Nonequivalent Statistical Equilibrium Ensembles and Refined Stability Theorems for Most Probable Flows
Statistical equilibrium models of coherent structures in two-dimensional and barotropic quasi-geostrophic turbulence are formulated using canonical and microcanonical ensembles, and the equivalence or nonequivalence of ensembles is investigated for these models. The main results show that models in which the global invariants are treated microcanonically give richer families of equilibria than models in which they are treated canonically. Such global invariants are those conserved quantities for ideal dynamics which depend on the large scales of the motion; they include the total energy and circulation. For each model a variational principle that characterizes its equilibrium states is derived by invoking large deviations techniques to evaluate the continuum limit of the probabilistic lattice model. An analysis of the two different variational principles resulting from the canonical and microcanonical ensembles reveals that their equilibrium states coincide only when the microcanonical entropy function is concave. These variational principles also furnish Lyapunov functionals from which the nonlinear stability of the mean flows can be deduced. While in the canonical model the well-known Arnold stability theorems are reproduced, in the microcanonical model more refined theorems are obtained which extend known stability criteria when the microcanonical and canonical ensembles are not equivalent. A numerical example pertaining to geostrophic turbulence over topography in a zonal channel is included to illustrate the general results.