Geoscience Reference
In-Depth Information
essentially no meridional or vertical circulation and no stratosphere-troposphere
exchange (Holton,
1992
; Andrews et al.
1987
), As just discussed, the existence of
a westerly vortex in winter and an easterly vortex in summer is qualitatively that
expected from radiative equilibrium, but the winter season actually shows consider-
able departures from the radiatively determined state. In the 30-60 km region, the
decrease in temperature from the winter pole to the summer pole is much smaller
than the radiatively determined gradient. This is attributed to eddy transports that
drive the flow away from a state of radiative balance. By contrast, the departure
from radiative equilibrium in summer is small, implying a reduction in transports.
In the troposphere, these eddy transports are largely associated with traveling,
synoptic-scale waves (which we associate with migrating cyclones and anticyclones
at the surface). By contrast, winter transports in the stratosphere are associated with
the long planetary waves, especially wavenumbers 1 and 2, which can penetrate
into the stratosphere under certain conditions. Wavenumber refers to the number of
atmospheric waves around a latitude circle. For any given day, Fourier Transform
methods can be used to break down the total circulation at a given level in the atmo-
sphere with respect to the relative contributions of long planetary waves (low wave-
numbers), which move only slowly or remain stationary with respect to the earth's
surface, and shorter traveling waves (higher wavenumbers). A strong wavenumber
2 component, for example, would have a pronounced expression of two ridges and
two troughs, each separated by 180° longitude. If we look at the circulation on a
typical winter day, we find that, whereas the troposphere has a strong contribution
from the higher wavenumbers, the circulation of the stratosphere is more symmet-
ric, indicating that the shorter waves are not readily penetrating into the strato-
sphere. For long time averages, the effects of the shorter waves cancel out. But even
with such time averaging, the winter circulation of the stratosphere (
Figure 4.2
) is
more symmetric than that of the troposphere (compare with
Figure 4.7
, described
later). At 60°N, the mean January field in
Figure 4.2
shows strong contributions
from wavenumbers 1 and 2. This can be related to the impacts of orography and
land - that is, sea thermal contrasts (Pawson and Kubitz,
1996
).
While a full explanation is beyond the scope of this textbook (see Holton,
1992
,
for further reading), it can be shown that waves will penetrate (vertically propagate)
into the stratosphere provided that the zonal-mean wind is positive (the wind aver-
aged around a latitude circle blows from west to east) but less than a critical value
that depends strongly on the length of the waves. The longer the wave (i.e., the
lower the wavenumber), the stronger the zonal wind can be and still that allows for
vertical propagation. Because the zonal wind tends to be at a maximum near the tro-
popause in middle and high latitudes (much lower in altitude than the winter strato-
spheric jet), the shorter waves in winter tend to be “trapped” in the troposphere. In
summer, the zonal-mean zonal winds in the stratosphere are negative (easterly, see
the July field in
Figure 4.2
). There can be no vertical propagation of waves in these
conditions. This is consistent with the fact that the summer stratospheric circulation
is highly symmetric with a thermal structure close to that expected from radiative
equilibrium.
