On the source of midlatitude low-frequency variability. Part II: nonlinear equilibration of weather regimes

The method is applied to a quasi-geostrophic channel model without topography and forced by a local baroclinic jet. Baroclinic perturbations grow and decay along a storm track which is linked with a maximum of low-frequency variability towards its exit, in agreement with the observations. The weather regimes are searched within the subspace spanned by the large scales only. They are identified through the resolution of a stationary problem in which the feedback of the transients is included as an ensemble average over analogs of the large-scale flow. The solution is obtained using a nonlinear optimization technique. Several regimes are identified corresponding to zonal and blocking situations. The blocking flow is characterized by a well-marked barotropic dipole at the end of the storm track of synoptic perturbations. It is shown that full nonlinearity is required to explain the observed behavior. -from Authors