High resolution climate and vegetation simulations of the Mid-Pliocene, a model-data comparison over western Europe and the Mediterranean region.

The Middle Pliocene (around 3 Ma) is a period characterized by a climate significantly warmer than today, at the global scale, as attested by abundant paleoclimate archives as well as several climate modelling studies. There we perform a detailed compari son between climate model results and climate reconstructions in western Europe and the Mediterranean area. This region is particularly well suited for such a comparison as several climate reconstructions from local pollen records covering the Mid-Pliocene provide quantitative terrestrial climate estimates. They show evidence for temperatures significantly warmer than today over the whole area, mean annual precipitation higher in northwestern Europe and equivalent to modern values in its southwestern part. To improve our comparison, we have performed high resolution simulations of the Mid- Pliocene climate using the LMDz atmospheric general circulation model (AGCM) with a stretched grid which allows a finer resolution over Europe. In a first step, we applied the PRISM2 (Pliocene Research, Interpretation, and Synoptic Mapping) boundary conditions except that we used modern terrestrial vegetation. Second, we simulated the vegetation for this period by forcing the Dynamic Global Vegetation Model ORCHIDEE with the climatic outputs from the AGCM. We then supplied this simulated terrestrial vegetation cover as an additional boundary condition in a second AGCM run. This gives us the opportunity not only to compare the generated vegetation cover to pollen records but also to investigate the model's sensitivity to the simulated vegetation changes in a global warming context. Model results and data show a great consistency for mean annual temperatures, indicating increases by up to 4°C in the study area. Comparison of the simulated winter and summer temperatures to pollen-based estimates show some disparities, in particular in the northern Mediterranean sector. The latitudinal distribution of precipitation depicted by pollen data over land is not reproduced by the model. Most excess Mid-Pliocene precipitation occurs over the North Atlantic but a slight weakening of the atmospheric transport does not allow for wetter conditions to establish in northwestern Europe, as suggested by the data. Continental moisture patterns predicted by the model are similar to those of the mean annual precipitation. Model results broadly underestimate the levels of available moisture indicated by the data. The biogeophysical effects due to the changes in vegetation simulated by ORCHIDEE, are weak, both in terms of the hydrological cycle and of the temperatures, at the regional scale of the European and Mediterranean mid-latitudes. In particular, they do not contribute to improve the model-data comparison. Their main influence concerns seasonal temperatures, with a decrease of the temperatures of the warmest month, and an overall reduction of the intensity of the continental hydrological cycle. Predicted climatic changes do not only arise from local processes but also result from an altered large-scale circulation initiated by regional-scale land cover changes. [ABSTRACT FROM AUTHOR]