Anthropogenic Changes of Compound Extreme Precipitation Preconditioned by Heatwaves Have Emerged From the Internal Climate Variability in China

Compound extreme precipitation preconditioned by heatwaves (CHEPs) are attracting people's attention nowadays, due to their devastating impacts on ecosystems and society. Although previous studies have widely investigated the historical changes of CHEPs, when these changes emerge from the internal climate variability remains unclear. In this study, we aim to understand the time of emergence (ToE) of CHEPs by utilizing multiple advanced Single Model Initial-condition Large Ensembles (SMILEs) in China. First, we investigate the spatiotemporal changes of CHEP characteristics over a 200-year period. Second, we detect ToE of CHEPs by adopting a signal-to-noise ratio framework. Third, we evaluate the added value of using SMILEs in the ToE analysis. We find that most areas have experienced increases in CHEP characteristic values in the historical period, which are projected to be more remarkable in the future under multiple projection scenarios. The agreement on the spatial distribution among SMILEs implies that anthropogenic forcings have already caused CHEP frequency to emerge from the internal climate variability over more than 84% of grid cells at least by 2020. In addition, earlier emergence can be found in CHEP hotspots. The temporal uncertainty largely derives from the model uncertainty in simulating signal-to-noise ratios for individual events rather than differences in ensemble sizes among SMILEs. Furthermore, ToE can be better constrained based on SMILEs compared to the climate models with fewer members, confirming the added value of using SMILEs. Our findings have implications for climate attribution and provide a scientific basis for mitigating and adapting to climate changes.