Yuanning Zheng

The diffuse attenuation coefficient for the downwelling irradiance $K_d\left(z,\lambda \right)$ is a critical parameter in terms of the optical properties of the ocean. In the northwestern South China Sea, there are complex physical processes, and the accurate estimation of $K_d\left(z,\lambda \right)$ in the northwestern South China Sea is critical for the study and application of the underwater light field and water constituents. In this study, using Hydrolight 6.0 (HL60) software, $K_d\left(z,\lambda \right)$ was simulated based on the inherent optical properties (IOPs) and chlorophyll a concentration dataset in the northwestern South China Sea. The simulations were in good agreement with the results calculated by the model of Lee (2005), and the spectral characteristics of $K_d\left(z,\lambda \right)$ were consistent with several oceanic types according to Jerlov’s classification. The horizontal and vertical distribution characteristics of $K_d(z,490)$ were studied in the two typical upwelling areas of eastern Hainan Island and eastern Vietnam. $K_d(z,490)$ in eastern Hainan Island exhibited an overall decreasing trend from west to east at the same depth, while the vertical depth of the maximum value of $K_d(z,490)$ in eastern Hainan Island was found to increase from west to east, which was significantly associated with the distribution trend of the temperature and salinity. $K_d(z,490)$ in eastern Vietnam exhibited unique horizontal and vertical distribution characteristics due to upwelling, with a low temperature and high salinity. A satisfactory linear relationship between $K_d(z,490)$ and $K_d\left(z,\lambda \right)$ was found from $420 \mathrm{t}\mathrm{o} 580 \mathrm{n}\mathrm{m}$ with $R^2$ > 0.76, root mean square (RMSE) $\le$ 0.010 ${\mathrm{m}}^{-1}$, and mean absolute percentage error (MAPE) < 9%, and this result indicated that $K_d\left(z,\lambda \right)$ from $420~580 \mathrm{n}\mathrm{m}$ could be estimated with $K_d(z,490)$. The regression accuracy sharply decreased after 580 nm, indicating that $K_d\left(z,\lambda \right)$ estimation based on $K_d(z,490)$ can be more suitably achieved from 420~580 nm and becomes inaccurate after 580 nm. Based on the simulations, an empirical relationship for $Z_{SDD}$ estimation involving $K_{d,10\%}\left(490\right)$ was developed, and $Z_{SDD}$ in the northwestern South China Sea was calculated, with a range of 5–23 m and a suitable agreement with $Z_{SDD}$ obtained via the method of Lee (2018).