Regional compound humidity-heat extremes in the mid-lower reaches of the Yangtze River: a dynamical systems perspective

Heat extremes including heatwaves have an adverse impact not only on ecosystems but also on human health. The impact can be seriously exacerbated when both spatial extension and compound factors (such as humidity) are included. However, a unified frame combining compound humidity-heat extremes with their regional extension has received little scientific attention. This study solves this problem by taking the evolution of daily mean 2 m air temperature (Tmean) and relative humidity (RH) over a large domain as two dynamical systems (DSs), then the instantaneous coupling index from the DS method combined with clustering analysis can sort out the regional compound humidity-heat extremes with distinct spatial organized structures. Among them, the compound humidity-heat extremes with dipole Tmean and RH patterns may be missed by the methods based on regional averaging or undiscerned by DS method. Moreover, the mechanisms behind these regional compound humidity-heat extremes with dipole pattern are distinctive on both dynamics and thermodynamics, with a dipole structure found in the atmospheric low-level circulation. These novel findings can contribute considerably to the in-depth understanding on the compound humidity-heat extremes and their mechanisms.