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.