Abstract
Ocean Thermal Energy Conversion (OTEC) could provide baseload power for tropical islands and reefs, but its thermal-resource suitability depends on the surface–deep seawater temperature difference (ΔT). This study assessed four South China Sea (SCS) sites—Yongshu Reef, Meiji Reef, Yongxing Island, and Huangyan Island—using monthly Copernicus Marine Environment Monitoring Service (CMEMS) reanalysis data for 2010–2023 within two site-centered averaging windows (±2° and ±0.5°), termed the large and small windows. CMEMS results were validated against in situ profiles from the Argo autonomous profiling-float network, yielding correlation coefficients of 0.66–0.94 and mean absolute errors of 0.44–0.59 °C. Mean ΔT changed little between windows, whereas differences in minimum ΔT ranged from 0.07 to 0.31 °C, with the large window generally producing slightly higher minima and a smoother lower tail. Meiji Reef showed the most stable thermal-resource conditions, whereas Yongxing Island had the highest monthly low-ΔT persistence. At the 22 °C threshold, the persistence index R was 1.429 and 1.339 at Yongxing Island for the large and small windows, respectively, but only 0.006 at Meiji Reef in both windows. Preliminary OTEC screening should therefore consider minimum ΔT and monthly low-ΔT persistence in addition to mean ΔT.