Atmospheric multiple equilibria and non-Gaussian behaviour in model simulations

Multiple equilibria from general circulation models (GCMs) are investigated via Gaussian mixture models. The cross-validation technique based on boots trap, used to find the possible number of clusters, is shown to have a tend ency to display a bias in the number of clusters when the sample is not sta tistically independent. The method is shown, however, to give satisfactory results when applied with a strategy based on statistical independence. The mixture model is successfully applied to a few dynamical systems and data from the UK Universities Global Atmospheric Modelling Programme (UGAMP) GCM . The same analysis is then carried out and applied to the wintertime 500 m b geopotential height field taken from a 50-year run of the Hadley Centre H adAM3 GCM. Three regions, the North Pacific-American, the North Atlantic, a nd the northern hemisphere are investigated for nonlinear circulation regim es. It is shown that the model display-a robust regime behaviour over the N oah Pacific-American sector, while only marginal regime behaviour is observ ed over the North Atlantic, and no trace of hemispheric regimes.