- Angell, J.K. ERL/Air Resources Laboratory
- Gelman, M.E. NWS/Climate Analysis Center
- Hofmann, D. ERL/Climate Monitoring and Diagnostic Lab.
- Lienesch, J. NESDIS/Satellite Research Laboratory
- Long, C.S. NWS/Climate Analysis Center
- Miller, A.J. NWS/Climate Analysis Center
- Nagatani, R.M. NWS/Climate Analysis Center
- Oltmans, S. ERL/Climate Monitoring and Diagnostic Lab.
- Planet, W.G. NESDIS/Satellite Research Laboratory
- Solomon, S. ERL/Aeronomy Laboratory
- Stowe, L. NESDIS/Satellite Research Laboratory
Concerns of possible global ozone depletion (e.g. WMO/UNEP, 1992) 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
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 Analysis Center
5200 Auth Road
Camp Springs, MD 20746-4304
Telephone: (301) 763-8000, ext. 7558
Fax: (301) 763-8125
The very low total column ozone values observed over Antarctica during September
and October 1994 were similar to the record-setting low values observed
in 1992 and 1993. Extremely low values of total ozone (near 100 DU, Dobson
Units) were observed in 1994. Ozone depletion of 20 percent to more than 50
percent was observed from 1979 levels over very large areas of the south
polar region. Vertical soundings over the South Pole showed nearly
complete destruction of ozone at altitudes between 15 and 20 km. The rate
of stratospheric ozone depletion over the South Pole in the 15-20 km region
was similar to the rates of destruction in 1992 and 1993, years influenced by
aerosols from the Pinatubo eruption, even though aerosol particles have
declined markedly in 1994 to near normal, background levels. However, in the
10-14 km region, where heterogeneous reactions on volcanic sulfate aerosol
particles dramatically reduced ozone in the springs of 1992 and 1993,
there was significant recovery in 1994, reflecting this diminished aerosol
loading. Lower stratosphere temperatures in the winter and spring of 1994 for
the Antarctic region were below the long-term average, near -80 C, and
sufficiently low for ozone destruction on polar stratospheric clouds within the
polar vortex to proceed during September and October. In addition, we have
examined long-term global total ozone changes since 1979 and have demonstrated
that the decline over mid-latitudes has been about 4 percent per decade, with
little or no long-term trend observed for the equatorial 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, temperature and aerosols.
||Komhyr et al., 1986
||Komhyr et al., 1986
|Total Ozone and
||Mateer et al., 1971
||Planet et al., 1994
||Gelman et al., 1986
||Stowe et al., 1992
||Long & Stowe, 1993
One particular element of data availability concerns that from the SBUV/2
instruments on the NOAA operational satellites. The first SBUV/2 was
launched on NOAA-9 in 1985, and the second on NOAA-11 in 1989.
Unfortunately, the NOAA polar orbiting satellites precess in their orbits such
that the equatorial crossing times trend to later in the day. For the SBUV/2
instruments this results in higher and higher solar zenith angles till,
eventually, the angles exceed the diffuser's calibrated range. In 1994, the
ozone data base was impacted in several ways. First, the NOAA-11 instrument,
which has been recently operating at relatively high solar zenith angles,
developed a failure in the diffuser mechanism causing operations to be
interrupted from mid-October to mid-December. Fortunately, the NOAA-9
instrument has migrated back into the preferred solar zenith angle range, and
colleagues at NASA GSFC and STX Corp. have developed an updated calibration.
After extensive comparisons with available ground-based observations and with
NOAA-11 (Crosby, personal communication), it was determined that the NOAA-9
data are, on average, within a few percent of NOAA-11. Consequently,
for analysis in the Antarctic Spring, we utilize the data from the NOAA-9
SBUV/2. For evaluation of the long-term trends, we have not yet included the
NOAA-9 data, and await a more complete final calibration.
Monthly mean NOAA-9 SBUV/2 total ozone amounts for October 1994 is shown in
Figure 1 . A region of high ozone (yellow and red colors) is seen equatorward
of the Antarctic region. Low total ozone values (green, 220 to 300 DU),
are normally seen in the tropics in all months. Antarctic "ozone hole" values,
below 220 DU (blue and purple), first began to appear in the 1980's (Farman et
al., 1985). During 1992 and 1993, extremely low values of total ozone, near
100 Dobson Units (DU), were seen in October over very large areas of the south
polar region. In October 1994, in general, the area covered by extremely low
ozone values was somewhat smaller than the record setting year of 1993, but
for a few days (not shown) the area covered by the ozone hole was larger than
in 1993.Figure 2 shows the percent difference between the SBUV/2 monthly
mean total ozone map for October 1994 minus the SBUV monthly mean for October
1979. Decreases since 1979 in total ozone of more than 50 percent (100 DU) are
shown by the purple colors, with decreases of greater than 20 percent (green
and blue) shown over a very large area of Antarctica. Small percent increases
are shown over some areas of the tropics and mid-latitudes, but these increases
are not representative of general, long-term changes of ozone.
The time series in Figure 3 shows total column ozone at the South
Pole integrated from balloon-borne ozonesonde observations during the July
to December period of 1992, 1993 and 1994. On 5 October 1994 (day 278), total
ozone fell to its lowest 1994 measured value of 102 DU (with an uncertainty
of 5 DU). The 1994 values at the south pole did not show the
long period of extremely low values of 1993, nor reach the record extreme
value of 91 DU observed on 12 October 1993. Recovery from low values began
earlier in 1994, and the extremely low total ozone values observed over the
south pole in 1993 and in 1992, were not sustained as long in 1994. As
suggested below, the record of extremely low (but not record setting) total
ozone values in 1994 may indicate some moderation in the ozone destruction in
the 10-14 km region, due to diminishing stratospheric aerosols.
Selected ozone profiles measured at the South Pole are shown in
Figure 4 .
The profile before significant depletion at the South Pole was present (2
September 1994) is compared with profiles at the time of minimum total column
ozone amount, on 5 October 1994 and also 8 October. The October profiles
show nearly complete destruction of ozone between 15 and 20 km.
A significant feature of stratospheric ozone destruction over the South Pole
in 1994 was the rate of decline during September of ozone in the 15-20 km
region. The rate was equal to that seen in the aerosol perturbed years of 1992
and 1993, even though aerosol particles declined markedly in 1994. The
increasing amounts of human-produced chlorine in the atmosphere may be
accelerating the long-term rate of ozone decline. In the 10-14 km region,
there was significant ozone recovery in 1994, reflecting the diminished
Temperatures in the lower stratosphere are closely coupled to ozone
through dynamics and photochemistry. Extremely low temperatures (lower
than -78 C) at about 50 mb over the Antarctic region are believed to lead to
depletion of ozone in that low temperatures contribute to the presence of
polar stratospheric clouds (PSCs), and in particular nitric acid trihydrate
(NAT), which is thought to be the dominant component of PSCs. PSCs
enhance the production and lifetime of reactive chlorine, leading to
ozone depletion (WMO/UNEP, 1992).
Daily minimum temperatures over the polar region, 65S to 90S at 50 mb
(approximately 19 km) is shown in Figure 5 .
We see that for
most of the winter and spring of 1994, the 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 mb layer derived from radiosonde data
(Angell, 1988) are shown in Figure 6 , and in
Figure 7 at 50 mb for
three latitude regions, 65S-90S, 25S-65S, and 25N-25S (Gelman et al., 1986).
For the Southern Hemisphere as a whole and specifically for mid latitudes
and equatorial regions, temperature anomalies for 1994 were near the 1993
record low values.
Aerosol concentration is another potentially important component of
stratospheric variation, and, as indicated above, has been suggested as a
possible source of ozone depletion (e.g. Hofmann et al., 1992). Aerosol
optical thickness from the NOAA/AVHRR instrument
( Figure 8) shows
that stratospheric aerosol concentrations continued to diminish from the
maximum values observed a few weeks after the eruption of Mount Pinatubo in
June 1991. Stratospheric aerosols were at such low levels in 1994, that it
was difficult to discern stratospheric aerosols from variations in
tropospheric values. AVHRR, however, may not measure all the sulfuric acid
aerosol that is present if the size of that aerosol is too small (Rao et al.,
1988). The NOAA 11 AVHRR instrument failed in September 1994.
Extending our evaluation of long-term total ozone changes to global scales, we
present in Figure 9 monthly average anomaly values
(Dobson Units) of
zonal mean ozone, as a function of latitude and time. The data base is that
of the SBUV on the NASA Nimbus -7 from 1979 to mid-1990 and the NOAA-11 SBUV/2
from January 1989 to September 1994. The anomalies are derived from each
month's long-term average. From Figure 9 it is
obvious in the extra-tropics
and polar regions that ozone is substantially lower in recent years than in
earlier years. Stolarski et al. (1992), and more recently Hollandsworth et
al. (1994) and Miller et al. (1994) have indicated that the trends in the
mid-latitudes are statistically significant and are about -2 to -4 % per decade
and that little or no significant trend exists over the equatorial
region. One other particular feature is the very large negative
anomaly in the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et
al., 1993) which is believed to be related to the Mt. Pinatubo eruption in
mid-1991. This feature has, in fact, disappeared in 1994 along with the
diminishing aerosol loading.
Stratospheric ozone values over Antarctica during September and October 1994
were extremely low, with the minimum values very near the extreme values of
around 100 DU observed in 1992 and 1993. In the 15-20 km region, the decline
in ozone during September 1994 was similar to that seen in the aerosol
perturbed years of 1992 and 1993, even though aerosol particles have
diminished markedly in this region since the eruption of Mt Pinatubo
in 1991. In the 10-14 km region, where heterogeneous reactions on the
volcanic sulfate aerosol particles dramatically reduced ozone in the springs
of 1992 and 1993, there was a significant recovery in 1994, reflecting the
diminished aerosol loading. We note that the lower stratosphere temperatures
over the Antarctic region in September and October 1994 were below the
long-term average, and sufficiently low (lower than -78 C) for polar
stratospheric clouds to form over a large region. A full explanation of ozone
and temperature anomalies must include all aspects of ozone photochemistry and
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