Open questions in turbulent stratified mixing: Do we even know what we do not know?

Understanding how turbulence leads to the enhanced irreversible transport of heat and other scalars in density-stratified fluids is a fundamental research challenge in geophysical and environmental fluid dynamics, although there are still leading-order open questions. One useful approach to addressing these questions is to consider carefully chosen idealized flow geometries. It is then possible to analyze in detail the energetic pathways and the subtle interplay between various characteristic time and length scales. Such analyses can lead to insight into at least some of the apparently mysterious emergent properties of turbulent stratified mixing.