[MITgcm-support] GM_Psi diagnostic when using HFacC and DeepFacC- quick question

[Flynn Ames]

Hi Jean-Michel,

No problem at all about the delay and thanks for clarifying this!

Cheers,
Flynn
________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Jean-Michel Campin <jmc at mit.edu>
Sent: 31 August 2025 9:02 PM
To: mitgcm-support at mitgcm.org <mitgcm-support at mitgcm.org>
Subject: Re: [MITgcm-support] GM_Psi diagnostic when using HFacC and DeepFacC- quick question

Hi Flynn,

Sorry for answering late, I was away when you posted this message and missed it
afterward.

To compute the bolus MOC ( MOC(j,k) at latitude yG(:,j), depth rF(k) ) which involves
the double sum over longitude width and from bottom to rF(k) of
   vVeli(i,j,k) * dxG(i,j)*deepFacC(k) * drF(k) * hFacS(i,j,k)
and using the model diagnostic GM_PsiY, I don't need the hFacS since GM_PsiY is already
vertically integrated from the bottom. But when I integrate along the longitudinal
direction I need to multiply dxG by deepFacF(k), to get:

 MOC_bolus(j,k) = Sum_(i1,i2){ GM_PsiY(i,j,k) *dxG(i,j)*deepFacF(k) }

Regarding the deep-geometry factor, there might be other ways to define GM_Psi,
but the expression above is consistent with how it's defined in MITgcm (and match
the code in gmredi_residual_flow.F, lines 61-98).

Cheers,
Jean-Michel

On Thu, Aug 07, 2025 at 04:23:58PM +0000, Flynn Ames wrote:
> Dear MITgcm people,
>
> Hello - I'm modelling an ocean using GMREDI with deep atmosphere mode turned on, and topography in the domain.
> I use the GM_Psi diagnostic to compute the bolus meridional overturning streamfunction.
>
> I'm wondering if I should be weighting GM_Psi for HFacC and DeepFacC when computing the bolus volume transport in my offline calculations, or whether the GM_Psi diagnostic has already been weighted by these factors so that this step isn't necessary?
>
> I had previously been doing this, but came across a section in the reentrant channel tutorial stating that weighting the GM_Psi diagnostics for HFacC isn't necessary (I haven't found any info about whether the same holds for deepFacC).
> To sanity check, I'm wondering if someone could confirm the correct approach here?
>
> Thanks in advance!
> Flynn
>
>

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