Generalized Patched Potential Density and Thermodynamic Neutral Density: Two New Physically Based Quasi-Neutral Density Variables for Ocean Water Masses Analyses and Circulation Studies

In this paper, two new quasi-neutral density variables generalized patched potential density (GPPD) and thermodynamic neutral density gamma(T)-are introduced, which are showed to approximate Jackett and McDougall empirical neutral density gamma(n) significantly better than the quasi-material rational polynomial approximation gamma(a) previously introduced by McDougall and Jackett. In contrast to gamma(n), gamma(T) is easily and efficiently computed for arbitrary climatologies of temperature and salinity (both realistic and idealized), has a clear physical basis rooted in the theory of available potential energy, and does not suffer from nonmaterial effects that make gamma(n) so difficult to use in water masses analysis. In addition, gamma(T) is also significantly more neutral than all known quasi-material density variables, such as sigma(2), while remaining less neutral than gamma(n). Because unlike gamma(n), gamma(T) is mathematically explicit, it can be used for theoretical as well as observational studies, as well as a generalized vertical coordinate in isopycnal models of the ocean circulation. On the downside, gamma(T) exhibits inversions and degraded neutrality in the polar regions, where the Lorenz reference state is the furthest away from the actual state. Therefore, while-gamma(T) represents progress over previous approaches, further work is still needed to determine whether its polar deficiencies can be corrected, an essential requirement for gamma(T) to be useful in Southern Ocean studies, for instance.