On spurious interactions among a mixed layer model, convective adjustment,and isopycnal mixing in ocean circulation models

Inconsistencies can arise in ocean circulation models when part of the phys ical processes responsible for vertical mixing is described in the usual di fferential form and part is formulated as adjustment precesses. Examples fo r the latter class are explicit convective adjustment and Kraus-Turner type models of the surface mixed layer. Implicit convective adjustment as well as various representations of interior-ocean mixing are normally described in differential form. All these schemes mix density, with a mixing intensit y that itself depends on stratification. This requires that information con cerning static stability is passed through the individual mixing routines i n a consistent sequence. It is shown that inconsistencies can arise when co upling a Kraus-Turner type model of wind-induced mixing with both a standar d implicit convective adjustment as well as with an isopycnal mixing scheme . This leads to considerably overestimated mixed layer depths, for example, by hundreds of meters in the subpolar North Atlantic. The problem is elimi nated first by ensuring that dissipation of potential energy during convect ion is included in the mixing scheme, even when considering wind-induced tu rbulence only, and second, by either calling the mixed layer routine before the differential vertical mixing scheme or tapering the vertical diffusivi ties to zero within the surface mixed layer.