Similarities and Differences in the Mechanisms Causing the European Summer Heatwaves in 2003, 2010, and 2018

The mechanisms causing European heatwaves in recent years, particularly their differences among several heatwaves, are poorly understood. Here atmospheric circulation anomalies and soil moisture-temperature coupling during the summer 2018 heatwave are comprehensively examined and compared with the 2003 and 2010 heatwaves using ERA5 reanalysis, model simulations, and eddy covariance flux measurements. We show that the 2018 heatwave successively affected northern and central Europe, and the peak temperature in Finland and northwest Russia broke historical records of the past 40 years. Although three heatwaves were all initially triggered by atmospheric circulation anomalies, the strong moisture-temperature coupling were found to further strengthen the 2003 and 2010 heatwaves. This coupling was also strong in central Europe during 2018 heatwave, but was weak in the northern European center of the heatwaves. The high temperature in 2018 was mainly due to increases in the amount of net surface radiation caused by the clear skies associated with reduced precipitation. Furthermore, we also find that land cover plays a critical role in determining the occurrence and strength of soil moisture-temperature coupling. Cropland/grassland depletes soil moisture more readily than forests, thereby triggering a more rapid release of sensible fluxes as observed during 2018 heatwave.