A heat wave defined as several consecutive days with positive temperature anomalies.Since the 1950s,heat waves have increased in number and intensified globally,exerting significant impacts on human health,theecosystem,and the social economy.The increase in heat waves,especially extreme events with longer duration and higher amplitude over North China after the late 1990s,is of particular note.However,relevant existing studies have mainly focused on unsorted events or typical cases,and the atmospheric backgrounds of heat waves with different severities are unclear.Therefore,using the CN05.1 daily maximum temperature and NCEP/NCAR reanalysis data for the period of 1961—2020,this study identifies severe and mild heat wave events over North China,then selects the years with high frequencies of such events.On this basis,the spatiotemporal variation and atmospheric background of severe and mild heat waves events over North China are investigated through composite analysis.The results show that high frequencies of severe and mild heat wave events mainly appear in the northern part and the middle and southern parts of North China,respectively.For both types of events,their frequencies averaged over North China have shown a significant increasing trend since the 1960s,with a higher occurrence rate of severe heat waves (0.09 events per decade) than mild ones (0.03 events per decade).Corresponding to high frequencies of severe heat waves in North China,an anomalous anticyclonic circulation with the quasi-equivalent barotropic structure dominates the mid-latitudes of East Asia,and the North China domain is located in its southeast flank.Against this background,anomalous subsidence and hence reduction of cloud cover and humidity appear in North China.In addition to the adiabatic heating of the subsidence,the clear-dry air over North China is conducive to stronger solar radiation reaching the land surface.Accordingly,the outgoing longwave radiation and sensible heat from the land surface are enhanced,thus warming the overlying atmosphere over the region.Incomparison,associated with high frequencies of mild heat wave events in North China is an anomalous dipole pattern with the anticyclonic and cyclonic anomalies respectively residing on the southeast and northwest sides of North China,and the target region is under the control of southwesterly anomalies.The southwesterly anomaly can heat the atmosphere through warm advection transport toward North China.At the same time,it also increases humidity over North China by transporting water vapor from the ocean to the region.The increased humidity may capture and reflect more longwave radiation onto the surface,thus favoring the warming of overlying atmosphere.Ingeneral,the occurrence of severe heat waves is affected by the joint contribution of adiabatic heating due to abnormal subsidence,solar radiation and sensible heating,exhibiting a dry-hot nature.The occurrence of mild heat waves is dominated by the greenhouse effect induced by abundant water vapor,and horizontal temperature advection,exhibiting a wet-hot nature.There is a discernible difference in the atmospheric background of severe and mild heat waves.