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

An area of extensive ozone depletion was again observed over Antarctica during the Southern Hemisphere winter/spring of 2002, with widespread total ozone anomalies of 30 percent or more below the 1979-1986 base period. However, the area covered by extremely low total ozone values of less than 220 Dobson Units, defined as the Antarctic "ozone hole", was the smallest and had the shortest duration of any since 1988. Limitation of the extent of the 2002 ozone hole is attributed to highly unusual meteorological conditions. The ozone hole reached maximum size in September, decreased in late September, and a small remnant persisted over a limited area of the Antarctic continent into early November. Vertical profiles of ozone amounts, measured by balloons over the South Pole, showed strongest destruction of ozone in the 15-20 km region. The minimum total ozone value of 152 Dobson Units, was observed on 21 October 2002 at the South Pole, when the center of the ozone hole was nearby. Minimum values were not as low as seen during other recent years. Lower stratosphere temperatures below -78 C (sufficiently low for polar stratospheric cloud formation) in the winter of 2002 occurred again over the Antarctic region, thus promoting chemical ozone loss. However, highly anomalous meteorological wind patterns and strong warming over Antarctica in late September limited the cold conditions and limited further severe ozone destruction, and the extent of ozone hole in 2002.

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 (80N to 80 S) and time (January 1979 to November 20020. The anomalies are derived relative to each month's 1979-2002 average. Certain aspects of long-term global ozone changes may be readily seen. In the polar regions, 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. However, in the winter and spring of 2002, average total ozone in the Antarctic region was more than10 percent higher than the long-term average.

Maps of monthly average Southern Hemisphere SBUV/2 total ozone for September and October 2002 are shown in Figure 2a, and Figure 3a. Lowest "ozone hole" values (defined as total ozone values less than 220 DU) appear near the south pole, and highest ozone are shown over the Antarctic sector near the international dateline. Figure 2b shows the difference in percent between the monthly mean total ozone for September 2002 and eight (1979-86) monthly means for September (Nagatani et al., 1988). Largest negative anomalies in total ozone of 20 to 50 percent are shown over the western sector of Antarctica, with very large positive anomalies over eastern Antarctica. The October average total ozone anomalies (Figure 3b) over the Antarctic region were strongly positive, with maximum anomalies of more than 30 percent higher than the base period.

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. However, the October/November 2002 average ozone hole area value was 3.5 million square kilometers, very much smaller than recent years, and smallest since 1988.

The center of the ozone hole, and associated lowest ozone, and polar vortex are often located close to the South Pole. However, for the winter/spring of 2002, the center of the ozone hole and the polar vortex were often displaced from the South Pole. Figure 5 shows a time series during 2002 of total ozone over the South Pole, measured using balloon-borne ozone instruments, compared with other selected years. Low ozone hole values appeared in early September 2002, with lowest values evident in mid-September and again in mid-October, when the center of the ozone hole was closest to the South Pole. Total ozone values rose at the end of September and again at the end of October, when the ozone hole was displaced from the South Pole. Figure 5 shows that 2002 was quite different from 1988 in terms of the progression of the ozone hole, even though both years had elevated minimum total ozone amounts.

Figure 6 shows the ozone profile measured at the South Pole on 27 September, when total ozone values rose dramatically to 375 DU. The high total ozone value was due to large ozone increases above 18 km (as compared to the 4 August profile), and despite large decreases in ozone shown between 14 and 18 km. The high 375 DU total ozone value on 27 September excludes the South Pole location from the ozone hole defined region, despite the ozone destruction shown in the 14-18 km region . On 21 October (Figure 6b) a total column ozone amount of 152 DU was observed at the South Pole, the minimum value for the year 2002. The 21 October profile shows the strong destruction of ozone between 15 and 20 km and very low total ozone values associated with classic ozone hole conditions.

The time series (Figure 7a) of ozone profiles at the South Pole during 2002 shows the dramatic September decreases in ozone between 14 and 20 km, associated with ozone hole conditions. The temporary ozone increases at the Pole in late September to early October coincided with a period of highly anomalous meteorological conditions, when the ozone hole moved away from the South Pole. Return of ozone hole conditions close to the South Pole is shown in October and early November. When the ozone hole moved back over the South Pole in October, the ozone amounts in the 12-20 km region remained the same as before the ozone hole had moved away from the Pole. This indicates that no further ozone loss occurred in the ozone hole area in the latter part of September. Figure 7b shows the South Pole ozone time series for the year 2001, when more extreme ozone losses prevailed for the winter/spring ozone hole time 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 8. For most of the Southern Hemisphere winter of 2002, minimum temperatures in the south polar region were below -78 C, but above long term average minimum values. The dramatic rise in temperatures in late September 2002, associated with anomalously warm meteorological conditions, limited the formation of polar stratospheric clouds and also limited ozone depletion.

Figure 9 shows heat flux by planetary waves over 30S to 90S for each month since 1979. The unusually strong positive heat flux shown in August and September 2002 preceded the late September stratospheric warming. Positive heat flux indicates transport of heat toward the South Pole and upward transport of wave energy. The magnitude of the heat flux in August and September 2002 was unprecedented in both the Southern Hemisphere (shown) and also the Northern Hemisphere (not shown). The exceptionally large, almost stationary waves, in the Southern Hemisphere troposphere effectively transported heat to the polar lower stratosphere, leading to unusually high temperatures in the south polar region. Those waves also transported ozone rich air from the mid-latitudes to the polar region, as the polar vortex and the ozone hole moved away from the South Pole.

Figure 10 shows monthly average temperature anomalies at 50 hPa for three latitude regions, 25N-25S, 25S-65S, and 65S-90S. For the south polar region, winter/spring temperatures were dramatically higher than the long-term average. In September and October 2002, the south polar anomalies were larger than any in the last 20 years. Negative temperature anomalies predominated over the middle latitudes of the Southern Hemisphere, with very large negative anomalies over tropical latitudes.

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 -78 C. The 2002 values are shown along with the average daily values and the maximum and minimum daily values for the most recent 10 years. During the spring of 2002, the area for all three of these indicators was smaller than average and indeed the smallest and of shorter duration than any recent year.

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 2002, the persistence of the ozone hole and the persistence of the Southern Hemisphere polar vortex were among the shortest of the years since 1980.

Very low ozone values were observed over Antarctica again in the Southern Hemisphere winter of 2002. Ozone depletion of 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 September and October 2002 again showed strong destruction of ozone at altitudes between 15 and 20 km. However, for the year 2002, the ozone hole declined rapidly in late September, and had the shortest duration of any year since 1988. Lower stratosphere temperatures in the winter and spring of 2002 over the Antarctic region were much higher than average values. Associated with this, the ozone hole area was among the smallest of recent years.

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. An intriguing aspect of recent observations of the Antarctic stratosphere had been the apparent trend towards a later breakup of the vortex in most recent years. However, the limited size and duration of the 2002 ozone hole is attributed to highly unusual meteorological conditions this year. 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.

Miller, A.J., dt al.,A cohesive total ozone data set from SBUV/(2) satellite system, in press, J.Geophys. Res., 2002.

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.