Escalating Coastal-Urban Heatwaves Preconditioned by Marine Heatwave–Tropical Cyclone Hazard Cascade

The tropical cyclone (TC)–heatwave compound hazards are two seemingly contrasting climate hazards that pose unprecedented recovery challenges to coastal communities when flood-induced power outages result in prolonged exposure of vulnerable populations to extreme humid heat. Landfalling tropical cyclones at the coast can cause catastrophic damage due to strong winds and flooding from storm surges and extreme precipitation-induced inundations. Further, storms often precede or follow extreme humid heat stress. However, a detailed investigation of the causal drivers of landfalling TCs and their subsequent impact on humid heatwave development in coastal cities remained unexplored. The eastern coastal regions in peninsular India, bordering the Bay of Bengal, frequently experience humid heat stress compared to other parts of the country due to large-scale subsidence, causing persistently high temperature and humidity levels. The densely populated metropolitan cities, Kolkata and Chennai, both with populations exceeding 10 million, experience risks of extreme heat stress, e.g., dangerous heat stroke events and higher likelihood of discomforts. Considering a 43-year (1982−2024) analysis period in a probabilistic framework, we analyze the spatiotemporal compounding patterns of marine heatwave (MHW)–TC–heatwave coupling for 259 landfalling TCs, including 37 rapidly intensified (intensity changes of 30 kts/24 hr) landfalling storms that move across the coast, within t ∈ [–15, +15] days of the occurrence of peak heatwave intensity over land and ocean (i.e., marine heatwave) when the storm passes within a 500 km radius distance. The MHWs favors the development of a warm thermal environment and increases the likelihood of rapid storm intensification. Over the Bay of Bengal, MHWs are spatially extensive, showing significant upper tail correlation between MHW and TC peaks over 86% of the ocean. Approximately 39% of the ocean shows a significant preconditioning of strong to severe MHWs on landfalling TCs. For selected urban sites across the coast with major economic activities (trade, finance, and maritime transportation), our results showed that the trend in TC-compounded daily maxima wet-bulb temperature (T_wmax) shows a statistically significant (at a 5% level) upward trend at the rate of 0.18/decade. The T_wmax anomalies even exceeded +3-standard deviations (s.d.) from the normal in ~12% (5/43) of years, and these events are compounded by tropical storms (maximum sustained windspeed < 63 kts). Moreover, the magnitude of temperature anomalies is dominant during the post-monsoon season. The 95^th quantile of T_wmax anomalies following a week of storms’ passage is ~24% greater than that of the corresponding T_wmax anomalies ahead of the storms, suggesting a significant heavy tail behaviour of temperature extremes after TC landfall. An event coincidence analysis of TC-heatwave coupling in heavily urbanized areas shows up to 50% coincidence of TC being followed by humid heatwaves, considering a few days of time lag after TC landfall, while during TC-heatwave coincidence (at a lag of zero days), such probability varies only from 5−20%. Recognizing the causal interaction between MHW−landfalling TC−and coastal urban heatwave event chain adds value to heatwave adaptation planning during post-TC landfall events, which is often overlooked in practice.     

Reference: Ganguli, P., Lin, N. npj Natural Hazards. 2(1), 1-15 (2025).