J-L.Dufresne, L.Bopp, P. Ciais, L. Fairhead, P. Friedlingstein, H.Le Treut, P.Monfray

EGS XXV General Assembly, Nice, France, 25-29 April 2000

In simulations carried out to estimate anthropogenic climate change, the atmospheric CO2 increase is commonly prescribed using different emission scenarii, like, for example, those assembled by the IPCC. The rate of atmospheric CO2 increase depends on the rate of these prescribed antropogenic emissions (changes in land use and fossil CO2 emissions) and on the uptake by the ocean and the biosphere. Carbon uptake by oceans and continents is not independent of climate change. Recent studies have suggested that over the next decades, the CO2 uptakes may be reduced by climate change thus introducing a positive feedback in the carbon-climate system. In order to study this feedback within the fully coupled carbon-climate system, we have recently added comprehensive, spatially resolved, oceanic and biospheric carbon models to the IPSL coupled AOGCM. The model configuration will be described and results from two 240 year long simulations carried out with it will be presented. The first simulation is a control simulation with no anthropogenic CO2 emissions. The second simulation is a scenario simulation using historical anthropogenic emissions from 1860 to 1990 and, from 1990 to 2100, emissions following the SRES98-A2 IPCC scenario. At the time of abstract submission the pertirbed experiment has reached year 1985, and the simulated CO2 content is realistic.

P. Friedlingstein, L.Bopp, P. Ciais, J-L.Dufresne, L. Fairhead, H.Le Treut, P.Monfray

EGS XXV General Assembly, Nice, France, 25-29 April 2000

In simulations carried out to study anthropogenic climate change, the atmospheric CO2 increase used has always been prescribed by different scenarii, like, for example, those suggested by the IPCC. The rate of CO2 increase depends on the rate of anthropogenic emissions and on the uptake by the ocean and the biosphere. However, recent studies suggest that those CO2 uptakes may be reduced by the climate change thus introducing a positive feedback in the carbon-climate system.
We will present here two estimates of this positive carbon-climate feedback. Our first estimation uses the results of a classical climate simulation carried out with the IPSL AOGCM where the CO2 increase is fixed at 1%/year (CMIP). These results are used to carry out two simulations of the carbon cycle: in the first, the carbon cycle models do not respond to climate change whereas in the second, they do. We can thus estimate the feedback between the carbon cycle and climate.
A second estimate of this feedback uses results from a fully coupled carbon-climate simulation now in progress.

L.Bopp, P.Monfray, O.Aumont, J.C.Orr, G.Madec, L.Terray, J-L.Dufresne, H.Le Treut.

EGS XXIV General Assembly, The Hague, The Netherlands, 19-23 April 1999

To investigate the effect of future climate change on marine biology and the ocean carbon cycle, we imbedded a prognostic model of ocean biogeochemistry in an oceanic general circulation model (OGCM). The same OGCM coupled to an atmospheric general circulation model, was used without flux correction to simulate climate response to increased greenhouse gases (+1% CO2/yr for 80 years). At 2xCO2 in 2060, both reduced nutrient supply and increased light efficiency resulted from increased stratification in the upper ocean. Both effects lead to significant large-scale changes in marine primary production (from -20% in the tropics to +5% in the Southern Ocean for the zonal mean). In regards to the increasing air-sea CO2 flux, we will differentiate increases due to rising atmospheric CO2 from reductions by climate change. Furthermore we will describe sensitivity studies made with two different prognostic biogeochemical schemes, and climate change forcing from two different coupled models.

Bopp L., Monfray P., Aumont O., Dufresne J.L., Le Treut H., Madec G. And Orr J.

Abstract for the 3d US-JGOFS SMP meeting, 1998

S. Bony, J.-L. Dufresne, L. Fairhead

EGS XXIII General Assembly, Nice, France, 20-24 April 1998

Interactions between sea surface temperature, clouds and radiation play a fundamental role in the natural climate variability and in the climate sensitivity to radiative perturbations. We will present an analysis of these interactions in the IPSL global ocean-atmosphere coupled model. First, this analysis will be based on seasonal and interannual variations, and a comparison will be done with satellite observations. Then, we will consider a transient CO2 experiment of 80 years. A comparison of the tropical clouds feedbacks involved in the CO2 experiment and in the seasonal or interannual variations will be presented.

L. Fairhead, J.-L. Dufresne, H. Le Treut, L. Li

EGS XXIII General Assembly, Nice, France, 20-24 April 1998

The IPSL coupled model has been used for two eighty-year simulations: a control simulation with constant CO2 concentration and a scenario simulation with a 1% yearly increase in CO2 concentration. The absence of any flux corrections implies that an initial drift, which stays reasonable, is present in the two simulations. The general characteristics of the control simulation will be presented as well as the perturbations on the atmospheric and oceanic circulations due to the CO2 increase and particularly the SST perturbation which is peculiarly strong along the equator in the Pacific. Comparisons with similar experiments using a simple oceanic mixed layer as the oceanic component of the coupled model will be made to illustrate the part of oceanic circulation in the perturbed simulations.

M.-A. Filiberti and J.-L. Dufresne

EGS XXIII General Assembly, Nice, France, 20-24 April 1998

Vintzileos A., Dufresne J.L., Le Treut H., Fairhead L.

EGS XXIII General Assembly, Nice, France, 20-24 April 1998

Filiberti M.A., Dufresne J.L., Madec G.

Atelier de Modélisation de l'Atmosphère, Toulouse, France, 8-9 décembre 1998

M.A. Filiberti, J.L.Dufresne, M.N. Houssais, M.Imbard, G. Madec

EGS XXII General Assembly, Viena, Austria, 21-25 April 1997

J.L.Dufresne, L. Fairhead,M.A. Filiberti, M.N. Houssais

EGS XXII General Assembly, Viena, Austria, 21-25 April 1997

In sea ice covered regions, the ocean surface is highly heterogeneous (open water, new ice, thick pack ice...). The sea ice GCM we developed takes this heterogeneity into account by allowing various sea ice classes, whereas atmospheric models used in coupled GCMs usually allow only homogeneus surfaces. We analyse how the sea ice distribution is modified by the following different sea ice / atmosphere interface models :
- the atmospheric model only considers one type of surface and fluxes over open water and over ice are similar
- the atmospheric model only considers one type of surface but the atmospheric heat flux is redistributed through an interface that approximates the spatial heterogeneity of the underlying surface
- the atmospheric model computes a separate flux over each of this type of surface
These results were obtained with the LODYC ocean/sea ice GCM coupled to a simple bulk atmosphere and with the full coupled IPSL GCM.

Dufresne J.L, Filiberti M.A., Grandpeix J.Y., Houssais M.N.

Atelier de Modélisation de l'Atmosphère, Toulouse, 2-3 décembre 1996

L'interface océan-glace de mer-atmosphère est extrêmement hétérogène. Le modèle de glace-océan du LODYC, utilisé par la communauté "GASTON", reproduit en partie cette hétérogénéité (mailles mixtes contenant de l'océan libre et différents types de glace) alors que les modèles d'atmosphère (Arpège, LMD 5) ne la reproduisent pas ou peu (mailles homogènes ou mailles mixtes océan libre - un seul type de glace). De plus, aux hautes latitudes, une maille atmosphérique recouvre un nombre élevé de mailles océaniques. Nous avons développé un modèle de raccordement qui répartit de façon différenciée les flux calculés par le modèle d'atmosphère sur les differents types de surface (oc\'ean libre ou vari\'et\'es de glace) en fonction de leur distribution statistique (fraction surfacique) et de leur caracteristiques individuelles (temp. de surface, albedo, etc...). Ce modèle de raccordement garantit la conservation des flux à l'interface. Nous présentons le modèle de raccordement et montrons la sensibilité de la fraction de glace à la méthode de répartition des flux. Nous abordons également le problème de la stabilité numérique du raccordement glace-atmosphère.

M.A. Filiberti, J.L. Dufresne, G. Madec, M.N. Houssais, M. Imbard

Atelier de Modélisation de l'Atmosphère, Toulouse, 2-3 décembre 1996, pp.63-68

Dufresne J.L, Grandpeix J.Y., Houssais M.N.

EGS XXI General Assembly, Den Hague, 6-10 May 1996

Two important issues of sea-ice model coupling with AGCM are considered. First the numerical oscillations induced by straight sea-ice/atmosphere coupling (in a fashion similar to the usual ocean/ atmosphere coupling) are investigated. A simple explanation is presented and exemplified : the oscillations are primarily due to the strong feedbacks between SST and surface flux beeing not properly taken into account. The second important issue is the quality of flux repartition between a homogeneous AGCM mesh and the amalgam of open water, thin ice, thick pack ice ... which make up a sea-ice model mesh. Practical solutions are given for both issues. A first implementation is tested in a thermodynamic sea-ice model coupled to a simple bulk formula atmosphere. The inclusion in the full coupled GCM of the IPSL is under way; first results are presented.

D. Cruete, A. Marillier, J.L. Dufresne, J.Y. Grandpeix
Atelier `` Expérimentation et Instrumentation '', Toulouse, 15-17 octobre 1996