CPC - Stratosphere: Winter Bulletins

Concerns of possible global ozone depletion (e.g. WMO/UNEP, 1994) 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) 763-8000, ext. 7558
Fax: (301) 763-8125
E-mail: mel.gelman@noaa.gov

Ozone values were extremely low over Antarctica during the Southern Hemisphere winter-spring of 1998. Minimum total ozone values of less than 100 Dobson Units, observed at the South Pole in October 1998, were near record low values. The area covered by extremely low total ozone values of less than 220 Dobson Units, defined as the "ozone hole", was larger in 1998 than in any previous year. The ozone hole persisted until the middle of December 1998, longer than in any previous year. Vertical profiles of ozone amounts over the South Pole in October 1998, showed nearly complete destruction of ozone in the 15-21 km region, similar to recent years. Lower stratosphere temperatures over the Antarctic region in 1998 were near record low values. Temperatures lower than -78 C occurred over a record large region, and were sufficiently low for formation of polar stratospheric clouds, and for enhanced ozone destruction to proceed.

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
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 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 August 1994 (Planet, et al., 1994); the NOAA-9 SBUV/2 instrument from September 1994 to June 1997; and the NOAA-14 SBUV/2 beginning in July 1997. Solar Backscatter Ultra-Violet instruments can only produce data 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 was caused by satellite precession from 1989 to 1994, resulting in SBUV/2 viewing during darkness at those high latitudes. NOAA-14 total ozone data have not yet been validated to the extent of NOAA-9 and NOAA-11 data. From preliminary comparisons of coincident data, however, we know that current operational NOAA-14 zonally averaged total ozone amounts are about 2 percent higher than those from NOAA-9. This impacts the trends determined for this period.

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 1998. The anomalies are derived relative to each month's 1979-1998 average. Certain aspects of the long-term global ozone changes may be readily seen. In the polar regions, ozone is 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 1998, South Polar anomalies exceeded 14 percent (more than 28 percent lower than in earlier years). The large negative anomalies in the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et al., 1993) were related to the Mt. Pinatubo eruption in mid-1991. The negative anomalies decreased in 1994 along with the diminishing aerosol loading, but large negative total ozone anomalies again developed in the Northern Hemisphere middle latitudes, peaking in early 1995. During 1998, the anomalies for the Northern Hemisphere were largely positive. Stolarski et al. (1992), Hollandsworth et al. (1995) and Miller et al. (1995), indicated that trends for the mid-latitudes were statistically significant and at about -2 to -4 % per decade. However, over the middle latitudes in both hemispheres the ozone decline has recently slowed. Little or no significant trend has been found over the equatorial region. Alternating years of positive and negative anomalies are seen as part of a quasi-biennial oscillation.

A map of monthly average Southern Hemisphere SBUV/2 total ozone for October 1998 is shown in Figure 2. The region of highest ozone (red and yellow colors) is seen equatorward of the Antarctic region. "Ozone hole" values (defined as total ozone values less than 220 DU, blue and purple) are shown over most 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 SBUV/2 data coverage for this time period. Figure 3 shows the difference in percent between the monthly mean total ozone for October 1998 and eight (1979-86) monthly means for October (Nagatani et al., 1988). Negative anomalies in total ozone of greater than 15 percent to more than 40 percent are shown over all of Antarctica (yellow to blue colors). Decreases of about 10 percent are evident over southern South America, and with some negative anomalies also evident over southern Africa, Australia and New Zealand.

Figure 4 presents a comparison for recent years of the Southern Hemisphere area covered by the ozone hole. In September 1998 the area reached values of grater than 25 million square kilometers, and in September and for most of October, the area of the ozone hole was larger than for the same period in any previous year. The ozone hole covered a large area even at the end of November, and persisted into the middle of December 1998, longer than for any previous year. In Figure 5 we show, for each year since 1979, the daily average of the ozone hole area for October to November. Extremely low ozone values first began to appear over the Antarctic region in the early 1980's (Farman et al., 1985). The growth in the average ozone hole area is quite apparent in Figure 5. From a very small area in 1982, values have increased dramatically, with some year to year variation, to a maximum in 1998 of about 17 million square kilometers.

The center of the ozone hole, and its associated lowest ozone, is often located close to the South Pole. Figure 6 shows a time series of ozone profiles measured over the South Pole during 1998. The appearance of anomalously low ozone values is seen at the end of September, with extremely low values evident in early October. The ozone destruction, especially in the 15 to 21 km region, is dramatically illustrated in Figure 7. The ozone profile measured at the South Pole on 3 October 1998, during the time of minimum total column ozone for 1998, is compared with the profile on 29 July. The October 1998 profile shows nearly complete destruction of ozone between 15 and 21 km, and with significant decreases down to 12 km. In this region of chemical ozone depletion that results from enhanced human produced chlorine, the 3 October profile shows 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 1998 were measured in early October with values below 100 DU. This was somewhat higher than the record low total ozone measurement of in 1993, associated with enhanced destruction on aerosols from the Pinatubo volcanic eruption (Hofmann et al., 1995). Late August and early September column ozone values were very low, 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 very similar to measured values for this time period in recent years. The changes from 250 DU in August to total ozone values near 100 DU represent decreases of about 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. The large depletion rate in mid-September 1998 was nearly identical to that seen in recent years. These values are expected for the next decade or more, after which declining stratospheric chlorine will result in slow recovery.

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/UNEP, 1994). 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 1998, the 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 (Gelman et al., 1986). For these regions, temperature anomalies during 1998 were 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 1998 values are shown along with the average daily values and the maximum and minimum daily values for the most recent 11 years. The area for all three of these indicators, during most of the period from September to December 1998, was larger and persisted longer than for any previous year. 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 3 out of the last 4 years have had the longest duration of winter vortex and ozone hole.

Very low ozone values were observed over Antarctica again in 1998. Ozone depletion of 10 percent to more than 50 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 again showed nearly complete destruction of ozone at altitudes between 15 and 21 km. Lower stratosphere temperatures in the winter and spring of 1998 over the Antarctic region were near record low values, and were sufficiently low for ozone destruction to proceed on polar stratospheric clouds within the polar vortex.

The Summary of the 1998 Scientific Assessment by the WMO and UNEP of the state of the ozone layer has recently become available. The Summary states that the abundance of ozone-depleting substances in the stratosphere is expected to peak by the year 2000. Even though international actions are working well to reduce the use and release of ozone depleting substances, chemicals already in the atmosphere will 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 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.

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