Box 1.2. Stratospheric transport, planetary waves and the Brewer-Dobson circulation
The stratospheric Brewer-Dobson circulation consists of rising motion in the tropics and sinking motion in the extratropics, together with an associated poleward mass flux. Its effect on chemical species is complemented by mixing (mainly
quasi-horizontal) that acts to transport air parcels both poleward and equatorward. Both processes are primarily driven by
mechanical forcing (wave drag) arising from the dissipation of planetary-scale waves in the stratosphere. These planetary
waves are generated in the troposphere by topographic and thermal forcing, and by synoptic meteorological activity, and
propagate vertically into the stratosphere. Because of filtering by the large-scale stratospheric winds, vertical propagation
of planetary waves into the stratosphere occurs primarily during winter, and this seasonality in wave forcing accounts for
the winter maximum in the Brewer-Dobson circulation. In the case of ozone, the Brewer-Dobson circulation (together with
the associated horizontal mixing) transports ozone poleward and downward and leads to a springtime maximum in extratropical ozone abundance (see figure).
Because air enters the stratosphere primarily in the tropics, the physical and chemical characteristics of air near the tropical
tropopause behave as boundary conditions for the global stratosphere. For example, dehydration of air near the cold tropical
tropopause accounts for the extreme dryness of the global stratosphere (Brewer, 1949). Overall, the region of the tropical
atmosphere between about 12 km and the altitude of the tropopause (about 17 km) has characteristics intermediate to those
of the troposphere and stratosphere, and is referred to as the tropical tropopause layer (TTL).
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Box 1.2, Figure. Meridional cross-section of the atmosphere showing ozone density (colour contours; in Dobson units (DU) per km)
during Northern Hemisphere (NH) winter (January to March), from the climatology of Fortuin and Kelder (1998). The dashed line denotes
the tropopause, and TTL stands for tropical tropopause layer (see text). The black arrows indicate the Brewer-Dobson circulation during
NH winter, and the wiggly red arrow represents planetary waves that propagate from the troposphere into the winter stratosphere.
port leads to significant variations of ozone in the extra-tropical
lower stratosphere, where the photochemical relaxation time is
very long (several months or longer) and ozone can accumulate
on seasonal time scales. Due to the seasonality of the BrewerDobson circulation (maximum during winter and spring), ozone
builds up in the extra-tropical lower stratosphere during winter
and spring through transport, and then decays photochemically
during the summer when transport is weaker. The columnozone distribution (measured in Dobson units, DU) is dominated by its distribution in the lower stratosphere and reflects
this seasonality (Figure 1.2). Furthermore, planetary waves
are stronger (and more variable) in the Northern Hemisphere
IPCC Boek (dik).indb 90
(NH) than in the Southern Hemisphere (SH), because of the
asymmetric distribution of the surface features (topography and
land-sea thermal contrasts) that, in combination with surface
winds, force the waves. Accordingly, the stratospheric BrewerDobson circulation is stronger during the NH winter, and the
resulting extra-tropical build-up of ozone during the winter and
spring is greater in the NH than in the SH (Andrews et al., 1987;
Figure 1.2).
Variations in the Brewer-Dobson circulation also influence
polar temperatures in the lower stratosphere (via the vertical
motions); stronger wave forcing coincides with enhanced circulation and higher polar temperatures (and more ozone trans-
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