Particle optical backscattering along a chlorophyll gradient in the upper layer of the eastern South Pacific Ocean

Abstract. The particulate scattering, b_p, and backscattering, b_bp, coefficients are determined by the concentration and physical properties of suspended particles in the ocean. They provide a simple description of the influence of these particles on the scattering of light within the water column. For the remote observation of ocean color, b_bp along with the total absorption coefficient govern the amount and spectral qualities of light leaving the sea surface. However, for the construction and validation of ocean color models measurements of b_bp are still lacking, especially at low chlorophyll a concentrations ([Chl]). Here, we examine the relationships between spectral b_bp and b_p vs. [Chl] along an 8000 km transect crossing the Case 1 waters of the eastern South Pacific Gyre. In these waters, over the entire range of [Chl] encountered (~0.02–2 mg m³), both b_bp and b_p can be related to [Chl] by power functions (i.e. b_p or b_bp=α[Chl]^β). Regression analyses are carried out to provide the parameters α and β for several wavelengths throughout the visible for both b_bp and b_p. When applied to the data, these functions retrieve the same fraction of variability in b_bp and b_p (coefficients of determination between 0.82 and 0.88). The b_bp coefficient fall within the bounds of previous measurements at intermediate and high [Chl] recently published. Its dependence on [Chl] below ~0.1 mg m⁻³ is described for the first time with in situ data. The backscattering ratio (i.e. b_bp/b_p) with values near 0.01 for all stations appears to be spectrally neutral and not significantly dependent on [Chl]. These results should foster the development of improved forward models of the mean optical properties for oceanic Case 1 waters as well as inverse models based upon them.