On the Production of Mean Zonal Currents in the Atmosphere by Large Disturbances

The problem of the production of mean zonal currents by large-scale horizontal disturbances in the atmosphere is attacked by computation of time-tendencies from the vorticity equation. It is shown that the earth's rotation, acting through the variable Coriolis parameter, generally produces a mean northward transport of westerly momentum, with the maximum transport occurring south of the middle of the zonal belt occupied by the disturbances, and therefore creating mean westerly and easterly currents in the central and outer parts respectively. Its effect on the disturbance may either be a damping or an amplification, depending on the asymmetry of the mean flow profile. Six different mean flow profiles have been investigated to determine the nature of the interaction between a disturbance and the mean flow. For three profiles with inflection points, the interaction depends upon the wave length of the disturbance. If the wave length is longer than a critical value, an amplifying effect on the disturbance is produced accompanied by a divergence of westerly momentum in the region of maximum mean flow, and if shorter, a damping effect with a convergence of westerly momentum. On the other hand, the interaction produces only a damping effect for three mean flow profiles without inflection point. Since the zonal currents in the atmosphere are seldom very strong and not far from symmetrical, the total effect of the earth's rotation and of the mean flow leads to horizontal damping, in which the contrast in the mean flow at different latitudes is increased and a mean northward transport of westerly momentum is produced. This is in qualitative agreement with observations, indicating that these factors are of real importance in the development and maintenance of the mean zonal flow.