Increase age tracer limit

[blimlim]

1. Summary:
In #703 we found that changing the restart date could result in the age tracer being overwritten. With the current settings, MOM5 limits the age tracer to be less than the model time. As a result, changing the restart date to 1/1/1 for example caused the age tracer to be reset to 0 everywhere.

It is possible to completely deactivate the age limiting in the namelist, by setting limit_age_tracer = .false.. However this also deactivates limiting of negative values, resulting in negative ages appearing in the output.

An alternative is to leave the age limiting active, and set a very high maximum age limit age_tracer_max_init that hopefully would never be reached. This PR sets the maximum age limit to 1000000 years, which I hope is large enough to not be reached in a model run.

2. Issues Addressed:

• Ocean age tracer gets reset when starting from year 1 #703

3. Dependencies (e.g. on payu, or model)

This change requires changes to (note pull request(s) where relevant):

• workflow manager (payu):
• model deployment (ACCESS-ESM1.6):
• model component or library dependency:
• input workflow:

4. Ad-hoc Testing

The following plot of the zonal maximum age tracer is from a run using /g/data/vk83/prerelease/configurations/inputs/access-esm1p6/modern/pre-industrial/restart/2026.02.20 which had its restart date reset to 1/1/1

There appears to be no limiting occuring:

What ad-hoc testing was done? How are you convinced this change is correct (plots are good)?

5. CI Testing

• !test repro has been run

6. Reproducibility

Is this reproducible with the previous commit? (If not, why not?)

• Yes
• No - !test repro commit has been run.

7. Performance

Has the model performance (say, throughput of model-years/wall-day) changed?

• Yes
• No
• N/A (if selected, please add a brief explanation why performance testing is not necessary for this PR)

This option should not affect performance
If yes, provide the numbers from your testing. Is the performance better or worse?

8. Manifests

Have you changed the executable, the input files and/or the restart files?

• Yes
• No

If yes, have you updated the manifests?

• Yes
• No

To update the manifests, run payu setup (in a cloned copy of your feature branch) with reproducibility tests turned off:

manifest:
  reproduce:
    exe: false
    input: false
    restart: false
runlog:
  enable: false

Then commit the newly created manifest files (under manifests/) only to the branch for this PR.

9. Documentation

Is the documentation updated?

• Yes
• N/A

10. Merge Strategy

• Merge commit
• Rebase and merge
• Squash

[blimlim]

!test repro

[dougiesquire]

Great work tracking this down @blimlim. LGTM

[github-actions]

❌ The Bitwise Reproducibility Check Failed ❌

When comparing:

• 702-max-age-tracer-limit (checksums created using commit d284c86), against
• dev-preindustrial+concentrations (checksums in commit d4b00ee)

Further information

The experiment can be found on Gadi at /scratch/tm70/repro-ci/experiments/access-esm1.6-configs/pr707/702-max-age-tracer-limit/d284c861b13e596ae4559c33b1fe053470920cb5, and the test results at https://github.com/ACCESS-NRI/access-esm1.6-configs/runs/72244102028.

The checksums generated by this !test command are found in the testing/checksum directory of https://github.com/ACCESS-NRI/access-esm1.6-configs/actions/runs/24700856899/artifacts/6545892918.

The checksums compared against are found here https://github.com/ACCESS-NRI/access-esm1.6-configs/tree/d4b00eebede999a167dce704367a49a90f2805ee/testing/checksum

Test summary:
❌ test_repro_historical
✅ test_repro_determinism
✅ test_repro_payu_setup

[blimlim]

!test repro commit

[github-actions]

❌ The Bitwise Reproducibility Check Failed ❌

When comparing:

• 702-max-age-tracer-limit (checksums created using commit d284c86), against
• dev-preindustrial+concentrations (checksums in commit d4b00ee)

🔧 The new checksums will be committed to this PR, if they differ from what is on this branch.

Further information

The experiment can be found on Gadi at /scratch/tm70/repro-ci/experiments/access-esm1.6-configs/pr707/702-max-age-tracer-limit/d284c861b13e596ae4559c33b1fe053470920cb5, and the test results at https://github.com/ACCESS-NRI/access-esm1.6-configs/runs/72245737500.

The checksums generated by this !test command are found in the testing/checksum directory of https://github.com/ACCESS-NRI/access-esm1.6-configs/actions/runs/24701417985/artifacts/6546083971.

The checksums compared against are found here https://github.com/ACCESS-NRI/access-esm1.6-configs/tree/d4b00eebede999a167dce704367a49a90f2805ee/testing/checksum

Test summary:
❌ test_repro_historical
✅ test_repro_determinism
✅ test_repro_payu_setup

[blimlim]

Thanks @dougiesquire for the review!

Just to note, the only checksum change is for the age tracer. I wasn't expecting this as the current restart file hasn't had it's date modified. However, it appears that the age tracer limiting still has an effect on the age tracer. The following show month mean, and first timestep differences for runs with and without this change:

[dougiesquire]

That's odd. So it was previously being limited even when the date wasn't modified?

[blimlim]

So it was previously being limited even when the date wasn't modified?

It's looking like it. The differences in the first time step are all one sign, and so it seems like the limiting was still somehow having an effect:

[blimlim]

In the discussion in #703, we saw that the numerics can lead to negative ocean age, if a lower limit is not applied. I suspect that the numerics may also cause increases in age which are greater than the model timestep. These errors can be much larger than noise (e.g. in the above plots, the limiting causes a difference of 1.3h after a 1h timestep).

However, removing the upper limit appears to be the only way to prevent problems when changing the date, and so we've decided that we will merge this change.

[blimlim]

!cherry-pick d284c86 into dev-4xCO2+concentrations dev-1pctCO2 dev-1pctCO2-bgc dev-1pctCO2-rad dev-flat10 dev-preindustrial+emissions dev-historical+emissions dev-historical+concentrations

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-1pctCO2-rad was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-historical+emissions was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-4xCO2+concentrations was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-flat10 was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-preindustrial+emissions was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-1pctCO2 was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-historical+concentrations was successful.

The new pull request can be reviewed and approved here.

[github-actions]

Automatic Git cherry-picking of commit(s) d284c86 into dev-1pctCO2-bgc was successful.

The new pull request can be reviewed and approved here.