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 γ T-are introduced, which are showed to approximate Jackett and McDougall empirical neutral density γ n significantly better than the quasi-material rational polynomial approximation γ a previously introduced by McDougall and Jackett. In contrast to γ n, γ 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 γ n so difficult to use in water masses analysis. In addition, γ T is also significantly more neutral than all known quasi-material density variables, such as σ2, while remaining less neutral than γ n. Because unlike γ n, γ 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, γ 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 γ T represents progress over previous approaches, further work is still needed to determine whether its polar deficiencies can be corrected, an essential requirement for γ T to be useful in Southern Ocean studies, for instance. [ABSTRACT FROM AUTHOR]