Barotropic Stability of Westward-Moving Waves in Realistic Stratospheric Zonal Flows

Abstract

The stability of a basic state composed of a westward-moving wave and a zonal mean jet is examined in a linearized barotropic nondivergent model on a sphere. The sensitivity of the stability to the strength and structure of the zonal jet is emphasized. For certain westward-moving basic state waves, inclusion of a very weak jet in the basic state can dramatically alter the stability of the flow. Examination of the energetics shows that some unstable disturbances depend almost entirely on zonal variations in the basic state for their existence. In cases where meridional variations of the basic state dominate the energy transfer, examination of basic state meridional vorticity gradients is useful in understanding the stability characteristics. At subcritical basic state wave amplitudes, addition of a weak jet, which by itself is stable, can change the meridional absolute vorticity gradient to resemble that for a supercritical basic state wave alone. Unstable disturbances then occur that have spatial structures and propagation characteristics similar to those for the supercritical wave alone. For a basic state wave resembling the observed ?2-day? wave, inclusion of an easterly (summer) jet in the basic state has a strong stabilizing influence. Unstable disturbances occur when a strong easterly jet is included that have structures similar to waves observed concurrently with the ?2-day? wave. Evidence is shown for seasonal dependence in the stability of several westward-moving basic state waves. Implications of these results on the observation of westward-moving waves in the stratosphere are briefly discussed.