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Equivalent Latitude

Figure 5.11 N.O abundance on February 20, 1992, at 800 K potential temperature versus equivalent latitude. N20 was measured by the CLAES on the UARS. The equivalent latitude was determined using UKMO meteorological fields from that day.

The barriers at 20°N and S isolate the tropics [152,182-184], Thus, air ascends in the tropics as part of the Brewer-Dobson circulation in relative isolation from the mid-latitudes—this has become known as the "tropical pipe" ]185|. The barrier at 65°N exists only in the northern hemisphere winter. A similar barrier exists in the southern hemisphere during its winter. These polar barriers slow mixing between the mid-latitudes and the polar regions [186,187] and play an import role in the formation and maintenance of the ozone hole (discussed in Chapter 7).

As a result of these transport barriers, air in the tropics, mid-latitudes, and polar region can have very different chemical compositions [185,188]. These transport barriers, therefore, have a profound impact on the distribution of gases in the stratosphere and their subsequent chemistry.

Finally, just as there is a seasonal cycle in the Brewer- Dobson circulation, there is also a seasonal cycle in the eddy transport. In fact, it is now understood that the Brewer-Dobson circulation and the eddy processes are physically related, and that it would be impossible to have one without the other. A thorough discussion of this is outside the scope of this book, but this point is discussed in detail elsewhere [146,148],

It is easy to see how the 2D circulation creates the observed distribution of long-lived trace gases in the stratosphere (Figure 5.12). The Brewer-Dobson circulation lifts the lines of constant mixing ratio (isopleths) of CH, in the tropics and pushes the isopleths down at mid- and high latitudes (indicated by the solid arrows in Figure 5.12). Thus, as one moves from the tropics to the poles at a constant altitude, the

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