- Angell, J.K. ERL/Air Resources Laboratory
- 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.
- Planet, W.G. NESDIS/Office of Research and Applications
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,
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:
Alvin J. Miller
NOAA Climate Prediction Center
5200 Auth Road
Washington D. C. 20233
Telephone: (301) 763-8000, ext. 7552
Fax: (301) 763-8125
Total ozone values over Antarctica during September and October 1996 were
extremely low, with the minimum values only slightly higher than the record low values observed
in 1993. The area covered by the lowest total ozone values was as large in 1996 as in 1993. The
"ozone hole" persisted till early December 1996, longer than in any previous year. Ozone profiles
in the 14-22 km region showed nearly complete destruction of ozone in October 1996, similar to
recent years. Lower stratosphere temperatures over the Antarctic region in 1996 were near
record low values, and sufficiently low (lower than -78 C) for polar stratospheric clouds to form
over a large region.
I. DATA RESOURCES
The data available and appropriate references are listed below. This combination of
complementary data, from different platforms and sensors, provides a strong capability to
monitor global ozone and temperature.
||Komhyr et al., 1986
||Komhyr et al., 1989
||Planet et al. 1994
||Mateer et al., 1971
||Planet et al., 1994
||Mateer et al., 1971
||Gelman et al., 1986
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, and the NOAA-9 SBUV/2 instrument beginning September 1994. 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 conditions. In addition, increasing data loss of NOAA-11 data at
high latitudes was caused by satellite precession over several years. Resulting changes of
SBUV/2 viewing times to later in the day and extremely high solar zenith angles, eventually
exceed the SBUV/2 instrument calibration range.
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 1996. The anomalies are derived
relative to each month's 1979-96 average. Certain aspects of the long-term global ozone changes
may be readily seen. In the extra-tropics and polar regions, ozone is substantially lower in recent
years than in earlier years. Largest anomalies are shown for the winter-spring months in each
hemisphere, 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 1996, south polar anomalies exceeded 14 percent (more than 28 percent lower than in
earlier years). Stolarski et al. (1992), Hollandsworth et al. (1995) and Miller et al. (1995), have
indicated that the trends in the mid-latitudes are statistically significant and are about -2 to -4 %
per decade. 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. In the Southern Hemisphere middle latitudes, a small positive total ozone anomaly
developed in the last few months of 1996, similar to the positive anomaly observed in 1991-92.
Little or no significant trend has been found over the equatorial region, but a weak negative
anomaly is seen in 1996, as part of a quasi-biennial oscillation.
A map of monthly average Southern Hemisphere SBUV/2 total ozone for October 1996 is
shown in Figure 2. The region of highest ozone (yellow and red colors) is seen equatorward of
the Antarctic region. Very low total ozone values (less than 220 DU, blue and purple) are shown
over the Antarctic continent. "Ozone hole" values, below 220 DU , first began to appear over the
Antarctic region in the 1980's (Farman et al., 1985). The most extreme low values of total ozone,
approaching 100 DU, are not shown on this map, because of lack of SBUV/2 data coverage for
this time period over the polar region (black area). Figure 3 shows the difference in percent
between the monthly mean total ozone for October 1996 and the eight monthly means for
October 1979-86. Decreases in total ozone of greater than 20 percent (green and blue) to more
than 50 percent are shown over a large area of Antarctica. The extreme low "ozone hole" values
developed in September, and reached lowest values in early October. The lowest values
periodically moved off the South Pole, with very low total ozone often extending over the
Weddell Sea, and sometimes as far north as the southern part of South America. The lowest
values moderated slowly in November, but "ozone hole values" persisted till early December
1996. The pattern of total ozone anomaly in Figure 3 shows lowest values over Antarctica, with
low values also extending over Australia and New Zealand, and with some positive percent
anomalies located over mid-latitudes. The shape of this pattern is very similar to the temperature
anomaly for October 1996 (map not shown), suggesting a significant meteorological dynamic
component for the source of the total ozone and temperature anomalies (Finger et al., 1995).
Figure 4 shows a comparison of the area covered by ozone values less than 220 DU, the value
generally used to denote the "ozone hole". In the Southern Hemisphere winter-spring of 1996,
the area of the "ozone hole" was similar to recent record setting years of 1995 and even 1993, and
substantially larger than the earlier years such as 1989. The ozone hole covered a large area even
at the end of November, and persisted into early December 1996, longer than for any previous
The time series in Figure 5 shows total column ozone at the South Pole integrated from
balloon-borne ozonesondes. Minimum ozone at the South Pole Station in 1996 occurred on
October 8, with a value of 114 DU (+/- 5 DU). This was somewhat lower than values in earlier
years (e.g. 1989), but higher than in 1995 (101 DU) and higher than the record low total ozone
measurement of 91 DU in 1993 (Hofmann et al., 1995). Late August and early September column
ozone was 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. Average total ozone at South
Pole in late September to early October was about 130 DU in 1996, very similar to measured
values for this time period in 1995.
Although the minimum total column ozone amount measured at the South Pole was not as low in
1996 as in 1995, this was not an indication of diminished ozone loss. The 12-22 km layer, the
region of ozone depletion that results from enhanced human produced chlorine, had ozone
amounts which were equal to or even less than in 1995 (Figure 6). In fact, through much of
the winter, both the total column ozone and the 12-22 km ozone values were at near their
historical minima. In mid-September the large depletion rate was nearly identical to that seen in
Enhanced ozone destruction, especially in the 14-22 km region, is dramatically illustrated in
Figure 7. The ozone profile measured at the South Pole on 8 October 1996, at the time of
minimum total column ozone for 1996, is compared with the 1995 minimum profile on 5
October. The October profiles show nearly complete destruction of ozone between 14 and 22
km. In this region of chemical ozone depletion that results from enhanced human produced
chlorine, the 8 October 1996 profile shows less ozone than the profile of 5 October 1995. The
higher total ozone amount in 1996 results from larger ozone concentrations above 25 km. This
clearly demonstrates the value of vertical profile information in helping understand the ozone
depletion phenomenon in Antarctica by allowing greater understanding of the processes
responsible for changes in the total column amounts.
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 8. For most of the southern hemisphere winter and spring of 1996, 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) for polar
stratospheric clouds to form and allow enhanced ozone depletion.
Temperature anomalies for the 100-50 hPa layer derived from radiosonde data (Angell, 1988) are
shown in Figure 9 for the Southern Hemisphere and for the South Polar area. Figure 10 shows
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 1996 were near record
III. CONCLUDING REMARKS
Very low total column ozone values (near 100 DU, Dobson Units) were observed over Antarctica
again in 1996. Ozone depletion of 20 percent to more than 50 percent was observed over
Antarctica compared to 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 14 and 22 km. Lower stratosphere temperatures in the winter and spring of
1996 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. Total ozone has
continued to decline over mid-latitudes at the rate of about 4 percent per decade since 1979.
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. Recovery could be expected with international adherence to the Montreal Protocol
and its amendments banning and/or limiting substances that deplete the ozone layer. 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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