On the Source of Midlatitude Low-Frequency Variability. Part II: Nonlinear Equilibration of Weather Regimes

Abstract

We present a new statistical-dynamical approach to the concept of weather regimes, including the effect of tralisients, without any assumption other than scale separation. 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. In this way, the feedback is a continuous function of the large-scale flow only, and the system of equations is closed, taking into account the whole coupling. 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. The feedback term is shown to act positively in both cases though there are major differences between zonal and blocking regimes. In particular we show that the dipole of the blocking flow is essentially maintained against dissipation by the small-scale fluxes. It is shown that full nonlinearity is required to explain the observed behavior. The efficiency of the method in this simple case allows us to discuss its extension to a more ambitious diagnostic of regimes in atmospheric observations as well as GCM simulations.