Offline tracer transport for BGC

[aekiss]

Apparently MOM6 has an offline tracer transport capability https://adcroft.github.io/MOM6/APIs/namespacemom__offline__main.html

This could be very useful for cheaply spinning up BGC and also doing BGC parameter tuning, so might be worth investigating.

[aekiss]

Note that this only works with standalone MOM6:

The subroutine step_tracers that coordinates this can be found in MOM.F90 and is only called using the solo ocean driver. This is to avoid issues with coupling to other climate components that may be relying on fluxes from the ocean to be coupled more often than the offline time step.

[aekiss]

@dougiesquire commented that WOMBAT needs atmospheric fields which we're getting from DATM via the NUOPC coupler in ACCESS-OM3, so this 1-way coupled mechanism won't work with the solo ocean driver. So this capability would need to be moved into the solo ocean driver.

[aekiss]

First step - Dale to try running MOM6 in offline mode with a simple tracer like age to see what's involved.

[aekiss]

Try a normal run for (say) a year, saving necessary volume fluxes etc, then re-run in offline mode using the saved fluxes.

[dougiesquire]

MOM6-example of offline tracer: https://github.com/NOAA-GFDL/MOM6-examples/tree/4ce152c8458ada43e1388568c1d37003be84b869/ice_ocean_SIS2/Baltic_ALE_z_offline_tracers

[aekiss]

Re. a normal run to save fluxes:

• MOM6-CICE6 1deg_jra55do_ryf is the most mature ACCESS-OM3 configuration and could run for multiple years last time I checked - see https://github.com/COSIMA/access-om3/wiki/Quick-start and https://github.com/COSIMA/MOM6-CICE6/tree/1deg_jra55do_ryf
• Another option could be this global MOM6-SIS2 configuration but it requires more resources and hasn't been run for years AFAIK https://github.com/COSIMA/mom6-om4-025

[dougiesquire]

MOM6-example of offline tracer: https://github.com/NOAA-GFDL/MOM6-examples/tree/4ce152c8458ada43e1388568c1d37003be84b869/ice_ocean_SIS2/Baltic_ALE_z_offline_tracers

I was wrong about what tracers are used in this example - looks like it's using an advection_test_tracer and ideal_age_example tracer

[dougiesquire]

Re. a normal run to save fluxes

I think the MOM6-example I linked above is set up to do both the online and offline runs

[access-hive-bot]

This issue has been mentioned on ACCESS Hive Community Forum. There might be relevant details there:

https://forum.access-hive.org.au/t/cosima-twg-meeting-minutes-2024/1734/6

[AndyHoggANU]

This offline capability would be pretty nice to have for BGC modelling. Actually, it would also be nice for our physics-only runs - because we could actually spin up the ideal age tracer for the first time ever!

I realise there is work to do to get WOMBAT in shape to do this spin up, but perhaps we should start testing this. Seems the best test might be to do something simple with ideal age itself...

I've heard a rumour that @dsroberts is already looking into this system? Dale, any interest in trying that test case? We can probably provide ACCESS-OM3 test cases if that is useful.

[adele-morrison]

I second this, spun-up ideal age would be really cool!!

[dsroberts]

@AndyHoggANU the rumours are true, confirmed here: #123 (comment). I've started digging into this test case, instructions are to run online, then offline, so its exactly what I was looking for. I've not run MOM6 at all before, so I'm getting up to speed with it before looking at BGC.