The drivers of summer extreme temperature trends in Europe

The frequency, duration and intensity of summer extreme temperatures over Europe have increased since the mid-20th century due to dynamic changes, thermodynamic factors, and their interaction via land—atmosphere feedbacks. However, a comprehensive analysis of all the mechanisms underlying their future trends, including an assessment of uncertainties due to inter-model differences and internal variability, is still lacking.

In this study, we investigate historical and future trends in the occurrence of atmospheric circulation patterns that triggered the three most intense heat waves during 1940—2022, identified using the Heat Wave Magnitude Index daily (Russo et al., 2015): the 2010 Russian, the 1972 Scandinavian and the 2003 French heat wave. To do that, we adopt the atmospheric flow analogue technique. We then decompose the trends of summer extreme temperature occurrences associated with these analogues in their thermodynamic, dynamic and interaction components, following Horton et al. (2015). The analyses are performed using large ensemble of climatic projections from six different models (three CMIP5 and three CMIP6), under the “business-as-usual" emission scenario. This approach allows us to investigate the role of the global warming, internal climate variability and model uncertainties on the European extreme temperature trends.

The results show a future increase in the occurrence of atmospheric circulation patterns similar to the 2003 French heat wave across all models. However, models generally underestimate observed historical trends, suggesting that future trends may be even higher. Furthermore, the results show that the extreme temperature occurrences associated with these analogues have increased in the historical period and will keep increasing in the future. In this context, trend partition analysis indicates that, while the historical trends were primarily driven by thermodynamic component, the future trends will be mainly driven by the interaction term. Interestingly, the interaction and dynamic components will explain a larger percentage of the total trend compared to the past, while the thermodynamic contribution will become less significant. Finally, the results suggest that land—atmosphere coupling processes will play a critical role in explaining the physical meaning of future interaction term and, thus, in driving projected increase in extreme temperature occurrences.

Results for the 2010 Russian and 1972 Finland heat waves generally align with those of the 2003 French heat wave. However, their dynamic trends are subjected to a certain degree of uncertainty due to inter-model differences, limiting the reliability of future dynamic projections and trend partition.

Bibliography

Horton, D. E., et al. (2015). Contribution of changes in atmospheric circulation patterns to extreme temperature trends. Nature, 522 (7557), 465-469.

Russo, S., et al., (2015). Top ten European heatwaves since 1950 and their occurrence in the coming decades. Environmental Research Letters, 10 (12), 124003.