What causes extreme hot days in Europe?

Days of temperature extremes are known to impact significantly on, among other things, human health and mortality, ecological systems, infrastructure and agriculture. It is therefore not surprising that in the presence of rapid climate change considerable research activity is being directed at documenting how these temperature extremes are changing at present and how they might be expected to change in the future, as well as at identifying the fundamental atmospheric mechanisms associated with such changes. The Russian heatwave in 2010, the worst event in Europe since at least 1950, led to a death toll of 55 000 people and some US$15 billion of total economic loss (Ragone et al 2018). The second most extreme heatwave, the 2003 western European event, resulted in the deaths of more than 70 000 people. The fact that these two extremes have occurred in the last two decades is consistent with the analysis of Li et al (2018) which shows that persistent changes to temperature extremes have already occurred over large parts of the Earth.

In recent times researchers have devised various ways of using climate models to identify the specific atmospheric factors which cause extreme events. In particular, attention has been devoted to estimating the effect of climate change on the roles played by the dynamics (i.e. the atmospheric circulation) and by the thermodynamics (the regional heat budget) (Sánchez-Benítez et al 2018). The impact of the latter is fairly well understood from the basic physics, whereas the part played by the atmospheric dynamics is much more complex and ambiguous. The paper of Jézéquel et al (2018) makes a significant contribution to the analysis of the 2003 and 2010 events, focussing particularly on the dynamics, by exploring atmospheric circulation patterns on specific hot days. They quantify the changes in the occurrence of circulation patterns related to such hot days, and investigate whether those circulation patterns are becoming more or less frequent. The very hot days they identify are related to atmospheric 'blockings', these being quasi-stationary, long-lasting anticyclonic circulations. A number of recent investigations have also pointed to various aspects of the key role played by blocking in affecting extremes and their trends (Xue et al 2017, Martineau et al 2017, Sousa et al 2018, Kim et al 2018, Fragkoulidis et al 2018, Schaller et al 2018).

Jézéquel et al's (2018) analysis also casts light on the contribution of increasing trends in anticyclonic circulations to hot extremes over portions of Eurasia, and the contribution of increasing trends in northerly flow to winter cold extremes over central Asia (Horton et al 2015), the region to the east of the block. This 'dipole' of temperature extremes is now also being intensively studied in the context of increasing blocking (Luo et al 2016, Yao et al 2017). Another valuable aspect of their discussion is that Jézéquel et al (2018) remind us of some of the potential dangers of over-interpreting the physical meaning of atmospheric composites associated with specific hot events, by pointing out that such composites identify necessary, but not sufficient, conditions for a heatwave to develop (Boschat et al 2016).

The neat method developed by Jézéquel et al (2018), and applied to individual days, is valuable in identifying the synoptic environment associated with dangerous heatwaves. (The authors comment that in the present paper they do not address the question of the attribution of these two extreme European summers to climate change, but flag that they will do so in further studies. This next step is to be welcomed, as there is now an appetite for new and insightful attribution methods (Lloyd and Oreskes 2018, Wilcox et al 2018).) To add to its utility the method involves little computation expense and is easy to implement. Given these advantages and the insights their method enables, one can imagine it being readily adopted by the weather and climate extremes community.