Insights into the Coupling of Turbulent Mixing and the Overturning Circulation

Turbulent mixing is thought to influence the ocean meridional overturning circulation (MOC), however a detailed physical understanding of the coupling between mixing and the MOC is yet to emerge. This coupling is examined through idealized laboratory experiments with a convective overturning driven by an applied salt flux at the surface.  Additional mechanical mixing was imposed using horizontal rods yo-yoed continuously through the water column. The overturning rate increased as the 1/4 power of the turbulent diffusivity in accordance with a theoretical model. The rate of vertical density transport is parameterized as a bulk diffusivity obtained from different methods, including one from a Munk-like advection–diffusion balance and another from the transport of buoyancy across the surface. For strong stirring, the bulk diffusivities returned by the various methods agree with the externally imposed mixing rate. However, the parameterizations implicitly include a nondiffusive component of vertical transport associated with advection of the density field and it is shown that, for weak stirring, the bulk diffusivities exceed the externally imposed mixing rate. For the oceans, results suggest that the primary effect of mixing (with energy sourced from winds, tides, and convection) is to deepen the thermocline, thereby influencing the entrainment and consequent vertical transport of density in the dense sinking regions. It is concluded that this advective transport of density, and not vertical mixing, is crucial for coupling the surface to the abyss.