We now turn to the way in which material actually crosses between the troposphere and stratosphere through the tropopause. The term is Stratospheric-Tropospheric Exchange (STE), and it refers to the transport of material across the tropopause. STE has direct implications on the distribution of atmospheric ozone, in particular, the decrease of lower stratospheric ozone and the increase of tropospheric ozone. STE also impacts the distribution of aircraft emissions and the vertical structure of aerosols and greenhouse gases. We divide the topic of STE into the movement of air into and exit of air from the stratosphere.

The transport of anthropogenic gases, like chlorofluorocarbons, from the troposphere into the stratosphere effects the chemical balance in both regions and provides the catalysts necessary for stratospheric ozone destruction. Stratospheric-tropospheric exchange also controls the rate of transport between source and sink regions for both tropospheric and stratospheric source gases. A consequence of this is the long lag time between the release of tropospheric trace gases and stratospheric ozone reduction.

5.1 Mean Meridional Circulation in the Overworld and the Lowermost Stratosphere

For time scales greater than several months, the mass flux through the tropopause is ultimately driven by large-scale processes related to the Brewer-Dobson circulation. To show this, it is helpful to further divide the stratosphere into the overworld and the lowermost stratosphere. The overworld is the region above the 380K isentropic surface. The lowermost stratosphere is the dark shaded region between the 380K isentropic surface and the tropopause. Figure 6.19 shows schematically these two layers of the stratosphere and the dynamical processes that occur in each.

In Figure 6.19, the tropopause is shown by the thick line using pressure coordinates. The tropopause is near 300 mb at the pole and 100 mb in the tropics. Transport in the overworld is controlled by the Brewer-Dobson circulation. That is, the strength of the upward STE in the tropics and downward STE in the extratropics is controlled by the hemispheric Brewer-Dobson circulation rather than by smaller scale, local transport processes at the tropopause boundary. For material descending into the troposphere, once it crosses from the overworld into the lowermost stratosphere, the time scale for it to cross the tropopause is on the order of a season. The actual transport from lowermost stratosphere into the troposphere is governed by smaller scale extratropical processes such as blocking anticyclones, cut-off lows and tropopause folds, discussed below.

5.1.1 Blocking Anticyclones (Highs) -- Large anticyclones or high pressure areas in the troposphere may persist for days or weeks. These are known as blocking anticyclones or blocking highs. The effect of such features is to lower column ozone in that region through two processes. First, the anticyclonic flow around the high brings lower latitude air poleward. This air will retain characteristics of its tropical or subtropical source region, including concentrations of trace gases, such as ozone. Tropical and subtropical air has a lower ozone mixing ratio, reflected in a lower total column ozone, than polar air. This lower ozone air is transported into a region where ozone amounts are usually higher. Secondly, the warmer temperatures associated with a blocking high cause isentropic surfaces to bend upwards, which has the effect of bending the tropopause upwards. Ozone density in the troposphere is lower than in the stratosphere, of course, so an increase in the vertical scale of the troposphere results in a lower column ozone density.

By pushing transporting tropospheric air poleward and bending the tropopause upwards, blocking highs can increase the time it takes STE processes to occur.

5.1.2 Cutoff Low Pressure Systems -- Cutoff lows are upper level cyclones which become cut off or separated from the main flow of the upper tropospheric jet stream. They are usually associated with blocking patterns. The majority of cutoff lows form during summer months and can last for several days. In general, they form as the jet stream becomes distorted as an upper tropospheric trough elongates meridionally. As the system becomes cut off, it isolates air with characteristics of its polar source region. That is, it will contain cold air, high in potential vorticity, and trace gases with concentrations characteristic of higher latitudes.

Cutoff lows are obvious mechanisms for horizontal transport and are potentially important for STE. Cut-off lows are capable of large scale cumulus convection. Within convective updrafts, tropospheric air can be transported across the tropopause. This action can eventually erode the tropopause itself, creating a vertically mixed region of stratospheric and tropospheric air. The tropopause may then reestablish itself at a higher altitude above the mixed layer, thereby capturing the stratospheric ozone below.

5.1.3 Tropopause Folds -- Another mechanism for STE is tropopause folding events. A stratospheric intrusion of air that sinks into the baroclinic zone beneath the upper tropospheric jet stream is known as a tropopause fold. They form by a steepening of the tropopause (and isentropes) at a jet core. Tropopause folds are the dominate and most efficient form of STE in the middle latitudes. Folds usually occur of the western flank of cutoff low systems. Clean, dry stratospheric air, rich in ozone and potential vorticity, is transported downward to tropospheric levels. Observations of the circulation near folding events reveal that tropospheric air is being advected upwards as well. This tropospheric air contains large amounts of water vapor, carbon monoxide, aerosols and low values of potential vorticity.