Temporal fluctuations in heat waves at Prague-Klementinum, the Czech Republic, from 1901-97, and their relationships to atmospheric circulation

Temporal fluctuations in heat wave occurrence and severity are analysed in long-term daily temperature series at Prague-Klementinum, the Czech Republic. Although the observations have been continuous since 1775, the period 1901-97 with the most credible data is mainly examined. Most of the warmest summers of the 20th century appeared within the periods 1943-52 and 1992-95; the temporal distribution of heat waves corresponds to this pattern and shows two maxima, in the 1940s to early 1950s and in the 1990s. A very low occurrence of heat waves was typical of the beginning of the 20th century and around 1980. The peak of heat wave severity in the 1940s-early 1950s, as well as their almost total absence in the first two decades of the 20th century, may be a common feature for a large area, as indicated by the comparison between two stations within central Europe, Prague-Klementinum and Basel (Switzerland), and by other studies. An extraordinary heat wave occurred in July and August 1994. It greatly exceeded other heat waves, mainly in the much higher cumulative temperature excess above 30degreesC and a record-breaking duration of a continuous period of tropical days. Relationships between heat wave characteristics in warm and cold decades and circulation conditions were analysed using the subjective Hess-Brezowsky catalogue of weather types (Grosswetterlagen). The link to the atmospheric circulation is evident, e.g. situations with an anticyclone or a ridge over central Europe were more (less) frequent during all the warm (cold) decades. Moreover, the occurrence of long and severe heat waves in the 1990s may reflect an enhanced persistence of the atmospheric circulation over Europe in the summer season because all groups of weather types have considerably increased residence times in 1988-97 compared with long-term means. Copyright (C) 2002 Royal Meteorological Society.