Salt fingering

Seawater density depends on both temperature and salinity. Warm water is generally less dense than cold water, while salty water is generally denser than fresher water. In a salt-fingering arrangement, warm, salty water lies above cooler, fresher water. The combined density profile can remain stable overall even though the salinity gradient is destabilizing. Since molecular heat diffusion is much faster than salt diffusion, a small parcel moving vertically can lose or gain heat more quickly than it exchanges salt with its surroundings. The resulting density differences support narrow rising and sinking motions called salt fingers.

Salt fingers transport salt and heat vertically, helping mix water masses. In some locations, this activity occurs at interfaces separating relatively uniform layers, producing step-like temperature and salinity profiles known as thermohaline staircases. Such structures have been observed in the tropical Atlantic and other ocean settings. Their presence and intensity depend on the local temperature and salinity gradients, as well as competing influences such as turbulence and internal-wave shear; salt fingering is not equally important throughout the ocean.

Ocean mixing by salt fingers is difficult to measure directly because the motions are small and can interact with other sources of mixing. A Woods Hole Oceanographic Institution–led tracer study in a tropical Atlantic staircase released a tracer near 400 metres depth and sampled the region again nine months later. The observed vertical spreading was consistent with appreciable salt-finger mixing at that site. Later observations in the northeastern Arabian Sea reported staircase-like microstructure and other indicators consistent with salt fingering in the daytime diurnal thermocline, showing that favorable conditions can also arise near the surface in a particular setting.

By modifying the vertical exchange of heat, salt and other water-mass properties, salt fingering can affect ocean structure and inform models of circulation and climate. However, research reviews emphasize unresolved questions about quantifying its fluxes and about how it interacts with shear, internal waves and turbulence. Individual measurements in regions favorable to fingering do not establish that it dominates mixing globally. Ocean models therefore need to represent such small-scale processes carefully, while remaining attentive to observational uncertainty.