Layering and layered states in single-component stratified flows

In stratified turbulent flows such as the ocean, layered features (in which well-mixed regions of fluid are separated by sharp interfaces where the density varies rapidly) are a common occurrence. Understanding how and when layers might arise, as well as how they evolve in time, is an ongoing area of study. Here, we discuss several mechanisms by which layers may form, including via an anti-diffusive instability of the background stratification, through a linear instability arising from vortical structures in stratified flows, and as a consequence of localized mixing caused by individual turbulent events. Because the resulting layers are frequently co-located with regions of strong shear, we briefly describe the different mixing mechanisms that may occur and impact the longevity of the layers. We then move from the formation and evolution of individual layers to the more general state of layered anisotropic stratified turbulence that has been proposed for strongly-stratified turbulent flows, and conclude by highlighting several important avenues for future study. This article is part of the theme issue 'Anti-diffusive transport and layering: from sub-cellular to astrophysical scales'.