REPRESENTATION OF CONVECTIVE PLUMES BY VERTICAL ADJUSTMENT

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).