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

Rémi Tailleux

doi:10.1175/jpo-d-16-0072.1

Abstract

AbstractIn this paper, two new quasi-neutral density variables—generalized patched potential density (GPPD) and thermodynamic neutral density γT—are introduced, which are showed to approximate Jackett and McDougall empirical neutral density γn significantly better than the quasi-material rational polynomial approximation γa previously introduced by McDougall and Jackett. In contrast to γn, γT 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 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 generalized vertical coordinate in isopycnal models of the ocean circulation. On the downside, γT exhibits inversions and degraded neutrality in the polar regions, where the 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.

Highlights

The problem of how best to construct a quasi-neutral density variable suitably corrected for pressure is a longstanding fundamental issue in oceanography whose answer is vital for many key applications ranging from the study of mixing to ocean climate studies
We have reexamined several issues pertaining to the construction of quasi-neutral density variables, starting with a fresh look at how best to define the continuous limit of patched potential density, a point of contention between JMD97 and de Szoeke and Springer (2000)
It was found essential to redefine patched potential density because even though the latter is commonly defined as potential density referenced to a piecewise constant reference pressure field s[S, u, pr(zi)], zi # z # zi11, this formula obscures the whole machinery of the actual patching process whereby different potential density surfaces are joined up at points of discontinuity

Summary

Introduction

The problem of how best to construct a quasi-neutral density variable suitably corrected for pressure is a longstanding fundamental issue in oceanography whose answer is vital for many key applications ranging from the study of mixing to ocean climate studies.

Results

Conclusion