A reasonable range for the background diffusion coefficient diff_cbt_back is from molecular values of 0.00134 cm2/sec to bulk values of about 0.1 cm2/sec. For background viscosity coefficient visc_cbu_back, a reasonable range is from molecular values of 0.0134 cm2/sec to bulk values of about 1.0 cm2/sec. Based on Pacanowski and Philander (1981), reasonable values for the maximum mixing coefficient fricmax range from about 50 cm2/sec to 100 cm2/sec. The Prandtl number is about 10 for stable regions and 1 for regions of strong mixing.
Choosing diff_cbt_back too large tends to erode the thermocline away. Values of visc_cbu_back occur in regions of high Richardson number and have an affect on limiting the speed of the Equatorial Undercurrent: lower (higher) values resulting in faster (slower) speeds. To work reasonably well, this scheme needs about 10 meter or finer resolution in the vertical. It may not do well off the equator if shortwave penetration isn't taken into account because vertical shear may not be enough to overcome buoyancy when all the heat flux is absorbed within the first vertical cell. In simulations, maximum values for mixing coefficients occur in regions of low Richardson number such as surface mixed layers. Philander speculates that fricmax should be a function of windspeed and this is being explored. The windmix parameter is arbitrary and is meant to simulate the high frequency wind bursts that are absent (due to the averaging process) from climatological forcing. Typically, windmix has been set at 15 cm2/sec.
Various parameters for this option may be changed through namelist. Refer to Section 14.4 for information on namelist variables.