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Equator

Latitude

FIGURE 12.3 Schematic of wave-driven extratropical pump that drives global-scale transport from the tropical troposphere to the stratosphere and then poleward. Three possible paths for stratosphere-troposphere exchange (STE) are shown. (Adapted from Holton et al., 1995.)

Pole

Equator

Latitude

FIGURE 12.3 Schematic of wave-driven extratropical pump that drives global-scale transport from the tropical troposphere to the stratosphere and then poleward. Three possible paths for stratosphere-troposphere exchange (STE) are shown. (Adapted from Holton et al., 1995.)

world," the "lowermost stratosphere," and the "underworld." The overworld and underworld are the regions of the stratosphere and troposphere, respectively, in which air moving along surfaces of constant potential temperature, i.e., isentropic surfaces (see Chapter 2), remains in those regions. The lowermost stratosphere is characterized by the fact that isentropic surfaces cross into the troposphere, as shown in Fig. f2.3. In these regions, the instantaneous tropopause can be strongly deformed, and diabatic heating or cooling and turbulent mixing can occur, providing another mechanism (Path II) for stratosphere-troposphere exchange of air (STE). This horizontal mixing along the isentropic surfaces in the lower stratosphere and the troposphere can be faster than mixing between the lowermost stratosphere and the overworld, due to the slow vertical mixing in the stratosphere. This mechanism for mixing between the lowermost stratosphere and the troposphere, which has been observed from SAGEtt measurements of 03 during the spring (Wang et al., 1998), can be particularly important for species found in, or emitted predominantly into, the lowermost stratosphere, e.g., emissions from supersonic aircraft (vide infra).

There appears to be a subtropical eddy transport barrier (of variable strength) to horizontal mixing of air between the tropical overworld stratosphere and the extratropical regions. This has been described by Plumb (1996) as a "tropical pipe," shown schematically in Fig. 12.4; in this model, meridional circulation from the tropics poleward by large-scale "pumping" described earlier occurs along with isentropic mixing within the Northern and Southern Hemispheres, respectively. The region of limited horizontal mixing around the equator is shown as the tropical pipe. Evidence for this barrier to horizontal mixing in the tropical overworld has been found in different correlations between the concentrations of trace gases in the tropical troposphere compared to the midlatitudes or Antarctica (Plumb, 1996) and in a "tape recorder effect" (see following) on the vertical distribution of water vapor in the tropics (Mote

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