Abstract:

This study aims to develop a mathematical model capable of accurately simulating waves in the island and reef areas, with the objective of investigating the wave climate and its changing trends in the Xuande Atoll. The third-generation numerical wave model SWAN (simulating waves nearshore) was employed. Grid refinement was carried out to the Xisha islands and reef areas, incorporating measured terrain data in some regions. The ERA5 reanalysis wind-field data (the fifth generation ECMWF atmospheric reanalysis of the global climate) from 1999 to 2023 was utilized as the driving force for wave simulations. Model performance was validated using measured data from the Xisha and Zhongsha Islands, with statistical metrics indicating its accuracy in simulating significant wave height. Analysis of the 25-year model results revealed distinct spatial and temporal patterns. Spatially, waves in the Xuande Atoll exhibited stronger intensities in the northeast and weaker intensities in the southwest, propagating generally from the northeast to the southwest. Due to the obstruction of the reef flat, a prominent wave-shadow zone was formed in the southwest. Temporally, wave intensity was the highest in December and the lowest in May, showing a gradual decrease from December to May and a subsequent increase from May to December. Quantitatively, the average significant wave height showed a slight decreasing trend (0 ~ 0.2 cm·a^-1) in most of the atoll, while it increased slightly (0 ~ 0.2 cm·a^-1) in some parts of the reef flat. The 99th percentile significant wave height increased in most areas (0 ~ 0.7 cm·a^-1) but decreased slightly in some reef flat areas (0 ~ 0.4 cm·a^-1), indicating a rising impact of extreme weather events like typhoons on the atoll. Monthly analysis showed that the average significant wave height exhibited an upward trend in six months and a downward trend in the other six. The maximum upward trend occurred in October (≤ 0.8 cm·a^-1), while the maximum downward trend was observed in November (≤1.7 cm·a^-1). The 90th percentile significant wave height decreased slightly in February and August, coinciding with the transition between the northeast monsoon and the southwest monsoon. Compared with the average significant wave height, the 99th percentile showed opposite trends in August and September, reflecting year-by-year variations in typhoon impacts. The Mann-Kendall (MK) test indicated relatively high significance levels for trends in some regions. The conclusions have a certain degree of reliability and hold great reference value for the future development and construction of island and reef areas, such as the Xuande Atoll.

Key words: simulation wave nearshore, islands and reefs, waves, wave climate, Xuande atoll