- Angell, J.K. OAR/Air Resources Laboratory
- Flynn, L.E. NESDIS/Center for Satellite Research and Applications
- Hofmann, D. OAR/Earth System Research Laboratory
- Johnson, B.J. OAR/Earth System Research Laboratory
- Long, C.S. NWS/Climate Prediction Center
- Oltmans, S.J. OAR/Earth System Research Laboratory
- Zhou, S. RS Information Systems, Inc.
Concerns about 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
after each April, and for the Southern Hemisphere, after each December. These Summaries are available
on the World-Wide-Web, at the site
Further information may be obtained from Craig S. Long
W/NP52, RM 806, WWB
NOAA Climate Prediction Center
5200 Auth Road
Camp Springs, MD 20746-4304
Telephone: (301) 763-8071, ext. 7557
Fax: (301) 763-8125
E-mail: Craig.Long at noaa.gov
Extensive ozone depletion was again observed over Antarctica during the Southern Hemisphere
winter of 2007, with widespread total ozone anomalies of 45 percent or more below the 1979 to 1986 base period. The area covered by extremely
low total ozone values of less than 220 Dobson Units, defined as the Antarctic “Ozone Hole” area, in September reached maximum size of greater
than 24 million square kilometers, the 7th largest over all 29 years of continuous satellite monitoring of Antarctic ozone. However, after
reaching its maximum size, the Ozone Hole diminished in size such that it was just the 17th largest for the month of October. Vertical profiles
of ozone amounts, measured by balloon-sondes over the South Pole, showed near-complete destruction of ozone in the 13 to 21 km region for a
relatively short period of time due to the Ozone Hole’s displacement off of the pole towards the Atlantic quadrant. Minimum total ozone values
observed at the South Pole were higher than those seen during recent years. Lower stratospheric temperatures over the Antarctic region in the
winter of 2007 were again well below -78ºC, and were sufficiently low for polar stratospheric cloud formation, promoting chemical ozone loss.
The size of the area of very low temperatures in 2007 was, however, average or below average for the past ten years. The polar vortex persisted
until mid-December when it and the remains of the Ozone Hole slowly diminished as the polar circulation changed over into its summer pattern.
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 Solar Backscatter
UltraViolet (SBUV) instrument from 1979 through 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 December 1998; NOAA-11 SBUV/2 from January 1999 to December
2000; NOAA-16 SBUV/2 from January 2001 to December 2005; and NOAA-17 from January 2006 to December 2007. Solar-backscatter ultraviolet
measurements are not available at polar latitudes during winter darkness.
Southern Hemisphere Winter/Spring Ozone Depiction
Maps of monthly average Southern Hemisphere SBUV/2 total ozone for August, September, October, and November 2007
are shown in Figures 1a, b,
c, and d (1a,b,c,d combined), respectively.
“Ozone Hole” values (defined as total ozone values less than
220 DU) appear over most of Antarctica with the area center slightly displaced from the South Pole, and the
highest total ozone amounts are found in the Pacific Ocean, poleward of Australia. Figures 2a,
c, and d (2a,b,c,d combined)
show the difference in percent between the monthly mean total ozone for each month, August-November 2007, minus the
respective average (1979-86) monthly means (Nagatani et al., 1988). Extreme negative anomalies in total ozone
of greater than 45 percent occurred in each of those months over almost all of Antarctica and adjacent ocean
areas, reaching toward the southern Atlantic Ocean. Also, of note is that in the region of high ozone values
towards Australia (yellows and oranges in Figs. 1b and 1c), ozone values are well above the 1979-1986 average
(oranges and browns in Figs. 2b and 2c).
Figure 3 compares the maximum Ozone Hole size of 2007 with those of the
previous 28 years. With a maximum Ozone Hole size of 24.3 million sq km, 2007 ranked 7th largest of all previous
Ozone Hole years back to 1979. It was also 7th largest of the past ten years as well, implying that
meteorological conditions were such that 2007 was an ‘average’ year.
Figures 4a, b,
and d (4a,b,c,d combined) show the individual monthly average Ozone Hole size for,
respectively, August, September, October, and November, 1980 to 2007. The Ozone Hole size was among the
largest in August and September, but decreased in October, and due to the persistence of the vortex remained
average size in November.
Figure 5 displays monthly average anomaly values (percent) of zonal
mean total ozone, as a function of latitude (80ºN to 80ºS) and time (January 1979 to November 2007). The
anomalies are derived relative to each month's 1979 to 2007 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. The
largest anomalies are found 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 the 1990s. 2007 shows anomalies that are nearly average in the SH for the 1979 to 2007 period.
These values are higher than were observed in 2006, which had large negative anomalies.
Ozone Measurements over the South Pole
This year’s polar circulation patterns, and consequently the Ozone Hole, were displaced towards the African
quadrant for most of September and October. Figure 6 shows a time series during 2007 of total ozone, measured
over the South Pole using balloon-borne ozone instruments, compared with other selected years. The lowest ozone
values are historically observed in late September and early October. The 2007 minimum total column ozone at
South Pole was 125 Dobson Units measured on 8 October 2007 (see Figure 7 ).
On a year by year basis, 2007 was the
15th lowest minimum observed over the 22 year record. However, the layer between 14 and 21 km showed a typical 95%
loss of ozone. In contrast, the 2006 minimum total column ozone was 3rd lowest at 93 Dobson units (9 October 2006)
and showed 99% ozone destruction in the 14-21 km layer. This year’s measurements showed that temperatures in the
critical layer of 20-24 km were below average in early September of 2007, but abruptly increased to above average
temperatures by mid-September. The warmer stratospheric temperatures indicated that the polar vortex center
shifted away from South Pole towards the Atlantic Quadrant. As the center of the Ozone Hole moved away from the
South Pole, total ozone values began to rise throughout October. As the polar vortex and the Ozone Hole moved
back over the South Pole in early November, we see a decline in the total ozone values. The total ozone values
then continue to rise as the Polar vortex began to dissipate.
The sequence of profiles in Figure 7 shows the gradual complete
destruction of ozone in the 14 to 21 km region, with the greatest depletion occurring on October 8. Unlike 2006
when the Ozone Hole was centered upon the South Pole for many days, this year’s Ozone Hole was not, thus limiting
the number of observations of vertical profiles with near complete destruction. The vertical time series in
Figure 8 from ozone profiles at the South Pole during 2007 also shows the
limited time of large depletion in
ozone between 14 and 21 km in late September and very early October.
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 9
Figure 9 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 as a result of variable meteorological conditions. This variability continues as 2007 had greater
ozone amounts for this period than the previous two years.
Temperature and Dynamical Relationships with the Ozone Hole
The dynamics of the middle and upper stratosphere strongly dictate the variability of the size of the Ozone Hole
and its longevity. Atmospheric waves can transport heat from lower latitudes into the polar latitudes thus
modulating the temperatures inside the polar vortex. Years with above average wave activity will result in
vortices with warmer temperatures, which then, will have smaller areas of Polar Stratospheric Clouds within
which chemical reactions destroy ozone. Additionally, years with greater wave activity also have shorter polar
vortex life times. Oppositely, years with quiet wave activity will lead to vortices with colder temperatures
and larger areas of PSC clouds and greater ozone destruction and tend to have prolonged life times. Examination
of the atmospheric wave structure also shows how much the polar vortex was off center and away from the South
Pole. Figure 10 shows a time history of the altitude dependence of the
amplitude of wave-one for 2007.
Figure 11 shows the time history of the meridional eddy heat flux.
Together they show periods of larger wave
activity and associated transport of heat southward (negative) toward the South Polar region. Episodic pulses
will result in rapidly decreased area size of cold temperatures and a smaller Ozone Hole. Such an event occurred
in mid-September just at the time of the expected peak Ozone Hole size. Following this episode the Ozone Hole
shrank in size to below average size for the last ten years and remained so until November. This also was the
period in which a persistent wave-one occurred in the lower stratosphere from late September through October.
This displacement from this persistent wave impacted the observations from the South Pole as explained in the
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 south polar region, averaged from 65S to 90S
are shown in Figure 12 . Climatologically, from May through October
minimum temperatures in the south polar region
are well below -78ºC. In 2007 these minimum temperatures were near-average until the middle of July when they
became slightly colder than average until mid-October. By this time the temperatures had risen above -78ºC
which means that PSCs would no longer be created and heterogeneous destruction of ozone in the stratosphere
would no longer take place. From this time onward, the depleted air inside the polar vortex
remains that way until the polar vortex begins to break down and ozone rich air entrains into the polar region.
From November 1 onward observed temperatures were below average coinciding with the prolonged lifetime of the
Figure 13 shows monthly average temperature anomalies for August,
September, and October (ASO) at 50 hPa for three latitude regions, 25ºS to 25ºN, 65ºS to 25ºS, and 90ºS to 65ºS.
For the south polar region, 2007 temperatures were slightly warmer than average, near average in the
mid-latitudes and colder than average in the tropics. Warmer temperatures coincide with greater amounts of
ozone, thus agreeing with the ozone anomalies shown in Figure 5 .
The tropical latitudes continue the tendency
toward lower temperatures after 1993 relative to higher temperatures in earlier years.
Figure 14 presents time series of the size of the Ozone Hole, the
size of the polar vortex, and the size of the polar area where lower stratosphere temperatures were below -78ºC
(polar stratospheric cloud, PSC area). The daily 2007 values are shown, along with the extreme and average
daily values for the most recent 10 years. The polar vortex size was average throughout the entire winter/spring
period. This and below average areas of PSC temperatures most likely led to a smaller peak ozone size in
mid-September and lower than the 10-year average in October. The persistence of the vortex in November and
early December prolonged existence of the Ozone Hole until mid-December.
Figure 15 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 2007, the persistence of the Southern Hemisphere polar vortex in the lower stratosphere extended
longer than many previous years. The persistence of the Ozone Hole to the beginning of December was the 8th
longest on record.
III. CONCLUDING REMARKS
Very low ozone values were again observed over Antarctica in the Winter of 2007. Ozone depletion greater than
45% was observed over Antarctica, compared to total ozone amounts observed in the early 1980's. Vertical
soundings over the South Pole during September and October 2007 showed near-complete destruction of ozone at
altitudes between 13 and 21 km. Lower stratosphere temperatures in the winter of 2007 over the Antarctic region
were near average levels. Associated with this, the peak size of the Ozone Hole was smaller than the record
breaking sizes seen this past decade. Other characteristics of the 2007 Ozone Hole are that it was near the
10-year average or below it for a major fraction of its existence, it was off center from the South Pole for
most of September and October, but due to the persistence of the polar vortex once it re-centered over the
South Pole, the Ozone Hole itself persisted well into December.
Observations of chlorofluorocarbons 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 amount for many
decades to come. The Antarctic Ozone Hole is expected to continue for decades. Antarctic ozone abundances
are projected to return to pre-1980 levels around 2060-2075, roughly 10-25 years later than estimated in
the 2002 Assessment. The projection of this later return is primarily due to a better representation of the
time evolution of ozone-depleting gases in the Polar Regions. In the next two decades, the Antarctic Ozone Hole
is not expected to improve significantly (WMO, 2006). Further, changing conditions (i.e. meteorological, solar,
and volcanic aerosols) 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 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 size and duration and size
of the 2007 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 ozone recovery.
Anderson, J., J. M. Russell III, S. Solomon, and L. E. Deaver, 2000: Halogen Occultation Experiment
confirmation of stratospheric chlorine decreases in accordance with the Montreal Protocol,
J. Geophys. Res., 105, 4483-4490.
Hofmann, D.J., S.J. Oltmans, J.M. Harris, B.J. Johnson, and J.A. Lathrop, 1997: Ten
years of ozonesonde measurements at the south pole: implications for recovery of
springtime Antarctic ozone. J. Geophys. Res., 102, 8931-8943.
Miller, A.J., R.M. Nagatani, L.E. Flynn, S. Kondragunta, E. Beach, R. Stolarsky,
R. McPeters, P.K. Bhartia, M. Deland, C.H. Jackman, D.J. Wuebbles, K.O. Putten, and R.P. Cebula, 2002,
A cohesive total ozone data set from SBUV/(2) satellite system, J.Geophys. Res., 107(0),doi:10.1029/200,D000853.
Nagatani, R.N., A.J. Miller, K.W. Johnson, and M.E. Gelman, 1988: An eight year
climatology of meteorological and SBUV ozone data, NOAA Technical Report NWS 40, 125 pp.
OFCM, 1988: National Plan for Stratospheric Monitoring 1988-1997. FCM-P17-1988. Federal
Coordinator for Meteorological Services and Supporting Research, U.S. Dept. Commerce,
Planet, W. G., J. H. Lienesch, A. J. Miller, R. Nagatani, R, D. McPeters, E.
Hilsenrath, R. P. Cebula, M. T. DeLand, C. G. Wellemeyer, and K. M. Horvath, 1994:
Northern hemisphere total ozone values from 1989-1993 determined with the NOAA-11 Solar
Backscatter Ultraviolet (SBUV/2) instrument. Geophys. Res. Lett., 21,
WMO, 1999: Scientific assessment of ozone depletion: 1998. World Meteorological
Organization Global Ozone Research and Monitoring Project - Report No. 44.
WMO, 2006: Scientific assessment of ozone depletion: 2006. World Meteorological
Organization Global Ozone Research and Monitoring Project.
VI. Web Pages of Interest