This study indicates a significant variation of humidity on extreme heat (EH) days over South Korea and southern–central Japan during the period 1979–2018. EH is therefore classified into three categories: type-A and type-B wet EH, and dry EH. Their statistical characteristics and formation mechanisms are investigated and compared. Our results suggest that the type-A wet EH is the most destructive, with the highest intensity, longest duration and broadest spatial scale covering most of mid-latitude East Asia. By contrast, type-B wet EH and dry EH are weaker, shorter and mostly confined to northeast Asia. Despite these differences in characteristics, both types of wet EH are caused by the poleward advance of tropical warm and humid air masses as a result of the northward displacement of the Asian westerly jet. By contrast, dry EH is primarily induced by an increase in adiabatic heating and solar radiation resulting from anomalous subsidence.
    The three types of EH are associated with distinct large-scale teleconnections over Eurasia. A stable and persistent tripole wave pattern is responsible for type-A wet EH. The activity of atmospheric blocking over northern Europe, where the pattern originates, plays a crucial role in maintaining this pattern. By contrast, type-B wet EH and dry EH are related to a quadruple pattern and a Silk Road pattern-like teleconnection, respectively, both lasting for a shorter time. These results highlight the diversity of EH, which suggests that multiple local and large-scale circulations should be considered to improve the forecast skills for EH.
 

extreme heat,large-scale circulation,northeast Asia