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Deep-Sea Research Part I: Oceanographic Research Papers




The Zhang et al. model [Optics Express,17, 5698-5710 (2009)] for calculating light scattering by seawater doesnot account for pressure, which should, theoretically, affect molecular scattering. While negligible in nearsurface waters, the error associated with this approximation could be significant when backscattering is mea-sured directly in the deep ocean, by deep CTD casts or biogeochemical-Argo floats, for example. We updated theparameterization in the Zhang et al. model using (1) the Millard and Seaver equation for the refractive index ofseawater [Deep Sea Research Part A,37, 1909-1926 (1990)] and (2) the Feistel equation for Gibbs free energyfor seawater thermodynamics [Deep-Sea Research I,55, 1639-1671 (2008)]. As these equations include theeffect of pressure as well as salinity and temperature, our new parameterization allows us to investigate thepotential effect of pressure on scattering. Increasing pressure suppresses the random motion of molecules, re-ducing the fluctuations in both density and concentration, which in turn causes an overall decrease in lightscattering by seawater. For pure water and seawater with a salinity of 34 PSU, the decreases are approximately13% and 12%, respectively, with a 100-MPa (approximately the pressure of seawater at 10000 m) increase inpressure. Below the thermocline and/or halocline where temperature and salinity change slowly, the steadyincrease of pressure is the dominant factor affecting the light scattering by seawater. At depths where back-scattering is typically dominated by molecular scattering by seawater, particulate backscattering would beunderestimated if the effect of pressure on molecular scattering were not considered.

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Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.