relies on the layer-thickness data. Once CO2 is included, they are reconstructed well with fewer thickness data, because mixed-layer development reflects entrainment of a high-carbonate content in the lower ocean.
JPM9v
Initial drift and atmospheric feedback processes in a coupled ocean-atmosphere model
H. Le Treut, L. Fairhead, Z. X. Li and M. Forichon
Laboratoire de Meteorologie Dynamique du CNRS and Institut Pierre Simon Laplace, Paris, FRANCE
The IPSL coupled model is the combination of the LMD atmospheric GCM, at a resolution of about 3.5 degrees, and of the LODYC ocean GCM at a resolution of about 1 to 1.5 degrees. The model has been run for multidecennal simulations without using any flux correction, starting from observed initial conditions. The drift of the model, in its first version, has been rather moderate in terms of temperature (with a slight general cooling in the Tropics and on the contrary a warming at high latitudes).
The dependence of these results on model parameterization is large, as was verified through a number of sensitivity experiments involving changes in the cloud optical or microphysical properties, in the vertical friction associated with cumulus clouds, or in the surface drag. We use the initial drift of the model to test the impact of those parameterization changes.
Some regional climate features, such as the warm pool over the Western Pacific, or the climatic conditions of the high latitude regions seems extremely sensitive, and are subject to a stronger initial drift. The key processes which seem to control the model behavior in those areas are detailed: they are also very often feedback processes which are essential in determining the climate sensitivity of the model.
JPM9Z
Simulation of the seasonal cycle and interannual variability in the Pacific and Atlantic Oceans
Carlos R. Mechoso, Jin-Yi Yu and Lishan Tseng
Department of Atmospheric Sciences, University of California Los Angeles, USA
This paper analyzes the performance of a coupled atmosphere-ocean general circulation model (GCM) in the simulation of the seasonal cycle in the Pacific and Atlantic Oceans; discusses some physical processes whose representations are key for a successful model performance; and investigates the interannual variability that is either intrinsic to each of those oceans and/or is connected to that of the other ocean. The work is based on the UCLA coupled GCM, which consists of the UCLA AGCM and the GFDL Modular Ocean Model (MOM). We use Tropical Pacific and Tropical Atlantic versions of MOM, whose domains extend from 30ƒS to 50ƒN. Concerning the Tropical Pacific, the sensitivity of the simulated seasonal cycle by the coupled GCM to cloud representations in its atmospheric component is examined. One sensitivity study focuses on the impact of cloud optical properties on the simulations. Other sensitivity studies address the impact of stratus clouds on the latitudinal asymmetry and seasonal cycle in the eastern part of the ocean. It is shown that (1) cloud radiative properties specified in the model can have a strong influence on the overall quality of the simulations, (2) Peruvian stratus decks have a significant influence on the local and remote SSTs in the tropical Pacific, and (3) the seasonal variations of these clouds contribute significantly to several asymmetric features of the seasonal cycle in the eastern equatorial Pacific. The physical mechanisms at work for these sensitivities are analyzed. Concerning the Tropical Alantic, it is shown that the coupled GCM simulates a realistic seasonal cycle, albeit there is an equatorial cold tongue that has similar deficiencies to that obtained in the Pacific.
The simulated interannual variability in the two oceans is described. Special attention is dedicated to the Atlantic. In this context, a comparison is made between multi-year long "twin experiments" with the model version in which atmosphere-ocean coupling is restricted to the tropical Atlantic and SSTs are prescribed elsewhere. In each 'twin' experiment, one component uses SST distributions corresponding to an observed climatology, and the other component uses SST distributions corresponding to observational analyses. The results of these experiments are used to demonstrate that the interannual variability in the Atlantic can be closely linked to that in the Pacific.
JPM9aa
The NCAR Climate System Model: Design and Control Climate Simulation
Frank O. Bryan
National Center for Atmospheric Research, USA
The NCAR Climate System Model (CSM) project formally began in January 1994 with the long-term goal of building, maintaining, and continually improving a comprehensive model of the climate system, including both physical and biogeochemical aspects. The initial version of the model, including only the physical aspects was released to the research community in May 1996. The CSM is presently composed of a set of four independent models for the basic system components: atmosphere, ocean, land surface, and sea ice, each communicating with a "Flux Coupler" using message passing. The component models are driven primarily by fluxes across interfaces at the Earth's surface. Those fluxes that directly depend on the state of more than one component model, e.g., turbulent fluxes of latent and sensible heat, are computed within the Flux Coupler, which is also responsible for interpolating and averaging between the differing grids of the component models while conserving local and integral properties. In this way (atmospheric model) sub-grid scale heterogeneity of surface types is directly accounted for. In this presentation we will describe the basic design of the system and its performance in a multi-century simulation of present day climate.
JPM9bb
Study of the Small Ice Cap Instability with a coupled atmosphere-sea ice-ocean-land ice model
M.A. Morales Maqueda, A.J. Willmott and J.L. Bamber
Department of Mathematics, Keele University,
UK
Centre for Remote Sensing, Dept of Geography, University of
Bristol, UK
Energy balance models (EBMs) in which the snow and ice albedos are parameterized as step functions of the air temperature have multiple equilibrium states for a given value of insolation, Q. When the area of an ice cap is less than a critical value, Qc, the ice cap becomes unstable. A slight increase in Q near Q_c leads to a sudden disappearance of the ice cap (Small Ice Cap Instability, or SICI). Decreasing Q from an initial ice free state leads to the reappearance of an ice cap for a value of Q below Q_c.
The SICI relies on the albedo--temperature feedback whereby an increase (decrease) in air temperature leads to a decrease (increase) in the surface albedo via a decrease (increase) in the ice area, reinforcing the initial temperature perturbation.
Here, we examine the existence of SICI events in a coupled atmosphere-sea ice-ocean-terrestrial ice model for the Southern Hemisphere. A two dimensional atmospheric EBM (AEBM), a thermodynamic sea ice model (TSIM), a uniform-depth ocean mixed layer model (OMLM), and a terrestrial ice sheet model (TISM) are coupled asynchronously. SICI events are studied for four different couplings: (1) AEBM alone; (2) AEBM-TSIM-OMLM; (3) AEBM-TISM; (4) fully coupled model. Cases (1) and (2) support SICI events, while cases (3) and (4) do not. This result casts doubts on the role of the SICI in the glaciation of Antarctic.
JPM9cc
The Effect of Interactive Sea-Ice on Seasonally Varying North Atlantic Thermohaline Circulation
W.D. Hibler, III, and Jinlun Zhang
Dartmouth College, Hanover, NH, USA
Polar Science Center, University of Washington, USA
Interdecadal oscillations of the thermohaline circulation are an intrinsic feature of many coupled Atmosphere-Ocean-Ice model simulations. In order to elucidate the essential role of interactive sea ice in such variations an idealized Atlantic Geostrophic ice-ocean circulation model has been constructed and used to carry out a series of approximately 5000 year simulations. Analysis of these simulations yields considerable insight into the role of interactive sea ice in coupled Atmosphere-Ocean-Ice numerical investigations. The character of these oscillations within the context of mean annual temperature and salinity forcing were examined in a previous paper (Hibler and Zhang, Annals of Glaciology, 21, 361-368, 1995). The results yielded interdecadal oscillations in both the ice thickness and northward oceanic heat transport that could be largely explained by the insulating effects of the sea-ice cover in the presence of Northward Thermohaline heat transport.
In this work a more realistic examination of the effect of interactive sea-ice on the North Atlantic thermohaline circulation is carried out by considering seasonally varying air temperatures to force a Geostrophic ice-ocean circulation model. Both sea ice dynamics and thermodynamics are included in the model which has a seasonally varying specified flux of sea ice at the Northern Boundary. In addition the effects of seasonally varying ice growth and melt on the salt fluxes is examined. The results yield interdecadal oscillations in Northward transport of heat and concomitant ice margin fluctuations in the case where interactive sea ice is included. In the absence of interactive sea ice these interannual oscillations either do not occur or are severely reduced in magnitude. Analysis of the results includes a determination of the relative roles of the insulating effects of sea ice versus salt flux effects on the interdecadal oscillations. The importance of ice transport into the overturning region is also determined.
Depending on progress made, simulations of a combined North and South Atlantic Geostrophic ocean circulation model with periodic boundary conditions in the Southern Hemisphere will also be examined to determine the relative roles of Southern versus Northern sea ice cover on interdecadal oscillations.
JPM9dd
The CSIRO ENSO Coupled General Circulation Model
Stephen Wilson
CSIRO Division of Atmospheric Research, Aspendale, Australia
An ENSO Coupled General Circulation Model (CGCM) consisting of a T63 version of the CSIRO atmospheric model and a high resolution tropical Pacific ocean model is presented. The ocean model is based on the GFDL Modular Ocean Model (MOM) code. To improve the ocean model's coupled performance it has an exponentially expanding vertical structure and a new vertical mixing scheme which both contribute to the maintenance of a "tight" thermocline in the east Pacific. In coupling these models wind stresses are anomaly coupled based on FSU climatology, whereas sea surface temperatures (SSTs) and heat fluxes are directly coupled with the addition of a 5 W/m2 K heat flux term relaxing SST towards observed SST climatology.
The CGCM produces irregular warm and cool events with a period of about 5 years. SST anomalies (SSTAs) have a similar magnitude and meridional extent to the observations and, like the observations, the amplitude of the cool events is slightly less than that of the warm events. SSTAs occur as a standing oscillation in the east Pacific and during some decades show westward propagation into the west Pacific. Like the observations, zonal wind stress anomalies show little inter-annual variability in the east Pacific but there are strong westerly anomalies in the west Pacific during model warm events. During model warm events, westerly wind stresses (as distinct from anomalies) which normally occur seasonally from about November to January in the west Pacific, extend to the dateline.
This model will now be used to investigate the influence of greenhouse conditions on ENSO and to do seasonal hindcast and prediction studies
JPM9EE
Human influence on the atmospheric vertical temperature structure: detection and observation
Simon F.B. Tett, John F.B. Mitchell, David E. Parker and Myles R. Allen
Hadley Centre, UK Met Office, UK
Space Science Department, Rutherford Appleton Laboratory, UK
Work by Santer et al (1996) suggests a discernible human influence on climate. At the Hadley Centre ensembles of coupled atmosphere ocean simulations, forced with changes in greenhouse gases, tropospheric sulphate aerosols and stratospheric ozone have been carried out.
Direct comparison of these ensembles and a control simulations of the same coupled model with a 1961-1995 dataset of radiosonde observations support the hypothesis of human influence on climate.
Results from these simulations are also compared, and found to be consistent with, with MSU temperature trends. Uncertainties remain due to imperfect knowledge of radiative forcing. Natural climate variability, and errors in observations and model response.
JPM9gg
AMIP Simulations using a Conformal-Cubic AGCM
John L. McGregor and Martin R. Dix
CSIRO Division of Atmospheric Research, Aspendale, Australia
A new two-time-level atmospheric general circulation model (AGCM) has been constructed on the conformal-cubic grid of Rancic et al. (1996). The model utilises semi-Lagrangian advection, as described by McGregor (1993, 1996) with variables cast on a staggered Arakawa C-grid. The model employs semi-implicit treatment for gravity waves; a simple tri-colour successive over-relaxation scheme has been developed to solve the associated Helmholtz equations. Major attractions of the model are quasi-uniform resolution, absence of any Gibbs’ phenomenon, scope for computational efficiencies, and potential ease of coding for massively parallel machines.
Some aspects of the model behaviour were presented by McGregor and Dix (1996) for the dynamical core test of Held and Suarez (1994). In this test, the model has zero orography and has specified forcing and dissipation. The jet structure and temperature distributions were found to be very satisfactory. The grid contains 8 vertices; there was no indication of strange behaviour near any of the vertices.
A full suite of physical parameterizations has now been incorporated into the model and multi-year simulations have been performed using AMIP prescriptions for sea-surface temperature and sea-ice extent. Aspects of the model climatology will be shown for various resolutions and compared with the climatology of the CSIRO spectral AGCM.
Held, I. M., and M. J. Suarez, 1994: A proposal for the intercomparison of the dynamical cores of atmospheric general circulation models. Bull. Amer. Meteor. Soc., 75, 1825-1830.
McGregor, J. L., 1993: Economical determination of departure
points for semi- Lagrangian models. Mon. Wea. Rev., 121,
221-230.
McGregor, J. L., 1996: Semi-Lagrangian advection on conformal-cubic grids. Mon. Wea. Rev., 124, 1311-1322.
McGregor, J. L., and M. R. Dix, 1996: A primitive equations model on a global conformal-cubic grid. In Proc. Eleventh Conference on Numerical Weather Prediction, Norfolk, Virginia, Amer. Meteor. Soc., 218-219.
Rancic, M., R. J. Purser, and F. Mesinger, 1996: A global shallow-water model using an expanded spherical cube: Gnomonic versus conformal coordinates. Quart. J. Roy. Meteor. Soc., 122, 959-982.
JPM9HH
Climate simulations with coupled ocean-atmosphere models in germany
Erich Roeckner
Max Planck Institute for Meteorology, Hamburg, GERMANY
During the last decade, several global coupled general circulation models of the atmosphere (including land surface processes) and the ocean (including sea ice) have been developed at the Max Planck Institute of Meteorology (MPI) in Hamburg. The atmospheric component (ECHAM, cycles 1 to 4) is a derivative of the ECMWF weather forecast model. For the ocean circulation, three fundamentally different models have been developed at MPI, denoted by LSG, HOPE and OPYC, respectively, and all of them have been coupled to different cycles of ECHAM. While LSG (Large Scale Geostropic) is especially designed for the study of slow climatic variations, the other ones are based on the primitive equations. However, they differ with respect to the vertical coordinates. While HOPE uses a z-coordinate system, OPYC is an isopycnal model.
The coupling strategy involves an initial spinup of the ocean model (for at least 1000 years) and different levels of flux adjustment. In the earlier attempts (ECHAM1/LSG and ECHAM3/LSG), the seasonal cycle of all surface fluxes (including wind stress) is adjusted. In the most recent model, ECHAM4/OPYC, flux adjustment is restricted to the annual means of heat and freshwater. The coupling of ECHAM and HOPE is done without flux adjustment. However, HOPE does not include sea ice, and temperature and salinity have to be adjusted to their observed values poleward of 60 degree.
The coupled models mentioned above have been applied for climate modelling studies of up to 1200 years. These simulations have been done in three different modes:
Results from some of these simulations will be presented.