Euro-Atlantic circulation types and modes of variability in winter
Extra-tropical atmospheric circulation variability is addressed in this study using two complementary approaches: circulation types and modes of variability. Principal component analysis (PCA) in T- and S-modes has been used to estimate the features. An objective synoptic classification of Euro-Atlantic atmospheric circulation is described. Eight circulation types have been identified and described in terms of their spatial features, mean frequency and lifetime, transitions and trends. The most persistent type is connected with the presence of a ridge over the British Isles and Iceland, while the less persistent type is related to a tilted ridge west of the continent. Increases in the persistence of some circulation types such as the Scandinavian and the Euro-Atlantic blockings and a ridge with axis over Western Europe have been found in the 1990s. Data from two independent re-analyses are used to test the robustness of the results. The main differences between the two datasets have been found in the distribution of events as a function of their duration. In a second step, the main modes of variability over the Euro-Atlantic area have been identified using daily data. An analysis of the relationship between these modes and the circulation types suggests that specific circulation types shift the phases of certain modes of variability.
KeywordsEmpirical Orthogonal Function North Atlantic Oscillation Zonal Flow Circulation Type Weather Regime
The authors acknowledge the scientists involved in producing the ERA40 and NCEP re-analyses. We would like to thank the two anonymous reviewers whose comments significantly contributed to improve the paper. Special thanks are due to the COST733 Action Harmonisation and Applications of Weather Type Classifications for European Region for their useful discussions. FJDR was supported by the EU-funded ENSEMBLES project (GOCE-CT-2003-505539).
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