Key differences of swells around China induced by two calculation domains

China’s vigorous development of the marine economy necessitates consideration of swell impacts in the design, construction, and maintenance of marine engineering projects, such as offshore wind farms and marine ranching. Wave computations for China Seas engineering projects, inclusive of swell components, typically select the Western North Pacific (WNP) as the computational domain, while swell may originate from more distant oceanic regions. This study establishes large and small computational domains (North Pacific, NP; Western North Pacific, WNP) to investigate whether swell in the China Seas is affected by more distant oceanic regions and what effects manifest. The study yields the following conclusions: (1) The significant differences in swell energy between computational domains demonstrate that larger computational domains are necessary for accurate swell modeling in the China Seas. (2) Spectral pattern shifts occur intermittently, with larger domains producing distinctly bimodal wave spectra. (3) The energy differences are predominantly concentrated in the low-frequency band (< 0.1 Hz), originating from more distant oceanic regions. During May, distant-source swells contribute >50 % of low-frequency energy for >29 days at maximum. (4) Swell backtracking results indicate that these distant-source swells potentially originate from the North Pacific trade wind belts or polar easterlies. This work provides a basis for future research on coastal swell forecasting and swell warning.