A state variable for characterizing water masses and their diffusive stability: spiciness

A state variable π(θ,s) most sensitive to isopycnal thermohaline variations and least correlated with the density field is proposed. π is constructed so that its diapycnal gradient dρπ is related to the density gradient ratio Rρ:dρπ≈(1+Rρ)/(1−Rρ). It is simultaneously a tracer conserved by isentropic motions, and an indicator of diffusive stability. It is useful for the combined description of interleaving and double-diffusive processes at the boundary between water masses.