Minimum-enstrophy and maximum-entropy equilibrium states in two-dimensional topographic turbulence

Abstract: In recent numerical simulations of two-dimensional (2D) topographic turbulence, the minimum-enstrophy equilibrium state (Bretherton & Haidvogel,J. Fluid Mech., vol. 78, issue 1, 1976, pp. 129-154) with an underlying linear relation between potential vorticity and streamfunction cannot generally describe the final state, due to the presence of long-lived and isolated vortices above topographic extrema. We find that these vortices can be described by a $\sinh$'' vorticity-streamfunction relation that corresponds to the maximum-entropy equilibrium state proposed by Joyce \& Montgomery (J. Plasma Phys., vol. 10, issue 1, 1973, pp. 107-121) and Montgomery \& Joyce (Phys. Fluids, vol. 17, issue 6, 1974, pp. 1139-1145) for 2D flat-bottom turbulence. Motivated by recent observations that a linear PV-streamfunction relation depicts the background flow of 2D topographic turbulence, we propose to model the final state of the total flow by additionally superposing a$\sinh$'' relation to account for isolated vortices. We validate the empirical ``$\sinh$'' relation and composition model against the final states in 2D turbulence simulations configured with two simple domain-scale topographies: a sinusoidal bump plus a dip, and a zonal ridge plus a trench. Good agreements are obtained for both topographies, provided that the empirical theory must be adapted to the zonally averaged flow states over the latter. For the first time, our empirical theory correctly describes the final flow structures and uncovers the coexistence of two equilibrium states in 2D topographic turbulence.