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Using field, numerical and laboratory studies, we consider the roles of both shear and convection in driving mixing in the interior of the density‐stratified ocean. Shear mixing dominates when the Richardson number , convective mixing dominates when , and in the intermediate regime when both shear and convection can contribute to mixing. For pure shear mixing the mixing efficiency approaches 0.5, while for pure convective mixing the mixing efficiency approaches 0.75. The diapycnal diffusivities for the two mechanisms are given by very different expressions. Despite these complexities, a simple mixing length model using the mean flow shear S provides robust estimates of diffusivity across the range . To account for the roles of both shear and convection over this range of , we also formulate a modified version of the empirical KPP model for parameterizing ocean mixing in numerical models.

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