Double-Diffusive Convection

Abstract

The density of seawater is determined by both its temperature and its salinity. Whereas added heat makes water lighter, added salt makes it denser, so both must be considered when evaluating the gravitational stability of the water column. In such situations where one component stabilizes the top-heavy distribution of the other component, small scale instabilities are possible which are known as double-diffusive convection. These arise because the molecular conductivity of heat is about 100 times larger than the diffusivity of salt in water. The faster diffusion of heat can release energy in an unstable distribution of salt in the ‘salt finger’ instability. It can also release the energy available when warm, salty water lies beneath cold, fresh water in ‘diffusive convection.’ Both types of instability can be manifested as alternating series of mixed layers and thin, high-gradient interfaces known as thermohaline staircases. Both forms of double diffusion will occur on the boundaries of water-mass intrusions that arise when temperature and salinity gradients exist on density surfaces. The distributions of temperature and salinity are often opposing in their effects on the density of seawater due to the ease with which water evaporates at high temperatures, creating a natural warm–salty and cold–fresh correlation in many places. Thus, the conditions for double-diffusion are common throughout the world ocean and their effects on mixing and water mass structure are estimated to be substantial in certain regions.