CPC - Stratosphere: Winter Bulletins

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

Further information may be obtained from Melvyn E. Gelman
NOAA Climate Prediction Center
5200 Auth Road
Camp Springs, MD 20746-4304
Telephone: (301) 763-8071, ext. 7558
Fax: (301) 763-8125
E-mail: melvyn.gelman@noaa.gov

An area of extensive ozone depletion was observed over Antarctica during the Southern Hemisphere winter/spring of 2001, as has been the case since about the mid-1980s. For 2001, the area covered by extremely low total ozone values of less than 220 Dobson Units, defined as the Antarctic “ozone hole”, was the third largest on record for October and November. The ozone hole reached maximum size in September and remained large through early October, then gradually decreased in size and ended in early December. October anomalies of greater than 40 percent below the 1979-1986 base period were observed over Antarctica, with negative anomalies of more than 10 percent also observed over southern South America. Vertical profiles of ozone amounts measured by balloons over the South Pole at the end of September and early October 2001 showed essentially total destruction of ozone in the 15-20 km region, similar to observations during other recent years. The minimum total ozone value of 101 Dobson Units, observed on October 4, 2001 at the South Pole, was not as low as the record low value of 86 DU observed in 1993. Lower stratosphere temperatures over the Antarctic region in 2001 were again low. Temperatures below -78 C ( sufficiently low for polar stratospheric cloud formation) occurred over a large region, thus promoting chemical ozone loss. At northern mid-latitudes, extensive ozone destruction was observed in the years following the massive eruption of the Pinatubo volcano (approximately 1992-1996), with smaller losses observed in the past few years. Uncertainties in the estimates of future ozone depletion include possible coupling to changes in water vapor and carbon dioxide. However, in the absence of further major eruptions, ozone depletion over much of the globe is not expected to worsen substantially in the coming decade, because international actions have been successful in reducing the release of ozone-depleting substances.

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 and temperature.

Parameter	Ground-Based	Satellite/Instrument
Total Ozone	Dobson	NOAA/ SBUV/2
		Nimbus-7/SBUV
Ozone Profiles	Balloon-Ozonesonde	NOAA/ SBUV/2
		Nimbus-7/SBUV
Temperature Profiles	Balloon - Radiosonde	NOAA/TOVS

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 1989; 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. Solar Backscatter Ultra-Violet (SBUV) instruments can produce data only for daylight-viewing conditions, so SBUV/2 data are not available at polar latitudes during winter darkness. Increasing loss of NOAA-11 data at sub-polar latitudes from 1989 to1993 was caused by satellite precession, resulting in SBUV/2 viewing high latitudes only in darkness. Recent NOAA-11 and NOAA-16 total ozone data have not yet been fully validated. This impacts trends determined for the recent period.

Figure 1 displays monthly average anomaly values (percent) of zonal mean total ozone, as a function of latitude and time, from January 1979 to November 2001. The anomalies are derived relative to each month's 1979-2001 average. Certain aspects of long-term global ozone changes may be readily seen. In the polar regions, ozone values have been 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. In September 2001, around 75 degrees south latitude, negative anomalies exceeded 26 percent (more than 52 percent lower than in earlier years), and were about 20 percent lower than average in October and November 2001. At midlatitudes, the anomalies also change from largely positive in the early years to negative in the 1990s. Little or no significant trend is seen over the tropical region, but alternating years of positive and negative anomalies are seen, as part of a quasi-biennial oscillation. At the end of 2001, positive anomalies were evident in the tropical region.

A map of monthly average Southern Hemisphere SBUV/2 total ozone for October 2001 is shown in Figure 2, with lowest ozone values displaced from the pole. "Ozone hole" values (defined as total ozone values less than 220) are shown over the South Atlantic sector of the Antarctic continent, with highest ozone over the Antarctic sector near the international dateline. Figure 3 shows the difference in percent between the monthly mean total ozone for October 2001 and eight (1979-86) monthly means for October (Nagatani et al., 1988). Negative anomalies in total ozone of up to 40 percent are shown over the South Atlantic sector of the Antarctic continent, and more than 10 percent below average values are also evident over southern South America.

Figure 4 compares for each year since 1979 the ozone hole area average for all days in October through November. The growth in the ozone hole area from the 1980s to the 1990s is quite apparent. From a very small area in 1982, October-November average values increased dramatically to a maximum in 1998 of 16.2 million square kilometers. The October-November 2001 average ozone hole area value was 15.9 million square kilometers, much larger than the ozone hole area in 2000, and only a little smaller than the two previous years. September data for all years were not included for this calculation because SBUV/2 data over the South Polar region were not available in early September for years 1992, 1993, and 1995.

The center of the ozone hole, and associated lowest ozone, is often located close to the South Pole. Figure 5 shows a time series during 2001 of ozone profiles over the South Pole, measured using balloon-borne ozone instruments. The appearance of anomalously low ozone hole values is seen to begin in mid August, with extremely low values evident at the end of September and in early October. The ozone destruction, especially in the 15 to 20 km region, is dramatic. Figure 6 illustrates the change in ozone profiles measured at the South Pole. On 4 October 2001, 101 DU total column ozone amount was observed, the minimum value for the year 2001. This is compared with the profile on 8 July, with total ozone amount of 271 DU. The decrease in total ozone between these two dates is 62 percent. The 4 October profile shows nearly complete destruction of ozone between 14 and 20 km. The figure also shows the region where temperatures on 4 October were lower than -78 C. In the region of low temperatures and chemical ozone depletion from enhanced human produced chlorine and bromine, the 4 October profile shows markedly less ozone than the profile of 8 July. This clearly demonstrates the value of vertical profile information in helping to understand the ozone depletion phenomenon and the processes responsible for changes in the total column amounts.

Figure 7 presents a time series at the South Pole of total column ozone, integrated from balloon-borne ozone measurements. Minimum ozone amounts at the South Pole Station in 2001 are seen at the end of September and in early October. Total ozone values were not as low during September as values for this time period for 2000, but values remained very low longer in 2001 than in 2000. The extremely low total ozone values in early September likely reflected ozone depleted air which had previously been exposed to sunlight prior to moving over the South Pole.

Antarctic ozone depletion has occurred primarily between the altitudes of 12 and 20 km. This is a region where polar stratospheric clouds form. Figure 8 shows 12-20 km column ozone integrated from the balloon-borne ozone measurements at the South Pole. In 2001 the values were generally as low as in any previous year. Large depletion rates are expected for the next decade or more, after which declining stratospheric chlorine amounts should result in slow recovery of stratospheric ozone.

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 are believed to 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 over the polar region, 65S to 90S at 50 hPa (approximately 19 km) are shown in Figure 9. For most of the southern hemisphere winter 2001, minimum temperatures in the polar region were low, but not near record low values. However, minimum temperatures in late September, October and November were near record lows. Minimum temperatures were sufficiently low (lower than -78 C) during May to November for polar stratospheric clouds to form and allow enhanced ozone depletion, in the presence of sunlight. Figure 10 shows monthly average temperature anomalies at 50 hPa for three latitude regions, 25N-25S, 25S-65S, and 65S-90S. For the polar region, temperatures for October and November were 3 to 4 C lower than the long-term average. Negative temperature anomalies also predominated over the middle and tropical latitudes of the Southern Hemisphere.

Figure 11 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 stratosphere temperatures were below -78C. The 2001 values are shown along with the average daily values and the maximum and minimum daily values for the most recent 10 years. During 2001 the area for all three of these indicators was larger than average. Indeed the ozone hole area and the polar vortex area were among the largest of recent years.

Figure 12 illustrates the direct relationship between the persistence of the ozone hole region 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 2001, the persistence of the ozone hole and the persistence of the Southern Hemisphere polar vortex were among the greatest for the years since 1982. Indeed, 5 out of the most recent 7 years have had the longest duration of winter vortex and ozone hole.

Very low ozone values were observed over Antarctica again in 2001. Ozone depletion of 10 percent to more than 40 percent was observed over Antarctica compared to total ozone amounts observed in the early 1980's. Vertical soundings over the South Pole during late September and early October 2001 again showed complete destruction of ozone at altitudes between 15 and 20 km. Lower stratosphere temperatures in the winter and spring of 2001 over the Antarctic region were below average values, and were sufficiently low for ozone production of polar stratospheric clouds within the polar vortex. The ozone hole area and the PSC area were again among the largest of all previous years. For the year 2001, the ozone hole and Southern Hemisphere polar vortex persisted into December, again among the longest duration of years since 1982.

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 deplete ozone 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 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. However, an intriguing aspect of recent observations of the Antarctic stratosphere is the apparent trend towards a later breakup of the vortex, as shown in Figure 12. 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. Russel 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.

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, 124pp.

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, 205-208.

WMO, 1999: Scientific assessment of ozone depletion: 1998. World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44.