Mixed Up

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Science  28 Feb 2003:
Vol. 299, Issue 5611, pp. 1283
DOI: 10.1126/science.299.5611.1283a

Diapycnal mixing in the ocean —mixing between two adjacent water masses of different densities across the surface that separates them—occurs at scales too small to be resolved in numerical models of ocean circulation. Improvement of these models depends in part on better parameterization of this type of mixing, which mixes both heat and salt, the two major determinants of seawater density. This is because slight differences in mixing coefficients for heat and salt can have important consequences for the thermohaline circulation. It is usually assumed that the eddy diffusivities of heat and salt are equal, but some work suggests that those two quantities diffuse at different rates in stratified turbulence such as that generated by breaking internal waves, a process which is thought to cause most of the mixing in the ocean interior. Hebert and Ruddick measured in the laboratory the vertical fluxes and diffusivities of two tracers with different molecular diffusivities for a wide range of breaking internal wave activity. They found that there is likely to be differential mixing in the ocean interior due to breaking internal waves because the ratio of the diffusivities of salt and heat is slightly less than unity. A more precise estimate of this ratio, and its impact on ocean circulation models, will require more work both in the laboratory and at sea. — HJS

Geophys. Res. Lett. 30, 1042, 10.1029/2002GL016250 (2003).

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