[2][3] Double-diffusive instability can be separated into two different regimes of statically stable density stratification: a salt fingering regime (warm salty overlying cool fresh) when the density ratio is greater than 1,[4] and a diffusive convection regime (cool fresh overlying warm salty) when the density ratio is between 0 and 1.
[5] Density ratio may also be used to describe thermohaline variability over a non-vertical spatial interval, such as across a front in the mixed layer.
is used, which is defined as[7] If the signs of both the numerator and denominator are reversed, the density ratio remains unchanged.
A related quantity which avoids this ambiguity as well as the infinite values possible when the denominator vanishes is the Turner angle,
[8][9] It is defined by The Turner angle is related to the density ratio by