Spice Variables and Their Use in Physical Oceanography

There have been a variety of definitions of a spice type variable in physical oceanography. These variables have been used to measure water-mass variations and to examine how the stability ratio varies vertically on a water column. Some authors have chosen to derive such an oceanographic variable, called potential spicity, so that its isolines are orthogonal to isolines of potential density on a suitably scaled salinity-temperature diagram. Here it is shown that isopycnal differences of potential spicity are more than a factor of 100 different to the corresponding isopycnal differences of other measures of water-mass variations such as spiciness, whose isopycnal differences are proportional to the compensating contributions of salinity and temperature to potential density. This behavior of potential spicity is caused by the imposition of the orthogonal constraint which lacks a physical oceanographic motivation. Such orthogonal potential spicity variables also depend on the arbitrarily chosen scaling factor that relates the scales of the salinity and temperature axes on the salinity-temperature diagram. A preferred alternative to using either of these two spice variables is (1) to plot variations of Absolute Salinity in approximately neutral surfaces and (2) to calculate and plot the stability ratio on vertical water columns. The anomaly of Absolute Salinity is best made with respect to a reference relationship between Absolute Salinity and Conservative Temperature that is typical of vertical casts in the oceanographic region of interest. Plain Language Summary The constraint of orthogonality (on the salinity-temperature diagram), which is the basis of a previously published potential spicity variable, causes the isopycnal gradient of potential spicity to be more than a factor of 100 different to the isopycnal gradient of physically based measures of water-mass properties, such as the compensating contributions of salinity and temperature to density along isopycnals. Hence such potential spicity variables, based as they are on the orthogonality principle, are not viable measures of water-mass variations along isopycnals.

Automated summary

There are various definitions of a spice type variable in physical oceanography, with differences in measuring water-mass variations and stability ratio. The potential spicity variable, based on the orthogonality principle, shows isopycnal differences more than a factor of 100 compared to other water-mass property measures. This makes potential spicity variables unsuitable for measuring water-mass variations along isopycnals.