[MITgcm-support] 2deg global experiment: advection schemes

[Jean-Michel Campin]

Hi Martin,

> I had expected that the additional diffusion introduced by these 
> schemes (in particular the flux limited schemes) would actually reduce 
> the circulation (as they mostly do with staggerTimeStep=.true.), but 
> not increase it.

I like simple explanations like this one:

You can connect the strength of the circulation with
some basic ideas about the energy:
- horizontal diffusion (on T & S) decrease the energy
- vertical diffusion (on T & S) create energy
When you change the advection scheme to a more diffuse one,
you don't know (or at least, I don't) before doing the test
which of those 2 effects will dominate.

You could try to separate those 2 effects by changing only
one of the 2 horizontal / vertical advection schemes
(setting tempVertAdvScheme, saltVertAdvScheme ..., but
you need to be careful, because mutidimAdvection & AB-2
are set according to the horizontal advection scheme).

The thing that is sure, is that when you use synchronous
time-stepping without AB-2 (= any time you use DST advection
scheme, with or without flux-limiter), you enter an unstable
regime for internal-waves, that will tend to increase
the variability of wVel, and results in more (or much more)
vertical diffusion, and the 2nd effect will dominate.

So, in a sense, this (very) simple argument seems
to fit with our results.

Jean-Michel