Contributors:
- Angell, J.K. ERL/Air Resources Laboratory
- Flynn, L. NESDIS/Climate Research and Applications
Division
- 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.
- Wang, J. ERL/Air ResourcesLaboratory
- Zhou, S. Research and Data Systems Corporation
Concerns of possible 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, 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_bulletins
Further information may be obtained from:
Melvyn E. Gelman
NOAA, Climate Prediction Center
5200 Auth Road Camp Springs, MD 20746-4304
Telephone: (301) 763-8071 ext.7558
Fax: (301) 763-8125
E-mail: mgelman@ncep.noaa.gov
ABSTRACT
For March 2000, over the
Arctic, total ozone values were 10 to 20 percent lower than the average since 1979, and
were lower than values observed during spring since 1997. Lower stratosphere temperatures
observed over the north polar region during December to March were also substantially
lower than average. These cold conditions led to widespread chemical destruction of ozone
in the lower stratosphere over the Northern Hemisphere polar region. At middle latitudes,
total ozone values ranged from near the long-term average to slightly lower than average.
Total ozone has generally decreased over the midlatitudes of the Northern Hemisphere at
the rate of 2 to 4 percent per decade, from 1979 to the early 1990s, but the downward
trend has not continued in recent years. The amounts of chlorine and other ozone
destroying chemicals in the stratosphere in recent years have been reported to have
reached peak values around 1997-98. Year to year differences in north polar winter-spring
stratospheric ozone destruction may be explained as being due to different conditions
associated with interannual 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 |
|
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 used the total
column ozone data from the NASA Nimbus-7 SBUV instrument from 1979 through 1984; NOAA-9
SBUV/2 from January 1985 to December 1988; the NOAA-11 SBUV/2 from January 1989 to
December 1993; the NOAA-9 SBUV/2 from January 1994 to December 1996; the NOAA-14 SBUV/2
from January 1997 to June 1999, and the NOAA-11 SBUV/2 since July 1999. Solar Backscatter
Ultra-Violet instruments can only produce data for daylight-viewing conditions, so no
SBUV/2 data are available at high polar latitudes during winter darkness. Also, NOAA
satellites have precessed in their orbit, so varying amounts of data at sub-polar
latitudes have been available, depending on the local time of viewing of each satellite
during winter months. NOAA-11 total ozone data since July 1999 have not yet been fully
validated. From comparisons of coincident data, however, we know that recent NOAA-11 total
ozone amounts may be about 2 percent too high. This impacts results determined for the
recent period.
II. DISCUSSION
Figure 1 shows
monthly average anomalies of zonal mean total ozone, as a function of latitude and time,
from January 1979 to March 2000. The percent anomalies are derived relative to each
month's 1979-2000 average. Largest anomalies are shown in winter and spring months for the
polar region of each hemisphere. In the north polar region, positive anomalies of more
than 10 percent prevailed in 1979 and the early 1980s, but mostly negative anomalies
predominated in the 1990s. Exceptions to this were in the winter and spring of 1997-98 and
1998-99, when positive zonal mean total ozone anomalies were seen. However, for March
2000, negative anomalies were again observed in high latitudes of the Northern Hemisphere.
In middle latitudes, near average conditions or slightly negative anomalies prevailed. The
Scientific Assessment of Ozone: 1998 (WMO, 1999) reported that total column ozone
decreased at northern midlatitudes (25-60N) between 1979 and 1991, with estimated linear
trend downward of 4 percent per decade. However, since the recovery from the 1991 Mt.
Pinatubo volcanic eruption, the downward trend of total ozone has not continued. The trend
for the middle latitudes, based on the SBUV and SBUV/2 data sets and updated from 1979
through March 2000, is -1.2 percent per decade for 30-40 N, and -2.4 percent per decade
for 40-50 N, with a 95 percent confidence estimate of 2 percent. No significant trend has
been found over the equatorial region. In the tropics, high anomaly is seen in 2000, part
of a quasi-biennial oscillation of total ozone.
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 from1979 through 1999. 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 means. Large negative
anomalies are shown for these stations in the winter and spring of 1998-99, but with near
average condition in autumn and winter 1999. These anomalies derived from ground-based
measurements are consistent with the anomalies shown in Figure 1,
from SBUV/2 satellite ozone measurements. Middle latitude total ozone values in the years
since 1993 have not continued to decline as they had declined from 1979 to 1993. The
implication of these changes needs to be examined in the context of changes in amounts of
ozone depleting gases in the atmosphere and varying meteorological conditions.
The map in Figure 4
shows Northern Hemisphere monthly mean total ozone amounts for March 2000. Low ozone
extends from low latitudes, over western Europe, to the Arctic region. High ozone
predominates from northern Canada, across Alaska to Siberia. Figure
5 shows the monthly mean total ozone percent difference for March 2000 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 are the negative anomalies of 10 to 18 percent over the Arctic and northern
European regions. Negative anomalies of this magnitude were previously seen three years
ago in March 1997, but not in the last two years. Positive anomalies appear throughout
sub-tropical latitudes. As mentioned in the previous section, however, total ozone data
from recent NOAA-11 are about 2 percent too high. 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. The temperature anomalies for the winter and spring of 1999-2000 were below
average values for north polar latitudes, and near average values for mid-latitudes and
the equatorial region. The pattern of zonal mean temperature anomalies closely corresponds
to the pattern of 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 linked 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, 1999). 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 1999-2000, daily minimum temperatures were much
below average values. In January and February, temperatures increased markedly at 10 hPa
and above, in association with stratospheric warming events. However the warming affected
the lower stratosphere only weakly, so the lower stratosphere polar vortex remained cold,
strong, and relatively undisturbed. This is discussed below.
Figure 8 compares
the average 100 hPa temperature for each March of the last 22 years with the date the
stratospheric polar vortex diminished below a specific threshold size. The size 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. March 2000 was similar to
1998, about mid-way among the years in persistence of the polar vortex and also for
average temperatures. Most other years in the 1990s had low temperatures, 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 persistence of the Arctic polar
vortex in the winter-spring of 2000 was similar to 1998, about average for all years. For
1999 the vortex size did not meet the size criteria, so it does not appear in Figure 9 and Figure 8.
Meteorological conditions of a weak polar vortex in March 1999 were associated with
limited ozone destruction and relatively high total ozone over the Arctic region. During
1996 and 1997, very low total ozone amounts were observed over northern latitudes due in
part to an unusually cold, persistent vortex.
Figure 10 shows the
average area, during February and March for each year since 1979, of low ozone (lower than
300 DU). For 2000, low total ozone was greater in extent than for the previous 2 years,
but near the average for years since 1990.
A time series, from 1979 to February 2000, of
normalized height anomalies from 1000 to 30 hPa, for the north polar region, is shown in Figure 11. Positive height anomalies were predominant in the
1980s, while negative anomalies appear in most of the 1990s. The winter and spring height
anomalies were positive for the years 1997-98 and 1998-99. But height anomalies for
1999-2000 were negative, consistent with total ozone anomalies for 2000. Note that the
height anomaly pattern in Figure 11 is very similar to the
downward propagating Arctic Oscillation signature shown by Baldwin and Dunkerton (1999).
Negative polar height anomalies are consistent with positive phase of the Arctic
oscillation. Negative height anomaly in 2000 is consistent with the observed positive
phase of the Arctic Oscillation. Investigations are underway concerning causes and effects
of stratospheric variability, as they may be related to tropospheric and surface changes.
For example, during the winter-spring of 1998-99, anomalies appear to propagate to the
surface, whereas the 1999-2000 anomalies did not indicate a continuous downward
propagation.
III. CONCLUDING REMARKS
In the spring of 2000, negative 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 were much below average
values. Temperatures were sufficiently low for ozone destruction to proceed on polar
stratospheric clouds within the polar vortex. The spring conditions in the Arctic region
in 2000 are in contrast to conditions in the Arctic region during 1999, when total ozone
was above the average. Chlorine and other ozone destroying chemicals in the lower
stratosphere reached peak values around 1997-98, and have remained at high levels. As a
consequence, lower stratosphere ozone destruction is strong when meteorological conditions
of a strong polar vortex and cold polar temperatures prevail. Low total ozone values in
the Arctic region in 2000 are attributed to meteorological conditions which were favorable
for ozone destruction, along with the continued presence of ozone destroying chemicals in
the stratosphere.
Total ozone declined over mid-latitudes of the
Northern Hemisphere at the rate of about 2 to 4 percent per decade from 1979 to 1993. In
recent years the strong rate of decline of Northern Hemisphere total ozone has not
continued, but current stratospheric ozone amounts continue to be below the amounts
measured before the early 1980s. 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.
IV. REFERENCES
Baldwin, M.P. and T.J. Dunkerton, 1999: Propagation
of the Arctic Oscillation from the stratosphere to the troposphere. J. Geophys. Res., 104,
30937-30946.
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, 125pp.
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.
WMO, 1999: Scientific assessment of ozone
depletion: 1998. World Meteorological Organization Global Ozone Research and Monitoring
Project - Report No. 44.
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