Abstract
Abstract. In the last decades, extremely hot summers (hereafter extreme summers) have challenged societies worldwide through their adverse ecological, economic and public-health effects. In this study, extreme summers are identified at all grid points in the Northern Hemisphere in the upper tail of the June–July–August (JJA) seasonal mean 2 m temperature (T2m) distribution, separately in ERA-Interim (ERAI) re-analyses and in 700 simulated years with the Community Earth System Model (CESM) large ensemble for present-day climate conditions. A novel approach is introduced to characterise the substructure of extreme summers, i.e. to elucidate whether an extreme summer is mainly the result of the warmest days being anomalously hot, of the coldest days being anomalously mild or of a general shift towards warmer temperatures on all days of the season. Such a statistical characterisation can be obtained from considering so-called rank day anomalies for each extreme summer – that is, by sorting the 92 daily mean T2m values of an extreme summer and by calculating, for every rank, the deviation from the climatological mean rank value of T2m. Applying this method in the entire Northern Hemisphere reveals spatially strongly varying extreme-summer substructures, which agree remarkably well in the re-analysis and climate model data sets. For example, in eastern India the hottest 30 d of an extreme summer contribute more than 65 % to the total extreme-summer T2m anomaly, while the colder days are close to climatology. In the high Arctic, however, extreme summers occur when the coldest 30 d are substantially warmer than they are climatologically. Furthermore, in roughly half of the Northern Hemisphere land area, the coldest third of summer days contributes more to extreme summers than the hottest third, which highlights that milder-than-normal coldest summer days are a key ingredient of many extreme summers. In certain regions, e.g. over western Europe and western Russia, the substructure of different extreme summers shows large variability and no common characteristic substructure emerges. Furthermore, we show that the typical extreme-summer substructure in a certain region is directly related to the region's overall T2m rank day variability pattern. This indicates that in regions where the warmest summer days vary particularly strongly from one year to the other, these warmest days are also particularly anomalous in extreme summers (and analogously for regions where variability is largest for the coldest days). Finally, for three selected regions, thermodynamic and dynamical causes of extreme-summer substructures are briefly discussed, indicating that, for instance, the onset of monsoons, physical boundaries like the sea ice edge or the frequency of occurrence of Rossby wave breaking strongly determines the substructure of extreme summers in certain regions.
Highlights
Numerous high-impact hottemperature extremes occurred on approximately seasonal timescales, including the extremely hot European summer in 2003 (Fink et al, 2004; Schär and Jendritzky, 2004), the 2010 Russian heat wave (Barriopedro et al, 2011), the hot and dry summer of 2015 in Europe (Dong et al, 2016; Hoy et al, 2017; Orth et al, 2016), the hot and humid summer of 2015 in western India and Pakistan (Wehner et al, 2016), and the concurrent heat waves across the Northern Hemisphere in the summer of 2018 (Vogel et al, 2019)
Extreme summers are defined in the upper tail of the June– July–August (JJA) seasonal mean T2m distribution at each grid point in the Northern Hemisphere and analysed with regard to their substructure
All days are ranked within their respective season and compared to the climatological T2m of all days with the same rank
Summary
Numerous high-impact hottemperature extremes occurred on approximately seasonal timescales, including the extremely hot European summer in 2003 (Fink et al, 2004; Schär and Jendritzky, 2004), the 2010 Russian heat wave (Barriopedro et al, 2011), the hot and dry summer of 2015 in Europe (Dong et al, 2016; Hoy et al, 2017; Orth et al, 2016), the hot and humid summer of 2015 in western India and Pakistan (Wehner et al, 2016), and the concurrent heat waves across the Northern Hemisphere in the summer of 2018 (Vogel et al, 2019). Extreme summers require a temporal organisation of the relevant synoptic flow features, which can occur either “by chance” (internal atmospheric variability) or favoured by more slowly varying processes Possible candidates for the latter are soil moisture fluctuations (Fischer et al, 2007; Lorenz et al, 2010; Seneviratne et al, 2010), sea ice dynamics (Cohen et al, 2014), or large-scale modes of variability in the ocean and atmosphere A summer might be an extreme summer because the hottest days of the season are anomalous, with the remainder of the summer days being only moderately warmer than or even close to climatology Such an extremesummer substructure was observed in large parts of Europe in the summer of 2015, when the anomalies of the seasonal hottest days exceeded those of the seasonal mean by almost a factor of 2 (Dong et al, 2016). Illustrate physical causes of the observed (and simulated) extreme-summer substructures in selected regions
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