Geoscience Reference
In-Depth Information
There is mounting evidence that sudden stratospheric warmings can have pro-
nounced effects on the circulation of the troposphere and surface. Various mecha-
nisms have been offered (Baldwin and Dunkerton,
1999
,
2001
; Hartmann et al.,
2000
; Ambaum and Hoskins,
2002
; Perlwitz and Harnik,
2003
). Although they
are still being elucidated, the point to be emphasized is the apparent existence of
two-way coupling - while upward propagation of planetary waves influences the
stratospheric circulation, changes in the stratospheric circulation can influence the
troposphere and surface, for example, by changing the conditions for vertical propa-
gation of waves. M. Baldwin and T. Dunkerton (
2001
) find that the surface signature
of downward propagating anomalies strongly resembles the surface signature of the
Northern Annular Mode (also referred to as the Arctic Oscillation), a mode of atmo-
spheric variability that has pronounced impacts on Arctic climate (
Section 4.72
).
4.2.4
Ozone Characteristics
Ozone measurements can provide three types of information. The first is the total
ozone in an atmospheric column. This is measured with the Dobson spectrophotom-
eter, which compares the solar radiation at a wavelength where ozone absorption
occurs with that in another wavelength where such effects are absent. Second is
the spatial pattern of total ozone. This is determined by satellite sounders such as
NASA's Total Ozone Mapping Spectrometer (TOMS) aboard Nimbus-7, Meteor-3
and ADEOS (Advanced Earth Observing Satellite). Third is the vertical distribution
of ozone. This can be measured by chemical soundings of the stratosphere, by pas-
sive microwave limb sounding from satellites, or calculated at the surface using the
Umkehr method. The last method determines the effect of solar zenith angle on the
scattering of solar radiation.
The annual cycle of mean ozone column totals is shown in
Figure 4.5
for two
periods: 1964-1980 and 1984-1993. There is a strong annual cycle in high latitudes
with values exceeding 400 DU (Dobson units = milli-atmosphere centimeters, or
mm column depth at standard temperature and pressure) in late winter and spring.
This compares to 300 DU in September. The late-winter to spring maximum is a
result of strong poleward transports of ozone by the mean residual meridional plane
circulation. However, winter variability is largely attributed to disturbances in the
circulation.
The recurrent stratospheric “ozone hole” that develops over Antarctica each
spring is well-known. This occurs because of photochemical catalytic reactions
during the return of sunlight in September and October (austral spring). The main
agents involved in such ozone destruction are chlorofluorocarbons (CFCs). If chlo-
rine atoms are freed, a catalytic chain reaction can occur (Chartrand, de Grandpere,
and McConnell,
1999
). It has been shown that a reactive chlorine compound, chlo-
rine monoxide (ClO), can be liberated from a reservoir species (such as hydrochlo-
ric acid or chlorine nitrate) through heterogeneous reactions in polar stratospheric
clouds (PSCs). PSCs or nacreous (mother-of-pearl) clouds form at about 20-30 km
