Abstract
Results from an integration of a hemispherics spectral model, starting from a Northern Hemisphere climatological zonal mean state plus a small amplitude white noise perturbation are presented. Although there is no external forcing in this model, upper tropospheric, midlatitude long-wave fields of amplitude comparable to those observed in the atmosphere are produced. Two distinct generation mechanisms which are important in consecutive phases of the integration are investigated. The first type of long-wave growth is due to the interaction of the synoptic-scale systems, which vary in amplitude with longitude, with the “mean” flow in which they are embedded. The second long-wave development, which takes the form of an intensifying jet stream extending over 60° of longitude, can be interpreted in terms of linear instability of the zonal-mean flow involving one normal mode for each of zonal wavenumbers 1, 2 and 3. These modes are slowly growing and slow-moving, their superposition yields a growing disturbance which is quasi-stationary, localized in space and coherent in time. It is suggested that such modes might be preferentially excited in the presence of realistic, large-scale orographic or diabatic forcing.
