  Climate models are often affected by long-term drift that is revealed by the
evolution of global variables such as the ocean temperature or the surface air
temperature. This spurious trend reduces the fidelity to initial conditions and
has a great influence on the equilibrium climate after long simulation times.
Useful insight on the nature of the climate drift can be obtained using two
global metrics, i.e. the energy imbalance at the top of the atmosphere and at
the ocean surface. The former is an indicator of the limitations within a given
climate model, at the level of both numerical implementation and physical
parameterisations, while the latter is an indicator of the goodness of the
tuning procedure. Using the MIT general circulation model, we construct
different configurations with various degree of complexity (i.e. different
parameterisations for the bulk cloud albedo, inclusion or not of friction
heating, different bathymetry configurations) to which we apply the same tuning
procedure in order to obtain control runs for fixed external forcing where the
climate drift is minimised. We find that the interplay between tuning procedure
and different configurations of the same climate model provides crucial
information on the stability of the control runs and on the goodness of a given
parameterisation. This approach is particularly relevant for constructing
good-quality control runs of the geological past where huge uncertainties are
found in both initial and boundary conditions. We will focus on robust results
that can be generally applied to other climate models.
