horizontal mixing options

[osean]

Hi All
I am trying to navigate through wiki roms to figure out what options I have for horizontal mixing of momentum and tracers.
Currently I find there are four - which are UV_VIS2, UV_VIS4 - these two are well commented - and perhaps also TS_DIF2 and TS_DIF4 that have zero information provided?

UV_VIS2
Option to activate/deactivate horizontal, harmonic viscosity of momentum. The hamonic mixing of momentum can be applied along terrain-following or geopotential levels. In ROMS, the viscosity is derived from the horizontal divergence of the deviatory stress tensor (Wajsowicz, 1993).
parameter = visc2(ng)
variable = visc2_p(:,:), visc2_r(:,:)
required = MIX_S_UV, MIX_GEO_UV
related = VISC_GRID
routine = uv3dmix2_s.h, uv3dmix2_geo.h, uv3dmix.F

UV_VIS4
Option to activate/deactivate horizontal, biharmonic viscosity of momentum. The bihamonic mixing of momentum can be applied along terrain-following or geopotential levels. In ROMS, the viscosity is derived from the horizontal divergence of the deviatory stress tensor (Wajsowicz, 1993).
parameter = visc4(ng)
variable = visc4_p(:,:), visc4_r(:,:)
required = MIX_S_UV, MIX_GEO_UV
related = VISC_GRID
routine = uv3dmix4_s.h, uv3dmix4_geo.h, uv3dmix.F

whereas TS_DIF2 and TS_DIF4 have absolutely no information -

TS_DIF2
Option to
parameter =
parent =
required =
related =
conflict =
routine =

TS_DIF4
Option to
parameter =
parent =
required =
related =
conflict =
routine =

Can someone explain what these are? Is there a reference to what they are? My suspicion is that they are 2nd order and 4th order schemes?

thanks

[jbettencourt]

TS_DIF2 and TS_DIF4 refer to the way in which the viscous diffusion of tracers is computed. UV_VIS2 and TS_DIF2 use the laplacian of the velocity field to compute the viscous diffusion term while UV_VIS4 and TS_DIF4 use the bilaplacian.