On the Links Between Neutral Directions, Buoyancy Forces, Energetics, Potential Vorticity, and Lateral Stirring in the Ocean: A First-Principles Approach
For over 80 years, lateral stirring in the ocean has been commonly assumed to occur preferentially along the N-neutral directions, a view at the origin of all forms of potential and quasi-neutral density variables used in oceanography. Yet, the validity of doing so has remained unclear as the theory...
| Main Authors: | , |
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| Format: | Article in Journal/Newspaper |
| Language: | unknown |
| Published: |
arXiv
2022
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| Subjects: | |
| Online Access: | https://dx.doi.org/10.48550/arxiv.2202.00456 https://arxiv.org/abs/2202.00456 |
| Summary: | For over 80 years, lateral stirring in the ocean has been commonly assumed to occur preferentially along the N-neutral directions, a view at the origin of all forms of potential and quasi-neutral density variables used in oceanography. Yet, the validity of doing so has remained unclear as the theory of N-neutrality is solely rooted in heuristic parcel-based arguments lacking firm theoretical foundations. Furthermore, the only form of neutrality deducible from the rigorous analysis of the equations of motion so far, called P-neutrality, is only able to justify N-neutrality theoretically near an arbitrary fixed reference pressure but not globally. In this paper, we succeed in eliminating this difficulty over most of the ocean by reformulating the theory of P-neutrality in terms of the spatially varying reference pressure entering Lorenz theory of available potential energy and generalising it to the full Navier-Stokes equations. The improved form of P-neutrality, however, remains irreconcilable with N-neutrality in regions strongly departing from Lorenz reference state, such as the Southern Ocean, where the use of neutral rotated diffusion might hence be potentially invalid and the cause of spurious diapycnal mixing. The new theory establishes that energetics, buoyancy forces, available potential energy, potential vorticity, the Bernoulli function, nonlinear balance, and small-scale mixing all play important roles in constraining the preferential directions of lateral stirring in the ocean. Its potential importance for numerical ocean modelling and the theory of density variables used in water mass analyses are also discussed. : 15 pages, 3 figures, submitted to JPO |
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