Energy Conversion, Mixing Energy, and Neutral Surfaces with a Nonlinear Equation of State
2011 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 41, no 1, 28-41 p.Article in journal (Refereed) Published
A local neutral plane is defined so that a water parcel that is displaced adiabatically a small distance along the plane continues to have the same density as the surrounding water. Since such a displacement does not change the density field or the gravitational potential energy, it is generally assumed that it does not produce a restoring buoyancy force. However, it is here shown that because of the nonlinear character of the equation of state (in particular the thermobaric effect) such a neutral displacement is accompanied by a conversion between internal energy E and gravitational potential energy U, and an equal conversion between U and kinetic energy K. While there is thus no net change of U. K does change. This implies that a force is in fact required for the displacement. It is further shown that displacements that are orthogonal to a vector P do not induce conversion between U and K, and therefore do not require a force. Analogously to neutral surfaces, which are defined to be approximately orthogonal to the dianeutral vector N. one may define P surfaces to be approximately orthogonal to P. These P surfaces are intermediate between neutral surfaces and surfaces of constant sigma(0) (potential density reference to the surface). If the equation of state is linear, there exists a well-known expression for the mixing energy in terms of the diapycnal flow. This expression is here generalized for a general nonlinear equation of state. The generalized expression involves the velocity component along P. Since P is not orthogonal to neutral surfaces, this means that stationary flow along neutral surfaces in general requires mixing energy.
Place, publisher, year, edition, pages
2011. Vol. 41, no 1, 28-41 p.
Oceanography, Hydrology, Water Resources
IdentifiersURN: urn:nbn:se:su:diva-68364DOI: 10.1175/2010JPO4250.1ISI: 000287222200002OAI: oai:DiVA.org:su-68364DiVA: diva2:472891
authorCount :12012-01-042012-01-032012-01-04Bibliographically approved