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Double diffusion in Powell Lake : new insights from a unique case study Scheifele, Benjamin

Abstract

High resolution measurements of temperature and electrical conductivity in Powell Lake, British Columbia provide an extensive set of layer and interface observations of a double diffusive staircase found between 325–350 m depth. Powell Lake is an ex-fjord with a quiescent salt layer at thermal steady state in which double diffusion is naturally isolated from turbulent and advective processes. Layers are coherent on the basin scale and their characteristics have a well defined vertical structure. The steady state heat flux is estimated from the large-scale temperature profile and agrees with an earlier estimate of the flux in the sediments. These estimates are compared to a 4/3 flux parameterization which agrees with the steady state flux to within a factor of 2. The discrepancy is explained by testing the scaling underlying the parameterization directly, and it is found that the assumed power law deviates systematically from the observations. Consequently, a different scaling which better describes the observations is presented. The assumption that interfacial fluxes are dominated by molecular diffusion is tested by comparing the interfacial gradient to that expected from the steady state heat flux; at low density ratios, the average interfacial gradient is not sufficiently large to account for transport by molecular diffusion alone, indicating that double diffusive fluxes cannot generally be estimated from bulk interface properties. Salinity interfaces are only marginally (9%) smaller than temperature interfaces, and a simple model to describe the observed difference is presented and shown to be consistent with the observations.

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