Abstract
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.
Highlights
Thirty-year periods are used and recommended in climatology as basic periods for the calculation of climate normals when it is supposed that a 30-year interval is long enough to express relatively representative and stable climatic patterns
From a comparison of circulation types and several climatic variables of the CR for two 30-year normal periods of the 1961–2020 period, the following conclusions can be shortly summarised: (i) Mean frequencies of anticyclonic and cyclonic circulation types according to the objective classification express generally significant changes between both 30-year normal sub-periods
An increase in annual, MAM, and JJA means in 1991–2020 compared to the preceding period was statistically significant; (iii) Mean, maximum, and minimum temperatures display different patterns in two 30year normal periods in accord with recent warming. They are reflected in statistically significant differences in means, characteristics of distribution, density functions, and significant linear trends through 1991–2020
Summary
Thirty-year periods are used and recommended in climatology as basic periods for the calculation of climate (climatological) normals when it is supposed that a 30-year interval is long enough to express relatively representative and stable climatic patterns. Given the recent climate warming starting in the late 1970s, which intensified from the 1980s [5,6,7], there is a question of how the recent normal period of 1991–2020 expresses its climatic patterns compared to that of 1961–1990. We are trying to document this by carrying out a comparison of both 30-year climate normals for the homogenised annual and seasonal series of several climatic variables calculated for the territory of the Czech Republic during the 1961–2020 period. Despite the fact that the previous studies in the Czech Republic dealt with incomplete 60-year periods (for example, Brázdil et al [9] with wind speed, Brázdil et al [10] with precipitation or Zahradnícek et al [11] with temperatures), the last study by Zahradnícek et al [12], which dealt with temperature extremes during the entire 60-year period 1961–2020, clearly demonstrates significant differences between both 30-year normal sub-periods
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