The density ratio of a column of seawater is a measure of the relative contributions of temperature and salinity in determining the density gradient. At a density ratio of 1, temperature and salinity are said to be compensated: their density signatures cancel, leaving a density gradient of zero. The formula for the density ratio, R, is:
R = αθz/βSz, where
θ is the potential temperature
S is the salinity
z is the vertical coordinate (with subscript denoting differentiation by z) ρ is the density α = −ρ−1∂ρ/∂θ is the thermal expansion coefficient β = ρ−1∂ρ/∂S is the haline contraction coefficient
When a water column is "doubly stable"—both temperature and salinity contribute to the stable density gradient—the density ratio is negative (a doubly unstable water column would also have a negative density ratio but does not commonly occur). When either the temperature- or salinity-induced stratification is statically unstable, while the overall density stratification is statically stable, double-diffusive instability exists in the water column. 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 between 0 and 1, and a diffusive convection regime (cool fresh overlying warm salty) when the density ratio is greater than 1.
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.
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, Tu.
See also
Spice (oceanography) Double diffusive convection