Transient-induced statistics in the atmosphere

The statistical effect of a time-changing zonal forcing on the large scale, barotropic components of the atmospheric circulation is considered. It is suggested that maxima of the statistical distribution of the zonal wind and traveling planetary wave intensity are the results of transitions between different, locally in time, dominating regimes. This interpretation is substantially different from that commonly adopted, which identifies the statistical maxima with the attractors of the theoretical models. To clarify this point, results from a numerical simulation, clearly showing the effects of transitions on distributions, are discussed. It is shown in particular how the properties of the basin of attraction of different regimes can deeply affect the resulting distributions of the relevant physical quantities. It is also shown how maxima of distributions have no simple connections with equilibria computed under the assumption of constant zonal forcing. These results suggest new interpretations of the statistics.