Large-scale circulation classification, weather regimes, and local climate over France, the Alps and Western Europe

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By applying the dynamical cluster algorithm to large-scale circulation patterns at vari- ous tropospheric levels (Z 700, Z500, and SLP (sea-level pressure)), we obtain the so-called weather regimes (WRs). WRs are the cluster central patterns; there is no subjectivity at all involved in the pro- cedure. A red noise test allows one to select the best number of clusters at each level. A comparison is performed between the different level classifications, and highly significant correlations are found. While previous attempts to classify daily circulations in a fully objective way were concerned with mid-tropospheric levels and could not go further back than 50 yr, here we classify 120 yr of SLP pat- terns. We arrive at the important conclusion that the same 5 WRs are found for the 3 periods 1880-1918, 1919-1957 and 1958-1997. The linkage between these WRs and the local tangible weather is then investigated for both temperatures and precipitation. It is found that the instanta- neous departure of local weather from average climate is highly correlated with the WRs, making this approach a challenging and coherent description of local climates. The atmosphere does not merely evolve around its mean state, but instead spends more time around a few peculiar (large-scale) states with specific consequences for local weather. As a result, the WRs may provide a reliable (and more- over fully objective) framework for building downscaling algorithms appropriate for local climate change studies.