I did not find as a new student how to calculate "Mean Absolute Error (MAE)" by CDO! Can you assist us with how to compute by CDO?
if you have your observations (or analysis?) in obs.nc and model in model.nc then you can calculate the MAE in the following way
# calculate the difference
cdo sub model.nc obs.nc diff.nc
# absolute value
cdo abs diff.nc abs.nc
# time mean
cdo timmean abs.nc MAE.nc
or piping it all in one line:
cdo -timmean -abs -sub model.nc obs.nc MAE.nc
If instead of the temporal mean you need the spatial MAE then of course you replace timmean
with fldmean