The Thermodynamic and Dynamic Cause Analysis of Three Extensive Compound Heatwaves from 2011 to 2024 in Mainland Spain

In recent years, frequent heatwaves (HWs) in Spain have increased mortality rates and impacted ecosystems. While most studies only investigate the causes of HWs in a single year, we analyzed the thermodynamic and dynamic causes of three extensive compound HWs (defined as concurrent daytime and nighttime high temperatures) over mainland Spain during the 2011–2024 summers using station and reanalysis data. In addition, we explained the differences in the duration of the three HWs in terms of thermodynamic processes and the evolution of large-scale circulation systems. For thermodynamic analysis, we applied the first law of thermodynamics to examine local temperature variations and the surface energy balance to assess solar radiation and soil moisture impacts on HWs. It was found that high temperatures occurred more frequently over mainland Spain during 2015–2024 compared with 2011–2014. The thermodynamic analysis indicates negative contributions from horizontal advection, positive contributions from adiabatic heating, and a dominant positive contribution from diabatic heating in the formation of the three HWs. Although we observed anomalously increased solar radiation during the three HWs, soil moisture deficit was the primary factor in HW formation. The dynamic analysis shows that a similar large-scale circulation configuration prevailed over mainland Spain during the three HWs. The region was simultaneously controlled by an anomalously intense Azores High and the ridge line of a warm high-pressure ridge, accompanied by a weak divergent flow. This work offers valuable insights for the study of HWs in Spain and helps to understand the universal mechanism behind the HWs.