Zhang,
I have reproduced on the NCAR computer the error that you are getting.
I have a fix for the error that works here. The attached tar file has code mods for a fix.
The mods are based on CESM 2.1.3.
If you are using that version, you can untar the tar file contents
by running the command "tar xf tavg_fix.tar" while in the subdirectory
SourceMods/src.pop of your case root.
If you are using a different version of CESM, you will need to modify the fix for your version.
The tar file has a diff of the mods versus the CESM 2.1.3 so you can see what was changed.
There are just a few lines changed, so I'm optimistic that you can do this.
Please let us know if these mods fix your problem.
Please note that because of the way that POP computes the MOC diagnostic,
that changing the frequency of just the velocity fields causes the MOC diagnostic to not be generated.
I wanted to be sure that you know this.
Also, adding the history_freq_opt change to user_nl_pop is not needed. This setting is related to
generating instantaneous snapshot output from POP.
I will move this question to the POP section of the forum, which is a more natural location for it.
Keith