Design wave heights and storm surge residuals around Taiwan under climate change

Global climate change has become a critical factor influencing marine and coastal safety. According to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6), even with global mitigation efforts, the trend of global warming remains irreversible. The increasing intensity of future extreme climate events is expected to affect wave dynamics, storm surges, and coastal hazards. This study aims to assess long-term variations in extreme waves and storm surges around Taiwan under climate change scenarios and to establish design parameters that reflect future conditions for coastal hazard mitigation and engineering applications.

Atmospheric data from the EC-Earth3 global climate model under the high-emission scenario (SSP5-8.5) were used to drive wave and ocean models simulating wave fields and storm surge residuals over the Northwest Pacific. To address the coarse resolution of global climate models, an artificial intelligence–based statistical downscaling framework was developed. This approach integrates Convolutional Neural Networks (CNNs) and Recurrent Neural Networks (RNNs) into a Convolutional Recurrent Neural Network (CRNN) architecture, improving spatial and temporal resolution and generating data representative of coastal-scale processes. The downscaled wave and surge results were analyzed using the Generalized Pareto Distribution (GPD) for extreme-value statistics to estimate design wave heights and storm surge residuals corresponding to return periods of 10, 25, 50, 75, 100, and 200 years. Statistical uncertainties were evaluated using a bootstrap resampling method.

Results show that under the high-emission scenario, significant wave heights and storm surge residuals exhibit an increasing trend with longer return periods. Wave analysis reveals marked changes in northern coastal waters, where design wave heights increase from 3.05 m to 5.65 m. The eastern coast shows moderate increases (4.08–4.22 m), while the Taiwan Strait and southern waters remain relatively stable at about 3.6 m and 5.2 m, indicating higher sensitivity of the north to extreme forcing. For storm surges, historical maximum residuals ranged from 0.36 m to 1.49 m, while future projections range from 0.31 m to 1.35 m, showing a slight decrease but similar spatial distribution, with larger deviations along western and island coasts. Design storm surge residuals increase with return period, from about 0.20 m for a 10-year event to 0.32 m for a 200-year event. Under future conditions, increases are projected mainly for western and island coasts, with southern and eastern shores also showing gradual rises.

Overall, extreme waves and storm surges around Taiwan exhibit long-term variations under climate change. Although short-term fluctuations remain moderate, both wave and surge intensities increase at longer return periods, implying that future coastal design standards should consider higher thresholds. The AI-based downscaling and extreme-value framework established in this study supports quantitative assessment of coastal hazards, engineering design, and adaptation planning in Taiwan.