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Relatively strongly stratified turbulent flows tend to self-organise into a'layered anisotropic stratified turbulence' (LAST) regime, characterised byrelatively deep and well-mixed density 'layers' separated by relatively thin'interfaces' of enhanced density gradient. Understanding the associated mixingdynamics is a central problem in geophysical fluid dynamics. It is challengingto study 'LAST' mixing, as it is associated with Reynolds numbers $Re := UL/\nu\gg 1$ and Froude numbers $Fr :=(2\pi U)/(L N) \ll 1$, ($U$ and $L$ beingcharacteristic velocity and length scales, $\nu$ being the kinematic viscosityand $N$ the buoyancy frequency). Since a sufficiently large dynamic range(largely) unaffected by stratification and viscosity is required, it is alsonecessary for the buoyancy Reynolds number $Re_{b} := \epsilon/(\nu N^{2}) \gg1$ where $\epsilon$ is the (appropriately volume-averaged) turbulent kineticenergy dissipation rate. This requirement is exacerbated for oceanicallyrelevant flows, as the Prandtl number $Pr := \nu/\kappa = \mathcal{O}(10)$ inthermally-stratified water (where $\kappa$ is the thermal diffusivity), thusleading (potentially) to even finer density field structures. We report here onfour forced fully resolved direct numerical simulations of stratifiedturbulence at various Froude ($Fr=0.5, 2$) and Prandtl numbers ($Pr=1, 7$)forced so that $Re_{b}=50$, with resolutions up to $30240 \times 30240 \times3780$. We find that, as $Pr$ increases, emergent 'interfaces' become finer andtheir contribution to bulk mixing characteristics decreases at the expense ofthe small-scale density structures populating the well-mixed 'layers'. However,extreme mixing events (as quantified by significantly elevated localdestruction rates of buoyancy variance $\chi_0$) are always preferentiallyfound in the (statically stable) interfaces, irrespective of the value of $Pr$.

Prandtl number effects on extreme mixing events in forced stratified turbulence