Attribution of the concurrent extreme heatwaves in Northern Europe and Northeast Asia in July 2018

Abstract

In July 2018, Northern Europe and Northeast Asia experienced unprecedented, concurrent extreme heatwaves. Utilizing the HadGEM3-A-N216 attribution model and dynamical adjustment method, we highlight that the internally-generated circulation was the predominant factor triggering these heatwaves both in Northern Europe and Northeast Asia. External forcings further amplify these heatwaves and contribute about 1 K surface air temperature (SAT) anomalies both in Northern Europe and Northeast Asia. Moreover, there is a statistically significant positive correlation of SAT between Northern Europe and Northeast Asia. An internal-generated zonal wave train spanning Eurasia was identified as a key circulation facilitating the concurrent heatwaves by instigating the simultaneous anomalous anticyclones in these two regions. Positive sea surface temperature (SST) anomalies in the North Atlantic plays a pivotal role in the formation of this zonal wave train. The negative SST anomalies in the northern Indian Ocean and tropical central Pacific can further amplify the magnitude of wave train, particularly in the Northeast Asia. In additions, the internal thermodynamic processes, especially soil moisture-SAT feedback can slightly damp the heatwaves in both regions. It is due to enhanced evaporative cooling and a reduction in upward sensible heat flux, linked to increased soil moisture in previous month.