variability in Tgav and using Hector to drive trained ESM emulators

[abigailsnyder]

Documenting points here:

• fldgen successfully deals with variability about the ESM_Tloc-ESM_Tgav regression.
• ESM Tgav trajectories have interannual and decadal variability. However, Hector's Tgav trajectories are significantly smoother, even when calibrated to a specific ESM.
• Work linking Hector and fldgen will need to take that into consideration and quantify or correct for that discrepancy

[claudiatebaldi]

Probably not a concern for the estimate of the intercept/slope but for the characteristics of the residuals. First thing to check, average globally the synthetic fields obtained using Hector and check that variability looks the same as original/real global average time series from ESM. If the inter-annual and multidecadal variability have lower amplitudes then we have a problem.