Disentangling internal and external impacts on increasing compound heat waves over the Yangtze River valley

The daytime-nighttime compound heat wave (CHW) exerts substantial impacts on human health due to its accumulative influence persisting all day, which draws increasing attention under current global warming (GW). The impact of anthropogenic activities on CHWs has been emphasized. Yet, how and to what extent internal climate variability influences their variation remains unclear. Here, we find the CHWs over the Yangtze River valley (YRV) show an oscillatory upward trend from 1961 to 2021, featuring a marked weakening during 1981–1999 and a pronounced intensification afterwards. This decadal fluctuation and upward trend are jointly modulated by GW and Pacific/Atlantic decadal variability. A quantified analysis shows that the GW contributes 36.9% while the decadal modes jointly contribute 63.1% to the total CHW changes. More interesting, this oscillatory increase in CHW is closely related to the distinct changes in seasonal-mean maximum and minimum temperatures. The daytime maximum temperature in the YRV primarily exhibits a warm-cold-warm decadal variation, notably induced by the Pacific Decadal Oscillation, while the nighttime minimum temperature is dominated by an increasing trend due to the GW and the subsequent influence of the Atlantic Multidecadal Oscillation. The large ensemble simulation also verifies the synergistic effects of external forcing and internal variability on the CHW increase. Our study sheds lights on CHW changes and its decadal prediction.