83rd Annual

Thursday, 13 February 2003
Recent changes in atmospheric circulation over Europe detected by objective and subjective methods
Jan Kysely, Institute of Atmospheric Physics, Prague, Czech Republic; and R. Huth
Poster PDF (177.2 kB)
Changes in atmospheric circulation over Europe during the past 40 years 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 for individual seasons, with emphasis on the differences between winter and summer.

Large majority of the most important changes in atmospheric circulation are the same or similar for the objective and subjective methods; namely the strengthening of the zonal flow in winter since the 1960s to the early 1990s; the rise in the frequencies of anticyclonic situations in winter from the late 1960s to the early 1990s, with a subsequent decline, and opposite changes for cyclonic conditions; and the sharp increase in the persistence (measured by the mean residence time) of all circulation "supertypes" in winter and cyclonic types in summer around 1990. Some differences between findings obtained using objective and subjective methods may result from the fact that the objective methods are based on 500 hPa height fields while the Hess-Brezowsky classification considers both the sea level and upper-air fields, and intrinsically from the different approach to classification (e.g., the Hess-Brezowsky weather types have a typical duration of at least 3 days while the objective types typically last 1-3 days).

Generally, changes in atmospheric circulation which took place since 1960s were more pronounced in winter than in summer. The most important change seems to be the considerable increase in the persistence of circulation types in the 1990s.

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