At the outmost grid points near the northern and southern boundaries of the
model, an artificial ocean boundary is imposed to reduce chances of
model 'blow-up' associated with numerical instability at the boundaries. We find
this greatly enhances the model's numerical stability and has little impact on
dynamics of the inner domain within 
.
Another way to deal with the boundary instability is to apply a sponge layer
at latitudes higher than (this cutoff value
is set by the local variable y0 in subroutine parinit).
There are five options of sponge layer functions.
The first function spngh0 acts to relax land surface temperature to climatology.
The sponge function spngh1 reduces heat and moisture flux and
spngh2 increase
diffusion in the boundary layer. spngh3 increases momentum damping,
while spngh4 reduces excessive precipitation in high latitudes.
In the standard release, only spngh0,spngh1 and spngh3
are active; sponge layer terms spngh2 and spngh4 are commented
out. The functional descriptions of the sponge layer are set in
subroutine parinit.
Sponge layer functions are defined as:
where 
is the latitude (in degrees) at the jth grid point.
In Figure 
, this is the location where the u
(and T) variables are located.
In the standard code v2.2, artificial ocean boundary is used and no sponge functions are turned on.