DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

the layers is small, saltier water will sink into
the lower layer in the form of thin columns (or
fingers, hence, “salt fingering”). Likewise, the
water below the interface gains heat faster than
it gains salt, resulting in penetration of fresher,
colder water into the upper layer.
On the other hand, if a layer of cold, fresh
water is above a layer of warm, salty water, the
water just above the interface gains buoyancy
from the heat transfer through the interface and
tends to rise. At the same time, the water below
the interface loses buoyancy and tends to sink.
This fluid motion leads to the development of
strata of homogeneous convective layers separated
by thin regions of high vertical gradients
of temperature and salinity. This phenomenon
is often called layering.
The degree of double diffusion is usually expressed
by the stability ratio Rρ, which is defined
by
Rρ =
stability due to the stabilizing component
instability due to the destabilizing component .
Two cases of double-diffusion occur:
Rρ = (α∂/∂z)/(β∂S/∂z) > 1
for the finger regime,
Rρ = (β∂S/∂z)/(α∂/∂z) > 1
for the diffusive regime.
Double diffusion is potentially active when 1 <
Rρ