Regime dependent instability as a transition mechanism in large-scale atmospheric flow

Two apparently distinct approaches to studying the observed low frequency variability in the atmosphere have evolved over the past few years. One approach invokes multiple, recurrent flow regimes to explain the observed variability. An alternative approach involves the linear stability properties of the climatological mean flow. In the present study, these approaches are merged to understand the different stability properties of the basic states associated with different flow regimes. In particular, the different zonally asymmetric components of the regime basic states lead to differing stability properties that in turn may explain the transition mechanism between regimes. This is particularly true for the transition between an amplified atmospheric planetary wave flow regime and a zonal regime. The observed transition streamfunction anomaly pattern compares very well to the most unstable stationary eigenmode of a linear stability calculation for both the barotropic and two-level baroclinic case studied. However, the growth rate of the barotropic case is too slow compared to observations and it is quite sensitive to the dissipation rate and the resolution of the calculation. In the baroclinic case, the same eigenmode appears but with a faster growth rate and more structural stability. Within the constraints of a two-layer model, the effect of baroclinicity is to remove the dependence on dissipation rate of the growth rate of the most unstable barotropic mode, allowing fast growth without sensitivity to chosen parameters. The most unstable stationary baroclinic eigenmode strongly resembles the anomaly pattern of the observed transition. The energetics of the growing mode involves extraction of energy from the zonally asymmetric flow in agreement with observations. Experiments with greatly increased dissipation reveal very little sensitivity of the growth rate of this stationary eigenmode to the rate of dissipation. Alternatively, the eigenmodes for the opposite transition considered, from the zonal to amplified wave regime, are different from this former case in terms of structure, growth rate and energetics. Therefore, we conclude that the linear stability properties of the atmospheric flow are a function of the amplitude of the zonal asymmetries in the antecedent regime basic state, and that the dynamics of the transitions between regimes might be understood within the context of the linear instability properties of the regime basic states