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300
30
350
Area of N.
America
No chlorine
300
25
250
250
20
With chlorine
200
200
15
Area of Antarctic
continent
150
150
10
100
100
5
50
50
0
0
0
100
200
300
400
1980
1990
2000
2010
10/1
1/7
4/17
7/26
11/4
Year
Day and date of simulation
Figure 11.15.
Minimum ozone column abundances
and areal extent of the ozone hole over the Antarctic
region between 1979 and 2010. For comparison, the
area of the Antarctic continent is about 13 million km
2
and the area of North America is about 24 million
km
2
.DatafromNASAGoddard Space Flight Center.
Figure 11.14.
Change in globally averaged ozone
column abundance during two global model
simulations in which chlorine (at 1989 levels) was
present and absent, respectively. In both cases,
ozone was removed initially from the model
atmosphere on October 1, 1988. Bromine was not
included in either simulation. From Jacobson
(2005a).
Figure 11.16 shows the extent of the Antarctic ozone
hole on October 3, 2010. Although the minimum was
within the polar vortex (Section 11.1), an ozone max-
imum was observed just outside the vortex. The prox-
imity of the maximum to the minimum is one factor
that allows the ozone hole to replenish itself from Octo-
ber to December, when the polar vortex breaks down,
allowing air outside the Antarctic to penetrate into the
Antarctic in the stratosphere.
Whereas the Antarctic ozone hole appears every year
during September through November (most of the SH
spring), a smaller
Arctic ozone dent
(reduction in
ozone to 240-260 DUs) appears from March to May,
or most of the NH spring. The greatest ozone dent to
date occurred in 2011, resulting in a March ozone col-
umn abundance reduction 60
◦
Nto90
◦
Nof24.5 per-
cent compared with March 1979 (Figure 11.11). The
second greatest dent occurred in 1997, when March
ozone decreased by 22 percent relative to 1979 (Figure
11.10a). During the 1997 event, the minimum ozone
value on April 1 was still 247 DUs. This minimum was
not low enough to qualify as an ozone hole (less than
220 DUs). In general, the dent is present during some
years (e.g., 2000, 2005, 2011) and absent during others
(1999, 2010) (Figure 11.11), with dents corresponding
to springs with lower Arctic stratospheric temperatures.
The hole and dent are caused by a complex set of
factors. One factor linking global ozone reductions to
polar ozone depletion is the presence of chlorine and
bromine in the stratosphere. Such compounds can cause
ozone depletion over the Antarctic or reduction over the
chlorine and bromine. In the second, ozone regeneration
wassimulated in the presence of 1989 levels of chlorine,
butinthe absence of bromine.
In both simulations, the
globally averaged column abundance of ozone regen-
erated to relatively steady values in less than 1 year
.
Regeneration during the simulation in which chlorine
wasinitially present was about 2 to 3 percent less than
that during the no-chlorine case, consistent with the
estimated global reduction in ozone of 2 to 3 percent
between the 1970s and 1989 due to chlorine-containing
compounds.
11.8. Antarctic Ozone Hole
The
Antarctic ozone hole
is defined as the area of
the globe over which the ozone column abundance
decreases below 220 DUs. Such decreases histori-
cally have occurred only over the Antarctic during the
SH spring (September-November). Between 1981 and
2000, the Antarctic ozone hole area increased from near
zero to 29.4 million km
2
in 2000 (Figure 11.15), an area
larger than North America. Between 2006 and 2010, the
area of the hole gradually declined slightly from 28 to
22 million km
2
.Between 1993 and 2010, the minimum
ozone anywhere over the Antarctic dropped below 100
DUs twelve times. The lowest minimum was 82 DUs
in 2003 (Figure 11.15). Most Antarctic ozone depletion
occurs between 14 and 18 km in altitude. Beginning in
1992, springtime ozone decreases were observed up to
24 km and down to 12 to 14 km.
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