Unveiling remote and local drivers for the interdecadal increase in summer extreme high temperature days over southern China

Summer extreme high temperature days (EHTD) over southern China undergoes a pronounced interdecadal increase in the early 2000s, but its drivers are unclear. Here, we show that the increased EHTD can be mainly attributed to two critical drivers: the phase transitions of both the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), and decreased local soil moisture. Analyses using observational data and model simulations indicate that a phase shift in the early 2000s—with the AMO turning positive and the PDO turning negative—triggered a Rossby wave train, which induced an equivalent barotropic high-pressure anomaly over southern China. Consequently, southern China received more incoming solar radiation, which heated land surface and the overlying air, increasing the likelihood of EHTD. Meanwhile, local soil moisture decreased, which further enhanced EHTD by intensifying the land‒air coupling strength and partitioning more net surface radiation into sensible heat flux. Based on the dynamic adjustment approach, the phase transitions of the AMO and PDO can lead to 47.4% of the soil moisture deficiency over southern China via reducing precipitation and accelerating evapotranspiration. Our finding can enrich the physical understanding and interpretation of the EHTD worldwide and expand the current theory on local soil moisture feedback.