4
of the layer that it occupies, this circulation cell is thermally direct, with
warm air rising and cold air sinking, but at levels above 65 km it is thermally indirect (Leovy 1964).
In a similar manner, the tropical (20N-20S) tropopause is remarkably
cold year round because of the upwelling associated with the Hadley Circulation, and the annual march of temperature at this level bears little
direct relation to the variations in radiative heating associated with the
earth's orbital geometry. Throughout this belt temperatures are higher in
July, when the earth is farthest from the sun, than they are in January.
Over the equator, where one would expect them to be zero, the differences
range as high as 10 K (Reed 1963, Reed and Vlcek 1968). Furthermore,
there is little indication of a semiannual cycle in response to the seasonal
variations in solar declination angle.
Yulaeva et al. (1994) have argued that this anomalous annual cycle is
a consequence of the fact that the broad mountain ranges in the Northern
Hemisphere (the Rockies, in particular) are much more effective in generating vertically propagating planetary-waves than their narrower counterpart, the Andes, in the Southern Hemisphere. The planetary waves in
the winter hemisphere disturb the stratospheric polar vortex, transferring
heat from low latitudes to higher latitudes. The same planetary-waves induce a time-mean poleward Lagrangian drift of air parcels in the winter
hemisphere, which is fed by ascending motion throughout the tropics. The
adiabatic cooling induced by this wave-driven ascent contributes to the remarkable coldness of the tropical tropopause. The more intense Northern
Hemisphere wintertime planetary-waves, which attain their peak amplitude in January induce a stronger high latitude warming, accompanied by
a stronger cooling of the tropical tropopause, than their Southern counterparts, which attain their peak amplitude in July. The almost perfect
compensation between the tropical and extratropical annual cycles, as illustrated in Fig. 1 reflects the constraint that the wave-driven Lagrangian
circulation cannot produce any net warming or cooling at any given level:
it can only move heat poleward (e.g., see Andrews et al. 1987).
Figure 2 shows an extended time series of total column ozone at Arosa,
Switzerland, as inferred from ground based measurements. During most
years the column ozone exhibits a pronounced springtime maximum, believed to be a consequence of the same wave-induced poleward Lagrangian
circulation mentioned in connection with Fig. 1, which carries ozone from
its photochemical source region at the 25-km level over the tropics, pole-
of the layer that it occupies, this circulation cell is thermally direct, with
warm air rising and cold air sinking, but at levels above 65 km it is thermally indirect (Leovy 1964).
In a similar manner, the tropical (20N-20S) tropopause is remarkably
cold year round because of the upwelling associated with the Hadley Circulation, and the annual march of temperature at this level bears little
direct relation to the variations in radiative heating associated with the
earth's orbital geometry. Throughout this belt temperatures are higher in
July, when the earth is farthest from the sun, than they are in January.
Over the equator, where one would expect them to be zero, the differences
range as high as 10 K (Reed 1963, Reed and Vlcek 1968). Furthermore,
there is little indication of a semiannual cycle in response to the seasonal
variations in solar declination angle.
Yulaeva et al. (1994) have argued that this anomalous annual cycle is
a consequence of the fact that the broad mountain ranges in the Northern
Hemisphere (the Rockies, in particular) are much more effective in generating vertically propagating planetary-waves than their narrower counterpart, the Andes, in the Southern Hemisphere. The planetary waves in
the winter hemisphere disturb the stratospheric polar vortex, transferring
heat from low latitudes to higher latitudes. The same planetary-waves induce a time-mean poleward Lagrangian drift of air parcels in the winter
hemisphere, which is fed by ascending motion throughout the tropics. The
adiabatic cooling induced by this wave-driven ascent contributes to the remarkable coldness of the tropical tropopause. The more intense Northern
Hemisphere wintertime planetary-waves, which attain their peak amplitude in January induce a stronger high latitude warming, accompanied by
a stronger cooling of the tropical tropopause, than their Southern counterparts, which attain their peak amplitude in July. The almost perfect
compensation between the tropical and extratropical annual cycles, as illustrated in Fig. 1 reflects the constraint that the wave-driven Lagrangian
circulation cannot produce any net warming or cooling at any given level:
it can only move heat poleward (e.g., see Andrews et al. 1987).
Figure 2 shows an extended time series of total column ozone at Arosa,
Switzerland, as inferred from ground based measurements. During most
years the column ozone exhibits a pronounced springtime maximum, believed to be a consequence of the same wave-induced poleward Lagrangian
circulation mentioned in connection with Fig. 1, which carries ozone from
its photochemical source region at the 25-km level over the tropics, pole-