THE STRATOSPHERE IS THE REGION OF THE ATMOSPHERE above the troposphere and below the mesosphere; thus, it extends from the tropopause at a height of about 8–15 km, or a pressure of around 200–300 hPa, to the stratopause at about 50 km or about 1 hPa (see Fig. 15.24 on page 574). The middle atmosphere is the somewhat larger region that also includes the mesosphere, and so that extends up to the mesopause at about 90 km or 2×10-3 hPa, but we won't consider the mesosphere here. Our goal in this chapter is to provide an introduction to the dynamics giving rise to the structure and variability of the stratosphere.

The outline of this chapter is roughly as follows. We begin with a rather descriptive overview of the stratosphere as a whole. Then, starting in Section 17.2, we discuss the Rossby and gravity waves that in many ways serve to drive the circulation. We come back to the circulation itself in Section 17.4, focusing mainly on the generation of zonal flows and the meridional residual overturning circulation. We round out the chapter with discussions of two striking examples of stratospheric variability, namely the quasi-biennial oscillation in Section 17.6, and extratropical variability and sudden warmings in Section 17.7, with these terms to be defined in the sections ahead.

A DESCRIPTIVE OVERVIEW

In the troposphere the stratification is determined by dynamical processes — largely by convection at low latitudes and additionally by baroclinic instability at high latitudes — and the tropopause is the height to which the dynamical activity reaches, as discussed in Chapter 15. In contrast, in the stratosphere the temperature is determined to a much greater degree by radiative processes and the dynamics are, compared to those in the tropopause, slow. Over much of the stratosphere the temperature actually increases with height, and this is due to a layer of ozone that absorbs solar radiation in the mid-stratosphere between about 20 and 30 km. If there were no ozone we would certainly have a tropopause and a stratosphere, but the temperature in the stratosphere would increase much less with height than it in fact does.

The radiative-equilibrium temperature for January is illustrated in Fig. 17.1.