全球变化知识资源中心

A large-scale high air temperature event in China during July 2017 is examined using observational reanalysis data. Based on this case study, summer extreme hot events (EHEs) over China are classified into five types using the self-organizing map (SOM) method. The five EHE types include those over northeast China (NEC type), northwest China (NWC type), north China (NC type), mid-eastern China (MEC type) and south China (SC type). All types are associated with sustainable anticyclones (EHE anticyclones). Composite analysis shows that the EHE anticyclone in northern China (corresponding to the NEC, NWC and NC types) is primarily formed by downstream wave train propagation from upstream blocking, while EHE anticyclone formation in southern China (corresponding to the MEC and SC types) is driven by the joint action of downstream wave train propagation and westward movement of the western Pacific subtropical high. Moreover, the upstream blocking and downstream EHE anticyclone embedded in the wave train play important roles in maintaining the wave train. The upstream blocking acts as an energy source for downstream EHE anticyclone intensification and leads to EHE formation, with the intensified zonal wind at certain latitudes affecting the wave train propagation direction. This finding indicates that the upstream blocking with intensified zonal wind at certain latitudes may serve as an indicator for the prediction of EHE anticyclones over China. As a consequence of the persistent EHE anticyclone, the land surface is primarily warmed by enhanced shortwave radiation through reduced regional cloud cover. For the NWC and NC types, heating of the lower atmosphere takes place via the sensible heat flux; whereas, for the NEC, MEC and SC types, the main heating terms are the latent heat flux, sensible heat flux and upward longwave radiation.