4.1 Increased HWs and EPEs in a warming climate
Our results in HWs are consistent with several previous studies that eastern China has experienced frequent HWs in recent decades, and such HWs are expected to increase in frequency, severity, and duration (Guo et al. 2017, Wang et al. 2017, Dosio et al. 2018, Z. Li et al. 2019, Ning et al. 2022). The number of EPEs significantly increases in eastern China and is projected to continue to rise by 2100 (Wang and Zhou, 2005, Feng et al. 2011, Liu et al. 2015, Zhang et al. 2017, Dong et al. 2020). Specifically, we find that the frequency of EPEs shows upward trends with 0.09±0.01 counts per decade and 0.21±0.01 counts per decade under SSP245 and SSP585 scenarios, the trends of HWN also increased from 0.31±0.01 counts per decade under SSP245 to 0.50±0.02 counts per decade under SSP585 (p < 0.01, Fig. 6a, c).
The increase in the atmospheric water-holding capacity associated with a temperature increase (the Clausius–Clapeyron relation) considerably influences the changes in extreme precipitation intensity at a rate of ~7%/℃ under warmer climates (Pall et al. 2007, Allan and Soden 2008, Utsumi et al. 2011). The increase in AEP and HWT reaches 0.14±0.03 mm per decade and 2.61±0.10 days per decade under the SSP245 scenario, under SSP585 scenario, the frequency of AEP and HWT rises at a rate of 0.37±0.03 mm per decade and 7.77±0.21 days per decade (p < 0.01, Fig. 6b, d). Notably, the trends of HWT and AEP under the SSP585 are 3.0-fold and 2.6-fold higher than those of SSP245, respectively. While the trends of HWN and EPE under the SSP585 are 1.6-fold and 2.3-fold higher than those of SSP245, respectively. This suggests that the intensity of HWs and EPEs changes faster than the frequency.
The HWs are becoming more frequent, longer lasting, and more intense in eastern China under global warming background (Fig. S7). On the other hand, global warming will alter atmospheric circulation and evaporation in some regions, providing a richer source of water for precipitation (IPCC 2018). The synergy of these two factors increases the probability of EPEs in humid regions (Allan and Soden 2008) (Fig. S8). Summer Tasmax anomalies in eastern China are shown to increase by to 1 ℃ in 2020 and are projected to be 2.5 ℃ and 4.8 ℃ warmer under SSP245 and SSP585 scenarios than the period of 1850-1900 (Fig. S9). Consequently, the Yangtze River Basin and its southern regions will face the combined disasters of HWs and EPEs in summer in the future projections, becoming a regional climate change hotspot.
4.2 Underlying mechanisms and connections between HWs and EPEs
Anthropogenic factors contributes to the rising occurrence of heavy precipitation and high-temperature extremes globally (Fischer and Knutti 2015, Dosio et al. 2018). The rapid increase in the risk of summer HWs and EPEs in easter China can also be partially attributed to global warming (Sun et al. 2014, Liu et al. 2015, Freychet et al. 2017). Furthermore, SST anomaly (SSTA) plays an important role in modulating HWs and EPEs in China (Zhu et al. 2011, Wang et al. 2017, Wei et al. 2020). When the SSTAs in the Indian Ocean and Tropical North Atlantic are positive, they contribute to the positive rainfall anomalies in South China and the southeast coast (Fig. 7c, d). The long-term climate variations in China in summer may be related to the warming trend of SST in the Indian Ocean (Hu et al. 2003) and teleconnections from the North Atlantic (Shang et al. 2020). Previous studies have shown that summer precipitation over central eastern and southern China are attributed to the warming trend of the ENSO-like SSTAs in the tropical Pacific, Indian Ocean, and North Atlantic, which will trigger anomalous anticyclonic circulation over Philippine Sea (Yang and Lau, 2004, Weijing Li et al. 2018, J. Liu et al. 2019).
When the SSTAs in the west Pacific, Tropical North Atlantic, and the Indian Ocean are positive, they can cause widespread warming in eastern China (Fig. 7e, g, h). The summer three-ocean SSTAs have a greater influence on Tasmax than on precipitation in eastern China, but the influences have significant variations from region to region. For example, the influence of Indian Ocean SST on precipitation and maximum temperature in eastern China is opposite (Fig. 7c, g). Notably, the west Pacific and Niño 3.4 SST anomalies alone have little influence on the summer precipitation in eastern China (Fig. 7a, b, f), which confirms that tropical Indian Ocean SST warming acts like a capacitor affecting summer climate anomalies over the Indo-western Pacific and East Asia, and the three-ocean interactions through ocean-atmosphere coupling can modulate climate variability (Xie et al. 2009, Cai et al. 2019, Wang 2019).
Moreover, summer EPEs and HWs in eastern China are closely related to the strength and location of the western North Pacific subtropical high (WNPSH), and their influences vary across space (Zhu et al. 2011, Freychet et al. 2017, Zhang et al. 2017). Summer Tasmax and frequency of HWs have a higher correlation with WNPSH than precipitation and frequency of EPEs (Fig. S10). The high value area of the correlation coefficient between anomalous Tasmax and 500-hPa geopotential height (GPH) appeared in the northern and eastern coasts of China, reaching above 0.6 (p < 0.05), the correlation coefficient between the anomalous frequency of HWs and GPH was mainly range from 0.4 to 0.6 (p < 0.05), and the spatial distribution of high value areas is scattered (Fig. S10 c, d). Notably, the intensification of the WNPSH is favorable for more summer HWs in eastern China under present climate, and more monsoon rainfall and HWs in future projections (Q. Liu et al. 2019, X. Chen et al. 2020, Li et al. 2021). In addition to WNPSH, other factors such as the tropical cyclones, Pacific Decadal Oscillation/Interdecadal Pacific Oscillation, and Atlantic Multidecadal Oscillation may contribute to the interdecadal or multidecadal variations in the EPEs and HWs in eastern China (Ding et al. 2009, Zhang et al. 2017). Detailed analysis of the impacts of these longer timescale factors on EPEs and HWs is beyond the scope of this manuscript.
Intuitively, we expect cooler summers when it rains, while heatwaves often accompany droughts (Trenberth and Shea, 2005). In the historical period, the correlation between mean anomalous precipitation and Tasmax is negative in eastern China, while the regions with the positive correlation area appear in the eastern coast of China under SSP245 scenario and expand rapidly under SSP585 scenario (Fig. 8a, c, e). Climate models suggest that EPEs and HWs will become more common in an anthropogenically warmed climate (IPCC, 2014). The correlations between the frequencies of EPEs and HWs shifted from negative in the historical period to positive under future projections (Fig. 8b, d, f). These results reveal a distinct link between future rainfall and temperature, with increased EPEs significantly associated with HWs during summer (Raghavendra et al. 2019). In addition, summer precipitation and maximum temperature were negatively correlated, consistent with the argument that warmer summers tend to be dryer, but this negative correlation is largely reduced in extreme cases.
HWs are strongly linked to global warming, and previous studies have shown a significant increase in global HW activity from present climate to future projections (Hu et al. 2003, Freychet et al. 2017, Dosio et al. 2018, Perkins-Kirkpatrick and Lewis 2020). However, the compound disasters of HWs and EPEs in a warming climate are rarely studied. In eastern China, as the correlations between the frequencies of EPEs and HWs shifts from negative to positive in summer, this will lead to future loss of life and property and enormous socioeconomic consequences. It further supports the urgent need for policymakers to take action to curb greenhouse emissions.