Salt fingering

From Glossary of Meteorology

salt fingering

A form of double-diffusive convection that occurs when warm, salty water overlies cold freshwater.

A parcel of freshwater moved upward will gain heat more quickly than it gains salt and so will become lighter than the surrounding water. It will rise unstably as a result. Salt fingering has been observed in the Caribbean Sea where it gives rise to stable layers hundreds of kilometers in extent. The process transports salt much more efficiently than heat.
See also Turner angle.