Particle optical backscattering along a chlorophyll gradient in the upper layer of the eastern South Pacific Ocean
The particulate scattering, <i>b<sub>p</sub></i>, and backscattering, <i>b<sub>bp</sub></i>, 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, <i>b<sub>bp</sub></i> 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 <i>b<sub>bp</sub></i> are still lacking, especially at low chlorophyll <i>a</i> concentrations ([Chl]). Here, we examine the relationships between spectral <i>b<sub>bp</sub></i> and <i>b<sub>p</sub></i> 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 m<sup>-3</sup>), both <i>b<sub>bp</sub></i> and <i>b<sub>p</sub></i> can be related to [Chl] by power functions (i.e. <i>b<sub>p</sub></i> or <i>b<sub>bp</sub></i>=a[Chl]<sup>ß</sup>) Regression analyses are carried out to provide the parameters a and ß for several wavelengths throughout the visible for both <i>b<sub>bp</sub></i> and <i>b<sub>p</sub></i>. When applied to the data, these functions retrieve the same fraction of variability in <i>b<sub>bp</sub></i> and <i>b<sub>p</sub></i> (determination coefficients between 0.82 and 0.88). The <i>b<sub>bp</sub></i> coefficient fall within the bounds of previous measurements at intermediate and high [Chl] recently published. Its dependence on [Chl] below ~0.1 mg m<sup>-3</sup> is described for the first time with in situ data. At these low and decreasing [Chl] a continuous trend with data at higher [Chl] is observed, i.e. a decrease in <i>b<sub>bp</sub></i>. The backscattering ratio (i.e. <i>b<sub>bp</sub></i>/<i>b<sub>p</sub></i>) with values averaging 0.008 is found to have a weak dependence 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.
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