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| Fig. 1 The Oceans 2.0 |
The simple meaning of "Potential Enthalpy" is "Ocean Heat Content".
What has been pointed out in this Dredd Blog series is that mainstream media and mainstream oceanographers are like the country singer who says he was "lookin' for love in all the wrong places".
The international standard for oceanographic nomenclature categorizes the scientific nomenclature designation for "potential enthalpy" as "ocean heat" but that is not in wide-spread use in either the "warming commentariat" (media) or the warming ocean main stream scientists (who like to comment in the media but not back it up with modern science).
It is usually the case that when you don't know what you are looking for you are not likely to find it.
So, here is a link and a quote from it that as been posted here previously:
"Potential temperature is used in oceanography as though it is a conservative variable like salinity; however, turbulent mixing processes conserve enthalpy and usually destroy potential temperature. This negative production of potential temperature is similar in magnitude to the well-known production of entropy that always occurs during mixing processes. Here it is shown that potential enthalpy—the enthalpy that a water parcel would have if raised adiabatically and without exchange of salt to the sea surface—is more conservative than potential temperature by two orders of magnitude. Furthermore, it is shown that a flux of potential enthalpy can be called “the heat flux” even though potential enthalpy is undefined up to a linear function of salinity. The exchange of heat across the sea surface is identically the flux of potential enthalpy. This same flux is not proportional to the flux of potential temperature because of variations in heat capacity of up to 5%. The geothermal heat flux across the ocean floor is also approximately the flux of potential enthalpy with an error of no more that 0.15%. These results prove that potential enthalpy is the quantity whose advection and diffusion is equivalent to advection and diffusion of “heat” in the ocean. That is, it is proven that to very high accuracy, the first law of thermodynamics in the ocean is the conservation equation of potential enthalpy. It is shown that potential enthalpy is to be preferred over the Bernoulli function. A new temperature variable called “conservative temperature” is advanced that is simply proportional to potential enthalpy. It is shown that present ocean models contain typical errors of 0.1°C and maximum errors of 1.4°C in their temperature because of the neglect of the nonconservative production of potential temperature ... and potential temperature, rests on an incorrect theoretical foundation ... it is perfectly valid to talk of potential enthalpy, h0,as the 'heat content' ..."
(Potential Enthalpy: Trevor J. McDougall, 2003, emphasis added). Two decades later the science is still there:
"While in situ temperature is an observed variable, its dependence on pressure (even for adiabatic variations of pressure at constant salinity) and its non-conservative nature under turbulent mixing processes have led to the adoption of Conservative Temperature in order to approximate the 'heat content' per unit mass of seawater."
(McDougall, T. J., Barker, P. M., Feistel, R., and Roquet, F.: A thermodynamic potential of seawater in terms of Absolute Salinity, Conservative Temperature, and in situ pressure, Ocean Sci., 19, 1719–1741, 2023).
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| Fig. 2 The Oceans 2.0 |
The turbulent mixing nomenclature is not presented well even in the relevant textbooks:
"Hence it is clear that the restrictions associated with use of the FTR ['fundamental thermodynamic relationship'] are not fulfilled when we combine it with the First Law and write the result using fluid dynamic notation and interpretation as though it might apply to the real ocean. We conclude that there are small thermodynamic inconsistencies involved with combining the FTR and the First Law into the forms of Eqs. (3) and (4). This same inconsistency is common to all advanced thermodynamics textbooks and is rarely discussed; a rare mention of the issue appears on the last page of Sect. 49 of Landau and Lifshitz (1959). Importantly, we point out below (in the paragraph that contains our Eq. 6) that in physical oceanography we do not need to use the evolution of entropy as it appears in Eqs. (3) and (4), but rather we exploit the fact that entropy is a function only of state variables and so can be expressed in the functional form.
This sidesteps the otherwise annoying conceptual issues that would arise when applying fluid mechanics concepts and fluid mechanical mathematical nomenclature (such as material derivatives) to the FTR where the same symbols have a different and more restrictive meaning.
A test of the conservative nature (or otherwise) of an oceanographic variable is to consider the turbulent mixing of two seawater parcels. If the total amount of the variable in the final mixed product is the sum of the amounts in the two original parcels, then the variable is conservative. This is rigorously true for enthalpy in an isobaric mixing process (apart from the dissipation of turbulent kinetic energy which needs to be budgeted separately) and is close to being true of Conservative Temperature (McDougall, 2003; Graham and McDougall, 2013)."
(ibid, emphasis added). So, in the appendices one can view a turbulent mixing of in situ measurements from around the globe that have been purposely mixed by latitude and longitude.
This mixes 'colder ocean water' locations with warmer ones in Conservative Temperature graphs and Potential Enthalpy graphs.
They are shown to have the same pattern or fingerprints (Appendix CT by Latitude, Appendix Potential Enthalpy by Latitude ... Fig. 2; Appendix CT by Longitude, Appendix Potential Enthalpy by Longitude ... Fig. 1).
Note that the values of CT and the potential enthalpy (ho) are at very different magnitude (e.g. 1 deg. Celsius vs 1000 J/kg), so that skews the graph lines some, but the pattern holds sufficiently to detail the point being made.
Yes, Conservative Temperature and Potential Enthalpy are constant in the sense of not being distorted but instead they hold to a pattern likeness "through thick and thin".
Dr. Rignot (video below) is well aware, after years of detailing the melting of tidewater glaciers, that the ocean heat is melting deep, melting at the grounding line, where it is often missed by surface skimming research ("In August 2023 and January 2024, ocean surface waters reached an unprecedented average of 21.1°C").
The next post in this series is here, the previous post in this series is here.
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