Growth Properties of Optimal Transition Perturbations

Abstract

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...