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This paper presents the projected changes in daytime-nighttime compound heat waves (HWs) (i.e. concurrent occurrence of HWs both in daytime and nighttime) and associated population exposure in China under the shared socioeconomic pathway (SSP)2-4.5 and SSP5-8.5 scenarios based on the Coupled Model Intercomparison Project phase 6 simulations. A comparison with the changes in daytime HWs (i.e. occurring only in daytime) or nighttime HWs (i.e. occurring only in nighttime) is also conducted. The results generally indicate an aggravated risk of compound HWs in China in the future under warmer scenarios. On the national average, the compound HWs are projected to increase persistently toward the end of the 21st century, with larger increase under SSP5-8.5 than that under SSP2-4.5. The greatest changes occur in northwest China and southern China. Compared with the daytime or nighttime HWs, the projected increase of compound HWs is the greatest. Accordingly, the proportion of compound HWs to the total HW events tends to increase and that of daytime HWs tends to decrease toward the end of the 21st century. The substantial increases in the frequency of compound HWs are expected to cause a significant increase in population exposure across the entire country. The projected increase of nationally averaged population exposure is 12.2-fold (7.9-fold) of the current in the mid-century (2046–2065) and further enhances to 16.3-fold (12.4-fold) in the end-century (2081–2100) under SSP5-8.5 (SSP2-4.5). The largest increases are distributed in western China and southern China. These findings raise the necessity and urgency for policy-makers and the public to develop measurements to address compound HW risks.

Heat waves, can be exacerbated by the co-occurrence of daytime heat waves and nighttime heat waves. Over China, the Yangtze–Huaihe River basin (YHRB) is the core region of the occurrence of such “compound heat waves”, which exert profound impacts on the society and ecosystems. However, the physical mechanisms responsible for the variability of the YHRB compound heat waves remain unclear. In this study, the interannual variability of YHRB compound heat waves in peak summer (July–August) and its possible causes are investigated based on station observations across China and global reanalysis datasets. A strong link is found between the previous winter Arctic Oscillation (AO) and these peak-summer compound heat waves. During a negative AO in winter, an anomalous tripolar pattern of sea surface temperature (SST) in the North Atlantic is induced by the AO-related atmospheric circulation. Such tripolar SST pattern can persist until the following summer. By that time, oceanic forcing dominates, and positive SST anomalies over the tropical North Atlantic can excite a Rossby wave train propagating eastward from the southwest coast of North America to East Asia. This results in a northwest-southeast tilted high pressure system over the YHRB, favoring the occurrence of peak-summer compound heat waves. Such a delayed influence of the winter AO, together with the North Atlantic capacitor effect, is clearly seen in a case study of the YHBR compound heat waves in 2010. The proposed mechanism is further verified based on numerical experiments with an atmospheric general circulation model.

Compound heat wave (CompoundHW) has attracted extensive attention for its prolonged extreme heat from daytime to nighttime during its process. However, the performance of identifying and characterizing CompoundHW across different datasets has not been systematically evaluated. Here, we compared the similarities and differences of the ERA5, Berkeley Earth, CHIRTS and CPC datasets in identifying and characterizing CompoundHW. Results showed that the match of CompoundHW identification between datasets was consistent in both temporal and spatial dimensions, with the highest match observed between the ERA5 and CHIRTS datasets. Match of CompoundHW identification exhibited significant correlation with the density of observation stations, with matching rates above 50% in regions with dense observation networks, but extremely low match in regions with sparse data coverage. The rising trends of the CompoundHW metrics were captured by all datasets, especially in parts of North America, Europe, western Russia and Asia. Despite differences in the amplitude of CompoundHW changes across the four datasets, over 42% of global regions concurred on the changes in CompoundHW frequency, duration, and magnitude, and more than 27% agreed on the changes in the proportion of CompoundHW occurrences. Inconsistencies of CompoundHW changes were predominantly observed in regions with low matching rates, indicating that precise identification of CompoundHW is the basis for characterizing the changes in CompoudHW characteristics accurately. This study highlights the importance of multiple datasets comparison in heat wave research, especially in metrics defined by multiple climate variables and regions with sparse observational data.

This study analyses the variability of daytime-only, nighttime-only, and compound heat waves (HWs) and their impact on population exposure across India using shared socioeconomic pathways (SSPs) scenarios (SSP126, SSP245, SSP370, and SSP585) from the Coupled Model Intercomparison Project Phase 6 experiment. The research questions addressed are: (1) what effects might compound heatwaves have under climate change scenarios? (2) How are compound heatwaves expected to impact the population in the future? The outcomes indicate that the compound HWs may increase by 4.6 events annually in Northwest India (NWI) under the SSP585 scenario. In contrast, daytime-only HWs are expected to decline after 2060, except in the Himalayan region, possibly due to changes in monsoon patterns and increased evaporative cooling. It is anticipated that nighttime-only heatwaves will uniformly increase across all regions and scenarios, with the most substantial rises observed in the Central Northeast India (CNI) and NWI. Under the SSP370 scenario during 2061–2100, the population exposure to compound heatwaves and nighttime-only heatwaves is projected to increase substantially across all regions. Specifically, exposure to compound heatwaves is anticipated to exceed historical levels by more than 30 times in most regions. Both the CNI and NWI regions show the highest rise in compound and nighttime-only heatwave extremes. The outcomes provide a substantial scientific foundation for policymakers to inform and enhance heat action plans at the national, state, and local levels.

Heat waves (HWs) with high humidity are dangerous to human health. However, existing studies on different types of HWs considering the effect of humidity are still limited. This study defines three types of wet summer HWs (that is, wet independent daytime and nighttime HWs and wet daytime-nighttime compound HWs) and investigates their spatial-temporal changes across China during 1961–2020. Results show significant upward trends of wet nighttime and compound HWs in terms of frequency, occurring days, duration, intensity and spatial extent, while changes for wet daytime HWs are weak and insignificant in nearly all sub-regions of China except for southwest and eastern northwest China. Compared with wet compound and daytime HWs, wet nighttime HWs accompanied by more elevated relative humidity exhibit larger growth rates in frequency, occurring days, duration and affected areas. Additionally, most wet nighttime and compound HWs with the longest duration and/or the maximum intensity are found to occur after the mid-1990s, compared to fewer than half for wet daytime HWs. Our findings emphasize the prominent intensifying trends in wet nighttime HWs across China for the last 60 years, and suggest more efforts on exploring humid HWs.

An increasing number of heatwaves have seriously threatened human production and life. Since nighttime heatwaves may prevent humans from effectively recovering from the damage to health caused by daytime heatwaves, the combination of daytime and nighttime heatwaves, known as compound heatwaves, may pose even greater risks to human health. Most studies have focused on the impacts of daytime heatwaves, with relatively few examining nighttime and compound heatwaves. Using the entropy weight method and the risk triangle assessment framework, along with heatwave indicators, socio-economic, and ecological data, we assessed the risks of daytime, nighttime, and compound heatwaves in the Yangtze River Delta (YRD) of China for the years 2000, 2005, 2010, 2015, and 2020. The results indicate that (1) the frequency and duration of heatwaves in YRD shows a continuous increasing trend, with an increase of about 20 days in the duration of heatwaves over the 30-year period, (2) most areas experiencing nighttime heatwaves also experience compound heatwaves, and their spatial distribution is essentially the same, (3) in 2020, Anhui had the overall highest heatwave risk, while Shanghai had a relatively lower heatwave risk. The multi-year risk assessment results suggest that nighttime heatwaves may \"amplify\" or \"diminish\" the risk of compound heatwaves. This study emphasizes the potential impact of nighttime heatwaves on the risk of compound heatwaves, which is crucial for addressing the risks of compound heatwaves in the context of global warming and accelerated urbanization.

Climate change has increased the frequency and intensity of abnormal weather, with current daytime and nighttime temperatures being higher than their historical counterparts. Previous studies have focused on exploring the health hazards of absolute heat (above the optimum temperature, often calculated on the basis of short periods of current data). However, the health hazards of climate change–induced relative heat (above the extremes of historical counterparts, often calculated on the basis of 30 years of temperature data or more) are unclear. Therefore, this study aims to explore the associations of different types (daytime only, nighttime only and combined daytime–nighttime) of heat and heat waves with stroke morbidity in consideration of human climate adaptation. The data of patients with stroke were obtained from Shenzhen, China, for the period of 2003–2018. Daytime and nighttime heat thresholds for specific calendar days in the study period were defined on the basis of hourly temperatures for long-term counterparts, which were the day and 7 d before and after each calendar day in the historical baseline (1973–2002). The associations of different types of heat and heat waves defined by bivariate heat thresholds with stroke morbidity were explored by using distributed lag nonlinear models. Relevant vulnerable populations and sensitive disease subtypes were identified through stratified analyses. Compound hot extremes and heat waves (combined daytime and nighttime heat and heat waves) were associated with stroke morbidity, with relative risks (RRs) of 1.279 (95% confidence interval (CI): 1.078, 1.519) and 1.500 (95% CI: 1.142, 1.969), respectively, and attributable fractions (AFs) of 1.658% (95% CI: 0.548%, 2.594%) and 0.970% (95% CI: 0.362%, 1.432%), respectively. Associations between heat and heat waves during daytime only and nighttime only with stroke morbidity were statistically insignificant. Males, females and adults aged under and over 65 years were vulnerable to compound hot extremes and heat waves, and the differences between subgroups were statistically insignificant. Ischaemic stroke was the subtype sensitive to compound hot extremes and heat waves with RRs of 1.338 (95% CI: 1.101, 1.626) and 1.553 (95% CI: 1.138, 2.119), respectively, and AFs of 1.956% (95% CI: 0.709%, 2.982%) and 1.064% (95% CI: 0.363%, 1.578%), respectively, whereas haemorrhagic stroke had statistically insignificant associations. Compound hot extremes and heat waves may lead to an increased risk of stroke morbidity in the context of climate change. Governments should emphasise the forecasting and warning of compound hot weather with temperatures higher than the extremes of long-term historical counterparts to reduce associated disease burdens. [Display omitted]

Heat waves, droughts, and compound drought and heat waves (CDHWs) have received extensive attention because of their disastrous impacts on agriculture, ecosystems, human health, and society. Here, we computed the heat wave magnitude index (HWMI), drought magnitude index (DMI), and compound drought and heat wave magnitude index (CDHMI) for Yangtze River Valley (YRV) from July to August during 1961–2022. We compared the large-scale atmospheric circulation characteristics of different extreme events based on these indexes. The results show that the positive center with sink motion in East Asia provides a favorable circulation background for heat wave events. Drought events are mainly affected by the zonal wave train dominated by a significant negative anomaly in Siberia and a high-pressure anomaly upstream, and a anticyclonic water vapor with strong divergence over the Yangtze River basin. During CDHW events, both anomalous systems that affect heat waves and droughts appear and strengthen simultaneously. Specifically, in the middle and upper troposphere, the positive height anomaly center in YRV expands abnormally, and the “+–+” wave train over the northern 50° N region of East Asia becomes more obvious. Therefore, the positive anomaly and water vapor anomaly brought by the two circulation patterns at different latitudes are superimposed over the YRV, leading to severe CDHWs. At the same time, the warm positive eddy center and cold negative eddy center in high latitudes exhibit more stable positive pressure features, which are conducive to the persistent development and strengthening of CDHWs. In addition, the anomalous warm sea surface temperature in western Pacific moderating the favorable circulation patterns may also promote the occurrence of CDHWs in the YRV during the same period.

Yangtze River basin (YZB) experienced record‐breaking heat in the summer of 2022. Here, we focused on daytime‐nighttime compound heat waves, and used the magnitude index that considers both duration and intensity to investigate the risk of the 2022 extreme heat. The magnitude of heatwaves in 2022 was much larger than the historical average level, which was estimated as a 1‐in‐64‐year event over 1979–2014 climate. Without mitigation efforts (SSP585), the record‐breaking heat would emerge as normal during 2050s, and would affect ∼70% of land and projected population in the basin before global mean temperature change reaches 3°C. Such an emergence could be progressively delayed and impacts could be reduced under lower warming levels. The affected area would be 60% lesser at 2°C warming, and the emergence could be avoided by limiting warming to 1.5°C. Our results call for urgent mitigation efforts for reducing the risk of compound heat extremes. Plain Language Summary Persistent extreme high temperature occurred in Yangtze River basin (YZB) during summer 2022, leading to widespread droughts, heat‐related disease and socioeconomic losses. The physical drivers and future risk of the heat extremes have attracted extensive attention. Focusing on daytime‐nighttime compound heat waves, which have more adverse effect on human health and ecosystem, we investigated the magnitude of compound heat waves occurred in 2022, and future risk under different warming levels and emission scenarios. Observations show that the magnitude was the strongest since records began in 1979, and the return period was 64 (95% CI: 30–223) years over the 1979–2014 climate. Projections show that such unprecedented heat in observed records would emerge as normal since 2053 (2081) under nonmitigation (moderate mitigation) scenario. Moderate climate mitigation efforts could delay the time by 28 years. Compared with the 2°C global warming level, about 10% of lands and population would be avoided to normally expose to such heat at 1.5°C warming level. However, the exposures would increase by 60% from 2°C to 3°C warming level. This study suggests that mitigation actions for accomplishing 2°C warming goal and further pursuing the ambitious goal of 1.5°C can substantially reduce the exposure risk of such unprecedented heat. Key Points The 2022 record‐breaking compound heat waves in Yangtze River basin would become normal during 2050s under a nonmitigation scenario Moderate climate mitigation efforts would delay the time of emergence by 28 years Compared to 3°C, limiting warming to 2°C and 1.5°C could avoid 60% and 70% of lands that are normally affected by such rare heat, respectively

Extreme events such as Marine Heat Waves (MHWs) and Low Chlorophyll-a (LChl-a) in the ocean have devastating impacts on the marine environment, particularly when they occur simultaneously (i.e., the compound of MHWs and LChl-a events). In this study, we investigate the spatiotemporal variability of MHWs and LChl-a events in the Arabian and Omani Gulf. For this purpose, we used satellite-based high-resolution observations of SST (0.05° × 0.05°; from 1982 to 2020) and chlorophyll-a concentration data (0.04° × 0.04°; from 1998 to 2020). Hourly air temperature, wind, and heat flux components from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) were used to explain the link between these extreme events and atmospheric forcings. Moreover, our results revealed that the annual frequency of MHW and LChl-a is related to the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). The results revealed an average SST warming trend of about 0.44 ± 0.06 °C/decade and 0.32 ± 0.04 °C/decade for the Arabian Gulf (AG) and the Gulf of Oman (OG), respectively. This warming rate was accompanied by MHW frequency and duration trends of 0.97 events/decade and 2.3 days/decade, respectively, for the entire study region from 1982 to 2020. The highest annual MHW frequencies were recorded in 2010 (6 events) and 2020 (5 events) associated with LChl-a frequency values of 4 and 2, respectively. La Niña events in 1999, 2010, 2011, and 2020 were associated with higher frequencies of MHW and LChl-a. The positive phase of IOD coincides with high MHW frequency in 2018 and 2019. The longest compound MHW and LChl-a event with a duration of 42 days was recorded in 2020 at OG. This extreme compound event was associated with wind stress reduction. Our results provide initial insights into the spatiotemporal variability of the compound MHW and LChl-a events that occurred in the AG and OG.