Regimes in the wintertime circulation over northern extratropics. II: Consequences for dynamical predictability

A classification of hemispheric flow patterns into clusters corresponding to circulation regimes is used to analyse the errors in geopotential fields predicted by the ECMWF models in the medium range during winter. It is found that the probability distributions of both the r.m.s. error over the northern hemisphere and the amplitude of large-scale planetary waves are bimodal in the region of phase space where the fields have negative or small projection onto an EOF (empirical orthogonal function) that partially resembles the Pacific-North-American pattern, and where the separation between clusters is strongest. In addition to a true systematic error, a regime-dependent component exists in the error fields, whose pattern is opposite to the anomaly of the observed flow regime. In fact, the ECMWF model (especially its earlier versions) tends to relax the flow towards its most densely populated regime. These results are supported by an analysis of the frequencies of transitions between different clusters, and are interpreted on the basis of simple analytical solutions of the Fokker-Planck equation for stochastically-perturbed dynamical systems. Statistics computed for different ECMWF models show that a considerable improvement in both the systematic and the regime-dependent component of the medium-range error occurred with the T106 model, owing principally to its improved physical parametrization. However, even though the bias towards states with low wave amplitude has been strongly reduced in the T106 model, a loss of predictability is still associated with transitions to regimes with large amplitude of planetary waves.

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