Observed Wave Characteristics and Tropical-Cyclone-Induced Spectral Evolution East of Hainan Island

A comprehensive understanding of the regional wave conditions has become increasingly important for structural design, operational safety, and marine hazard prevention. Continuous directional wave measurements in the deeper offshore waters east of Hainan Island remain limited. This study examined the wave characteristics and tropical cyclone-induced spectral evolution east of Hainan Island using nine months of directional wave observations collected at a water depth of approximately 120 m. The monthly mean significant wave height H1/3 ranged from 0.80 to 2.81 m, with a maximum observed H1/3 of 4.56 m, while the monthly mean significant wave period T1/3 ranged from 5.01 to 8.61 s; waves from the ENE and E waves were dominant. The baseline ERA5-based classification indicated a predominance of swell-dominated mixed seas (58%), although ±1 RMSE wind-speed perturbations substantially altered the class proportions. In contrast, pure wind seas consistently showed the strongest wind–wave directional agreement across all sensitivity scenarios, with 68.4–75.5% of cases within 22.5° and 95.2–97.1% within 45°. Among the two tropical cyclones affecting the study region, Tropical Storm Maliksi produced a stronger local wave response than that of Severe Tropical Storm Prapiroon, with mean H1/3 values of 2.06 and 1.49 m, respectively. During Maliksi, the spectral density increased markedly near 0.10–0.12 Hz, while the pre-storm double-peaked spectrum evolved into a stronger, directionally concentrated dominant component. These findings provide site-specific observational evidence of wave variability and tropical cyclone-induced spectral evolution east of Hainan Island, offering a reference for regional wave characterization and wave-model evaluation.