Ideally, the new difference scheme should be tested with no lateral diffusion
of density. This is due to the fact that the density diffusion along 
surfaces generates horizontal gradients wherever the surfaces are not
flat, and then produces horizontal pressure gradients which drive currents in
much the same way as the pressure gradient errors (McCalpin, 1994).
Unfortunately, the absence of the horizontal diffusion keeps the small-scale
pressure disturbances generated by topographic scale density advection, and
the induced small-scale velocity fields, which in turn cause the computational
instability problem. Thus, some lateral viscosity on surfaces is
required in the momentum equations to maintain stability. A constant
coefficient (
) biharmonic formulation is used here for the lateral
viscosity, which varies from 10
m
s
in this study.