WOMBAT: issue with units of ice-to-ocean algae and nitrate fluxes

[dougiesquire]

In MOM5, the units of the prognostic tracer stf field (surface tracer flux) should be "rho * m/s * tracer conc". E.g., WOMBAT prognostic tracers carry units of xmol/m3 (x=m for all tracers except iron which has x=u), so their stf units should be kg*xmol/m5/s. This is the case for DIC, aDIC, O2 and iron (I think, though the provenance of the dust.nc input file is unknown).

However, phytoplankton and nitrate surface tracer fluxes are set directly to the ice-to-ocean algae and nitrate fluxes received from the coupler, which I think have units of mmol/m2/s. I think these fields should be multiplied by rho0. I.e. the surface fluxes are approximately ~1000 too small at the moment.

[pearseb]

That's a good catch Dougie. I really haven't looked closely at these fluxes at all, but I know that there is a lot of room for improvement here.

pearse

[aekiss]

ping @hakaseh

[hakaseh]

I had a quick look and I agree that I had assumed incorrect units for those fluxes, as mmol/m2/s.
I had noticed in the past that these ice-to-ocean fluxes had negligible effects on surface ocean phy or no3 and now it makes sense why. Thanks @dougiesquire for careful check.
Should I fix this?

[aekiss]

Is this error only in the ice->ocean flux or also in the ocean->ice flux?

[aekiss]

Should density be rho0 or ice density? rho0 seems right to me, but I thought I should ask.

[dougiesquire]

Should density be rho0 or ice density? rho0 seems right to me, but I thought I should ask.

Yeah I think rho0. That's what's done for the other surface flux terms in WOMBAT.

[dougiesquire]

Is this error only in the ice->ocean flux or also in the ocean->ice flux?

Sea surface concentration of nitrate and phytoplankton in mmol/m^3 are passed back to the ice (see here and here). This looks correct.

[dougiesquire]

Should I fix this?

@hakaseh, if you have time to submit a PR that would be welcome. I'd be happy to review.

[aekiss]

Is this error only in the ice->ocean flux or also in the ocean->ice flux?

Sea surface concentration of nitrate and phytoplankton in mmol/m^3 are passed back to the ice (see here and here). This looks correct.

Hm, so exchange with sea ice acts as a sink of ocean nutrient and phyto, since 99.9% of it won't come back to the ocean (if we neglect productivity in the ice).

[dougiesquire]

Hm, so exchange with sea ice acts as a sink of ocean nutrient and phyto, since 99.9% of it won't come back to the ocean (if we neglect productivity in the ice).

I may be misunderstanding, but I don't think the ice removes any nitrate/phytoplankton from the ocean. An amount 1000 times too small is fluxed into the ocean from the ice, but nothing is fluxed out. I'm not sure what happens on the CICE side.

[pearseb]

I think it’s likely that if the coupling to sea-ice is done properly, it is as Andrew says, that there is a sink of nutrient and phytoplankton to the sea ice. The phytoplankton in sea ice has to come from somewhere and I would be surprised if Hakaseh didn’t seed the sea ice phytoplankton with phytoplankton from the ocean. From: Dougie Squire ***@***.***> Date: Thursday, 16 May 2024 at 9:47 AM To: COSIMA/access-om2 ***@***.***> Cc: Pearse J Buchanan ***@***.***>, Comment ***@***.***> Subject: Re: [COSIMA/access-om2] WOMBAT: issue with units of ice-to-ocean algae and nitrate fluxes (Issue #282) Hm, so exchange with sea ice acts as a sink of ocean nutrient and phyto, since 99.9% of it won't come back to the ocean (if we neglect productivity in the ice). I may be misunderstanding, but I don't think the ice removes any nitrate/phytoplankton from the ocean. A amount 1000 times too small is fluxed into the ocean from the ice, but nothing is fluxed out. I'm not sure what happens on the CICE side. — Reply to this email directly, view it on GitHub<#282 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AA7GKGGXASF43I63YNSEHLLZCPXX5AVCNFSM6AAAAABGTZWBKGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDCMJTGY3DAOBSGI>. You are receiving this because you commented.Message ID: ***@***.***>

[aekiss]

Ocean nutrient and phyto is coupled to ice (as well as the reverse) https://github.com/COSIMA/1deg_jra55_ryf/blob/0d412448f6ff5a4ce74987e3e8e0d20d3043334b/namcouple#L322-L331

[ofa001]

@aekiss that suggests that the correction needs to be applied both ways, if it was in error that way too.

[anton-seaice]

Hm, so exchange with sea ice acts as a sink of ocean nutrient and phyto, since 99.9% of it won't come back to the ocean (if we neglect productivity in the ice).

Does this mean it accumulates in an unrealistic way in the sea-ice? Possibly not because the sea-ice will melt and so the nutrient/phyto disappears over summer?