- Angell, J.K. OAR/Air Resources Laboratory
- Flynn, L.E. NESDIS/Office of Research and Applications
- Gelman, M.E. NWS/Climate Prediction Center
- Hofmann, D. OAR/Climate Monitoring and Diagnostic Lab.
- Long, C.S. NWS/Climate Prediction Center
- Miller, A.J. NWS/Climate Prediction Center
- Oltmans, S. OAR/Climate Monitoring and Diagnostic Lab.
- Zhou, S. RS Information Systems, Inc.
Concerns of global ozone depletion (e.g. WMO, 1999) have led to major international
programs to monitor and explain the observed ozone variations in the stratosphere. In
response to these, as well as other long-term climate concerns, NOAA has established
routine monitoring programs utilizing both ground-based and satellite measurement
techniques (OFCM, 1988).
Selected indicators of stratospheric climate are presented in each Summary from
information contributed by NOAA personnel. A Summary for the Northern Hemisphere is issued
each April, and for the Southern Hemisphere, each December. These Summaries are available
on the World- Wide-Web, at the site
with location: products/stratosphere/winter_bulletins.
Further information may be obtained from Melvyn E. Gelman
W/NP52, RM 806, WWB
NOAA Climate Prediction Center
5200 Auth Road
Camp Springs, MD 20746-4304
Telephone: (301) 763-8071, ext. 7558
Fax: (301) 763-8125
Extensive ozone depletion was again observed over
Antarctica during the Southern Hemisphere winter-spring of 2005, with widespread total ozone anomalies of 45 percent or more below the
1979-1986 base period. In September the area covered by extremely low total ozone values of less than 220 Dobson Units, defined as the Antarctic
“ozone hole” area reached maximum size of 25 million square kilometers, with an average size of more than
22 million square miles, among the largest sizes of recent years. For the winter, the size of the ozone hole was about average for years
since 1987. Vertical profiles of ozone amounts, measured by balloons over the South Pole, showed strongest destruction of ozone in the
15-20 km region, with minimum values as low as seen during other recent years. At the South Pole, the minimum total ozone value of
121 Dobson Units was observed on 23 September 2005, when the center of the ozone hole was nearby. Lower stratospheric temperatures
over the Antarctic region in the winter of 2005 were again below -78 C, and were sufficiently low for polar stratospheric cloud formation,
promoting chemical ozone loss. The size of the area of very low temperatures was about the same in 2005 as others in the last 10 years.
In the middle of October 2005, Antarctic lower stratospheric temperatures warmed above -78 C, limiting further severe ozone destruction
and also limiting the extent and duration of the ozone hole in 2005.
I. DATA RESOURCES
The data used for this report are listed below. This combination of complementary data,
from different platforms and sensors, provides a strong capability to monitor global ozone
Method of Observation
||Balloon - Radiosonde
We have used total column ozone data from the
NASA Nimbus-7 SBUV instrument from 1979 through February1985; NOAA-9 SBUV/2 from March 1985 to
December 1988; NOAA-11 SBUV/2 from January 1989 to December 1993; NOAA-9 SBUV/2 from January 1994 to
December 1995; NOAA-14 SBUV/2 from January 1996 to June 1998; NOAA-11 SBUV/2 from July 1998 to September 2000;
and NOAA-16 SBUV/2 from October 2000 to November 2005. Solar Backscatter Ultra-Violet (SBUV) data are not available at
polar latitudes during winter darkness.
Maps of monthly average Southern Hemisphere SBUV/2 total ozone for August, September, October, and November 2005 are
shown in Figures 1a, 2a,
3a, and 4a, respectively.
“Ozone hole” values (defined as total ozone values less than 220 DU) appear over most of Antarctica slightly displaced
from the South Pole, and highest total ozone is shown in the Pacific Ocean, poleward of Antarctica and Australia.
Figures 1b, 2b,
3b, and 4b
show the difference in percent between the monthly mean total ozone for each month, August-November 2005, minus
the respective average (1979-86) monthly means (Nagatani et al., 1988).
Extreme negative anomalies in total ozone of greater than 45 percent are shown
in Figures 1b, 2b, and
3b, for August, September, and October over most of Antarctica and adjacent ocean areas, reaching to southern Argentina.
Figure 4b shows that in October the ozone anomaly was located over
Antarctica and adjacent ocean area south of Africa.
Figure 5a compares, for each year since 1979, the ozone hole area
average for all days in October through November. The growth in the October-November ozone hole in the years
from the 1980s through the 1990s is quite apparent. From a very small area in 1982, October-November average
values increased dramatically to a maximum in 1998 and 1999 of 16.4 million square kilometers. The October-November
2005 average size of the ozone hole was 10.5 million square kilometers, about average for years since 1987. Figures
show the individual monthly average ozone hole size for, respectively, August, September, October, and November,
1980-2005. The size increased from August to September 2005, decreased from September to October, and
substantially diminished in November (see Figure 13 for daily time
series of the 2005 ozone hole area values). The size of the ozone hole for August 2005 was second largest on
record, about average for September and October, and smaller for November than most recent years. It should be noted
that due to the precesion of the NOAA satellites there are some years in which the south polar area was not adequately
observed until late August early September. See Figure 6 for indication of these years.
Figure 6 displays monthly average anomaly values (percent) of zonal
mean total ozone, as a function of latitude (80N to 80 S) and time (January 1979 to November 2005). The
anomalies are derived relative to each month's 1979-2005 average. Long-term ozone changes may be readily
seen in the polar regions, where ozone values were substantially lower in the 1990s than in the 1980s.
Largest anomalies are shown for the polar regions in each hemisphere in winter-spring months, with positive
anomalies of more than 10 percent in the earlier years changing to negative anomalies of greater than 10
percent for most recent years. For 2005 the lowest ozone anomalies over south polar latitudes were about
average for recent years, but well below the 10% or larger positive anomalies in the early 1980s.
The center of the ozone hole and associated lowest ozone, and polar vortex are often located close to the
South Pole. Figure 7 shows a time series during 2005 of total ozone, measured over the South Pole using
balloon-borne ozone instruments, compared with other selected years. Low ozone hole values appeared in
September 2005, with lowest values evident at the end of September and early October , when the center of
the ozone hole was closest to the South Pole. Total ozone values rose dramatically in November, when the
ozone hole diminished and was displaced from the South Pole.
On 23 September (Figure 8) a total column ozone amount of 121 DU
was observed at the South Pole, the minimum South Pole sonde value for the year 2005. This profile shows strong destruction of
ozone between 15 and 20 km associated with classic ozone hole conditions. The time series in
Figure 9 of ozone profiles at the South Pole during 2005 shows the
time sequence of dramatic decreases in ozone between 15 and 20 km in September. Extremely low values of ozone
associated with ozone hole conditions continued in October, but moderated in November.
One of the longest records of ozone measurements in Antarctica is the total column ozone amount obtained with
the Dobson spectrophotometer at South Pole Station. Consistent observations can be obtained beginning on
October 15 of each year when sufficient sunlight is available for these optical measurements that use the
sun as a light source. This record of average October 15-31 column amounts shown in
Figure 10 indicates declines that accelerated in the 1980s and
reached consistently low values from 1993-1999. Since 2000 there has been greater variability in this average
with the suggestion of a tendency toward larger column amounts than observed during the 7-year minimum period.
Ozone amounts in the lower stratosphere are closely coupled to temperatures through dynamics and photochemistry.
Extremely low stratospheric temperatures (lower than -78 C) over the Antarctic region contribute to depletion of
ozone, in that low temperatures lead to the presence of polar stratospheric clouds (PSCs). PSCs enhance the
production and lifetime of reactive chlorine, leading to ozone depletion (WMO, 1999). Daily minimum
temperatures at 50 hPa (approximately 19 km) over the polar region, averaged from 65S to 90S are shown in
Figure 11. For all of the Southern Hemisphere winter of 2005, minimum
temperatures in the south polar region were below -78 C. These minimum temperatures were below long-term values
and near the lowest since regular satellite observation began in 1979. The rise in temperatures above -78 C in
late October 2005 limited the further
formation of polar stratospheric clouds and thus also limited further extreme ozone destruction. Temperatures in
the winter and spring of 2005 were lower than in 2004, and about the same as in 2003. This is consistent with
the size of the ozone hole that was larger than in 2004 and about the same as in 2003.
Figure 12 shows monthly average temperature anomalies at 50 hPa for
three latitude regions, 25S-25N, 65S-25S, and 90S-65S. For the south polar region, 2005 temperatures were
mostly lower than the long-term average. Negative temperature anomalies also predominated over the middle
latitudes of the Southern Hemisphere and over tropical latitudes. Both the tropical and middle latitudes
of the Southern Hemisphere continue the tendency toward lower temperatures after 1993 relative to earlier years.
Figure 13 presents time series of the area of the ozone hole, the
size of the polar vortex, and the size of the polar area where lower stratospheric temperatures were below -78 C
(polar stratospheric cloud, PSC area). The daily 2005 values are shown, along with the extreme and average
daily values for the most recent 10 years. During 2005, the ozone hole was larger than average from August through October.
The decrease of the ozone hole in late October coincided with the decrease in size of the area of very low
temperatures. The size of the ozone hole correlates well with the size of the PSC area during the formation
stages of the ozone hole in August and the first part of September.
Figure 14 illustrates the direct relationship between the persistence
of the ozone hole and the persistence of the Antarctic polar vortex. In years when the winter polar vortex
persisted later in the season, the duration into the spring season of the ozone hole also tended to be extended.
For the year 2005, the persistence of the ozone hole and the persistence of the Southern Hemisphere polar vortex
extended longer than most years since 1980, but shorter than most years since 1990.
III. CONCLUDING REMARKS
Very low ozone values were again observed over Antarctica in
the winter of 2005. Ozone depletion of more than 45 percent was observed over Antarctica, compared to total
ozone amounts observed in the early 1980's. Vertical soundings over the South Pole during August, September
and October 2005 again showed strongest destruction of ozone at altitudes between 15 and 20 km. Lower
stratospheric temperatures in the winter of 2005 over the Antarctic region were lower than in 2004, and about
the same as in 2003. Associated with this, the ozone hole was larger than in 2004 and about the same as in 2003.
In late October, the ozone hole in 2005 diminished in size and depth along with warming stratospheric conditions,
and by mid-November, total ozone over Antarctica had increased to levels above ozone hole values.
Observations of chloroflourocarbons and of stratospheric hydrogen chloride support the view that international
actions are reducing the use and release of ozone depleting substances (WMO, 1999; Anderson et al., 2000).
However, chemicals already in the atmosphere are expected to continue to impact the ozone amounts for many
decades to come. Further, changing atmospheric conditions that modulate ozone can complicate the task of
detecting the start of ozone layer recovery. The eruption of the Pinatubo volcano provided an example of
such a complication in the 1990s. Based on an analysis of 10 years of South Pole ozone vertical profile
measurements, Hofmann et al., (1997) estimated that the start of the recovery in the Antarctic ozone hole may be detected
as early as the coming decade. Indicators include: 1) an end to springtime ozone depletion at 22-24 km,
2) 12-20 km mid-September column ozone loss rate of less than 3 DU per day, and 3) a 12-20 km ozone column
of more than 70 DU on September 15. An intriguing aspect of recent observations of the Antarctic stratosphere
had been the apparent trend towards a later breakup of the vortex in years since 1990, relative to the 1980s.
The limited duration and size of the 2005 ozone hole is attributed in part to meteorological conditions. A full
explanation of such meteorological anomalies is not yet available. Continued monitoring and measurements,
including total ozone and its vertical profile, are essential to achieving the understanding needed to identify
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confirmation of stratospheric chlorine decreases in accordance with the Montreal Protocol,
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years of ozonesonde measurements at the south pole: implications for recovery of
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Planet, W. G., J. H. Lienesch, A. J. Miller, R. Nagatani, R, D. McPeters, E.
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VI. Web Pages of Interest