Dynamics of Weather Regimes: Quasi-Stationary Waves and Blocking

Abstract

We hypothesize that periods of quasi-stationary behavior in the large scales are integrally associated with an organized behavior of the synoptic scales, thus the terminology ?weather regime.? To investigate our hypothesis, we extend the model of Charney and Straus (1980) to include an additional wave in the zonal direction which is highly baroclinically unstable and can interact directly with the externally forced large-scale wave. We find that such a model aperiodically vacillates between two distinct weather regime states which are not located near any of the stationary equilibria of the large-scale state; thus, we cannot ascertain the qualitative behavior of the large-scale flow in our model knowing only the large-scale equilibria and their respective stabilities to perturbations on the scale of the equilibria. The state of the model flow may remain in either one of the two regime states for several synoptic periods. During each of the two regimes, the net transports by the transient disturbances are found to have consistent, zonally inhomogeneous structure, the form of which depends upon the regime. This result implies that the transports appear as a net additional external forcing mechanism to the large-scale wave, accounting for the differences between the time-mean regime state and the stationary equilibria. Following the analysis procedure of Frederiksen (1979), we show that the observed structure of these net transports can be accounted for by the spatial modulation of the baroclinically most unstable eigenmodes by the large-scale wave. We then consider only the tendency equations of the large-scale variables where the effects of the transients are parameterized by solving the stability problem at each time step. We find that such a dynamical system possesses two absolutely stable ?regime-equilibria? which are very close in phase space to the time mean states of the regimes appearing in the full model. We then demonstrate that the instantaneous component of the transients are also capable of transferring the state of the model flow from the attractor basin of one of the stable regime-equilibria to the attractor basin of the other. Our experiments thus indicate that the transients are important in determining the qualitative behavior of both the instantaneous and time-mean components of the large-scale flow in our system, and suggest that the very different short-range climates in the atmosphere can result from entirely internal processes.