Differential Diffusion

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The density of seawater is principally a factor of its temperature and salinity. One difference between salt and heat in seawater are the tremendous differences in their molecular diffusion rates (a factor of 100). This can have important consequences for large-scale ocean stratification. One way we study this is by turbulence simulations. Seen here are distributions of salinity (top) and temperature (bottom) following incomplete mixing. The finer-scale structure in the salinity is because it diffuses so much more slowly.

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Another way we study differential diffusion is with sensors such as this. This very fine temperature/conductivity sensor permits an evaluation of the small scale fluctuations of both salinity and temperature in the ocean. It has been deployed on both our vertical profiler, Chameleon, and our horizontal profiler, Marlin.

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Measurements using this sensor show the high-wavenumber extension of the salinity above temperature in coincident spectra (upper panel). But the lower panel shows that, when appropriately scaled, temperature and salinity spectra collapse, indicating that the same physics governs the diffusion of each - the difference being that salinity is simply slower.

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Combined results from numerical simulations, laboratory experiments and geophysical measurements indicate that, at low (but non-zero) turbulence levels (small Reb), there is a strong tendency for incomplete mixing of salinity when temperature is completely mixed (small d). If this is widespread in the deep ocean, models that mix both constituents at the same rate must be reconsidered.
More about our work on differential diffusion:

Differential diffusion in breaking Kelvin-Helmholtz billows, J. Phys. Oceanogr., 35, 1004-1022, 2005 (W.D. Smyth, J.D. Nash and J.N. Moum) [pdf]

Microstructure estimates of turbulent salinity flux and the dissipation spectrum of salinity, J. Phys. Oceanogr., 32, 2312-2330, 2002. (J.D. Nash and J.N. Moum) [pdf]

Estimating salinity variance dissipation rate from microstructure conductivity measurements, J. Oceanic Atmos. Technol., 16, 263274, 1999 (J.D. Nash and J.N. Moum) [pdf]