Several horizontal mixing options are available for mixing tracers and momentum. With a few exceptions (discussed later), the choice of tracer mixing can be made independently of the choice of momentum mixing. As with vertical mixing, the main namelist input only selects the choice of mixing options; the actual mixing parameters associated with each option are read from a namelist specific to that option. The del2 (Laplacian) and del4 (bi-harmonic) mixing options are ad hoc level-oriented parameterizations that mix water-mass properties across sloping isopycnic surfaces. The Gent-McWilliams  parameterization remedies this shortcoming by forcing the mixing (of tracers only) to take place along isopycnic surfaces. The principal drawback of the gent option is cost; it nearly doubles the running time. For momentum mixing, an anisotropic viscosity parameterization (aniso) is also available which assigns different values of viscosity parallel and perpendicular to a given direction, where the direction can be specified as described in a later section. Under the aniso option, a Smagorinsky form of viscosity can be specified.
|horizontal mixing methods|
|hmix_momentum_choice||'del2'||'anis'||'del4'||'del2', 'del4', 'anis'||method for horizontal mixing of momentum (Laplacian, biharmonic or anisotropic)|
|hmix_tracer_choice||'del2'||'gent'||'del4'||'del2', 'del4', 'gent'||method for horizontal mixing of tracers (Laplacian, biharmonic or Gent-McWilliams)|
|N/A||.true.||.false.||.true., .false.||flag for activating submesoscale mixing|