Double diffusion

(fluid mechanics) A type of convective transport in fluids that depends on the difference in diffusion rates of at least two density-affecting components. Also, it is necessary to have an unstable or top-heavy distribution of one component. For example, in oceanography, the two components are heat and dissolved salts, and the unstable component is the slower-diffusing salt.

A type of convective transport in fluids that depends on the difference in diffusion rates of at least two density-affecting components. This phenomenon was discovered in 1960 in an oceanographic context, where the two components are heat and dissolved salts. Besides different diffusivities, it is necessary to have an unstable or top-heavy distribution of one component.

In the oceanographic context, if the unstable component is the slower-diffusing one (salt), with the overall gravitational stability maintained by the faster-diffusing component (heat), then “salt fingers” will form. Since warm, salty tropical waters generally overlie colder, fresher waters from polar regions, this is a very common stratification in the mid- to low-latitude ocean. Salt fingers arise spontaneously when small parcels of warm, salty water are displaced into the underlying cold, fresh water. Thermal conduction then removes the temperature difference much quicker than salt diffusion can take effect. The resulting cold, salty water parcel continues to sink because of its greater density. Conversely, a parcel of cold, fresh water displaced upward gains heat but not salt, becoming buoyant and continuing to rise. The fully developed flow has intermingled columns of up- and downgoing fluid, with lateral exchange of heat but not salt, carrying advective vertical fluxes of salt and to a lesser extent heat.

Another form of double-diffusive convection occurs when the faster-diffusing component has an unstable distribution. In the ocean, this happens when cold, fresh water sits above warmer, saltier and denser water. Such stratifications are common in polar regions and in local areas above hot springs at the bottom of the deep sea. See also Ocean circulation; Oceanography; Seawater.

The importance of double diffusion lies in its ability to affect water mass structure with its differential transport rates for heat and salt. This is believed to play a significant role in producing certain oceanic water types with well-defined relationships between temperature and salinity. See also Ocean circulation; Seawater.