The effects of sills and mixing on the meridional overturning circulation

The meridional overturning circulation of the global oceans is thought to be a result of an interplay, not yet well understood, of surface buoyancy fluxes, wind-driven upwelling and turbulent mixing. One factor believed to be important to this interplay is the latitudinal distribution of the surface buoyancy fluxes. The resulting convective circulation can be modeled in idealized laboratory experiments and numerical solutions. The convection has a number of features in common with the meridional overturning circulation and is used in this thesis to examine the effects of topography and turbulent mixing on the global oceans. A set of laboratory experiments is used to explore the effect of a partial barrier (modeling a sill in an ocean basin) on an overturning circulation. The experiments are forced by a gradient in either surface temperatures or heat fluxes, show that the sill will influence the density field when the sill depth is less than twice the boundary layer (or thermocline) depth. Application of the results to the North Atlantic circulation suggests that the Greenland-Scotland Ridge is shallow enough to significantly reduce the density of North Atlantic Deep Water, and this is consistent with the conclusions from an analysis of water mass properties. A set of numerical solutions is used to investigate the effect of a sill on an overturning circulation from an energetics viewpoint. Specifically, the numerical solutions provide a means to test various methods of defining a reference potential energy state of no motion, an exercise complicated by topographic barriers. Ignoring the topography overestimates of available potential energy of the circulation. However, it is found that the method used to determine the reference state does not significantly affect the calculated rate of energy input from surface buoyancy fluxes, which is the quantity of dynamic importance to the maintenance of the circulation. A second set of laboratory experiments is used to examine the effects of externally imposed rates of small-scale mixing on the overturning circulation forced by differential surface buoyancy fluxes. Sources of stabilizing and destabilizing buoyancy were applied at the surface and mechanical stirring was imposed throughout the depth. When the added stirring (and consequent mixing) provides the dominant contribution to the total vertical transport, measurements of the equilibrated circulation show a dependence of boundary layer thickness and overturning transport on the mixing rate in accord with a theoretical model. For weak or no external stirring, internal processes maintain a level of vertical transport approximately 200 times larger than that expected by molecular diffusion and it is argued that this includes a component that is not irreversible mixing but which is instead advection due to turbulent entrainment into the sinking plume. With reference to the oceans, it is shown that the primary effect of mixing (with energy sourced from winds, tides and convection) is to deepen the thermocline, thereby influencing the buoyancy entrained by the plume. It is concluded that the advective transport of buoyancy by the plume, and not vertical mixing, is crucial for coupling the surface to the abyss. -- provided by Candidate