Modeling European hot spells using extreme value analysis

Atmospheric blocking in mainland Europe is often cited as the cause of extremely high temperatures lasting several days. By definition, extreme temperatures are rare, and yet the theory of extreme value statistics has seldom been applied to quantify the influence of atmospheric blocking on hot spells. Similarly, a comparison of the relative influence of other well-known atmospheric drivers, such as the North Atlantic Oscillation (NAO) and the El Nino-Southern Oscillation (ENSO), has seldom been explored. We applied a novel combination of extreme value and geometric distributions to observed daily temperature maxima from 74 stations across Europe, covering 1951-2010, to establish a stationary model of the expected magnitude, frequency and duration of hot spells that did not explicitly account for atmospheric drivers. Monthly time series of NAO, ENSO and 4 coherent atmospheric blocking regions were then incorporated as non-stationary covariates in the distribution parameter estimates to assess the dependence of hot spells on atmospheric covariates. We concluded that ENSO does not have a significant influence on hot spell magnitude or frequency; the NAO is a significant driver of hot spell magnitude (maximum attained temperature), frequency (annual event count) and duration (length of event) in northern Europe and Atlantic bordering stations; and atmospheric blocking is a significant driver of all aspects of hot spells in all parts of Europe. While NAO may increase peak temperatures by 2-4 degrees C only in the north, relatively strong atmospheric blocking could result in increased temperatures of at least 4 degrees C higher across Europe, with a commensurate increase in hot spell duration of 2-4 d.