Observational Evidence Reveals Compound Humid Heat Stress‐Extreme Rainfall Hotspots in India.

Sequential climate hazards, such as "warm and wet" compound extremes, have direct societal implications for highly urbanized regions and agricultural production. While typically extreme temperatures and rainfall are inversely correlated during the summer, extreme humid heatwaves often lead to atmospheric instability and moisture convection, increasing the likelihood of extreme precipitation (EP). Little is known about how heatwave characteristics, such as peak intensity and duration, influence EP at a regional scale. Using high‐resolution, sub‐daily station‐based observational records over five decades (1971–2021) across India, we find a robust increase in the frequency of compound humid heat‐peak precipitation events in all seasons. Our sensitivity analysis of the impact of humid heatwave characteristics on the subsequent sub‐daily rainfall extremes reveals that, with an increase in peak heatwave intensity for a given heatwave duration, >50% of sites show an increase in the magnitude of rainfall; conversely, with an increase in heatwave duration for a given peak heatwave intensity, around 67% sites show a decline in sub‐daily rainfall extremes. An asymmetrical shift toward above‐average precipitation extremes in response to humid heat stress is mainly clustered around low‐elevation, densely populated coastal areas and the irrigation‐intensive Indo‐Gangetic Plains. Plain Language Summary: Compound humid heatwave‐extreme rainfall events substantially impact society, as the sequential occurrence of such events has immense damage potential due to the limited recovery time compared to the situation where these events occur in isolation. Detecting spatiotemporal patterns in compound hot‐wet weather extremes is essential for disaster management, projecting future changes, and devising an early warning system for vulnerable populations. Using gauge‐based observations of more than five decades across India, we find a robust increase in the number of compound humid heatwave‐extreme precipitation events over climatologically homogeneous regions of India across all seasons. We analyze how the rainfall extremes depend on the characteristics of the preceding heatwave: we find that rainfall extremes increase with increasing heatwave intensity for more than 50% of sites; in contrast, rainfall extremes decrease with increasing heatwave duration for around 67% of sites. A shift toward higher rainfall in response to humid heat characteristics is apparent in densely populated coastal areas and irrigation‐dominated regions of the country. Key Points: We quantify the exceedance probability of sub‐daily rainfall peaks conditioned on preceding heatwavesA significant upward trend in the frequency of compound heatwave‐extreme precipitation events is detected across all seasonsSub‐daily rainfall peaks shows notable changes in their exceedance probability in response to changing heatwave properties [ABSTRACT FROM AUTHOR]