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transfers mass and trace chemicals upward across the tropopause in the tropics
and downward in the extratropics. This vertical circulation is closed in the lower
stratosphere by a poleward meridional drift balanced by EP flux convergence.
That this schematic picture gives a qualitatively correct view can be ascertained by
examination of the zonal mean mixing ratio distribution of any long-lived vertically
stratified trace species. As an example, the distribution of N 2 O is shown in Fig. 12.9.
N 2 O is produced at the surface, is mixed uniformly in the troposphere, but decays
with height in the stratosphere due to photochemical dissociation. Thus, as shown
in Fig. 12.9, the mixing ratio decreases upward in the stratosphere. Note, how-
ever, that surfaces of constant mixing ratio are displaced upward in the tropics and
downward at higher latitudes, suggesting that the mean meridional mass transport
is upward in the tropics and downward in the extratropics as suggested in Fig. 12.9.
In addition to the slow meridional drift by the residual meridional velocity shown
in Fig. 12.9, tracers in the winter stratosphere are also subject to rapid quasi-
Fig. 12.9
October zonal mean cross section of methane (ppmv) from observations by the Halogen
Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS).
Note the strong vertical stratification due to photochemical destruction in the stratosphere.
The upward bulging mixing ratio isopleths in the equatorial region are evidence of upward
mass flow in the equatorial region, whereas the downward sloping isopleths in high latitudes
are evidence of subsidence in the polar regions. The region of flattened isopleths in the
midlatitudes of the Southern Hemisphere is evidence for quasi-adiabatic wave transport
due to wintertime planetary wave activity. (After Norton, 2003.)
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