On the Theory of the Long-Term Variability of the Atmosphere

Abstract

Much of the atmosphere's long-term variability is contained in the planetary modes with zonal wavenumber m≤5. It is proposed that a considerable fraction of this variability is induced by the nonlinear interaction of synoptic-scale modes (m>5) with the planetary-scale modes. TO test this hypothesis, the synoptic-scale forcing of the planetary-scale 500 mb streamfunction is determined from data. It is found that this forcing can be fitted approximately to a Markov process of first order, and that it strongly depends on locality. The response of the planetary motions to this forcing is calculated within the framework of the linear barotropic vorticity equation on the sphere and for β-plane flow. The power spectra and the low-frequency variance of the induced planetary-scale motions are presented. It is shown that there is a good correspondence between the observed low-frequency variance and the result of the computations on the sphere. The results for the β-plane channel flow are less satisfactory.