- 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/Climate Research and Applications Division
- Zhou, S. Research and Data Systems Corporation
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, and other long-term climate concerns, NOAA has
established routine monitoring programs using 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_bulletin
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
Total ozone values for the high latitudes of the Northern Hemisphere
during the winter/spring of 1998-1999 were higher than the average of the last 21 years.
For March 1999, over the Arctic, total ozone values were about 10 percent higher than
average, and were higher than values observed during these months since the early 1980s.
At middle latitudes, total ozone values were about 5 percent lower than the long-term
average. Lower stratosphere temperatures observed over the north polar region were
generally near average or above average. Temperatures were especially high during the
stratospheric warming episodes in December 1998 and February 1999. Warm conditions did not
allow for widespread chemical destruction of ozone over the Northern Hemisphere polar
region in 1999. This is in contrast to the Antarctic region in 1998, where record low
temperatures and stratospheric ozone destruction caused the ozone hole to reach a record
large size. Total ozone generally decreased over the mid-latitudes of the Northern
Hemisphere at the rate of 2 to 4 percent per decade, from 1979 to the early 1990s. Since
1997, Northern Hemisphere total ozone in the winter and spring has not continued to
decrease. However, the amount of chlorine and other ozone destroying chemicals in the
stratosphere in recent years have been reported to be near peak values. The differences
between Antarctic and Arctic stratospheric ozone destruction in recent years may be
explained as being due to differences in meteorological conditions over the regions and
natural year to year meteorological variability.
I. DATA RESOURCES
The data available 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
||Balloon - Ozonesonde
||Balloon - Radiosonde
We 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; the NOAA-9 SBUV/2 instrument from September 1994 to June 1997; and the NOAA-14
SBUV/2 beginning 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
also at those 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
Figure 1 shows monthly average anomalies of zonal
mean total ozone, as a function of latitude and time, from January 1979 to March 1999. The
percent anomalies are derived relative to each month's 1979-99 average. Long-term
decreases of ozone, from largely positive anomalies in 1979 and the early 1980s to recent
negative anomalies, may be readily seen in the extra-tropical regions. The largest
anomalies are shown in winter-spring months for the polar region of each hemisphere. In
the north polar region, positive anomalies of more than 10 percent in the earlier years
change to negative anomalies for many of the most recent years. In contrast, for the
winter and spring of 1998-99, positive zonal mean total ozone anomalies of up to 12
percent are seen for the high latitudes of the Northern Hemisphere. In the tropical region
also, a high anomaly is seen in 1999, as part of the quasi-biennial oscillation of total
ozone. However, negative anomalies of up to 7 percent are apparent in the middle latitudes
of the Northern Hemisphere.
The large negative anomalies seen in Figure 1 for
the Northern Hemisphere extra-tropics during 1992-1993 were related to the Mt. Pinatubo
eruption in mid-1991 (Gleason et al., 1993 and Solomon et al., 1996). Those negative
anomalies decreased in 1994 with the diminishing aerosol loading, but large negative total
ozone anomalies again developed in the Northern Hemisphere middle latitudes, peaking in
early 1995. Stolarski et al. (1992), Hollandsworth et al. (1995) and Miller et al. (1995),
have reported trends of total ozone in mid-latitudes of about -4 % per decade. No
significant trend has been found over the equatorial region. The trend for the middle
latitudes, based on the SBUV and SBUV/2 data sets and updated from 1979 through March
1999, is -1.6 percent per decade for 30-40 N, and -3.3 percent per decade for 40-50 N,
with a 95 percent confidence estimate of 2 percent.
The NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) operates a
16-station global Dobson spectrophotometer network for total ozone trend studies. Figure 2 shows the total ozone data for four central U.S. stations.
The large annual variation is a result of ozone transport processes which cause a
winter-spring maximum and summer-fall minimum at northern mid-latitudes. Figure 3 shows the four-station average percent deviation from their
long-term monthly mean. The values in the years since 1993 have not continued to decline
in recent years as they had declined from 1979 to 1993. The implication of these trends
and their changes needs to be examined in the context of changes in the ozone depleting
gases and varying meteorological conditions.
A map of Northern Hemisphere monthly mean total ozone amounts for March
1999 is shown in Figure 4. The map shows highest ozone (red and
brown colors) located over middle to high northern latitudes. Lowest values appear in the
tropical region. Figure 5 shows the monthly mean total ozone
percent difference of March 1999 from the mean for eight March monthly means, 1979- 1986
(Nagatani et al., 1988). The 1979 to 1986 base period is chosen because these values are
indicative of the early data record. Most notable is the absence of large negative
anomalies over the Arctic region. In fact, positive anomalies of up to 15 percent are
shown over the Arctic, from Greenland to northern Europe. Negative anomalies are evident
in a band at sub-tropical to high latitudes, peaking at up to 16 percent over the mid
Pacific. Small positive anomalies appear over the tropical region. As mentioned in the
previous section, however, total ozone data from NOAA-14 are biased by about 2 percent
higher than the NOAA-9 data. That bias must be taken into account when interpreting
results using the preliminary NOAA-14 data.
Figure 6 shows monthly mean temperature anomalies
at 50 hPa for three latitude regions, 90N-65N, 65N-25N, and 25N-25S (updated from Gelman
et al., 1986). The temperature anomalies for much of the winter and spring of 1998-99 were
above average values for north polar latitudes, near record low values for mid-latitudes,
and somewhat below the long term average in the equatorial region. The pattern of zonal
mean temperature anomalies closely corresponds to the zonal mean ozone anomalies at middle
and high latitudes.
Extremely low temperatures (lower than -78 C) over the Arctic region in
the lower stratosphere are believed to lead to depletion of ozone. Temperatures in the
lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Low
temperatures contribute to the presence of polar stratospheric clouds (PSCs). PSCs enhance
the production and lifetime of reactive chlorine, leading to ozone depletion in the
presence of sunlight (WMO/UNEP, 1994). Daily minimum temperatures over the polar region,
65N to 90N at 50 hPa (approximately 19 km) are shown in Figure 7.
During the winter and spring, the daily minimum temperatures were mostly above average
values. In December 1998 and again in late February 1999, temperatures increased markedly,
in association with stratospheric warming events, and weakening of the stratospheric polar
Figure 8 compares for each March of the last 21
years the average 100 hPa temperature with the date the stratospheric polar vortex
diminished below a threshold size. The area of the vortex was defined by the maximum in
the gradient of potential vorticity contours at the 450 K isentropic surface, based on the
NCEP/NCAR reanalyses. For 1999, the polar vortex was among the least persistent and also
recorded March average temperatures which were among the highest. This is in contrast to
most other years in the 1990s, which had low temperatures during late winter and spring,
along with extended persistence of the polar vortex.
Figure 9 shows the relationship between the
persistence of the polar vortex and the persistence of high latitude total ozone values of
less than 300 DU. The Arctic polar vortex in the winter of 1998- 1999 was less persistent
than in any year since 1991. Also there was no region of low ozone (less than 300 DU)
observed in 1999. During the 1996-97 winter/spring period, record-low total ozone amounts
were observed over high northern latitudes due in part to an unusually cold, persistent
vortex (Newman et al., 1997). The persistence of both the vortex and low values of total
ozone were greatest in the years 1997, 1996 and 1995. Other years in the late 1980s and
1990s were only somewhat less strongly persistent. However for 1999, meteorological
conditions of a weak polar vortex were associated with limited ozone destruction and
relatively high total ozone over the Arctic region.
Figure 10 shows the average area, during February
and March for each year since 1979, of low ozone (lower than 300 DU). For 1999, low total
ozone was practically not seen over the north polar region. The persistence and extent of
low total ozone values for 1999 were among the least of all years since 1979.
III. CONCLUDING REMARKS
The Arctic polar vortex in the winter and spring of 1998-99 was less
persistent than in any year since 1991. Also there was no region of low ozone (less than
300 DU) observed in 1999. In the winter- spring of 1998-99, positive anomalies of total
ozone were prevalent in the high latitudes of the Northern Hemisphere. Lower stratosphere
temperatures over the Arctic region during the winter and spring 1998-99 were near average
or above average values. Temperatures were not sufficiently low for ozone destruction to
proceed on polar stratospheric clouds within the polar vortex. The conditions in the
Arctic region in 1999 are in contrast to conditions just a few months previous in the
Antarctic region during 1998. Record low temperatures and stratospheric ozone destruction
over the Antarctic region in 1998 caused the ozone hole to reach a record large size. Thus
the high total ozone values in the Arctic region in 1999 are attributed to meteorological
conditions, and occurred despite chlorine and other ozone destroying chemicals in the
stratosphere being near their expected peak values. Total ozone has declined over
mid-latitudes of the Northern Hemisphere at the rate of about 2 to 4 percent per decade
since 1979. However, since 1997, the decline of Northern Hemisphere total ozone has not
continued. A full explanation of ozone and temperature anomalies must include all aspects
of ozone photochemistry and meteorological dynamics. Continued monitoring and measurements
are essential toward this end.
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