Open-ocean deep-water formation involves the interplay of two dynamica
l processes; plumes (less than or equal to 1 km wide), driven by ''upr
ight'' convection, and geostrophic eddies (greater than or equal to 5
km wide), driven by baroclinic instability. Numerical ''twin'' experim
ents are used to address two questions about the plumes: Can they be r
epresented by a simple mixing process in large-scale models? If so, is
it important that the mixing occurs over a finite time t(mix), or wou
ld instantaneous mixing produce the same effect on large-scale propert
ies? In numerical simulations which resolve the geostrophic eddies, we
represent the plumes with a ''slow'' convective adjustment algorithm
which is broadly equivalent to an enhanced vertical diffusivity of den
sity in statically unstable regions. The diffusivity kappa depends on
t(mix), the mixing timescale. The fidelity of the plume parameterizati
on is then evaluated by comparison with plume-resolving simulations of
open-ocean deep convection. Integral properties of the plumes, such a
s the temperature census of the convected water and the strength of th
e rim current that encircles the convecting region, are all accurately
reproduced by the slow adjustment scheme. The importance of choosing
an appropriate finite value for t(mix) is explored by setting t(mix) =
12 hours in some experiments, in accordance with scaling consideratio
ns, and t(mix) = 0 in others, corresponding to instantaneous adjustmen
t, the conventional assumption. In the case of convection into a moder
ately or strongly stratified ocean the behavior does not significantly
depend on t(mix). However, in neutral conditions the slow adjustment
does improve the parametric representation. Our experiments confirm th
e picture of plumes homogenizing the water column over a time t(mix).