The budgets of heat and salinity in NEMO

• We investigate the heat and salinity budgets of NEMO.
• Isoneutral diffusion gives a dominant contribution to these budgets.
• The isoneutral diffusive flux of heat and salt is nearly always upward.
• Geothermal heating is an important heat source in the deep ocean.
• Penetrative shortwave radiation dominates the heat budget of the mixed layer.

The near steady state heat and salinity budgets under surfaces of constant depth are examined in the Nucleus for European Modelling of the Ocean (NEMO) model. It is seen that the heat fluxes in NEMO are difficult to reconcile with the idea of a deep ocean in advection–diffusion balance. Some reasons for this are that the resolved heat advection is downward above 2000 m, and that geothermal heating is, in fact, a major heat source in the deeper parts of the domain. It is also seen that isoneutral diffusion gives a very large contribution to the budgets and that the fluxes from isoneutral diffusion is in general upward. It is explained how the sign of these fluxes depends on the stratification. The heat budget for the upper 100 m of the ocean is seen to be dominated by penetrative shortwave radiation, which is so influential that we would have a mixed layer of considerable thickness even in the absence of other sources of turbulent mixing. Penetrative shortwave radiation is therefore a considerable source of potential energy.