In this Chapter, we have taken a detailed qualitative look at the stratosphere and the effects on the transport of ozone and other trace gases. As we shall see in Chapter 12, understanding stratospheric dynamics and tracer transport is critical for modeling and predicting stratospheric ozone amount and distribution. In this chapter, we have highlighted some of the basic properties of the stratosphere that are relevant to the transport of ozone. Modeling stratospheric ozone concentrations is critically dependent on transport because of the long life time of ozone. Because of the need for long term modeling of the stratosphere, ozone transport rates must be very well understood. In fact, the greatest uncertainties associated with modeling ozone is a result of transport uncertainty. The dynamics of the stratosphere is a critical research area, and will remain so for the next few decades.
We began Chapter 6 with an overview of the stratosphere: its composition, structure, and the dynamics of motion. This included density and temperature of stratospheric air and how these change with altitude. Temperatures actually rise with altitude in the stratosphere, due to the radiative properties of ozone there, causing it to be a very stable layer of air. Indeed, ozone is the reason for the existence of the stratosphere.
The next topic was potential temperature and its relationship to the stability (or buoyancy) of air, as well as to the vertical motion of air. Potential temperature is the temperature a parcel of air at some altitude (pressure level) would have if compressed adiabatically to a reference pressure of 1000 mb. Motions in the stratosphere are primarily in the horizontal along surfaces of constant potential temperature called isentropic surfaces. Vertical motions are very small.
We then looked at the well-mixed nature of air (molecular nitrogen and molecular oxygen) in the stratosphere, and indeed through a large vertical depth of the atmosphere known as the homosphere. This lead to a discussion of how and why certain minor gaseous elements of the air display the variability observed.
Returning to the stratosphere, we identified four regions of the stratosphere, the tropics, the middle latitudes or "surf zone," the polar vortex, and the lowermost stratosphere, defining and explaining each one.
Section 3 was on the Brewer-Dobson Circulation, a circulation pattern that sets up between equator and pole in the winter hemisphere. We first described how the circulation works. First, air is lifted out of the tropics from the troposphere to the stratosphere, where it acquires a high ozone content in the photochemical source region of the tropical stratosphere. Next, this high-ozone air moves poleward and downward, descending into the middle latitudes upper troposphere and polar latitudes lower stratosphere. It is the reason for the observed column ozone distribution: low in the tropics and high in the polar regions.
Turning to why the Brewer-Dobson Circulation exists, it involves the existence of so-called planetary waves which propagate vertically into the stratosphere and slow up the stratospheric polar night jet. This is the jet stream that sets up in the winter time at the edge of the polar night and isolates the polar vortex region inside of it. Planetary waves are generated by large scale topography and land-sea temperature contrasts, and they are much more numerous in the northern hemisphere than the southern hemisphere. The Brewer-Dobson Circulation sets up in response to a meridional (pole to equator) mass imbalance which is brought on by a radiative imbalance. This radiative imbalance occurs when the polar night jet is slowed down by planetary waves, resulting in deformation of the polar vortex and stratospheric sudden warmings. Because of significant topographical differences between the northern and southern hemisphere, there are differences in the amount of wave activity and hence the strength of the Brewer-Dobson cell. These differences show up in differences in the distribution of trace gases like ozone and methane between the northern and southern hemisphere.
The next topic was the Quasi-Biennial Oscillation, a periodic shifting of stratospheric winds in the tropics from westerly (warm phase) to easterly (cold phase) that occurs every 22 to 34 months. We discussed the QBO circulation and how it affects the Brewer-Dobson Circulation, and hence how it affects ozone distribution both in the tropics and extratropics.
Circulation patterns in the troposphere and the mesosphere, below and above the stratosphere, respectively, was the next topic. This was followed by a discussion of the "age of air" in the stratosphere, a concept that gives us an idea of how long it takes for various trace gases to cycle between the surface and top of the stratosphere. This is particularly important for chlorofluorocarbons, which are responsible for stratospheric ozone destruction in spring over Antarctica and the resulting "ozone hole." By knowing the age of air, we know how long before a given CFC molecule will cycle through the stratosphere.
The topic of atmospheric waves, in particular, Rossby waves, was addressed in Section 4. The special type of Rossby wave, the standing planetary wave induced by topography, was introduced in our discussion of the Brewer-Dobson Circulation. These are atmospheric waves with very long wavelengths, on the order of 10,000 km, and are induced by topography and land-sea temperature differences. We discussed how such waves grow, move, and dissipate into the stratosphere.
The last section was on Stratospheric-Tropospheric Exchange (STE) in the context of meridional circulations. The effects of blocking highs, cutoff lows, and tropopause folds on the exchange of material and tracer transport was also included.