Both the Held and Larichev and Visbeck et al. schemes employ the same time scale. They differ only in their choice of length scale. Held and Larichev (1996) use the Rhines' length, whereas Visbeck et al. use a length determined by the width of the baroclinic zone. Treguier et al. suggest that the Larichev and Held (1995) and Held and Larichev (1996) studies indicate that an appropriate choice is the smaller of the two length scales. In MOM, if both options hl_diffusivity and vmhs_diffusivity are employed, then the smaller of the two length scales will be used.