Spiciness

We define and present algorithms for spiciness, which is an oceanographic variable whose isopycnal variations reflect isopycnal water-mass contrasts in density units. Discussion of spiciness in the oceanographic literature has often concentrated on its supposed orthogonality to isopycnals on the salinity-temperature diagram and how this orthogonal nature means that spiciness is a passive thermodynamic variable. Here we show that this orthogonal property is devoid of physical meaning. Moreover, it is emphasized that the notion of orthogonality on the salinity-temperature diagram does not give rise to a passive thermodynamic variable. Rather, the passive nature of variations of any thermodynamic variable is gained by evaluating those variations along isopycnals so that, for example, the isopycnal variations of both Absolute Salinity and Conservative Temperature are passive. The advantage of using isopycnal variations of our definition of spiciness is that this measures the passive spatial variations of water-mass properties in density units. The spiciness variables presented here have been derived using the equation of state from the International Thermodynamic Equation of Seawater - 2010.