The numerical schemes used for tracer subgridscale physics are written such that the heat and salt contents are conserved (equations in flux form). Since a flux form is used to compute the temperature and salinity, the quadratic form of these quantities ( their variance) globally tends to diminish. As for the advection term, there is conservation of mass only if the Equation Of Seawater is linear.
constraint of conservation of tracers:
In fact, this property simply results from the flux form of the operator.
constraint on the dissipation of tracer variance:
Gurvan Madec and the NEMO Team
NEMO European Consortium2017-02-17