[MITgcm-support] Re : Water at the bottom of NA below freezingpoint

[Jeff Scott]

>
> I have run many different (uncoupled) configurations where the heat flux is 
> prescribed at the surface (or computed from atmospheric parameters and bulk 
> formulae and then prescribed) and I have never seen this type of behavoir, 
> even in 1000-9000 year runs. My suspicion is that your system is loosing heat 
> somewhere, probably in the coupling. I suggest that you diagnose your net 
> heat flux at the surface and then you'll see how big the imbalance is. Is it 
> possible that your atmosphere is loosing the heat and transfering that 
> imbalance to the ocean? I know for sure that if the MITgcm (ocean model) 
> conserves heat very accurately and global mean temperature drifts are only 
> possible, if you have a non-zero net heat input.
>
> I would try to find out whether your cold temperatures come from the surface 
> (I bet they do) and are "convected" downwards.
>

Gus,

By usual course the atmosphere is going to be losing heat through 
radiative processes... but I think Martin is suggesting there might be a 
problem/bug with atmosphere or the coupling here and the system cannot 
come to balance. That is certainly one possibility.

But I think it is also certainly possible that the problem is with the 
ocean. What is your vertical diffusivity? If it is low (.2e-4 or 
especially if less), I had exactly the same happen -- specifically, 
problems with advection-dominated flow causes overshoots and undershoots, 
the later being handled by convection and/or GM to fill the ocean with 
cold temps, which often keep getting colder the longer the run.

(this general problem was very clear when I was using standard MOM in a 
single hemisphere, coarse-res box configuration with very low diffusivity: 
the whole deep tropical ocean was convecting near the bottom, and the deep 
ocean temp kept dropping)

Curiously, I also had most problems with my similar 4x4x22 (coupled) 
configuration (I had KPP on, with vertical layers starting at 10m at the 
top); rather than having the (spurious) deep cold temps confined in the 
NA, the problem occurred near the ACC and it filled the whole deep ocean 
with water below the freezing point. I do not have a problem using 
2.5x2x22 in the SH, but the spurious cold temps were back again in the NA. 
Very odd... but I thought perhaps that 22 layers/10m was too fine given 
the super-coarse horizontal resolution (although I have no real basis to 
make this statement).


My advice - definitely do not use partial cells. If you can stand a little 
diffusion in your system (I agree with Christopher Wolfe's earlier post 
about SOM giving +1e-5 or more) and can be a little more patient, use SOM, 
as this should pretty much solve your problem (assuming this is the 
problem). For other reasons, I'd turn off the CD scheme too, but as Martin 
said it might take some retuning of the viscosities.

And if by chance you are interested in playing with my setup or 
topography, and/or the coupled atm2d package, I'd be happy to help.

Cheers,
Jeff