INTEGRAL EFFECTS OF DEEP CONVECTION

The large-scale, integral effect of convective elements (plumes) const ituting an open-ocean chimney is investigated both theoretically and w ith a plume-resolving numerical model. The authors consider an initial ly homogeneous ''patch'' of ocean of depth H, with Coriolis parameter f, in which buoyancy is lost from the surface at a rate B. Both vortic ity constraints on the convection patch and model analyses imply that, irrespective of the details of the plumes themselves, the mean vertic al transport resulting from their action must be vanishingly small. Pl umes are best thought of as mixing agents, which efficiently homogeniz e properties of the chimney. Scaling laws are derived from dynamical a rguments and tested against the model. Using an expression for the ver tical mixing timescale, they relate the chimney properties, the streng th of the geostrophic rim-current setup around it, and its breakup tim escale by baroclinic instability to the external parameters B,f, and H . After breakup, the instability eddies may merge to form larger ''con es'' of convected water, which offset the buoyancy loss at the surface by laterally incorporating stratified fluid. Properties of the plumes only enter the scaling results by setting the vertical mixing timesca le. The authors argue that the plume scale may be parameterized by a m ixing scheme if this implies the appropriate mixing timescale. Finally , the authors suggest that for the estimation of deep-water formation rates the volume of convectively modified fluid processed by a chimney should be computed rather than the mean vertical transport during the convection phase.