I believe this is true, but I don't have as much experience here. The FC5 is a 1850 constant forcing, while the FAMIPC5 is time-varying forcing. Perhaps the former is more in line with your SOM experiment.
I'm attempting to generate a SOM Q-flux file using CESM1-POP2 data from a Year 2000 simulation (last 100 years of a 500 year equilibrium simulation at this time). The climate in this simulation appears to be mostly equilibrated, so I thought it would be suitable for generating a SOM Q-flux. I started a timeslice run using Q-flux generated from this simulation using the pop_frc_mlt.ncl script. However, I found that about ~10 years into the simulation, the Y2000 Qflux case has a dramatic loss of summertime sea ice in the central Arctic (first attached figure for aice at ~81N,220E), rendering the Arctic ice free in summer. The sea ice cover is larger in the original Y2000 coupled simulation, so it seems like there is something wrong with the Qflux I've generated.
Do you have any advice on how to deal with this issue? Something I notice when comparing the September Y2000 to PI Qflux is that there is a small negative anomaly that extends into the Central Arctic (second attached figure). If I understand correctly, this means that there is an anomalous input of heat into the slab ocean layer. Could this be large enough to melt the ice entirely as I'm seeing?
I would expect that a year 2000 SOM forcing file would be very warm from a climate state that is not in balance. That is why we tend to stick with 1850 picontrol runs as these are better equilibrated with a top of atmosphere of near zero. There is a Meehl et al. paper that talks about the issues of a 2000 control run.