Transport Into and Out of the Stratosphere

The stratosphere is not a closed system but exchanges mass with its neighbors, the troposphere (below) and mesosphere (above). We will discuss these interactions in this section.

Since the late 1940s, when it was recognized that troposphere-to-stratosphere transport was important for regulating the abundance of water vapor in the stratosphere [147], stratosphere-troposphere exchange (STE) has been a subject of great interest in stratospheric science. Recent research into the loss processes of O, has emphasized the need for understanding STE because the vast majority of the precursors of the 0,-destroying radicals (such as chlorofluorocarbons (CFCs) and N:,0) originate in the troposphere and are transported into the stratosphere. STE is also important because it leads to the injection of Or and NOv-rich stratospheric air into the troposphere, affecting the chemistry of that region. Our present understanding of STE is reviewed in detail by Holton et al. [148].

Following the nomenclature of Hoskins [149], we divide the atmosphere into three regions: the "overworld," "middleworld," and "underworld" (Figure 5.4). The overwork! is defined to be the region above the -380 K potential temperature surface (about 16 km altitude), and contains surfaces of constant potential temperature (isentropes) that are within the stratosphere at all latitudes. The middleworld is defined to be the region between the -380 and -310 K potential temperature surfaces, and contains isentropes that are in the troposphere at low latitudes and in the stratosphere at high latitudes. The stratospheric part of the middleworld is often called the "lowermost stratosphere". The underworld is defined to be the region below the -310 K potential temperature surface, and it contains isentropes that are within the troposphere at all latitudes.

Mass flux into the stratosphere occurs on some combination of the three paths shown in Figure 5.4. Path A is the mean meridional circulation, often referred to as the Brewer-Dobson circulation [147,150]. It consists of air rising through the tropopause into the stratosphere at low latitudes and descending back into the

Figure 5.4 Latitude-height schematic cross-section of the atmosphere. The heavy line is the tropopause. Overworld-middleworld and middleworld-underworld boundaries are shown as dotted lines. The middleworld stratosphere is hereafter referred to as the "lowermost stratosphere." See text for details about paths A. B, and C. (After Dessler et at. 1164]. Figure 1.)

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