CPC - Stratosphere: Winter Bulletins

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_bulletins

Further information may be obtained from Alvin J. Miller
NOAA, Climate Prediction Center
5200 Auth Road
Camp Springs, MD 20746-4304
Telephone: (301) 763-8000 ext.7552
Fax: (301) 763-8125
E-mail: alvin.miller@noaa.gov

Ozone measurements during the Northern Hemisphere winter of 1996-1997 indicate that total column ozone values were substantially lower than values observed during these months in 1979 and the early 1980's. Over the north polar region, total ozone for March 1997 was lower by up to 40 percent than during the earlier period. Total ozone has decreased since 1979 over the mid-latitudes of the Northern Hemisphere at the rate of about 4 percent per decade. Little or no significant long-term trend is observed for the equatorial region. Lower stratosphere temperatures over the north polar region in March 1997 reached record low values. Temperatures observed were sufficiently low within the polar vortex for chemical destruction of ozone on polar stratospheric cloud particles.

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.

GROUND-BASED OBSERVATIONS
Parameter	Method	Reference
Total Ozone	Dobson	Komhyr et al., 1986
		CMDL, 1990
Ozone Profiles	Balloons	Komhyr et al., 1989 CMDL, 1990

SATELLITE OBSERVATIONS
Parameter	Method	Reference
Total Ozone	NOAA/SBUV/2	Planet et al., 1994
	Nimbus-7 SBUV	Mateer et al., 1971
Ozone Profiles		Miller, 1989
		Planet et al., 1994
		Mateer et al., 1971
Temperature Profiles	NOAA/TOVS	Gelman et al., 1986

Total column ozone data from the NASA Nimbus -7 SBUV instrument were used 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 provide data for daylight-viewing conditions, so no SBUV/2 data are available at polar latitudes during winter darkness. In addition, increasing data 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 at those high latitudes.

Northern hemisphere monthly mean total ozone amounts for March 1997 are shown in Figure 1. Lowest monthly mean values over the north polar region (blue, less than 300 Dobson Units, DU), extend from Greenland, to northern Europe and northern Siberia. While total ozone values of less than 250 DU are typical for tropical values, those low monthly mean values are unusual for the Arctic region. Indeed, during March 1997, extremely low daily values of total ozone (lower than 230 DU) were reported. The March 1997 monthly mean map also shows a region of high ozone (yellow and orange colors) located over middle to high northern latitudes, typical for this region and season. Figure 2 shows the monthly mean total ozone percent difference of March 1997 and the mean for eight March monthly means, 1979-1986. The 1979 to 1986 base period is chosen because these values are indicative of the early data record. Decreases of 25 to 40 percent (blue) cover a very large area over the Arctic, from Greenland, over the Baltic Sea, to northern Europe and northern Siberia. Also over the United States and northwest Europe, March 1997 values were lower than for the base period by 10 to 20 percent. Indeed, decreases of total ozone are shown over almost the entire extra-tropical region of the Northern Hemisphere. Small percent increases are shown in Figure 2over only very limited areas in extra- tropical latitudes.

Dobson spectrophotometer readings are made at Point Barrow, Alaska (71.3N), on the Alaskan north slope, the closest U.S. territory to the polar low ozone region. Figure 3 shows daily total ozone values for March 1997. The March average was 388 DU, about 6% below the previous ten year average of 413 DU. On March 17 (307 DU) and March 18 (316 DU), Barrow was on the edge of the polar low ozone region and record low ozone values for that location were observed for a March day. The previous daily low had been 326 DU on March 22, 1993. However, lower March averages were recorded in both 1990 (369 DU) and 1993 (355 DU). Owing to the normal springtime high pressure system which builds up in the stratosphere over the Aleutians and moves the cold polar low towards Europe and Asia, the Alaskan Arctic is generally spared the very low springtime ozone values which have been observed over northern Europe and Siberia during the past several years. This high pressure system appears to have been weaker than average in the years that Barrow experienced low March ozone, for example 1997.

Temperatures in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Extremely low temperatures (lower than -78 C) over the Arctic region in the lower stratosphere are believed to lead to depletion of ozone. 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 mb (approximately 19 km) are shown in Figure 4. We see that during December and January of 1996-97, the daily minimum temperatures were above or near the long-term average minimum temperatures. However, in February, temperatures decreased markedly, and reached sufficiently low values (lower than -78 C) for polar stratospheric clouds (PSCs) to form and allow enhanced ozone depletion. Indeed, during February and March 1997, minimum 50 mb temperatures were at record low values. The polar vortex during the winter of 1996-97 was very stable, and persisted through the end of March, with no substantial stratospheric warming in the lower and middle stratosphere. While PSCs are commonly observed during winter, PSCs are not usual in late March in the northern hemisphere. The unusually persistent vortex of 1997 allowed PSCs to form in March.

Figure 5 shows monthly mean temperature anomalies at 50 mb for three latitude regions, 90N-65N, 65N-25N, and 25N-25S (updated from Gelman et al., 1986). The temperature anomaly for March 1997 was a record low value for north polar latitudes, and also below the long term average for mid-latitudes, while the equatorial region anomaly was slightly positive in March for the first time since 1993. Temperature anomalies for winter seasons (December, January, February) since 1958, for the 100-50 mb layer derived from radiosonde data (updated from Angell, 1988), are shown in Figure 6. Temperature anomalies for the Northern Hemisphere as a whole were similar to the record low values of 1995-96 and 1994-95. Temperatures for the north polar region were also below the 30-year average.

Anomalies of zonal mean total column ozone are shown in Figure 7, as a function of latitude and time, from January 1979 to March 1997. The monthly mean anomalies (percent difference) are derived relative to each month's 1979-1997 average for each latitude. In the tropical region, a weak high anomaly is seen in 1996-97 (green area), as part of the quasi-biennial oscillation of total ozone. For the winter of 1996-97, negative total ozone anomalies are seen for the extra-tropics of the Northern Hemisphere. Again in 1997, as in recent years, zonal mean total ozone at high-latitudes has been about 10 percent lower than the long-term average, and more than 20 percent lower than in the earlier years. At mid-latitudes, the anomaly was negative during 1996-97, in contrast to the positive anomalies in 1995-96. The large negative anomalies in the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et al., 1993 and Solomon et al., 1996) have been related to the Mt. Pinatubo eruption in mid-1991. Stolarski et al. (1992), Hollandsworth et al. (1994) and Miller et al. (1994) have indicated that middle latitude Northern Hemisphere total ozone trends of about -2 to -4 % per decade are statistically significant, and that little or no significant trend exists over the equatorial region. For the region 30N-50N (the basic latitude range of the coterminous United States), the trend, based on the SBUV and SBUV/2 data sets, and updated from 1979 through March 1997 is about -4 percent per decade, with a 95 percent confidence estimate of about 2 percent.

The NOAA Climate Monitoring and Diagnostics Laboratory operates a 16-station global Dobson spectrophotometer network for total ozone trend studies. Figure 8 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. The average value for 1979 has been subtracted from each individual monthly value for 1979-1996. These are shown in Figure 9 as a four-station average percent deviation, and the resulting trend is -3.9 percent per decade.

Observed total ozone values continued to be very low over high latitude regions of the Northern Hemisphere during the winter of 1996-97. Lower stratosphere temperatures over the north polar region also reached record low values. A full explanation of ozone and temperature anomalies must include all aspects of ozone photochemistry and meteorological dynamics.

Angell, J. K., 1988: Variations and trends in tropospheric and stratospheric global temperatures. J. Climate, 12, 1296-1313.

Climate Monitoring and Diagnostic Laboratory (CMDL), 1990: Summary Report 1989. 141pp. Available from National Technical Information Service, 5285 Port Royal Rd., Springfield, Va. 22161.

Gelman, M.E., A.J. Miller, K.W. Johnson and R.M. Nagatani, 1986: Detection of long-term trends in global stratospheric temperature from NMC analyses derived from NOAA satellite data. Adv. Space Res., 6, 17-26.

Gleason, J., P.K. Bhartia, J.R. Herman, R. McPeters, P. Newman, R.S Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C. Wellemeyer, W.D. Komhyr, A. J. Miller, and W. Planet, 1993: Record low global ozone in 1992. Science, 260, 523-526.

Hofmann, D.J., S.J. Oltmans, J.M. Harris, S. Solomon, T. Deshler, and B.J. Johnson, 1992: Observation and possible causes of new ozone depletion in Antarctica in 1991. Nature, 359, 283.

Hollandsworth S.M., R.D. McPeters, L. Flynn, W.G. Planet, A.J. Miller, and S. Chandra, 1994: Ozone trends deduced from combined Nimbus 7 SBUV and NOAA-11 SBUV/2 data. Geophys. Res. Lett., 22, 905-908.

Komhyr, W.D., R.D. Gross and R.K. Leonard, 1986: Total ozone decrease at South Pole, Antarctica, 1964-1985. Geophys. Res. Lett., 13, 1248-1251.

Komyhr, W.D., R.D. Grass, P.J. Reitelbach, S.E. Kuester, P.R. Franchois and M.L. Fanning, 1989: Total ozone, vertical distributions, and stratospheric temperatures at South Pole, Antarctica, in 1986 and 1987. J. Geophys. Res., 94, 11429-11436.

Mateer, C.Y., D. F. Heath and A. J. Krueger, 1971: Estimation of total ozone from satellite measurements of backscatter ultraviolet earth radiance. J. Atmos. Sci., 28, 1307-1311.

Miller, A. J., 1989: A review of satellite observations of atmospheric ozone. Planet. Space Science, 37,1539-1554.

Miller, A.J., G.C. Tiao, G.C. Reinsel, D. Wuebbles, L.Bishop, J. Kerr, R.M. Nagatani, J.J. DeLuisi, C.L. Mateer, M.E. Gelman, S.Oltmans, W.G. Planet, and R. McPeters, 1994: Consideration of the 1994 Antarctic ozone hole and an update of stratospheric ozone trends, Proceedings of the Nineteenth Annual Climate Diagnostic Workshop, U.S. Department of Commerce NOAA, 13-17.

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.

Planet, W. G., J. H. Lienesch, A. J. Miller, R. Nagatani, R, D. McPeters, E. Hilsenrath, R. P. Cebula, M. T. DeLand, C. G. Wellemeyer, and K. M. Horvath, 1994: Northern hemisphere total ozone values from 1989-1993 determined with the NOAA-11 Solar Backscatter Ultraviolet (SBUV/2) instrument. Geophys. Res. Lett. 21, 205-208.

Solomon, S., R.W. Portmann, R.R. Garcia, L.W. Thomason, L.R. Poole, and M.P. McCormick, 1996: The role of aerosol variations in anthropogenic ozone depletion at northern midlatitudes, J. Geophys. Res., 6713-6727.

Stolarski, R., R. Bojkov, L. Bishop, C. Zerefos, J. Staehelin and J Zawodny, 1992: Measured trends in stratospheric ozone, Science, 256, 342-349.