Geography Reference
In-Depth Information
Fig. 10.2
Schematic Eulerian mean meridional circulation showing the streamfunction for a thermally
direct Hadley cell.
Now, diabatic heating in the Northern Hemisphere decreases for increasing y.
Thus, the
fi
rst term on the right is positive and tends to force a mean meridional
cell with χ>0. This is referred to as a
thermally direct
cell, as warm air is ris-
ing and cool air sinking. It is this process that primarily accounts for the Hadley
circulation of the tropics as illustrated in Fig. 10.2. For an idealized Hadley cell
in the absence of eddy sources, the differential diabatic heating would be bal-
anced only by adiabatic cooling near the equator and adiabatic warming at higher
latitudes.
In the extratropical Northern Hemisphere, poleward eddy heat fluxes due both
to transient synoptic-scale eddies and to stationary planeta
ry wa
ves tend to transfer
heat poleward, producing a maximum poleward heat flux
v
T
in the lower tropo-
sphere at about 50
◦
l
atitud
e as shown in Fig. 10.3. Because
χ is p
roportional to the
second derivative of v
T
, which should be negative where v
T
>
0
, the heat flux
forcing term should tend to produce a mean meridional cell with χ<0 centered
in the lower troposphere at midlatitudes. Thus, the eddy heat flux tends to drive an
indirect meridional circulation.
The existence of this indirect meridional circulation can be understood in terms
of the
need
to maintain geostrophic and hydrostatic balance. North of the latitude
where v
T
is a maximum there is a convergence of eddy heat flux, while equa-
torward of that latitude there is a divergence. Thus, eddy heat transport tends to
reduce the pole-to-equator mean temperature gradient. If the mean zonal flow is
to remain geostrophic, the thermal wind must then also decrease. In the absence
of eddy momentum transport, this decrease in the thermal wind can only be pro-
duced by the Coriolis torque due to a mean meridional circulation with the sense
of that in Fig. 10.4. At the same time, it is not surprising to find that the vertical
mean motions required by continuity oppose the temperature tendency associated
