Contributors:

• Angell, J.K. OAR/Air Resources Laboratory
• Flynn, L. NESDIS/Climate Research and Applications Division
• Gelman, M.E. NWS/Climate Prediction Center
• Hofmann, D. OAR/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. OAR/Climate Monitoring and Diagnostic Lab.
• Wang, J. OAR/Air Resources Laboratory
• 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 the Northern Hemisphere winter and spring of 2000-2001, total ozone values observed over the Arctic region were substantially higher than average. Anomalously low total ozone values were observed over the Arctic region only intermittently during January and the first part of February. Total ozone values in February were more than 20 percent higher over portions of the Arctic region than comparable values during the early 1980s, and December, January, and March values over some Arctic areas averaged 10 to 15 percent higher. Temperatures observed in the lower stratosphere over the Arctic region were above their long term average for most of the winter and spring of 2000-2001. Only during January and the beginning of February, were lower stratosphere temperatures colder than average, reaching below minus 78 C, allowing formation of polar stratospheric clouds which promote the chemical destruction of ozone. Temperatures in February and March rose dramatically, after significant stratospheric warming throughout the stratosphere and associated circulation effects. These conditions were also associated with the prevalence of high ozone in the polar region during the winter and the absence of the very low total ozone values after mid February. At middle latitudes of the Northern Hemisphere, total ozone values were predominately lower than average. Total ozone 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. Much of the recent year-to-year differences in north polar winter-spring stratospheric ozone destruction may be explained as being due to the varying 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

We used the total column ozone data from the NASA Nimbus-7 SBUV instrument from November 1978 through February 1985; NOAA-9 SBUV/2 from March 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 1995; the NOAA-14 SBUV/2 from January 1996 to June 1998, and the NOAA-11 SBUV/2 since July 1998. 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. During winter months, varying amounts of data at sub-polar latitudes have been available, depending on the local time of viewing of each satellite. NOAA-11 total ozone data since July 1998 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 2001. The percent anomalies are derived relative to each month's 1979-2001 average. SBUV/2 data (in this figure only) have been adjusted for long term consistency (Miller et al., 2001, in review). Largest anomalies are shown in winter and spring months for the polar region of the Southern Hemisphere. In the north polar region, positive anomalies prevailed in 1979 and the early 1980s, but mostly negative anomalies predominated in the 1990s. However, during the winter and spring of 2000-2001, positive zonal mean total ozone anomalies prevailed over the north polar region, as was also the case in1997-98 and 1998-99. In middle latitudes, near average or slightly negative anomalies prevailed in 2000-2001. 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 after 1993 from the 1991 Mt. Pinatubo volcanic eruption, the downward trend of total ozone has not continued. In the tropics, small negative anomaly is seen in 2001, 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 2000. 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. These anomalies, derived from ground-based measurements, are consistent with the anomalies from SBUV/2 satellite ozone measurements, shown in Figure 1 . Middle latitude total ozone values in the years since 1993 have not continued to decline as they had declined from 1979 to 1993. However ozone values have also not recovered to their higher 1980 values. 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 2001. High ozone extends over the entire Arctic region. Figure 5a shows the monthly mean total ozone percent difference of March 2001 from the mean for eight March monthly means, 1979-1986 (Nagatani et al., 1988). The 1979 to 1986 base period is chosen because 1979-1986 average values are indicative of the early data record. Most notable are the positive anomalies of more than 12 percent over the Arctic region. Negative anomalies appear throughout middle latitudes. Even larger positive anomalies are shown over the Arctic region in the maps for December 2000 (Figure 5b ), January 2001 (Figure 5c ) and February 2001 (Figure 5d ).

Figure 6 shows monthly mean temperature anomalies at 50 hPa for three latitude regions, 90N-65N, 65N-25N, and 25S-25N. The temperature anomalies for the winter and spring of 2000-2001 were above average values for north polar latitudes, and negative 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 shown in Figure 1

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 much of December 2000 and January 2001, daily minimum temperatures were lower than -78 C. At the end of January, temperatures increased markedly at 50 hPa, in association with a large stratospheric warming. Without the extremely low temperatures, enhanced ozone destruction would not occur.

Figure 8 compares the average 100 hPa temperature in the polar region for each March of the last 23 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 2001 was among the years with the shortest duration of the polar vortex and with the highest 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. In the winter-spring of 2001 there was low persistence of both the Arctic polar vortex and of the region of anomalously low ozone. Meteorological conditions in the winter of 2000-2001 were directly related to the 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 2001, the average area of low total ozone was smaller than for the previous year, and among the smallest values of all the years.

A time series, from 1979 to March 2001, 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 for 2000-2001 were strongly positive, consistent with total ozone anomalies. 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). Positive height anomaly in 2000-2001 is consistent with the observed negative 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 2000-2001, anomalies appear to propagate to the surface, whereas the1999-2000 anomalies did not indicate a continuous downward propagation.

III. CONCLUDING REMARKS

In the winter and spring of 2000-2001, positive anomalies of total ozone were prevalent in the high latitudes of the Northern Hemisphere. Lower stratosphere temperatures over the Arctic region were above average values. Only for only short periods were Arctic temperatures sufficiently low for the formation of polar stratospheric clouds and consequent chemical ozone depletion within the polar vortex. The conditions in the Arctic region in 2000-2001 are in contrast to conditions during 1999- 2000, when total ozone in the Arctic region was below 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. High total ozone values in the Arctic region in the winter and spring of 2000-2001 are attributed to meteorological conditions which were not favorable for ozone destruction, even 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.

Miller, A.J., et al., A cohesive total ozone data set from SBUV/(2) satellite system, to be submitted to J. Geophys. Res., 2001.

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.