Predictability of the Onset of Blocking and Strong Zonal Flow Regimes

Abstract

Abstract Flows with high and low sensitivity with respect to the initial conditions for onset of blocking (BL) and strong zonal flow (SZF) regimes have been analyzed. The author has considered BL and SZF regimes at 20°W (Atlantic region) and at 150°W (Pacific region). The BL and SZF regimes are characterized by the same dipolelike anomaly pattern but with opposite signs. Experiments have been performed with a three-level quasigeostrophic model triangularly truncated at wavenumber 21 (T21QG) and its tangent linear and adjoint versions. The sensitivity is calculated by perturbing the reference flow with perturbations that optimally trigger the onset of a BL or SZF regime after a prescribed forecast time. For forecast times larger than 3 days an iterative technique is used to take into account nonlinear growth of the perturbations. The flows with a high sensitivity show an intensified jetstream to the west of a diffluent flow. The strong jetstream by itself results in large perturbation growth. The presence ...