Planetary-Scale Waves in the Southern Hemisphere Winter and Early Spring Stratosphere: Stability Analysis

Abstract

Eastward-traveling waves 2 and 3 are frequently observed to grow in the Southern Hemisphere stratosphere during late winter and early spring. Observations show times when wave 2 growth appears to be confined to the stratosphere. This suggests that instability is one in situ mechanism that should be considered. The stability of stratospheric flows derived from data is examined during some of these times, using several linear models of quasigeostrophic instability. Unstable modes of both wave 2 and wave 3 have periods and spatial structures similar to observations. Wave 2 and wave 3 momentum fluxes are similar in observations and model results and are consistent with the transfer of kinetic energy from the zonal-mean flow to the wave. When a barotropic model with a zonally symmetric basic flow is used, wave 3 is usually most unstable. Including a stationary wave 1 in the basic flow destabilizes both wave 2 and wave 3, but has little effect on their periods or spatial structures. When a zonally symmetric flow with realistic meridional and vertical structure is used, resulting unstable modes have shorter periods and slower growth rates than for barotropic flows. Wave 2 is usually more unstable than wave 3 when realistic vertical structure is included. The similarity between observed fields and model results in a number of cases when wave 2 appears to grow within the stratosphere suggests that in situ instabilities play a role in the evolution of the eastward-traveling wave 2 characteristic of the Southern Hemisphere winter and early spring stratosphere.