Barotropic Instability of the Polar Night Jet Stream

Abstract

An eigenvalue analysis of the nondivergent barotropic model on a sphere and an initial value analysis of a baroclinic, quasi-geostrophic model on a sphere are used to study barotropically unstable zonal flows similar to those observed in the wintertime stratosphere. The unstable modes fall into two categories. Polar modes associated with regions of negative potential vorticity gradient on the polar flank of the stratospheric westerly jet are most unstable for zonal wavenumbers 1 and 2 which have significant growth rates and have periods on the order of 3?4 days and 1.5?2 days, respectively. Thew polar modes correspond to a wave observed by Venne and Stanford (1982). Mid-latitude modes associated with regions of negative potential vorticity gradient on the equatorward flank of the stratospheric westerly jet have much longer periods. For wind profiles near marginal stability the most unstable modes occur for the lowest zonal wavenumbers 1?3 and have periods which are on the order of a week or more for wavenumbers 1 and 2. It is suggested that these instabilities may interact strongly with planetary waves propagating upward from the troposphere producing an additional in situ source of energy for these waves.