Multivariate Approach Reveals a Higher Likelihood of Compound Warm-wet Spells in Urban India

Floods, heatwaves, and humid heat stress often lead to extreme consequences that threaten human health, economic stability, and resilience to natural and built environments. According to the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change, heat stress has steadily increased over the Indian subcontinent. Along with the increase in heat stress, the frequency of heavy precipitation and flood events has grown over multiple locations across South Asia. Most studies assessed trends in heatwave and dry spells over South Asia, emphasizing sensible heat (dry bulb temperature) only and mostly performed attribution and risk analyses considering one driver at a time. A few studies have presented a climate-informed pluvial flood risk model accounting for sensible heat flux, neglecting the influence of humidity. However, very few studies have explored the compound role of humid heat stress followed by extreme precipitation within a limited time window. Here we show the concurrence of humid heat stress (i.e., heatwave compounded by humidity, hereafter HHS) and peak rain events in major urban locations across climatologically disparate monsoon sub-regions of India using ground-based observations. The observational evidence reveals the cities across the western half of the country show positive dependence between humid heat stress and extreme precipitation, whereas those located over the eastern half showed negative correlations. Our findings suggest the role of moisture transport in amplifying precipitation intensity preconditioned by HHS. Further, our joint hazard assessment model identifies potential hotspots where the humid heat stress is likely to intensify the precipitation extremes and consequently have extreme impacts of consecutive disasters (close succession of heatwave and heavy rainfall) over densely populated urban locations. The derived insights provide a clear rationale for assessing heatwave-induced pluvial flood response in a multi-hazard framework, which has implications for climate adaptations, ensuring science-policy cooperation.