Analyses of numerical experiments of climate changes due to the increase of greenhouse gases: sensitivity of the response to the model formulation and the oceanic forcing

The response of a climate model to the expected greenhouse gas increase is studied using the successive general circulation models developed at Meteo-France: EMERAUDE and ARPEGE. These models extend to 70 km in altitude having the original feature of solving the dynamic of the atmosphere up to the top of the stratosphere with ozone ratio as a prognostic variable. A preliminary step was to compare the results of the control simulation with observations of the actual climate. Thereafter, a sensitivity experiment study with uniform prescribed anomalies of the sea surface temperature was done in the framework of an international intercomparison project. Three double CO₂ experiments were, then, performed, one with EMERAUDE and two with ARPEGE, which differ in the lower boundary conditions. Anomalies, calculated by coupled ocean-atmosphere models during transient CO₂ experiments, were added to the sea surface temperature prescribed during the control run, in order to take into account the oceanic influence. We also used the sea-ice extent calculated by these models. We used MPI (Hamburg) results in EMERAUDE and in the first experiment with ARPEGE, and Hadley-Centre (UKMO) results in the second. Thus, both the sensitivity of the results to the formulation of the model and the oceanic forcing could be studied. The results obtained show that the geographical repartition of the oceanic warming has only an influence within the boundary layer. The strength of the forcing controls the amplitude of all the tropospheric anomalies obtained (the warming increasing with the altitude, the acceleration of the westerly jets, the rise of clouds height). Meanwhile, in the stratosphere, apart from gravity waves which influence the dynamic flow, the anomalies such as the increase of ozone ratio and the cooling increasing with the altitude are strictly linked to the change of radiative properties. The global warming at ground level (between 1.4 deg. C and 1.9 deg. C) shows similar patterns in our three experiments. The warming increases with latitude having maxima over continents at mid-latitudes in the northern hemisphere. The enhancement of the water cycle is proportional to the warming at the surface and is around 2.5%.K^-1 for rainfall and 2%.K^-1 for evaporation, whereas snowfall is strongly decreased. The ARPEGE model also simulated a decrease in the cloud cover. All these anomalies are discussed over the entire earth surface but some areas are more specifically studied in order to assess the continents which are more influenced by the oceanic forcing. A simple analysis (the Koppen's classification) is used to translate to climate types and vegetation covers, the anomalies obtained on precipitation and temperature. Special attention is given to the storm-track activity and the frequency of blocking events. Other specific studies performed with the models output such as regionalization, down-scaling and impact studies are presented. Especially two techniques are used with the ARPEGE output to examine changes in the precipitation and snow cover over the Alps. (author)