A research team at The Hong Kong Polytechnic University (PolyU) has found that the area affected by tropical cyclone (TC) rainfall is extending inland along Northern Hemisphere coasts at an average rate of approximately 3.8 kilometres per decade. Driven by rising nearshore sea-surface temperature (SST), this shift is increasing disaster risks for inland regions.
While extensive research has examined TC rainfall intensity, accumulation and frequency under a warming climate, comprehensive global-scale quantitative analysis of whether the area affected by TC rainfall is extending inland and the role of nearshore SST warming in this shift has remained limited.
To address this gap, Prof. Yi-Qing NI, Yim, Mak, Kwok & Chung Professor in Smart Structures and Chair Professor of Smart Structures and Rail Transit of the PolyU Department of Civil and Environmental Engineering, together with Research Fellow Dr E DENG and their research team, analysed global TC and rainfall data from 1980 to 2023. Combining these data with numerical simulations and climate model analyses, the team assessed trends in the inland extension of TC rainfall.
Prof. Ni said, “Our research shows that heavy TC rainfall, defined as at least 30 millimetres within three hours, is extending further inland along Northern Hemisphere coasts at a rate of approximately 3.8 ± 1.8 kilometres per decade, affecting areas more than 100 kilometres from the coastline. These areas have historically been less affected by such disasters and often lack adequate flood protection infrastructure and emergency preparedness. Comprehensive measures to strengthen disaster preparedness and infrastructure planning should therefore be taken at an early opportunity.”
The team found a close relationship between the inland extension of heavy TC rainfall and rising nearshore SST. Experiments using the Weather Research and Forecasting model showed that warmer nearshore waters increase moisture in the lower atmosphere while strengthening differences in airflow between land and sea. These changes alter atmospheric conditions from coastal to inland areas after TC landfall, making it easier for heavy rainfall to persist and develop further inland.
The study also found that when the influence of nearshore SST warming was statistically removed from the data analysis, the trend towards inland extension of heavy TC rainfall almost entirely disappeared, highlighting the critical role of ocean warming. Analysis of global climate simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), coordinated by the World Climate Research Programme, indicated that the warming trend in nearshore SST is primarily associated with human-induced greenhouse gas emissions.
The inland extension of heavy TC rainfall is particularly evident in the western North Atlantic, the Bay of Bengal and the western North Pacific. Stronger signals were also identified in the southeastern United States, southeastern Mexico, eastern and southeastern India, Bangladesh, eastern and southeastern China, and the Indochina Peninsula. These findings highlight the need to monitor more closely changes in TC-related flood risks affecting inland communities and infrastructure in these regions.
Dr Deng said, “By establishing a clear link between the inland extension of TC rainfall and rising nearshore SST, this research provides a physical basis for predicting changes in TC rainfall patterns under a warming climate. The team will next investigate how factors such as human-induced heat emissions and aerosols interact with ocean warming to influence TC rainfall and flood risks across different regions.”
The findings have direct implications for climate adaptation, urban planning and disaster management. As flood risks associated with TC extend inland, relevant authorities should review flood hazard zoning, drainage and flood protection design standards, land-use planning, and early warning and emergency response arrangements. The team emphasised that future disaster preparedness planning should account for the combined effects of climate warming and urban expansion to strengthen the resilience of cities and other communities to extreme weather.
The study has been published in the international journal Nature Communications and is supported by the Research Grants Council’s Theme-based Research Scheme 2023/24, with the aim of fostering sustainable urban development.
