Changes in atmospheric circulation over Europe detected by objective and subjective methods

Changes in atmospheric circulation over Europe since 1958 were examined using both objective (modes of low-frequency variability and objective classification of circulation types) and subjective (Hess-Brezowsky classification of weather types) methods. The analysis was performed with an emphasis on the differences between the winter (DJF) and summer (JJA) seasons, and between objectively and subjectively based results. Majority of the most important changes in atmospheric circulation are same or similar for the objective and subjective methods: they include the strengthening of the zonal flow in winter since the 1960s to the early 1990s; the increase (decrease) in frequency of anticyclonic (cyclonic) types in winter from the late 1960s to the early 1990s, with a subsequent decline (rise); and the sharp increase in the persistence (measured by the mean residence time) of all groups of circulation types in winter around 1990 and of anticyclonic types in summer during the 1990s. Differences between the findings obtained using the objective and subjective methods may result from the intrinsically different approach to the classification (e.g. the Hess-Brezowsky weather types have a typical duration of at least 3 days while objective types typically last 1-3 days). Generally, changes in atmospheric circulation which have taken place since the 1960s were more pronounced in winter than in summer. The most conspicuous change seems to be the considerable increase in the persistence of circulation types during the 1990s, which may be also reflected in the increase in the occurrence of climatic extremes observed in Europe during recent years.