Circulation Regimes due to Attractor Merging in Atmospheric Models

Abstract

From a dynamical systems theory perspective, the mechanisms of atmospheric regime behavior in a barotropic model, a pseudobarotropic model, and a baroclinic three-level model, where all of them show quite realistic regimes, are unveiled. Along with this, the role played by multiple equilibria for the emergence of regimes in barotropic models is critically reexamined. In the barotropic model, a sequence of bifurcations is observed, which leads to the merging of coexisting attractors and results in two pronounced regimes corresponding to high- and low-index flow. The pseudobarotropic model is constructed from the three-level model by introducing a strong internal friction between the levels and switching off the interfacial diabatic forcing, and it has essentially the same bifurcation properties and regimes as the truly barotropic model. A continuous metamorphosis between the pseudobarotropic and the original baroclinic three-level model is accomplished by a linear interpolation of parameters and forcing fields between these two models. Both local and global bifurcations occurring during this transition to baroclinicity are analyzed in detail, yielding two main results. First, almost all of the multiple steady states of the pseudobarotropic model owe their existence merely to the fact that the surface friction has generally to be chosen unphysically weak in barotropic models in order to obtain chaotic behavior. Second, the circulation regimes in both the pseudobarotropic model and the baroclinic three-level model are proven to emerge from the unification of multiple attractors, which coexist at intermediate strength of baroclinicity and correspond to low- or high-index flow configurations, respectively.