To help keep the model numerically stable, an explicit viscosity is
added to the system through variables `viscx`

and `viscy`

(set in subroutine `DriveInit`

). This viscosity is equivalent to the
coefficient in the advection-diffusion operators (e.g. equation
5.11 and 5.12 in Neelin and Zeng [1]), with one
subtle difference. In the model, instead of making isotropic,
is specified separately for the *x* and *y*-directions. Thus,
the diffusion term is actually represented as
in the model.

Wed Aug 26 16:58:31 PDT 1998