[MITgcm-devel] seaice

[Martin Losch]

Hi there,
I have (again) questions about the seaice-pkg. I am working on a  
2x2*cos(phi) deg quasi global (no arctic) model approximately 30 to  
50km resolution in the Weddell Sea (and Ross Sea). I am forcing with  
the CORE fields (with daily winds, tair and humidity), but my  
problems probably not connected to the forcing. I use the exf-pkg and  
kpp and seaice (SEAICE_MULTILEVEL undefined).
My basic problem is that there is not enough sea-ice in the Weddell  
Sea (and the Ross Sea, but that's a different story). One of the  
reasons is that the ACC "breaks" through the South Scotia ridge (the  
extension of the Antarctic pensinsula) into the Weddell Sea and  
directly imports warm water instead of staying further north and  
introducing the warm water further east into the Weddell Gyre. As a  
consquence (or a cause?) there is no Weddell Gyre to speak of and too  
little ice (the distribution is OK, but the thickness is too small  
towards the north-east of the Weddell Sea).
The funny thing is that, when I turn off the seaice-model, things  
look much better (except that the surface is now too cold, which I  
would have expected in the absence of ice growth): I have a strong  
Weddell Gyre and the ACC stays completely north of the Weddell Sea as  
it should. The same is true, when I use ncep heat fluxes and surface  
field restoring (as in global_ocean.90x40x15). So topography (which  
was my first guess) cannot be the reason for this behavior, but it is  
most likely the seaice that makes the Weddell Gyre disappear and lets  
the ACC into the Weddell Sea. Candiates are:
1. not likely: buoyancy fluxes: I don't see how the buoyancy fluxes  
can cause this problem. I have SEAICE_EXTERNAL_FLUXES defined, so  
that exf does all the work over open water.
2. more likely: momentum forcing: But ... I tried a physically  
sensible stress formulation (seaice_concentration*ocean_ice_stress +  
(1-seaice_concentration)*ocean_air_stress, see ostres.F with  
SEAICE_TEST_ICE_STRESS_1 defined); and I tried using only  
ocean_air_stress as precomputed by exf_bulkformulae (no influence of  
ice on stress, the default). Both give similar results, which I don't  
understand.

In order to understand what I am doing when I mess up the code, I  
need to know what the different fields are. As far as I can see, the  
seaice-velocities UICE and VICE have three time levels, with UICE(:,:, 
1,:,:) being the most current, and UICE(:,:,2,:,:) being the previous  
timestep and level 3 probably some intermediate step/auxiliary  
variable used in lsr.F. But what are UICEC and VICEC, in what sense  
are they different from the first time-level of UICE/VICE?

The SEAICE_ORIGINAL_BAD_ICE_STRESS in ostres.F uses a mixture of UICE 
(:,:,1,:,:), UICE(:,:,3,:,:) and UICEC to compute the stresses, which  
I do not understand at all. Which is the ice-velocity to use for  
computing ocean-seaice stresses?

Puzzled,

Martin