Variation in downwelling diffuse attenuation coefficient in the northern South China Sea

The diffuse attenuation coefficient for downwelling irradiance (K _d(λ)) is an important parameter for ocean studies. Based on the optical profile data measured during three cruises in the northern South China Sea in autumn from 2003 to 2005, variations in the K _d(λ) spectra were analyzed. The variability of K _d(λ) shows much distinct features in both magnitude and spectra pattern, it is much higher in coastal waters than that of open oceanic waters; and the blue-to-green (443/555) ratio of K _d(λ) tends to increase with chlorophyll a concentration ([Chl-a]) from open ocean to coastal waters. These characteristics can be explained most by the increase of a _w+p(443)/a _w+p(555) with [Chl-a]. In short waveband, the relation between K _d(λ)-K _w(λ) and [Chl-a] can be well described by a power law function, indicating the large contribution of phytoplankton to the variations in K _d(λ). As for the spectral model of the diffuse attenuation coefficient, there are good linear relationships between K _d(490) and K _d(λ) in other wavelengths with own slope and intercept of a linear functions in the spectral range 412–555 nm. K _d(490) is well correlated with the spectral ratio of remote sensing reflectance; and should enough measurement data are given, this empirical algorithm would be used in the K _d(λ) retrieval from ocean color satellite data. The variation in K _d(λ) provides much useful information for us to study the bio-optical property in the northern South China Sea. Supported by the Chinese Academy of Sciences (No. KZCX2-YW-215) and the National Natural Science Foundation of China (No. 40476019), and the Project of Knowledge Innovation of South China Sea Institute of Oceanology (No. LYQY200701).