Wave simulation and characteristic analysis of South China Sea islands and reefs: Jiuzhang Atoll

The wave field around South China Sea islands and reefs exhibits significant spatiotemporal variations due to monsoons, tropical cyclones, offshore swells, and complex reef topography. Focusing on Jiuzhang Atoll, Chigua Reef, and Dongmen Reef, this study establishes an unstructured-mesh SWAN model driven by the ERA5-CMA-Holland blended wind field and Copernicus Marine open-boundary wave forcing for multi-year wave simulations. Model performance is evaluated using satellite observations from Sentinel-6A, CFOSAT, Sentinel-3A/3B, and other missions. Validation against deep-water samples from six satellite missions yields a bias of 0.015 m, an RMSE of 0.256 m, and a CC of 0.933. Wave conditions at Jiuzhang Atoll are generally stronger in winter and weaker in summer, while Dongmen Reef exhibits greater wave energy and a stronger long-period contribution than Chigua Reef. Sensitivity experiments indicate that open-boundary wave forcing mainly maintains regional background wave energy and low-frequency swell, whereas Holland wind-field correction primarily affects short-duration high-wave events during tropical cyclones. The 2024 bathymetry-mesh representation sensitivity experiment shows that combined bathymetry-mesh changes mainly influence local wave transformation over reef flats and reef slopes. The results provide a reference for marine environmental assessment and navigation safety in the Nansha reef region.