The Reddest Atmospheric Modes and the Forcings of the Spectra of These Modes

Abstract

The reddest atmospheric modes are studied using a 500-yr integration performed with the coupled ECHAM1/LSG general circulation model. By fitting a first-order autoregressive process to the considered time series, a simple measure of the spectral shape is obtained that allows an objective intercomparison of the spectra and the time series. Two modes, the tropical and the Southern Hemisphere modes, are identified as the reddest modes of the model atmosphere. Both are strongly anisotropic. The Southern Hemisphere mode, characterized by a dipole of zonal wind anomalies at Southern Hemispheric mid- and high latitudes, is related to a mass redistribution and a well-defined meridional circulation, whereas no significant anomalies of surface pressure and meridional velocity are found for the tropical mode, which is characterized by a maximum of zonal wind anomalies in the upper tropical troposphere. On timescales longer than a month, the two modes make, relative to other motions, the largest contributions to the global axial relative and Ω angular momenta Mr and MΩ (i.e., the portions of the angular momentum that are related to the relative motions and the distributions of mass, respectively) and control the variations of Mr and MΩ. Based on these relationships, the separated budgets of Mr and MΩ are used to study the forcings of the spectra of the modes. The results suggest that the forcings of the low-frequency parts of the spectra are extremely feeble, whereas the forcings of the high-frequency parts of the spectra can be easily captured. For the Southern Hemisphere mode, the forcings of the high-frequency variations involve processes that are related to large meridional velocity near the surface.