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Interpretation of net surface heat fluxes and Meridional Overturning Circulations in global coupled UK-HadGEM3 climate simulations
  • Michael J Bell,
  • A J George Nurser,
  • David Storkey
Michael J Bell
Met Office, Met Office

Corresponding Author:mike.bell@metoffice.gov.uk

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A J George Nurser
National Oceanography Centre, National Oceanography Centre
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David Storkey
Met Office, Met Office
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The annual mean net surface heat fluxes (NSHFs) from the ocean to the atmosphere play an important role in driving both atmospheric circulations and oceanic meridional overturning circulations. Those generated by historical forcing simulations using the UK HadGEM3-GC3.1 coupled climate model are shown to be relatively independent of resolution, for model horizontal grid spacings between 1 and 1/12 degree, and to agree well with those based on the DEEPC analyses for the period 2000-2009. Interpretations of the geographical patterns of the NSHFs are suggested that are based on relatively simple dynamical ideas. As a step toward investigation of their validity, we examine the contributions to the rate of change of the active tracers (potential temperature, salinity and potential density) from the main terms in their prognostic equations as a function of the active tracer and latitude. We find that the main contributions from vertical mixing occur in “near surface” layers and that, except at high latitudes, the time-mean advection of potential temperature and density is well anti-correlated with the sum of the surface fluxes and vertical diffusion. By contrast, the tracer budget for the salinity has at least four terms of comparable magnitude. The heat input by latitude bands is shown to be dominated by the NSHFs, the time-mean advection, and the equatorial Pacific. Expressions for global integrals of the salt and heat content tendencies due to advection as a function of salinity and potential temperature respectively are derived and shown to make contributions that should not be neglected.