Abstract

Stratified flows are common place in geophysics, and understanding how such flows become and remain turbulent is a research area of major significance, with relevance to heat and pollutant transport over a huge range of scales. Parameterising such turbulence, and its associated irreversible scalar mixing in terms of larger scale flow parameters is needed to capture the key effects of turbulence in larger scale models of the earth’s (rapidly changing) climate system. Unfortunately, constructing reasonable and robust parameterisations is exceptionally challenging, not least because of the widely observed  tendency for sufficiently `strongly’ stratified flows to form (and maintain) density staircases, with relatively deep, relatively well-mixed `layers’ being separated by thinner `interfaces’ of enhanced density gradient. Here the initial `strength’ of the stratification is quantified in terms of the magnitude of the time scales and/or length scales of the background stratification relative to the equivalent scales of the turbulence and/or a larger scale background flow, and traditionally it has been assumed that much of the world’s oceans can be considered to be in a `strongly stratified turbulence’ regime, with an associated predicted highly anisotropic flow dynamics.  Unfortunately, the existence of such  spatio-temporally variable `staircases’ and layers calls into question the very concept of a flow being `strongly’ stratified, due not least to the spatio-temporal variation of the relevant length scales and time scales. I will attempt to discuss various aspects of this interesting issue in terms of real-world observations, laboratory experiments and numerical simulations.