The seasonal cycle of the stratospheric circulation

Up until this point, we have concentrated almost exclusively upon the troposphere, which is characterized by a relatively weak stratification, with a temperature lapse rate of around 6–7 K km–1. At the tropopause, the lapse rate becomes close to zero; the lower stratosphere is nearly isothermal. The corresponding change in stratification, as measured by the Brunt-Väisälä frequency, is by a factor of around two, from values of 10–2 s–1 in the troposphere to values of 2 × 10–2 s–1 in the lower stratosphere. In the upper stratosphere, from heights of 30 km to around 50 km, the temperature actually increases with height. The transition to stably stratified conditions is called the tropopause, which is extremely sharp in the tropics and midlatitudes. It is rather more gradual in polar latitudes, especially in winter when there is no incoming sunlight. The abrupt increase of stratification at the tropopause means that the stratosphere is dynamically very different from the underlying troposphere. Baroclinic instability is virtually suppressed and disturbances are mainly forced from below. The stratification acts as a filter, removing the smaller scale disturbances and allowing only the longest waves to propagate out of the troposphere to great heights in the stratosphere. Shorter wavelength disturbances are thereby trapped in the troposphere, which behaves as a waveguide, the upper boundary of which is the tropopause.