horizontal diffusivity depth dependent hack

[riquitorres]

In mod_fabm_3D.F in the advection subroutine around lines 1660 there is a modification of the horizontal viscosity that is dependent on depth. More specifically, it is reduced on the shelf (5% of original value) and increases linearly between 200-800 to its nominal value.

!*******************************Jadon*************************************
VISCOF=(FACT*0.5_SP*(VISCOFF(IA)*NN_HVC(IA)+VISCOFF(IB)*NN_HVC(IB)) +
FM1*0.5_SP*(NN_HVC(IA)+NN_HVC(IB)))
      if (min(h(ia),h(ib))>800.0) then
      elseif (min(h(ia),h(ib))<200.0) then
        VISCOF=VISCOF*0.05
      else
        VISCOF=VISCOF*(1-(800.0-min(h(ia),h(ib)))/600.0*0.95)
      endif
!*****************************************************************************

I believe this was done to solve instabilities issues in the South China Sea implementation shelf break for nutrients but this shouldn't be there by default... and it should certainly be documented otherwise. There is no equivalent treatment in the salinity advection code