The distribution of ozone in a particular region is determined by three processes, in situ creation (production), in situ destruction (loss), and transport into or out of the region. The first two processes, ozone production and loss, are termed photochemical processes. We've examined these in detail in Chapters 5 and 11. In the upper stratosphere, where there is plenty of the necessary ultraviolet light, photochemical processes control most of the ozone budget. Transport processes play only a minor role. Transport processes refer to winds and broad circulation patterns that move parcels of air containing ozone. In the upper stratosphere, the lifetime of an ozone molecule is brief, meaning that the time between creation and destruction of an ozone molecule is very short compared to how long it takes to move ozone around by transport processes. Yet transport still plays an indirect role in the ozone budget of the upper stratosphere because of the impact it has on the temperature structure, which in turn effects ozone amounts. It is in the lower stratosphere where there is much less ultraviolet light and ozone lifetimes are much longer that transport is fundamental for determining the basic ozone distribution. In the lower stratosphere, photochemistry plays a large role only in certain geographic locations and at certain times of the year, such as in the Antarctic ozone hole during the spring. In general, transport of ozone dominates over photochemistry in the lower stratosphere.
In this chapter we will explore the motion of air in the stratosphere. The central concept in stratospheric motions is the Brewer-Dobson circulation. The chapter is divided into four sections. In these sections, we will investigate (1) some fundamental transport concepts in the stratosphere; (2) the average motion of air, which defines the Brewer-Dobson circulation; (3) the mixing of air by atmospheric waves; and (4) how air enters and exits the stratosphere in a process known as Stratospheric-Tropospheric Exchange.
As we explain the motion of this air, we will also illustrate key aspects of the physics, radiation, and chemistry of the stratosphere. We will show how motions can directly affect the chemistry of the stratosphere, and ultimately we will show how these motions influence the ozone distribution of the stratosphere.