= Salt fingering
<Salt fingering> is <double-diffusive convection> in which narrow rising and sinking fluid structures develop because <heat> and dissolved salt diffuse at different rates. Warm, salty water above colder, fresher water can have a stable total <mass density> stratification: the stabilizing <temperature> difference outweighs the destabilizing <salinity> difference.<The Salt-Fountain and Thermohaline Convection by Melvin E. Stern>[Tellus12(2),172–175: warm saline water above cooler fresh water and unequal heat/salt transport]{ref}
A small upper parcel displaced downward enters colder surroundings. It loses <heat> relatively quickly while retaining more of its salt, becoming denser and continuing downward. A fresher parcel displaced upward can gain <heat> while remaining relatively fresh and light. The distinction between <thermal diffusivity> and <solutal diffusivity> supplies the feedback; stability of the initial combined <mass density> profile does not exclude this mechanism. The <density ratio (double-diffusive convection)> records the competing contributions, while the <salt-finger instability criterion> specifies the parameter regime.
A laboratory analog can replace <heat> and salt by two dissolved substances. <David Osmond (double diffusion researcher)> describes a 1820-by-80-by-120 <millimetre> tank with salt and sugar solutions entering at opposite ends and an overflow at the centre. The supplied solutions had matching <mass density>, but salt diffused faster than sugar. The salt-rich plume could lose salt faster than it gained sugar, producing a lighter core and denser surroundings. Fingering and diffusive layers appeared in different regions, showing that identical source densities need not remain dynamically equivalent when composition diffuses.<Some analysis of a two-dimensional double diffusion experiment by David Osmond>[Section 2 and Figure1: apparatus and plume evolution; subsequent profiles identify the fingering regions]{ref}
In this analog, salt plays the faster-diffusing role played by <heat> in the ocean, while sugar plays the slower-diffusing role of oceanic salt. Keeping that correspondence explicit prevents reversing the physical explanation.
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