Frequency and within-type variations of large-scale circulation types and their effects on low-frequency climate variability in Central Europe since 1780

This study investigates the importance of large-scale atmospheric circulation changes for Central European climate variations during the last two centuries. On the basis of an objective classification of monthly mean sea level pressure (SLP) grids reconstructed back to 1780 and monthly historical station data for the same period, temperature and precipitation changes in Central Europe since 1780 are decomposed into two parts; one part due to frequency changes of large-scale circulation types, and the other part caused by (dynamic and climatic) changes within these circulation types. This is achieved by applying a particular decomposition scheme for moving 31-year time windows during the 1780-1995 period. Results indicate that large parts of the long-term variations in Central European climate cannot be explained sufficiently by frequency changes of circulation types. Roughly one half of these variations - even up to 80% during July - can be ascribed to varying internal properties of some major circulation types. Percentages of frequency-related and within-type-related climate changes are seen to vary on decadal to multidecadal time scales, thus implying that relationships between large-scale circulation patterns and regional climates are characterised by distinct instationarities. Furthermore, regional climate variations, being attributable to within-type changes of major circulation types, can only partly be explained by corresponding variations in dynamic properties (vorticity, intensity) of these circulation types. This points to the importance of further sources for within-type variability, including subgrid-scale processes, synoptic-scale variations, and modifications of the climatic boundary conditions. Copyright (c) 2006 Royal Meteorological Society.