| description abstract | The development of perturbations that optimally trigger the onset of Euro?Atlantic blocking (BL) and strong zonal flow (SZF) regimes has been investigated. These perturbations will be called optimal transition perturbations. First, a T21 three-level quasigeostrophic model (T21QG) including a forward and adjoint tangent propagator, is used to compute the sensitivity in the initial conditions for onset of BL and SZF regimes. The evolution of an optimal transition perturbation during a sensitive 72-h period is extensively studied. Barotropic and baroclinic mechanisms are distinguished by displaying the results in terms of the barotropic and baroclinic modes of the system. Next, the perturbation is decomposed in normal modes. The evolution can be divided in two phases. During the first rapid phase, the growth is strongly nonmodal and baroclinic. After that, the growth is still nonmodal but not as strong and almost barotropic. In the second part of this paper, the barotropic evolution is studied using a leading-order WKB approximation adopted for nonzonal smooth background flows. This approach is based on the assumptions that the perturbations may be represented by wave packets and that a scale separation between the perturbations and the background flow can be made. The WKB approach is used as a diagnostic tool to interpret the evolution of the optimal perturbations qualitatively. This paper focuses on the evolution of zonally elongated wave packets that are located in or near the jet stream, and propagate into a diffluent area. Because the background flow is nonzonal, total wave action of a packet is not conserved. However, under certain conditions total wave enstrophy of a packet is conserved. The WKB equations predict reasonably well the evolution of the perturbations, although the assumptions are violated in the final stage of the integration period. | |
