Study of the Profile Distribution of the Diffuse Attenuation Coefficient and Secchi Disk Depth in the Northwestern South China Sea

Abstract

The diffuse attenuation coefficient for the downwelling irradiance (Formula presented.) 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 (Formula presented.) 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, (Formula presented.) 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 (Formula presented.) were consistent with several oceanic types according to Jerlov’s classification. The horizontal and vertical distribution characteristics of (Formula presented.) were studied in the two typical upwelling areas of eastern Hainan Island and eastern Vietnam. (Formula presented.) 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 (Formula presented.) 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. (Formula presented.) 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 (Formula presented.) and (Formula presented.) was found from (Formula presented.) with (Formula presented.) > 0.76, root mean square (RMSE) (Formula presented.) 0.010 (Formula presented.), and mean absolute percentage error (MAPE) < 9%, and this result indicated that (Formula presented.) from (Formula presented.) could be estimated with (Formula presented.). The regression accuracy sharply decreased after 580 nm, indicating that (Formula presented.) estimation based on (Formula presented.) can be more suitably achieved from 420~580 nm and becomes inaccurate after 580 nm. Based on the simulations, an empirical relationship for (Formula presented.) estimation involving (Formula presented.) was developed, and (Formula presented.) in the northwestern South China Sea was calculated, with a range of 5–23 m and a suitable agreement with (Formula presented.) obtained via the method of Lee (2018).