Preprints
https://doi.org/10.5194/gmd-2020-426
https://doi.org/10.5194/gmd-2020-426

Submitted as: methods for assessment of models 19 Jan 2021

Submitted as: methods for assessment of models | 19 Jan 2021

Review status: this preprint is currently under review for the journal GMD.

The interpretation of temperature and salinity variables in numerical ocean model output, and the calculation of heat fluxes and heat content

Trevor J. McDougall1, Paul M. Barker1, Ryan M. Holmes1,2, Rich Pawlowicz3, Stephen M. Griffies4, and Paul J. Durack5 Trevor J. McDougall et al.
  • 1School of Mathematics and Statistics, University of New South Wales, Sydney, NSW 2052, Australia
  • 2Australian Research Council Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, NSW 2052, Australia
  • 3Dept. of Earth and Ocean Sciences, University of British Columbia, Vancouver, B.C. V6T 1Z4, Canada
  • 4NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA
  • 5Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, California, USA

Abstract. The 2010 international thermodynamic equation of seawater, TEOS-10, defined the enthalpy and entropy of seawater, thus enabling the global ocean heat content to be calculated as the volume integral of the product of in situ density, ρ, and potential enthalpy, h0 (with reference sea pressure of 0 dbar). In terms of Conservative Temperature, Θ, ocean heat content is the volume integral of ρcp0Θ, where cp0 is a constant isobaric heat capacity.

However, several ocean models in CMIP6 (as well as all of those in previous Coupled Model Intercomparison Project phases, such as CMIP5) have not been converted from EOS-80 (Equation of State - 1980) to TEOS-10, so the question arises of how the salinity and temperature variables in these models should be interpreted. In this article we address how heat content, surface heat fluxes and the meridional heat transport are best calculated in these models, and also how these quantities should be compared with the corresponding quantities calculated from observations. We conclude that even though a model uses the EOS-80 equation of state which expects potential temperature as its input temperature, the most appropriate interpretation of the model's temperature variable is actually Conservative Temperature. This interpretation is needed to ensure that the air-sea heat flux that leaves/arrives-in the atmosphere is the same as that which arrives-in/leaves the ocean.

We also show that the salinity variable carried by TEOS-10 based models is Preformed Salinity, while the prognostic salinity of EOS-80 based models is also proportional to Preformed Salinity. These interpretations of the salinity and temperature variables in ocean models are an update on the comprehensive Griffies et al. (2016) paper that discusses the interpretation of many aspects of coupled model runs.

Trevor J. McDougall et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gmd-2020-426', Remi Tailleux, 16 Feb 2021
    • AC1: 'Reply on RC1', Trevor McDougall, 24 Apr 2021
  • RC2: 'Comment on gmd-2020-426', Baylor Fox-Kemper, 25 Feb 2021
    • RC3: 'Further clarification...', Baylor Fox-Kemper, 25 Feb 2021
  • RC4: 'Further comment on gmd-2020-426', Remi Tailleux, 28 Feb 2021
    • RC5: 'Reply on RC4', Baylor Fox-Kemper, 01 Mar 2021
      • RC6: 'Reply on RC5', Remi Tailleux, 01 Mar 2021
        • RC7: 'Reply on RC6', Baylor Fox-Kemper, 01 Mar 2021
          • RC8: 'Reply on RC7', Baylor Fox-Kemper, 01 Mar 2021
  • AC2: 'Comment on gmd-2020-426', Trevor McDougall, 24 Apr 2021

Trevor J. McDougall et al.

Trevor J. McDougall et al.

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Latest update: 31 Jul 2021
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Short summary
We show that the way that the air-sea heat flux is treated in ocean models means that the model's temperature variable should be interpreted as being Conservative Temperature, irrespective of whether the equation of state used in the model was EOS-80 or TEOS-10.