A mechanism for layer formation in stratified geophysical flows

We discuss the possibility that steplike structures are formed in subcritic al regions of vertically stratified shear flow. The mechanism we propose es sentially consists of localized intense mixing in highly stratified and she ared flows, probably following frontogenesis. Its main assumption is that t he vertical density flux increases monotonically with decreasing gradient R ichardson numbers, which corresponds to enhanced stratification and/or diap ycnal shear. This differs from Phillips' [1972] mechanism, which we argue m ay not apply to vertically stratified shear flow. An essential condition fo r the formation of constant density steps is the incorporation of a Langevi n type equation which takes into account that turbulence must last for some finite characteristic time. We present numerical computations for the case of approximately constant diapycnal shear which lead to the formation of a staircase depth-density structure.