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/products/stratosphere/winter_bulletins

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
E-mail: mel.gelman@noaa.gov

For the Northern Hemisphere winter of 2001-2002, total ozone values observed over the Arctic region were again, as last year, substantially higher than average. Anomalously low total ozone values were observed over the Arctic region only intermittently during December. Total ozone values over portions of the Arctic region in December, January and February averaged from 15 to 25 percent higher than comparable values during the early 1980s. Temperatures observed in the lower stratosphere over the Arctic region were also above their long term average for most of the winter of 2001-2002. Only during a few days in December did lower stratosphere temperatures reach below minus 78 C, allowing the possibility for formation of polar stratospheric clouds which promote the chemical destruction of ozone. Lower stratosphere temperatures in December and January rose dramatically, with significant 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. 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. High total ozone values in the Arctic region in the winter of 2001-2002 are attributed to meteorological conditions which were not favorable for ozone destruction, even with the continued presence of ozone destroying chemicals in the stratosphere.

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 have used total column ozone data from the NASA Nimbus-7 SBUV instrument from 1979 through February 1985; NOAA-9 SBUV/2 from March 1985 to December 1988; NOAA-11 SBUV/2 from January 1989 to December 1993; NOAA-9 SBUV/2 from January 1994 to December 1995; NOAA-14 SBUV/2 from January 1996 to June 1998; NOAA-11 SBUV/2 from July 1998 to September 2000; and NOAA-16 SBUV/2 from October 2000. Solar Backscatter Ultra-Violet (SBUV) instruments can produce data only for daylight-viewing conditions, so SBUV/2 data are not available at polar latitudes during winter darkness. Increasing loss of NOAA-11 data at sub-polar latitudes from 1989 to1993 was caused by satellite precession, resulting in SBUV/2 viewing high latitudes only in darkness. Recent NOAA-16 total ozone data have not yet been fully validated. This impacts trends determined for the recent period.

Figure 1 shows monthly average anomalies of zonal mean total ozone, as a function of latitude and time, from January 1979 to March 2002. The percent anomalies are derived relative to each month's 1979-2002 average. SBUV/2 data (in this figure only) have been adjusted for long term consistency (Miller et al., 2002, in press). The largest anomalies occur 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 of 2001-2002, positive zonal mean total ozone anomalies prevailed over the north polar region, as was also the case in1997-98, 1998-99, and 2000-2001. In middle latitudes, negative anomalies prevailed in 2001-2002. 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, a small positive anomaly is seen in 2001-2002, consistent with 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 mid-latitude U.S. stations from1979 through 2001. 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 2002. High ozone extends over northern latitudes, with a relative low area extending over the Arctic region. Figure 5a shows the monthly mean total ozone percent difference of March 2002 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. For March 2002, regions of small positive as well as some negative anomalies are evident over the Arctic region. Negative anomalies appear throughout middle latitudes. However large positive anomalies of more than 15 to 25 percent are shown over the Arctic region in the maps for December 2001 (Figure 5b ), January 2002 (Figure 5c ) and February 2002 (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 north polar latitudes for December 2001 and January and February 2002 were above average values and slightly below average for March 2002. Temperature anomalies were strongly negative for mid-latitudes and also negative over the equatorial region. The pattern of zonal mean temperature anomalies in the winter of 2001-2002 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. Very 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 November, December and January, daily minimum temperatures only for a very few days were lower than -78 C. Temperatures increased markedly in association with stratospheric warmings during each of the months. Without extremely low temperatures, enhanced ozone destruction does not occur.

Figure 8 compares the average 100 hPa temperature in the polar region for each March of the last 24 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 2002 was among the years with the higher average temperatures and shorter duration of the polar vortex. Most 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 of 2001-2002 there was low persistence of both the Arctic polar vortex and no defined region of anomalously low ozone. Meteorological conditions in the winter of 2001-2002 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 2002, there was no area of anomalously low total ozone, comparable to north polar winter conditions in 4 out of the last 5 years.

A time series, from 1979 to March 2002, of normalized height anomalies from 1000 to 30 hPa, for the north polar region 65-90N, is shown in Figure 11 . Positive height anomalies were predominant in the 1980s, while negative anomalies appear in most of the 1990s. The winter height anomalies for 2001-2002 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 anomalies in 2001-2002, as well as 2000-2001 are 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 (Zhou et al., 2002). For example, during the winter of 2001-2002 and 2000-2001, anomalies appear to propagate to the surface, whereas the 1999-2000 anomalies did not indicate a continuous downward propagation.

In the winter of 2001-2002, positive anomalies of total ozone were prevalent in the high latitudes of the Northern Hemisphere. Lower stratosphere temperatures over the Arctic region were also predominantly above average values. Only for 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 2001-2002 and 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 of 2001-2002 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.

Baldwin, M.P., X. Cheng, and T.J. Dunkerton, 1994: Observed correlations between winter-mean tropospheric and stratospheric circulation anomalies. Geophys Res. Lett., 21, 1141-1144.

Miller, A.J., R.M. Nagatani, L.E. Flynn, S. Kondragunta, E. Beach, R. Stolarsky, R. McPeters, P.K. Bhartia, M. Deland, C.H. Jackman, D.J.Wuebbles, K.O.Putten, and R.P. Cebula., 2002, A cohesive total ozone data set from SBUV/(2) satellite system, press, J. Geophys. Res., 107(0), doi:10.1029/200,D000853.

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.

Zhou, S., A.J. Miller, J. Wang, and J.K. Angell, 2002: Downward-propagating temperature anomalies in the preconditioned polar stratosphere. J. Climate, 15, 781-792.