[MITgcm-devel] monthly, inter-annually varying forcing with exf

[Oliver Jahn]

Hi Developers,

I have some code for extending the exf package to read calendar-monthly
forcing fields that vary from year to year.  I think other people could
find this useful too, see for instance

http://mailman.mitgcm.org/pipermail/mitgcm-support/2019-October/012213.html

I'd like to get some input on how to enable this feature.  Right now the
magic value of *period = -12 is used for monthly climatologies.  I have
talked to Jean-Michel about this and we think the most logical extension
would be to use the same value for inter-annually varying monthly fields
and distinguish by *RepCycle.  A *RepCycle of 0 would be for
non-repeating forcing, and 12 could be for climatology.  One could also
support monthly forcing that repeats after a different number of months
(well, years, usually), and *RepCycle could be in units of months - as
suggested by *period = -12.  So, *RepCycle = 120 would be monthly
forcing that repeats every 10 years.

We do not want to break existing setups, of course, so we were thinking
of defaulting *RepCycle to 12 when *period is -12 and to repeatPeriod in
all other cases.  This would be accomplished by an UNDEF default value
that is then changed to either 0 or repeatPeriod once *period has been set.

Any thoughts on this would be very welcome!  Maybe this breaks someone's
use case we didn't think of.  Please let us know!

Cheers,
Oliver