Diapycnal Mixing Deductions From the Large-Scale, Time-Mean, World Ocean Temperature-Salinity Distribution

The turbulent diapycnal mixing in the ocean is intimately linked to the strength of the oceanic circulation and is currently obtained from microstructure measurements, dye experiments and large scale hydrographic budgets. We use a large scale view point here and show how the numerous observations of temperature and salinity (which allow to construct a meaningful time-mean of these quantities) can be used to obtain the diapycnal mixing through the water column. The new method makes use of a general circulation model to compute a World Ocean Circulation forced by the climatology of temperature and salinity through restoring terms and surface winds. At steady state the large scale advection of these tracers is balanced by the divergence of turbulent fluxes which are given by the restoring terms. After interpolating these restoring terms on neutral density coordinates, we obtain diapycnal fluxes that can be interpreted as diapycnal diffusion and henceforth the diapycnal mixing coefficient throughout the water column. The geography of diapycnal mixing for selected water masses in the various basins along with the errors associated is discussed. In all ocean basins, diapycnal diffusivities increase with depth and are stronger in western boundary current regions with values reaching O(10^-3) m²s^-1. In the ocean interior, the pattern of diapycnal diffusivities is more disorganised with local patches of positive and negative diffusivities. On the global average, effective diffusivities increase downward, peaking at about 5x10^-4 m²s^-1 near the bottom, in agreement with previous estimates based on inverse modelling or direct measurements. In the upper ocean, heat is pumped downward resulting in negative global average effective diffusivities there.