A baroclinic laminar state for rotating stratified flows

A baroclinic laminar model is developed as the late-time equilibrium state in the free decay of rotating stratified turbulence under low-Froude-number scaling. Vertical motions are suppressed by stratification and ambient rotation, and in the laminar end state the flow assumes a quasi-two-dimensional form. Geometric analyses of this nonlinear conservative model reveal an f-plane baroclinic topology characterized by vertical alignment, vanishing nonlinearity, and the complete absence of helicity. Equivalent-barotropic flow is the only nonunique baroclinic solution, and its horizontal topology is restricted to a unidirectional jet and a circular vortex. Such a depleted geometry results from the constraint of basic equations where density is advected as an active scalar. It provides a baroclinic mechanism for the formation of coherent structures in geophysical flows.