Geography Reference
In-Depth Information
As in the case of the dynamical variables discus
se
d in Section 12.2, it is useful
to define a longitudinall
y
averaged mixing ratio χ , and a disturbance or eddy
ratio χ
χ
.
A
gain, it proves useful to utilize the residual
such that χ
=
χ
+
mean meridional circulation
v
∗
, w
∗
defined in (10.16ab). The zonal mean tracer
continuity equation in the TEM framework can then be written as
∂χ
∂t
+
∂χ
∂y
+
∂χ
∂z
=
1
ρ
0
∇
·
v
∗
w
∗
S
+
M
(12.47)
Here,
M
represents the diffusive effects of the eddies, plus advective effects not
represented by the residual meridional circulation. In models the term involving
M
is often represented by meridional and vertical eddy diffusion, with empirically
determined diffusion coefficients.
In order to appreciate the role of the wave-induced global circulation in deter-
mining the distribution of long-lived tracers in the middle atmosphere, it is useful
to consider a hypothetical atmosphere in which there are no wave motions, and
hence no wave-induced zonal force. In that case, as argued in Section 12.2, the
middle atmosphere would relax to radiative equilibrium, the residual circulation
would vanish, and the distribution of the tracer would be determined at each alti-
tude by a balance between slow upward diffusion and photochemical destruction.
Thus, tracer mixing ratio surfaces would, in an annual mean, tend to be close to
horizontal. This is to be contrasted to observed distributions, which are charac-
terized by mixing ratio surfaces that are bowed upward in the tropics and slope
downward toward both poles (e.g., Fig. 12.9).
As discussed in Section 12.2, the wave-induced global-scale circulation consists
of upward and poleward motion across the isentropes in low latitudes, accompanied
by diabatic heating, and downward motion across the isentropes at high latitudes,
accompanied by diabatic cooling. Actual parcel trajectories, of course, do not
follow the zonally averaged motion, but are influenced by the three-dimensional
wave motion. Nevertheless the diabatic circulation defined by the mean diabatic
heating and cooling closely approximates the global transport circulation. For
seasonal and longer timescales the TEM residual circulation generally provides a
good approximation to the diabatic circulation and is generally simpler to compute
from standard meteorological analyses. For shorter period phenomena in which
the temperature tendency is large, the residual circulation is no longer a good
approximation to the diabatic circulation.
The above conceptual model of global transport is clearly supported by long-
lived tracer observations as shown in Figure 12.9. In middle latitudes there are
regions in which tracer mixing ratio isopleths are nearly horizontal, reflecting
the horizontal homogenizing role of meridional dispersion by planetary wave-
breaking in the surf zone. The same processes also tend to homogenize the PV
distribution on isentropes in the so-called surf zone in midlatitudes where Rossby
wavebreaking tends to occur. The surf zone is bounded at both low and high
