Add capability to exchange carbon content of runoff from Land to Ocean biogeochemistry component for ESM4.5

[nikizadehgfdl]

CMIP7/ ESM4.5 needs capability to exchange carbon content of runoff from Land to Ocean COBALT biogeochemistry component.
Many components need to be updated to make this happen (in the current coupler design):

• Land component calculates the DOC (Dissolved Organic Carbon) content flux in runoff (KgC/m2/s)
• COBALT needs access to that runoff flux (currently done via data_override from file)
• Ocean component needs to make the flux available to COBALT
• Ice component needs to make the flux available to Ocean
• Coupler land_ice_flux_exchange needs to exchange the flux from Land to Ice

Here's the rough summary of what needs to be done:

FMScoupler: Exchange the carbon content flux
main...nikizadehgfdl:FMScoupler:exchange_land2ice_runoffDOC

MOM6: Introduce/initialize/Update a new boundary member to receive the Ice flux ice_ocean_boundary%runoff_carbon
NOAA-GFDL/MOM6@dev/gfdl...nikizadehgfdl:MOM6:exchange_land2ice_runoffDOC

SIS2: Introduce/initialize/Update a new boundary member to receive the land flux from coupler land_ice_boundary%runoff_carbon and pass it to Ocean in ice_ocean_boundary%runoff_carbon
NOAA-GFDL/SIS2@dev/gfdl...nikizadehgfdl:SIS2:exchange_land2ice_runoffDOC

lm4P: Introduce/initialize/update a new boundary member to send the land flux to coupler
https://gitlab.gfdl.noaa.gov/fms/lm4P/-/merge_requests/new/diffs?merge_request%5Bsource_project_id%5D=764&merge_request%5Bsource_branch%5D=exchange_land2ice_runoffDOC&merge_request%5Btarget_project_id%5D=764&merge_request%5Btarget_branch%5D=user%2Fslm%2Fesm4p5

COBALT: Add the land-calculated runoff carbon content (contemporary) to the background (geological) flux
NOAA-CEFI-Regional-Ocean-Modeling/ocean_BGC@dev/cefi...nikizadehgfdl:cefi_ocean_BGC:exchange_land2ice_runoffDOC

[laurenchilutti]

The null components will need to be updated as well.

[wfcooke]

This will increase the memory footprint of all coupled models with the allocation in full/ice_ocean_flux_exchange.F90 ?

Any chance of having a switch to allocate the space only if you are running with carbon_runoff? Or is that a rabbit hole we do not want to descend into?

[slm7826]

This works only for DOC tracer; I think there is a high probability that in the future more tracers may be added, e.g. nitrogen-related components. River code -- the source of "runoff" fields and DOC -- already has a generic mechanism for tracers.

I wonder if it is possible to make the exchange mechanism in the flux exchaneg generic too? Than might address the @wfcooke comment about the memory footprint: if we do not have tracers we do not allocate/pass data for them.

[Hallberg-NOAA]

This proposed set of changes could work. However, please be aware that because the various component models are subject to independent version control, it will take a long time to propagate these changes through the components one-by-one, starting with SIS2 because that is where the new flux being added to the ice-ocean boundary type. Once that new flux is in SIS2, it could be used in MOM6 perhaps 6 months later, after we update the version of SIS2 that the MOM6 driver points to (and MOM6 will no longer be able to use any previous versions of SIS2). Some time after MOM6 is updated, the driver could be updated to use new fields in MOM6, but once again all previous versions of MOM6 (and SIS2) will be unusable with the updated coupler. It is fortunate that the MOM6 FMS driver is not widely used outside of GFDL, otherwise these change would require coordinated updates across the entire MOM6 community, and that would itself take many months. As one can imagine, adding the new field this way is an extremely disruptive process, and in the past the time it has taken to go from proposing changes to the coupler types to having them fully realized is measured in years. Moreover, given the maturity of many of our models and a very natural reluctance to change them when they are in production, this process can make it much harder to support such models or to propagate other, less disruptive model improvements into models at a natural pace.

We have some analogous changes in which we would like to pass additional fields between SIS2 and MOM6, but we have found the coordination of versions to be so onerous that the proposed changes have literally languished for years and we are probably going to move SIS2 and MOM6 into the same git repository to allow for more graceful additions of future coupling fields between the two components.

It should be noted that if the ice-ocean boundary type and other exchange types were made opaque with an object-oriented extractor routine, we could at least compile with older versions of the various components, and the massive disruption associated with adding a single field would be dramatically reduced. Such a style change might be an important objective for the FMS coupler redesign project.

[thomas-robinson]

@nikizadehgfdl have you considered this feedback? Is there a PR for this?