Sponge layers are no longer necessary as of version 2.3 to stabilize the model. If
need arises they can be turn on by defining the preprocessor macro SPONGE.
In the standard code v2.2, artificial ocean boundary is used and no sponge
functions are turned on. To deal with the boundary instability a sponge layer can be
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 version 2.1 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 3.1, this is the location where the u
(and T) variables are located.