Compounding heat waves and extreme precipitation in India using event coincidence analysis

The increasing frequency of compound extremes, such as heat waves followed by heavy rainfall, present significant challenges due to their amplified regional impacts. This study quantifies the association between heat waves and heavy rainfall across India from 1951 to 2020, using Event Coincidence Analysis to measure the Precursor Coincidence Rate (PCR) and Trigger Coincidence Rate (TCR). PCR assesses the likelihood of heat waves preceding rainfall, while TCR indicates proportion of rainfall events following heat waves. Results reveal decline in TCR for shorter windows (ΔT = 2, 5) but notable increases (50%) over longer windows (ΔT = 7) in recent period, with approximately 70% of grid points exhibiting this trend. This suggests enhanced atmospheric moisture capacity and delayed convective processes drive rainfall after heat waves. PCR increased consistently across all windows, with 27% of grid points recording a heat wave within seven days before heavy rainfall in recent period, compared to 13% in base period. Seasonal analysis highlights stronger heat wave-rainfall coupling during winter and pre-monsoon, with heightened variability in recent decades. During monsoon, abundant moisture moderates this coupling, though sporadic extreme events persist. These results underscore the importance of understanding seasonal and temporal variations in compound extremes under climate change.