Estimating salinity variance dissipation rate from conductivity microstructure measurements

Abstract

At the smallest length scales, conductivity measurements include a contribution from salinity fluctuations in the inertial-convective and viscous-diffusive ranges of the turbulent scalar variance spectrum. Interpreting these measurements is complicated because conductivity is a compound quantity of both temperature and salinity. Accurate estimates of the dissipation rate of salinity variance χ(S) and temperature variance χ(T) from conductivity gradient spectra Ψ(C)(k) require an understanding of the temperature-salinity gradient cross spectrum Ψ(S(z)T(z))(k), which is bounded by ζΨ(S(z)T(z))ζ ≤ √Ψ(S(z))Ψ(T(z)). Highly resolved conductivity measurements were made using a four-point conductivity probe mounted on the loosely tethered vertical profiler Chameleon during cruises in 1991 and 1992. Thirty-eight turbulent patches were selected for homogeneity in shear, temperature gradient, and salinity gradient fluctuations and for clear relationship between temperature and salinity. Estimates of χ(T) and χ(S) from the conductivity probe are found to agree with independent estimators from a conventional thermistor probe.