Age tracer reset in each IAF cycle

[aekiss]

The IAF cycles are run as an ensemble, restarting each cycle using the initial condition from the end of the previous cycle (to avoid leap year issues - see #149).

Unfortunately this resets the age tracer to zero at the start of each cycle, e.g.

import cosima_cookbook as cc
session = cc.database.create_session()
ageiaf = cc.querying.getvar('01deg_jra55v140_iaf_cycle2', 'age_global', session, decode_coords=False, n=-1)
ageiaf.max().load()

returns 60.5968 even though it's 120 years into the run.

This age resetting issue does not affect the RYF runs, e.g.

import cosima_cookbook as cc
session = cc.database.create_session()
ageryf = cc.querying.getvar('01deg_jra55v13_ryf9091', 'age_global', session, decode_coords=False, n=-1)
ageryf.max().load()

returns 270.7675, consistent with being 271 years into the run.

[rmholmes]

Just a note: This affects the 1-degree and 1/4-degree OMIP-2 simulations to be submitted to the ESGF as well.

[aekiss]

The solution is to use something like

&ocean_tracer_nml
     age_tracer_max_init = 61.0
     limit_age_tracer  = .true.

where age_tracer_max_init is the total length of the run at the start of a new cycle (so it will need to be updated at the start of each cycle).
See mom-ocean/MOM5#339 (comment)