Abstract/Summary

In this paper, two new quasi-neutral density variables — generalised patched potential density (GPPD) and thermodynamic neutral density γT — are introduced, which are showed to approximate Jackett and McDougall (1997) empirical neutral density γn significantly better than the quasi-material rational polynomial approximation γa previously introduced by McDougall and Jackett (2005b). In contrast to γn, γT is easily and efficiently computed for arbitrary climatologies of temperature and salinity, both realistic and idealised, has a clear physical basis rooted in the theory of available potential energy, and does not suffer from non-material effects that makes γn so difficult to use in water masses analysis. In addition, γT is also significantly more neutral than all known quasi-material density variables, such as σ2, while remaining less neutral than γn. Because unlike γn, γT is mathematically explicit, it can be used for theoretical as well as observational studies, as well as a generalised vertical coordinate in isopycnal models of the ocean circulation. On the downside, γT exhibits inversions and degraded neutrality in the polar regions, where Lorenz reference state is the furthest away from the actual state. Therefore, while γT represents progress over previous approaches, further work is still needed to determine whether its polar deficiencies can be corrected, an essential requirement for γT to be useful in Southern Ocean studies for instance.