Abstract
We use the 100-member Max Planck Institute Grand Ensemble (MPI-GE) to disentangle the contributions from colocated dynamic atmospheric conditions and local thermodynamic effects of moisture limitation as drivers of variability in European summer heat extremes. Using a novel extreme event definition, we find that heat extremes with respect to the evolving mean climate increase by 70% under a moderate warming scenario during the twenty-first century. With a multiple regression approach, we find that the dynamical mechanisms representing blocking and anticyclonic conditions are the main driver of variability in extreme European summer temperatures, both in past and future climates. By contrast, local thermodynamic drivers play a secondary role in explaining the total variability in extreme temperatures. We also find that considering both dynamical and thermodynamical sources of variability simultaneously is crucial. Assessing only one type of drivers leads to an overestimation of their effect on extreme temperatures, particularly when considering only thermodynamical drivers. Lastly, we find that although most past and future heat extremes occur under favorable dynamical atmospheric conditions; this occurs 10–40% less frequently over Central Europe in the twenty-first century. By contrast, heat extremes over Central Europe occur 40% more frequently under concurrent extreme moisture limitation in the twenty-first Century. Our findings highlight a new type of neutral-atmosphere, moisture-driven heat extremes, and confirm that the increase in European heat extremes and associated variability increase are dominated by the local thermodynamic effect of moisture limitation.
Highlights
The frequency and intensity of extreme summer heat events are projected to increase over Europe as a response to rising global mean temperatures (Stott et al 2004; Christidis et al 2014; Suarez-Gutierrez et al 2018)
To understand the background conditions that lead to heat extremes and which mechanisms dominate the variability of extreme summer temperatures over Europe, we develop a multiple regression model based on the variables describing the large-scale dynamic atmospheric state (Z500, sea level pressure (SLP)) and local thermodynamical effects of moisture availability (SM, ET), as well as the annual global mean surface temperature (GMST), as predictors of extreme European summer temperatures (EuSTs)
Our results indicate that the colocated dynamical mechanisms combining the effects of Z500 and SLP are the dominating driver of extreme European summer temperature variability
Summary
The frequency and intensity of extreme summer heat events are projected to increase over Europe as a response to rising global mean temperatures (Stott et al 2004; Christidis et al 2014; Suarez-Gutierrez et al 2018). Some studies argue that the distribution of European summer extreme temperatures in a warmer climate mostly follows the mean summer warming (Ballester et al 2009, 2010), the variability in European summer temperatures is large, and the evidence indicating that this variability could increase under global warming, potentially resulting in extremes that warm more than the mean summer climate, is robust (Fischer et al 2012; Lustenberger et al 2014; Bathiany et al 2018) It remains unclear which of the two main driving mechanisms of extreme European summer temperatures controls this increase in variability. Other studies point to the large-scale dynamical atmospheric state as
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