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 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: melvyn.gelman@noaa.gov

Ozone measurements for the Northern Hemisphere middle and high latitudes during the winter/spring of 1997-1998 showed positive zonal mean total ozone anomalies of up to 8 percent. However, total column ozone values for February and March 1998, over the Arctic, from Greenland, to northern Europe and northern Siberia, were about 10 percent lower than values observed during these months in the early 1980s. Lower stratosphere temperatures over the north polar region in November and December 1997 reached record low values. Temperatures observed were sufficiently low within the polar vortex, in the regions of low ozone, for chemical destruction of ozone on polar stratospheric cloud particles. Total ozone has decreased since 1979 over the mid-latitudes of the Northern Hemisphere at the rate of 2 to 4 percent per decade.

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 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 period.

Figure 1 shows monthly average anomalies (percent) of zonal mean total ozone, as a function of latitude and time, from January 1979 to March 1998. The anomalies are derived relative to each month's 1979-98 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 most recent years. However, for the winter-spring of 1997-98, positive zonal mean total ozone anomalies are seen for the extra-tropical regions of the Northern Hemisphere, with positive anomalies of up to 8 percent shown for the middle and high latitudes. In the tropical region, a low anomaly is seen in 1998, as part of the quasi-biennial oscillation of total ozone.

The large negative anomalies seen in Figure 1 for the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et al., 1993 and Solomon et al., 1996) were related to the Mt. Pinatubo eruption in mid-1991. 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. For the latitude range of the coterminous United States, the trend, based on the SBUV and SBUV/2 data sets and updated from 1979 through March 1998, 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 resulting trend is about -3.4 percent per decade, with a 95 percent confidence estimate of 1.4 percent. In addition, Figure 3 shows the monthly anomalies observed at 30-50 degrees North latitude by the SBUV and SBUV-2 satellite instruments, previously shown in Figure 1. There is good agreement in the anomalies measured independently by the satellite and ground-based measurements, although the recent NOAA-14 satellite measurements indicate slightly higher values than the CMDL measurements. Both systems also show a flattening of the trend in the anomalies in the most recent years. The implication of these trends and their changes needs to be examined in the context of changes in the ozone depleting gases.

A map of Northern Hemisphere monthly mean total ozone amounts for March 1998 is shown in Figure 4. Lowest monthly mean values over the north polar region (green, less than 380 Dobson Units, DU), extend from Greenland, to northern Europe and northern Siberia. The March 1998 monthly mean map also shows highest ozone (red colors) located over middle to high northern latitudes, the location typical for this season. Figure 5 shows the monthly mean total ozone percent difference of March 1998 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. Negative anomalies of about 10 percent (yellow ) are shown over the Arctic, from Greenland to northern Europe and northern Siberia. The largest positive anomalies are shown over eastern Siberia and the eastern European-Mediterranean region. Indeed, positive anomalies of total ozone are shown over almost the entire extra-tropical region of the Northern Hemisphere. 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, as seen from coincident data in the months immediately preceding July 1997. That bias must be taken into account when interpreting results using the preliminary NOAA-14 data.

Temperatures in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Extremely low temperatures (lower than -78^oC) 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 hPa (approximately 19 km) are shown in Figure 6. During November and December 1997, the daily minimum temperatures were near record low values. However, in January, temperatures increased markedly, in association with a stratospheric warming event. Low temperatures returned only for a short time at the end of January and mid-February.

The Arctic polar vortex in the winter of 1997-98 was warmer and less stable than in the previous year. 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). Coy et al. (1998) showed that reduced poleward heat transport by planetary-scale waves during late winter and early spring was likely responsible for the cold vortex in 1996-97. Figure 7 compares values of the poleward eddy flux at the 50 hPa level, between 45-75 N latitude, during the January-March period, for the current year (1998) and for 1997. The 1998 eddy heat flux averaged over this three-month period is approximately 50 percent higher than in 1997. Similar results were also obtained for the 30 and 10 hPa levels. The dominant contribution to the eddy heat flux through the 50-150 hPa region comes from stationary planetary-scale waves that are forced in the troposphere by topography and land/sea temperature differences. During the first three months of 1998, stationary wave patterns in the lower stratospheric circulation show enhanced (relative to the long-term mean) wave number 1 amplitudes and diminished wave number 2 amplitudes throughout the northern extra-tropics. During the same period, the tropospheric circulation was under the influence of unusually warm sea surface temperatures in the eastern Pacific, commonly referred to as El Niño. Observational studies have shown that the year-to-year variability in the wintertime stratospheric circulation is correlated with the quasi-biennial oscillation (QBO) in the lower stratospheric equatorial wind, with the tropospheric circulation pattern known as the North Atlantic Oscillation (NAO), and with the Pacific/North American (PNA) teleconnection pattern (e.g. Baldwin et al., 1994; Perlwitz and Graf, 1995). The latter is related to the strengthening of the Aleutian low in response to El Niño conditions. The enhanced wave number 1 amplitude observed in the lower stratospheric circulation this winter is consistent with the deeper Aleutian low caused by the 1997-98 El Niño event. Therefore, the winter stratospheric circulation does exhibit a response to prevailing El Niño conditions, although these conditions are not necessarily the only cause of the warmer, more disturbed polar vortex observed this winter. Factors such as the QBO and NAO also need to be considered.

Figure 8 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 1997-98 were near average values for north polar latitudes, somewhat below the long term average for mid-latitudes, but near record low values in the equatorial region.

In the winter-spring of 1997-98, positive anomalies of total ozone were observed in the middle and high latitudes of the Northern Hemisphere. However, ozone depletion of about 10 percent in total ozone was observed in February and March 1998 over limited areas of the Arctic region. Lower stratosphere temperatures over these areas of the Arctic region during November and December 1997 were near record low values. Temperatures were sufficiently low for ozone destruction to proceed on polar stratospheric clouds within the polar vortex.

Total ozone declined over mid-latitudes at the rate of about 2 to 4 percent per decade since 1979.

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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