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here does provide a useful framework for considering the role of weather distur-
bances in maintenance of the general circulation. The observed energy cycle as
summarized in Fig. 10.13 suggests the following qualitative picture:
1. The zonal-mean diabatic heating generates mean zonal available potential
energy through a net heating of the tropics and cooling of the polar regions.
2. Baroclinic eddies transport warm air poleward, cold air equatorward, and
transform the mean available potential energy to eddy available potential
energy.
3. At the same time eddy available potential energy is transformed into eddy
kinetic energy by the vertical motions in the eddies.
4. The zonal kinetic energy is maintained primarily by the conversions from
eddy kinetic energy due to the correlation u v . This is discussed further in
the next section.
5. The energy is dissipated by surface and internal friction in the eddies and
mean flow.
In summary, the observed atmospheric energy cycle as given by the Eulerian
mean formulation is consistent with the notion that baroclinically unstable eddies
are the primary disturbances responsible for the energy exchange in midlatitudes.
It is through the eddy motions that the kinetic energy lost through turbulent stresses
is replaced, and it is the eddies that are primarily responsible for the poleward heat
transport to balance the radiation deficit in the polar regions. In addition to transient
baroclinic eddies, forced stationary orographic waves and free Rossby waves may
also contribute substantially to the poleward heat flux. The direct conversion of
mean available potential energy to mean kinetic energy by symmetric overturning
is, however, small and negative in middle latitudes, but positive in the tropics where
it plays an important role in the maintenance of the mean Hadley circulation.
10.5
LONGITUDINALLY DEPENDENT TIME-AVERAGED FLOW
So far in this chapter we have concentrated on the zonally averaged component
of the general circulation. For a planet with a longitudinally uniform surface the
flow averaged over a season should be completely characterized by the zonally
averaged component of the circulation since for such a hypothetical planet the
statistics of zonally asymmetric transient eddies (i.e., weather disturbances) should
be independent of longitude. On the earth, however, large-scale topography and
continent-ocean heating contrasts provide strong forcing for longitudinally asym-
metric planetary scale time-mean motions. Such motions, usually referred to as
stationary waves , are especially strong in the Northern Hemisphere during the
winter season.
Observations indicate that tropospheric stationary waves generally tend to have
an equivalent barotropic structure; that is, wave amplitude generally increases
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