Discussion and Conclusions

This paper has reported a preliminary analysis of 15 simulations of IPCC AR-4 coupled models for the 20th century in term of a cyclogenesis index, CYGP, based on a combination of large-scale environment factors, among which the simulated convective precipitation plays the part of a thermal potential. For the current climate the geographical distribution of the cyclogenesis patterns simulated by the different models has some degree of realism, although the absolute number of cyclogenesis in the different ocean basins differ widely from model to model. The cyclogenesis index presents large interdecadal fluctuations and trends. In scenario A2 the trends in the cyclogenesis response differ according to the ocean basins and models. While in some ocean basins like the Indian Ocean, the majority of models compute an increasing trend in cyclogenesis, the response is much less coherent in other basins where some models give a decreasing trend. In the global mean, nearly all models give an overall increase of cyclogenesis in the 21st century, except for the INGV model. Interestingly this is the model with the highest resolution (T106). The CYGP response in this model appears in good agreement with a direct analysis of the cyclone tracks (Gualdi et al. 2008). This would seem to add another piece of evidence to support the idea that models with a resolution of T106 or above produce a rather different response of the tropical cyclones to climate change than lower resolution models.

The lack of coherence of the TC genesis response to future climate change can be associated to the different response patterns of SST in the coupled models, particularly over the Equatorial Pacific and their differences in the simulation of ENSO. How the ENSO will change in a warmer climate is still a very uncertain and debated issue. A study of ENSO in different CGCMs has shown that most models simulate different characteristics of the ENSO pattern, intensity and period (van Oldenborgh et al. 2005, Guilyardi 2006, Lin 2007b). As the ENSO phenomenon has a large impact on the distribution of TCs over the Pacific, and even over the Atlantic, it seems useful to extend this study by analyzing the CYGP at higher temporal resolution (seasonal) and relating it to an ENSO index in each of the model simulation. In this way one could attempt to relate the response of each model in the future climate to its response to ENSO SST patterns. However it seems that the large spread of model responses in term of SST patterns precludes drawing precise conclusions on the future TC genesis until some better convergence of the model SST response has been achieved. The question of the horizontal resolution of the coupled models may also be a crucial one, since it seems that the higher resolution models may have a different response than the lower resolution models in term of convective precipitation response.

Acknowledgments We thank the modeling groups and the PCMDI for making the model data available. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the U.S. Department of Energy (DOE) Office of Sciences. We acknowledge support from the European Commission 6th Framework Programme (Project ENSEMBLES, contract GOCE-CT-2003-505539), and the French Agence Nationale de la Recherche (ANR Project Cyclones&Climate)

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