Circulation and Climate Variability in the Czech Republic between 1961 and 2020: A Comparison of Changes for Two “Normal” Periods

Thirty-year periods are treated in climatology as spans with relatively representative and stable climatic patterns, which can be used for calculating climate normals. Annual and seasonal series of circulation types were used to compare two 30-year sub-periods, 1961–1990 and 1991–2020, the second one being strongly influenced by recent global warming. This analysis was conducted according to the objective classification of circulation types and the climatic characteristics of sunshine duration, temperature, humidity, precipitation, and wind speed as calculated for the territory of the Czech Republic during the 1961–2020 period. For both sub-periods, their statistical characteristics were calculated, and the statistical significance of differences between them was evaluated. There was a statistically significant increase in the annual frequencies of anticyclonic circulation types and a significant decrease in cyclonic circulation types during 1991–2020 compared with 1961–1990. Generally, in both 30-year periods, significant differences in means, variability, characteristics of distribution, density functions, and linear trends appear for all climatic variables analysed except precipitation. This indicates that the recent 30-year “normal” period of 1991–2020, known to be influenced more by recent climate change, is by its climatic characteristics unrepresentative of the stable climatic patterns of previous 30-year periods. Annual and seasonal sunshine duration series do no express significant changes between two 30-year normal periods in terms of their variability, characteristics of distribution, density functions and linear trends. Only increase in annual, MAM and JJA means in 1991–2020 compared to the preceding period was statistically significant. Mean, maximum and minimum temperatures display quite different patterns in two 30-year normal periods in accord with recent warming. They are reflected in statistically significant differences in means, characteristics of distribution, density functions and in significant linear trends in 1991–2020 (annual, JJA, SON). It is pronounced particularly for JJA series. Statistically significant decreases in means of relative humidity between two 30-year normal periods (except SON) are reflected in a significant shift of density functions to lower values. But decreasing linear trends were significant only for MAM. Increasing variability in relative humidity was significant for annual and MAM series. Precipitation totals of the both 30-year periods are represented well by insignificant linear trends, no substantial changes in mean and variability as well as in the character of their distribution represented by the density functions. Wind speeds in two 30-year normal periods represents quite different patterns expressed by statistically significant decreasing linear trends in 1991–2020, significant differences in means, partly in variability and skewness (annual, MAM) and in density functions.