1999

National Oceanic and Atmospheric Administration

Contributors:

Angell, J.K. ERL/Air Resources Laboratory
Flynn, L.E. NESDIS/Office of Research and Applications
Gelman, M.E. NWS/Climate Prediction Center
Hofmann, D. ERL/Climate Monitoring and Diagnostic Lab.
Long, C.S. NWS/Climate Prediction Center
Miller, A.J. NWS/Climate Prediction Center
Nagatani, R.M. NWS/Climate Prediction Center
Oltmans, S. ERL/Climate Monitoring and Diagnostic Lab.
Zhou, S. Research and Data Systems Corporation

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

        http://www.cpc.ncep.noaa.gov

with location: products/stratosphere/winter_bulletins.

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: mel@climon.wwb.noaa.gov

ABSTRACT

Ozone values were extremely low over Antarctica during the Southern Hemisphere winter-spring of 1999. The area covered by extremely low total ozone values of less than 220 Dobson Units, defined as the "ozone hole", was larger than in any previous year except for 1998. The ozone hole persisted until the middle of December 1999, longer than in any previous year except for 1998. Vertical profiles of ozone amounts over the South Pole, at the end of September and early October 1999, showed nearly complete destruction of ozone in the 15-21 km region, similar to values observed during other recent years. The minimum total ozone value of 90 Dobson Units, observed on September 29, 1999 at the South Pole, was second only to the record low value of 86 DU observed in 1993. Lower stratosphere temperatures over the Antarctic region in 1999 were again near record low values. Temperatures lower than -78 C occurred over a large region, and were sufficiently low for formation of polar stratospheric clouds, and for enhanced ozone destruction to proceed. The rate of decline in stratospheric ozone at midlatitudes has slowed during the 1990s. The fact that ozone depletion appears to have stabilized supports the conclusion that international actions are working well to reduce the use and 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.

Method of Observation

We have used the total column ozone data from the NASA Nimbus-7 SBUV instrument from 1979 through 1988; the NOAA-11 SBUV/2 from January 1989 to December 1993 (Planet, et al., 1994); the NOAA-9 SBUV/2 instrument from January 1994 to December 1996; the NOAA-14 SBUV/2 from January 1997 to June 1999, and the NOAA-11 SBUV/2 from July 1999. Solar Backscatter Ultra-Violet (SBUV) instruments can produce data only for daylight-viewing conditions, so no SBUV/2 data are available at polar latitudes during winter darkness. Increasing loss of NOAA-11 data at sub-polar latitudes from 1989 to1994 was caused by satellite precession, resulting in SBUV/2 viewing high latitudes only in darkness. NOAA-9, NOAA-14 and recent NOAA-11 total ozone data have not yet been fully validated. From comparisons of coincident data, however, we know recent total ozone amounts may be 2 to 3 percent too high. This impacts trends determined for the recent period.

II. DISCUSSION

Figure 1 shows monthly average anomaly values (percent) of zonal mean total ozone, as a function of latitude and time, from January 1979 to November 1999. The anomalies are derived relative to each month's 1979-1999 average. Certain aspects of long-term global ozone changes may be readily seen. In the polar regions, ozone values have been substantially lower in recent years than in earlier years. 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 consistent negative anomalies of greater than 10 percent for recent years. In October and November 1999, South Polar anomalies exceeded 14 percent (more than 28 percent lower than in earlier years). At midlatitudes, the anomalies also change from largely positive in the early years to negative in the 1990s. The Scientific Assessment of Ozone Depletion: 1998 (WMO, 1999) reported that the rate of decline in stratospheric ozone at southern midlatitudes has slowed during the 1990s. The anomalies shown in Figure 1 are consistent with that finding. Little 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. In 1999, positive anomalies are evident in the tropical region.

A map of monthly average Southern Hemisphere SBUV/2 total ozone for October 1999 is shown in Figure 2 . The region of highest ozone (green and yellow colors) is seen equatorward of the Antarctic region. "Ozone hole" values (defined as total ozone values less than 220 DU, purple and magenta) are shown over an area the size of the Antarctic continent. The most extreme low values of total ozone are not able to be seen on this map over the polar region (black area), because of lack of solar illumination for SBUV/2 data in this time period. Figure 3 shows the difference in percent between the monthly mean total ozone for October 1999 and eight (1979-86) monthly means for October (Nagatani et al., 1988). Negative anomalies in total ozone of greater than 10 percent to more than 40 percent are shown over all of Antarctica (purple and magenta colors). Decreases of about 10 percent are also evident over southern South America.

Figure 4 presents a comparison for recent years of the Southern Hemisphere area covered by the ozone hole. In September 1999 the area of the ozone hole was greater than 21 million square kilometers, somewhat smaller than in 1998. For October and November 1999, the size of the ozone hole was similar to1998. Also in 1999, the ozone hole persisted into the middle of December, not as long as in 1998, but longer than any previous year. In Figure 5 we show, 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 16.7 million square kilometers. The October- November 1999 average value was 16.2 million square kilometers, almost as large as in 1998. An average for September through November would show a larger difference between 1999 and 1998. However, September data were not included in this figure 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 6 shows a time series during 1999 of ozone profiles over the South Pole, measured using balloon-borne ozone instruments. The appearance of anomalously low ozone values is seen in mid September, with extremely low values evident at the end of September and in early October. The ozone destruction, especially in the 15 to 21 km region, is dramatic. Figure 7 illustrates the ozone profiles measured at the South Pole on 29 September 1999, with the minimum total column ozone for 1999 of 90 DU. This is compared with the profile on 29 July. The 29 September profile shows nearly complete destruction of ozone between 15 and 21 km. The figure also clearly shows the region where temperatures are lower than -78 C. In the region of low temperatures and chemical ozone depletion from enhanced human produced chlorine and bromine, the 29 September profile shows markedly less ozone than the profile of 29 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 8 presents a time series of total column ozone at the South Pole integrated from balloon-borne ozone measurements. Minimum ozone amounts at the South Pole Station in 1999 are seen at the end of September and in early October. Late August and early September column ozone values were lower than previous weeks, and likely reflected the presence of ozone depleted air which had previously been exposed to sunlight prior to moving over the South Pole. Total ozone values at South Pole in late September to early October were similar to measured values for this time period in recent years. The changes from 250 DU in August to total ozone values below 100 DU represent decreases of more than 60 percent.

Antarctic ozone depletion has occurred primarily between the altitudes of 12 and 20 km, and for recent years up to 22 km. This is a region where polar stratospheric clouds form. Figure 9 shows 12-20 km column ozone integrated from the balloon-borne ozone measurements at the South Pole.In early September 1999 the values were lower than in any previous year, but October and November values were similar to recent years. Large depletion rates are expected for the next decade or more, after which declining stratospheric chlorine should result in slow recovery of stratospheric ozone.

Temperatures in the lower stratosphere are closely coupled to ozone 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 10 . For most of the southern hemisphere winter and spring of 1999, minimum temperatures in the polar region were substantially lower than average and near record low values. Minimum temperatures were sufficiently low (lower than -78 C) during May to October for polar stratospheric clouds to form and allow enhanced ozone depletion. Figure 11 shows monthly average temperature anomalies at 50 hPa for three latitude regions, 65S-90S, 25S- 65S, and 25N-25S . For these regions, temperature anomalies during 1999 were again substantially below the long-term average, and often near record low values.

Figure 12 presents time series of the area of the ozone hole, the size of the polar vortex, and the size of the area over Antarctica where lower stratosphere temperatures were below -78C. For comparison, the 1999 values are shown along with the average daily values and the maximum and minimum daily values for the most recent 13 years. The area for all three of these indicators, during most of the period from September to December 1999, was larger than average. The persistence into December 1999 of the ozone hole was similar to 1998, longer than any previous year. However, the area of the polar vortex diminished quickly in 1999, much earlier than in 1998. Figure 13 shows a direct relationship in 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 ozone hole also tended to be extended. We note that 4 out of the last 5 years have had the longest duration of winter vortex and ozone hole.

III. CONCLUDING REMARKS

Very low ozone values were observed over Antarctica again in 1999. Ozone depletion of 10 percent to more than 40 percent was observed over Antarctica compared to total ozone amounts observed previous to the early 1980's. Vertical soundings over the South Pole during October 1999 again showed nearly complete destruction of ozone at altitudes between 15 and 21 km. Lower stratosphere temperatures in the winter and spring of 1999 over the Antarctic region were near record low values, and were sufficiently low for ozone production of polar stratospheric clouds within the polar vortex.

The Scientific Assessment Of Ozone Depletion: 1998 (WMO, 1999) states that the abundance of ozone-depleting substances in the stratosphere is expected to peak by the year 2000. International actions are working well to reduce the use and release of ozone depleting substances. The fact that Antarctic ozone depletion appears to have stabilized, supports this conclusion. However, chemicals already in the atmosphere are expected to continue ozone depletion for years to come. Recovery could be expected with international adherence to the Montreal Protocol and its amendments banning and/or limiting substances that deplete the ozone layer. Changing atmospheric conditions and natural ozone variability complicate the task of detecting the start of the ozone layer recovery. Only over the middle latitudes in both the Northern and Southern Hemispheres has the ozone decline recently slowed. Based on an analysis of 10 years of South Pole ozone vertical profile measurements, Hofmann et al. (1997) estimated that recovery of the Antarctic ozone hole may be conclusively detected as early as the year 2008. The indicators in the vertical ozone profile that will allow the early detection of the recovery 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 ozone column of more than 70 DU on September 15. A full explanation of ozone and temperature anomalies must include all aspects of ozone photochemistry and meteorological dynamics. Continued monitoring and measurements including total ozone and its vertical profile are essential toward this end.

IV. REFERENCES

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,!988: 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.