[MITgcm-support] how to compute water flow through ocean sections with gmredi / kpp ?

[gael forget]

Hi Alexandre,

regarding methods to avoid interpolation in computing transports, please refer to earlier email (http://mitgcm.org/pipermail/mitgcm-support/2017-March/010943.html) pointing to the grid-independent "broken lines” approach as implemented in gcmfaces. 

Regarding the GM / bolus velocity contribution you are correct — it can induce an overturn component through any section. To demonstrate how one computes this contribution from GM_PsiX, GM_PsiY model output I just added a call to gcmfaces_calc/calc_bolus.m in, previously mentioned, example_transport.m 

Like the rest of gcmfaces, calc_bolus.m is designed to work with all MITgcm grids so that you should be able to use it without any alteration.

Hope this helps,
Gael

ps: to demo these computations it should suffice to download the up to date gcmfaces code (from cvs or github) and model output from ECCO v4 r2 (nctiles_grid and nctiles_climatology) as explained in section 1 of the gcmfaces documentation (https://github.com/gaelforget/gcmfaces/blob/master/gcmfaces.pdf or http://wwwcvs.mitgcm.org/viewvc/MITgcm/MITgcm_contrib/gael/matlab_class/gcmfaces.pdf?view=co) and then follow directions provided by `help example_transports'


On Apr 11, 2017, at 10:49 AM, Alexandre Pohl <pohl at cerege.fr> wrote:

> Hi David,
> 
> Thank you for your answer.
> 
>> GM has a zero net contribution because the streamfunction is zero at top and bottom.
> 
> Actually, I want to compute the water flow (in Sv) through arbitrary sections (E-W, N-S, SE-NW…), at each level of the ocean. If I understand well, the depth-integrated contribution of GM is null, but it has an impact throughout the water column. Am I wrong ? If not, is it possible to compute the latter based on the diagnostics provided by the model ?
> 
>> Last remark, the interpolated/rotated velocities are not appropriate to compute the net flux, it is very inaccurate. The computation must to be done on the native grid.
> 
> Thanks for mentioning this. Now I avoid interpolating U and V on grid cells centres. Computed depth-integrated fluxes seem to be much more robust, that’s great. However, I did not find any way to skip the rotation. I have to quantify the flux which is eastward or northward, and it does not seem easy to do without rotating the fields. Any idea is welcome :)
> 
> Best,
> Alexandre
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