General circulation model simulation of the structure and energetics of atmospheric normal modes

Abstract

Atmospheric simulations produced by the Canadian Climate Centre general circulation model (GCM) were examined to detect Rossby normal modes such as the familiar “5-day” and “16-day” waves. Space-time spectral analysis of the simulated geopotential fields showed a concentration of power near the scales and frequencies predicted for the two gravest Rossby modes of both zonal wavenumbers one and two. By filtering the time series of simulated geopotential and wind fields in narrow bands around these spectral peaks, the vertical and meridional structures of these prominent oscillations were revealed. These structures were in good agreement with those theoretically predicted for the Rossby normal modes. The overall amplitudes of these modes appear to be realistic and even the day-to-day evolution of the modal amplitudes in the GCM is similar to that observed in the real atmosphere. A more complicated picture emerged when the same analysis was applied to the lower frequency “16-day” wave (i.e., the second gravest symmetric Rossby mode). The spectra produced from the GCM fields failed to reproduce the peak in power in westward propagating zonal wavenumber one variance near 15–20 days that has been observed by Speth and Madden (1983). Examination of the geopotential field after it had been filtered to include only low frequency westward-propagating zonal wavenumber one variance did reveal at least one isolated episode when the meridional structure displayed some resemblance to the second gravest symmetric Rossby mode of theory. However, amplitudes are much stronger in the winter hemisphere than in the summer. A similar hemispheric asymmetry in the “16-day” wave has been found in observations. The strength of the gravest symmetric zonal wavenumber one Rossby mode (“5-day” wave) was examined in a series of GCM simulations performed with climatological sea surface temperatures and in simulations that had anomalously warm sea surface temperatures imposed in the tropical Pacific. There is a clear tendency for the 5-day wave to be stronger in the simulations with warm ocean temperatures, in agreement with observations of the interannual variability of this mode in real data. The excellent simulation of the gravest Rossby modes in the model suggests that the forcing and dissipation mechanisms for the modes in the GCM may also be realistic. An attempt was made to examine the forcing and dissipation of the modes in the model through a diagnostic study of the energetics of the simulated fields. The interaction with topography acts as a net sink for the modal energy. The dissipation arising from the parameterized mechanical friction in the GCM is much more important for these modes than that due to radiative heating and cooling. DOI: 10.1111/j.1600-0870.1987.tb00320.x