Unveiling the dynamics of sequential extreme precipitation-heatwave compounds in China

The escalating impacts of global warming are intensifying the frequency and severity of compound extreme weather events, posing significant threats to human lives and property. Traditional response strategies often focus on individual events, neglecting the cumulative and complex effects of compound events. This study addresses this gap by focusing on SEPHCE in China. Heatwaves and extreme precipitation are two of the most widespread and impactful extreme weather events, individually causing substantial damage. The combined effect of these events, as compound events, is often far greater than the sum of their individual impacts. The concept of compound events has been increasingly recognized in recent IPCC reports, highlighting the need to understand their characteristics and changes for effective disaster prevention and mitigation. Since the 1950s, high-temperature events and extreme precipitation have increased globally, resulting in significant economic losses and fatalities. While research on compound events has increased, studies focusing on China, a nation experiencing rapid urbanization and increased vulnerability to extreme weather, are limited. This research aims to fill this gap by systematically investigating the occurrence and evolution of SEPHCE in China to inform effective response strategies and promote sustainable development.

Existing literature highlights the increasing frequency and severity of heatwaves and extreme precipitation worldwide. Studies in countries like the United States and Japan have shown the high incidence of compound flood-heatwave events, demonstrating their devastating consequences. Previous research explores the probability and characteristics of compound heatwave and flood events, noting that heavy rainfall is more likely after shorter, hotter heatwaves. While heatwaves and extreme precipitation events are often isolated, the intensification of climate change increases the probability of their convergence. However, previous research has primarily focused on countries such as the United States and Japan, neglecting the significant impact of urbanization and climate change in China. Various indicators, including frequency, duration, and intensity, have been used to examine extreme weather characteristics. Studies have defined heatwaves based on temperature thresholds, while extreme precipitation is often defined using percentile-based thresholds. Recent research has also highlighted the importance of studying the start and end times of these events to improve prediction and warning systems and reduce loss of life and property.

This study utilized daily maximum temperature, minimum temperature, and precipitation data from 1975 to 2020 from the National Meteorological Information Center (NMIC) of China Meteorological Administration. Data from 1929 meteorological stations across mainland China were used. Data quality control measures included checks for climate extremes, internal consistency, temporal consistency, and spatial consistency. Missing values were interpolated, with the number of missing values kept below 5% annually. Heatwaves were defined as periods of at least three consecutive days where daily maximum and/or minimum temperatures exceeded the 90th percentile of a 15-day moving window (7 days before and after), calculated from a 30-year reference period (1975–2004). Extreme precipitation events were defined as daily precipitation exceeding the 95th percentile of all rain days during the same reference period. A SEPHCE was defined as the occurrence of extreme precipitation followed by a heatwave within specific time intervals (1, 3, 5, and 7 days). The study analyzed SEPHCE frequency, duration, start and end times, and the joint probability distribution of extreme precipitation and heatwave intensities. The Theil-Sen Median trend analysis and Mann-Kendall (MK) test were used to assess temporal trends in SEPHCE frequency and duration. Copula functions were employed to model the joint probability distribution and return periods of extreme precipitation intensity and heatwave intensity. Specifically, the Gaussian copula was selected based on OLS test results.

The study found a significant and consistent increase in SEPHCE frequency and duration from 1975 to 2020. The increase was particularly notable after 1993. Shorter interval SEPHCE events (1-day interval) showed the most pronounced increase in frequency and duration. Spatially, higher SEPHCE frequencies and durations were concentrated in southwestern and southern coastal regions of China. Analysis of SEPHCE start and end times revealed a clear trend of advancing start times and delaying end times, with the advancement rate exceeding the delay rate. The joint probability distribution of extreme precipitation and heatwave intensities increased as both intensities increased. However, a weak negative correlation (−0.049) was observed between extreme precipitation and heatwave intensities using the Gaussian copula model. The joint return period of extreme precipitation and heatwave intensities increased with increasing intensities, indicating that more intense events have longer return periods. Figures 1-8 provide detailed graphical representation of these findings, illustrating spatial distribution, temporal trends, and joint probability distributions.

The findings confirm the increasing frequency and duration of SEPHCE in China, particularly since 1993, and their concentration in southern China. This aligns with previous research showing an increase in sequential flood-heatwave events in the southeast and western regions. The earlier onset and delayed cessation of SEPHCE, especially those within shorter time intervals, highlights the intensified pressure from these events, posing significant threats to agriculture, infrastructure, and human health. The observed increases are closely linked to global warming and human activities. While the study focuses on China, the methodology can be applied globally. The weak negative correlation between extreme precipitation and heatwave intensities, while counter-intuitive, warrants further investigation. Potential physical mechanisms, such as tropical intraseasonal oscillations and the influence of typhoons, may play a role, although further research is needed to establish robust connections.

This study provides the first comprehensive assessment of SEPHCE in China, demonstrating a significant upward trend in their frequency and duration. The spatial concentration in southern China highlights regional vulnerability. The earlier onset and delayed cessation of these events necessitate urgent action. Future research should focus on refining the understanding of underlying physical mechanisms, improving prediction models, and developing targeted adaptation strategies for vulnerable regions.

The study is limited by the data availability, using data only from 1975 to 2020. The definition of heatwaves and extreme precipitation events, while widely used, may influence the results. The analysis of the physical mechanisms driving SEPHCE is still preliminary, requiring further investigation. Future work should consider high-resolution climate simulations and downscaled data for more localized analysis.