Planetary waves and the extratropical winter stratosphere
The Stratosphere: Dynamics, Transport, and Chemistry
Planetary-scale Rossby waves dominate the dynamics of the winter stratosphere. In their classic analysis of the propagation of such waves on a mean state that varied only with height, Charney and Drazin (1961) concluded that deep vertical propagation is permitted only around the equinoxes, when the mean winds are westerly and sufficiently weak. Subsequent developments, especially by Matsuno, incorporated spherical geometry and latitudinal variations of the mean state into the analysis. A
... analysis. A refractive index for the waves can be determined from the mean state; it has been widely used to diagnose wave propagation characteristics, and usually leads one to conclude that the winter westerlies, even in southern winter when the westerlies are strongest, are transparent to such waves. However, such conclusions rest on Wentzel-Kramers-Brillouin (WKB) assumptions, which are often inappropriate in the presence of realistic latitudinal variations of the mean state. It is here argued that the original conclusions of Charney and Drazin are qualitatively correct and that in its undisturbed, radiative equilibrium state, the winter stratosphere does not permit deep wave propagation. Such propagation requires the westerlies to be weakened by the waves themselves; it is argued that the consequent feedback between waves and the mean flow is at the heart of the strong variability of the stratospheric circulation, including the occurrence of major warmings, and may be central to stratospheretroposphere interactions.