Circulation vs. climatic changes over the Czech Republic: A comprehensive study based on the COST733 database of atmospheric circulation classifications

We have analyzed the effect of changes in the frequency of circulation types on significant seasonal trends of eight climatic variables (maximum, minimum, and average daily temperature, precipitation amount and occurrence, relative humidity, cloudiness, and sunshine duration) at 21 Czech stations in the period 1961-1998 using 24 objective and two subjective atmospheric circulation classifications collected and developed within the COST733 Action. Using two independent methods, we show that recent climatic trends can be only partly explained by changes in the frequency of circulation types. In winter, such changes in the objective catalogues explain around 30% of long-term linear temperature trends and around 50% of temperature changes that took place between the first and the second half of the study period. Changing internal properties of individual circulation types (i.e. within-type climatic changes) are responsible for a major part of the observed climatic trends in spring, summer, and autumn; but also in winter for variables other than temperature. The subjective catalogues are usually more tightly connected with the observed climatic changes than the objective ones. These results are consistent with the fact that except for winter, very few circulation types in the objective catalogues yield significant long-term trends in their seasonal occurrence. On the contrary, both used versions of the subjective Hess-Brezowsky catalogue bear a substantial proportion of significant trends in the occurrence of circulation types in all seasons. Very high variability in the results, caused both by the variability within stations and within the circulation classifications used, suggests that such a comparative approach is highly desirable in synoptic-climatological studies. (C) 2009 Elsevier Ltd. All rights reserved.