Extreme precipitation events (EPEs) are devastating natural disasters causing significant societal, economic, and environmental damage. The World Meteorological Organization reports that heavy precipitation-flood disasters have been the most frequent and costly weather-related events since 1970. The IPCC Sixth Assessment Report highlights that over 700 million people now experience significantly more intense EPEs than in the 1950s. Understanding the interplay between climate extremes, especially compound events involving heat and precipitation, is critical. Compound heat stress and heavy precipitation events (CHPEs), where extreme precipitation immediately follows heat stress, are particularly impactful. While research exists on the occurrence and characterization of CHPEs, comparative studies directly contrasting EPEs with preceding heat stress (EPE-Hs) and those without (EPE-NHs) are lacking. This study aims to fill this gap, providing crucial insights for developing climate change adaptation strategies.

Global warming significantly alters climate variability, leading to changes in the frequency, intensity, and impact of extreme precipitation. Most regions are projected to experience increased heavy precipitation and pluvial floods. Earth System Models (ESMs) from CMIP6, using SSP-RCP emission scenarios, are valuable tools for projecting future climate conditions. Previous CMIP6 studies have shown significant global increases in future extreme precipitation, especially under high emission scenarios. The frequency of CHPEs has also increased since the 1960s and is projected to continue rising. The increase in extreme precipitation is linked to rising temperatures, with the Clausius-Clapeyron (C-C) relation suggesting a 7%/°C increase in atmospheric moisture capacity. However, observed precipitation-temperature (P-T) scaling relationships vary regionally, from strong super C-C scaling in mid-latitudes and dry regions to weak sub-C-C or negative rates in the tropics and wet regions. Studies have observed a "P-T hook" structure in mid-latitudes, where extreme precipitation increases with temperature up to a threshold and then decreases. The future scaling rate may also increase with regional disparities. Understanding the differences in P-T relationships between EPE-Hs and EPE-NHs is crucial for climate change adaptation.

This study analyzes global extreme precipitation using reanalysis data (ERA5) and CMIP6 ESM simulations from six models (CNRM-CM6-1, EC-Earth3-Veg, KACE-1-0-G, MPI-ESM1-2-HR, MRI-ESM2-0, NorESM2-MM). The analysis focuses on the warmest five months of the year. EPEs are identified using the 90th percentile of daily precipitation in the warm season (1979-2014). Heat stress events are defined when the daily lethal heat stress temperature (Th) exceeds its 90th percentile for at least three consecutive days. Th is calculated using a formula incorporating wet-bulb temperature and relative humidity. CHPEs are identified as heat stress events followed by an EPE within a 3-day time interval; the EPEs within CHPEs are classified as EPE-Hs, and others as EPE-NHs. EPEs are characterized by frequency, duration, and magnitude (normalized daily precipitation anomalies). The Quantile delta Mapping (QDM) method is used to bias-correct ESM daily variables. The temperature sensitivity of extreme precipitation is calculated by dividing the relative change in total EPE precipitation by the change in global surface air temperature (GSAT). P-T scaling relationships are examined using quantile regression and LOWESS smoothing, with linear regression fitted on the logarithm of running averaged precipitation intensity and GSAT/regional surface air temperature (RSAT). A composite analysis of atmospheric variables (CAPE, specific humidity, surface sensible heat flux, vertically integrated moisture convergence, and total column water vapor) one day before EPE-Hs and EPE-NHs is performed to explore underlying mechanisms.

Analysis of historical (1979-2014) data reveals that EPE-Hs are less frequent globally but have greater magnitude and longer durations than EPE-NHs, particularly in high-latitude regions. EPE-NHs are more frequent in Southeast Asia, the Amazon, northeastern America, and northeastern Asia, while EPE-Hs are more common north of 45°N, in the central United States, and the southern Tibetan Plateau. Future projections (2015-2099) under four SSP-RCP scenarios show a significant increase in the frequency, duration, and magnitude of EPE-Hs, especially under high emission scenarios. In contrast, EPE-NHs show relatively stable characteristics. EPE-Hs demonstrate substantially higher temperature sensitivity than EPE-NHs globally, exceeding 200%/°C, particularly pronounced in low latitudes. The spatial distribution of temperature sensitivity shows large positive values in several regions for EPE-Hs, while EPE-NHs exhibit predominantly negative values in mid-latitudes. The P-T scaling relationships differ significantly between EPE-Hs and EPE-NHs. Globally, all EPEs show a monotonic linear increase with GSAT, while EPE-Hs exhibit a "hook" structure (initial increase followed by a decrease), and EPE-NHs show an initial decrease followed by an increase. Regionally, the P-T relationship varies, with EPE-Hs showing a peak precipitation intensity at a specific temperature in low latitudes and a continuous rise elsewhere; EPE-NHs initially decrease and then increase in low latitudes and monotonically decrease in other regions. Composite analysis suggests that higher CAPE, specific humidity, surface sensible heat flux, and total column water vapor are observed one day prior to EPE-Hs in high northern latitudes, contributing to their greater intensity.

The findings highlight the amplified sensitivity of EPE-Hs to global warming compared to EPE-NHs. The distinct P-T scaling structures of EPE-Hs and EPE-NHs underscore the complexity of climate responses to warming and the importance of region-specific considerations. The increased intensity and frequency of EPE-Hs projected in the future pose significant risks requiring tailored adaptation strategies. The observed differences in atmospheric conditions prior to EPE-Hs and EPE-NHs provide insights into the underlying physical mechanisms, suggesting that increased atmospheric instability and moisture content associated with heat stress contribute to the enhanced intensity and duration of EPE-Hs. The contrasting responses of EPE-Hs and EPE-NHs to warming emphasize the need for detailed regional assessments and the development of accurate, high-resolution climate models.

This study reveals the distinct characteristics and temperature sensitivities of extreme precipitation events with and without preceding heat stress. EPE-Hs show amplified sensitivity to global warming, particularly in low latitudes. The differing P-T scaling relationships highlight regional variations in climate responses. These findings emphasize the need for region-specific early warning systems and adaptation strategies to mitigate the increasing risks of extreme precipitation under climate change. Future research should focus on investigating the underlying physical mechanisms in more detail and on improving high-resolution climate models to better simulate compound climate events.

This study utilizes a specific set of CMIP6 ESMs, and the results may vary depending on the model selection. The analysis focuses on the warmest five months of the year, potentially neglecting important seasonal variations. The identification of CHPEs and EPE-Hs relies on a 3-day time interval, which might not capture all relevant compound events. Finally, the study primarily focuses on the observed differences and projections of extreme precipitation and their response to warming, with limited exploration of the full underlying physical mechanisms at work.