Geography Reference
In-Depth Information
the tropics. Evidence from a number of studies indicates that for conditions existing
in the earth's atmosphere a symmetric hemispheric-wide Hadley circulation would
be baroclinically unstable. If such a circulation were to become established by
some mechanism, it would quickly break down outside the tropics as baroclinic
eddies developed and modified the zonal-mean circulation through their heat and
momentum fluxes.
The observed general circulation thus cannot be understood purely in terms of
zonally symmetric processes. Rather, it can be thought of qualitatively as devel-
oping through three-dimensional interactions among radiative and dynamical pro-
cesses. In the mean the net solar energy absorbed by the atmosphere and the earth
must equal the infrared energy radiated back to space by the planet. The annually
averaged solar heating is, however, strongly dependent on latitude, with a max-
imum at the equator and minima at the poles. The outgoing infrared radiation,
however, is only weakly latitude dependent. Thus, there is a net radiation surplus
in the equatorial region and a deficit in the polar region. This differential heating
warms the equatorial atmosphere relative to higher latitudes and creates a pole-to-
equator temperature gradient. Hence it produces a growing store of zonal-mean
available potential energy. At some point the westerly thermal wind (which must
develop if the motion is to be balanced geostrophically in the presence of the
pole-to-equator temperature gradient) becomes baroclinically unstable. As shown
in chapter 8, the resulting baroclinic waves transport heat poleward. These waves
will intensify until their heat transport (together with the heat transported by plan-
etary waves and ocean currents) is sufficient to balance the radiation deficit in the
polar regions so that the pole-to-equator temperature gradient ceases to grow. At
the same time, these perturbations convert potential energy into kinetic energy,
thereby maintaining the kinetic energy of the atmosphere against the effects of
frictional dissipation.
From a thermodynamic point of view, the atmosphere may be regarded as a
“heat engine,” which absorbs net heat at relatively warm temperatures in the trop-
ics (primarily in the form of latent heat due to evaporation from the sea surface)
and gives up heat at relatively cool temperatures in the extratropics. In this man-
ner net radiation generates available potential energy, which is in turn partially
converted to kinetic energy, which does work to maintain the circulation against
frictional dissipation. Only a small fraction of the solar energy input actually
gets converted to kinetic energy. Thus, from an engineer's viewpoint the atmo-
sphere is a rather inefficient heat engine. However, if due account is taken of
the many constraints operating on atmospheric motions, it appears that the atmo-
sphere may in fact generate kinetic energy about as efficiently as dynamically
possible.
The above qualitative discussion suggests that the gross features of the general
circulation outside the tropics can be understood on the basis of quasi-geostrophic
theory since, as we have previously seen, baroclinic instability is contained within
