ALBERT

Description: Dynamical mechanisms of atmospheric regime behavior are investigated in the context of a quasigeostrophicthree-level T21 model of the wintertime atmospheric circulation over the Northern Hemisphere.The model, driven by realistic orography and using a thermal forcing determined by a newly developedtuning procedure, is shown to possess a reasonable climatology and to simulate the Arctic Oscillation quiterealistically. It exhibits pronounced internally generated interannual and decadal variability and, in particular,circulation regimes that agree fairly well with observed ones. Two known hypotheses about theorigin of regime behavior, as it occurs in the model herein are addressed: (i) multiple equilibria and (ii)chaotic itinerancy between attractor ruins. The first hypothesis is falsified at very high probability, while thesecond is likely to be true.

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