Baroclinic Instability of Realistic Zonal-Mean States to Planetary Waves1

Abstract

It is demonstrated that realistic zonal mean states on a sphere are unstable to perturbations of zonal wavenumbers 2 and 3. Moreover, these waves have significant growth rates and are of such a narrow meridional scale that they fall into the class of unstable modes first discovered by Charney. For zonal mean wind cross sections characteristic of the Southern Hemisphere, wavenumber 2 has an unstable mode with an e-folding time of about 9 days and a period of about 2 weeks. The growth rate of the unstable wave 2 is relatively insensitive to changes in wind structure and to topography representative of the Antarctic continent. The unstable waves extend well into the stratosphere, and it is suggested that the eastward traveling wave 2 and wave 3 components observed in the Southern Hemisphere may be baroclinically unstable modes of the Charney type. The relatively large growth rates of these waves are made possibly by a relatively narrow meridional scale in the troposphere. and their propagation into the stratosphere is enhanced because their meridional scale increases with increasing height in the stratosphere by a factor of 2 or more over its value in the troposphere.