Geography Reference
In-Depth Information
eddy forcing maintains the observed temperature above its radiative equilibrium in
the extratropical stratosphere, there is radiative cooling and from (12.3) the resid-
ual vertical motion must be downward. By mass continuity it is then required that
the residual vertical motion be upward in the tropics, implying that the temperature
must be below radiative equilibrium in that region. Note that it is the dynamical
driving by extratropical eddies, rather than local forcing, that is responsible for the
upward residual motion and net radiative heating in the tropical stratosphere.
12.2.3
Zonal-Mean Transport
Viewed from the TEM perspective the overall meridional circulation in the win-
ter stratosphere is qualitatively as pictured in Fig. 12.8. The residual circulation
35
10
WAVE-DRIVEN
30
"PUMP"
25
30
20
100
15
10
300
5
1000
60
0
30
90
Latitude
Fig. 12.8
Schematic cross section of the wave-driven circulation in the middle atmosphere and its
role in transport. Thin dashed lines denote potential temperature surfaces. Dotted line is the
tropopause. Solid lines are contours of the TEM meridional circulation driven by the wave-
induced forcing (shaded region). Wavy double-headed arrows denote meridional transport
and mixing by eddy motions. Heavy dashed line shows an isopleth of mixing ratio for a
long-lived tracer.
