CPC - Stratosphere: Winter Bulletins

Concerns of possible global ozone depletion (e.g. WMO/UNEP, 1992) have led to major international programs to monitor and explain the observed ozone variations in the stratosphere. In response to these, as well as other long-term climate concerns, NOAA has established routine monitoring programs utilizing 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 Analysis 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

The very low total column ozone values observed over Antarctica during September and October 1994 were similar to the record-setting low values observed in 1992 and 1993. Extremely low values of total ozone (near 100 DU, Dobson Units) were observed in 1994. Ozone depletion of 20 percent to more than 50 percent was observed from 1979 levels over very large areas of the south polar region. Vertical soundings over the South Pole showed nearly complete destruction of ozone at altitudes between 15 and 20 km. The rate of stratospheric ozone depletion over the South Pole in the 15-20 km region was similar to the rates of destruction in 1992 and 1993, years influenced by aerosols from the Pinatubo eruption, even though aerosol particles have declined markedly in 1994 to near normal, background levels. However, in the 10-14 km region, where heterogeneous reactions on volcanic sulfate aerosol particles dramatically reduced ozone in the springs of 1992 and 1993, there was significant recovery in 1994, reflecting this diminished aerosol loading. Lower stratosphere temperatures in the winter and spring of 1994 for the Antarctic region were below the long-term average, near -80 C, and sufficiently low for ozone destruction on polar stratospheric clouds within the polar vortex to proceed during September and October. In addition, we have examined long-term global total ozone changes since 1979 and have demonstrated that the decline over mid-latitudes has been about 4 percent per decade, with little or no long-term trend observed for the equatorial region.

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, temperature and aerosols.

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

SATELLITE OBSERVATIONS
Parameter	Method	Reference
Total Ozone and	NOAA/SBUV/2	Mateer et al., 1971
Ozone Profiles		Miller, 1989
		Planet et al., 1994
Temperature Profiles	NOAA/TOVS	Gelman et al., 1986
Aerosols	NOAA/AVHRR	Stowe et al., 1992
		Long & Stowe, 1993

One particular element of data availability concerns that from the SBUV/2 instruments on the NOAA operational satellites. The first SBUV/2 was launched on NOAA-9 in 1985, and the second on NOAA-11 in 1989. Unfortunately, the NOAA polar orbiting satellites precess in their orbits such that the equatorial crossing times trend to later in the day. For the SBUV/2 instruments this results in higher and higher solar zenith angles till, eventually, the angles exceed the diffuser's calibrated range. In 1994, the ozone data base was impacted in several ways. First, the NOAA-11 instrument, which has been recently operating at relatively high solar zenith angles, developed a failure in the diffuser mechanism causing operations to be interrupted from mid-October to mid-December. Fortunately, the NOAA-9 instrument has migrated back into the preferred solar zenith angle range, and colleagues at NASA GSFC and STX Corp. have developed an updated calibration. After extensive comparisons with available ground-based observations and with NOAA-11 (Crosby, personal communication), it was determined that the NOAA-9 data are, on average, within a few percent of NOAA-11. Consequently, for analysis in the Antarctic Spring, we utilize the data from the NOAA-9 SBUV/2. For evaluation of the long-term trends, we have not yet included the NOAA-9 data, and await a more complete final calibration.

Monthly mean NOAA-9 SBUV/2 total ozone amounts for October 1994 is shown in Figure 1 . A region of high ozone (yellow and red colors) is seen equatorward of the Antarctic region. Low total ozone values (green, 220 to 300 DU), are normally seen in the tropics in all months. Antarctic "ozone hole" values, below 220 DU (blue and purple), first began to appear in the 1980's (Farman et al., 1985). During 1992 and 1993, extremely low values of total ozone, near 100 Dobson Units (DU), were seen in October over very large areas of the south polar region. In October 1994, in general, the area covered by extremely low ozone values was somewhat smaller than the record setting year of 1993, but for a few days (not shown) the area covered by the ozone hole was larger than in 1993.Figure 2 shows the percent difference between the SBUV/2 monthly mean total ozone map for October 1994 minus the SBUV monthly mean for October 1979. Decreases since 1979 in total ozone of more than 50 percent (100 DU) are shown by the purple colors, with decreases of greater than 20 percent (green and blue) shown over a very large area of Antarctica. Small percent increases are shown over some areas of the tropics and mid-latitudes, but these increases are not representative of general, long-term changes of ozone.

The time series in Figure 3 shows total column ozone at the South Pole integrated from balloon-borne ozonesonde observations during the July to December period of 1992, 1993 and 1994. On 5 October 1994 (day 278), total ozone fell to its lowest 1994 measured value of 102 DU (with an uncertainty of 5 DU). The 1994 values at the south pole did not show the long period of extremely low values of 1993, nor reach the record extreme value of 91 DU observed on 12 October 1993. Recovery from low values began earlier in 1994, and the extremely low total ozone values observed over the south pole in 1993 and in 1992, were not sustained as long in 1994. As suggested below, the record of extremely low (but not record setting) total ozone values in 1994 may indicate some moderation in the ozone destruction in the 10-14 km region, due to diminishing stratospheric aerosols.

Selected ozone profiles measured at the South Pole are shown in Figure 4 . The profile before significant depletion at the South Pole was present (2 September 1994) is compared with profiles at the time of minimum total column ozone amount, on 5 October 1994 and also 8 October. The October profiles show nearly complete destruction of ozone between 15 and 20 km. A significant feature of stratospheric ozone destruction over the South Pole in 1994 was the rate of decline during September of ozone in the 15-20 km region. The rate was equal to that seen in the aerosol perturbed years of 1992 and 1993, even though aerosol particles declined markedly in 1994. The increasing amounts of human-produced chlorine in the atmosphere may be accelerating the long-term rate of ozone decline. In the 10-14 km region, there was significant ozone recovery in 1994, reflecting the diminished aerosol loading.

Temperatures in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Extremely low temperatures (lower than -78 C) at about 50 mb over the Antarctic region are believed to lead to depletion of ozone in that low temperatures contribute to the presence of polar stratospheric clouds (PSCs), and in particular nitric acid trihydrate (NAT), which is thought to be the dominant component of PSCs. PSCs enhance the production and lifetime of reactive chlorine, leading to ozone depletion (WMO/UNEP, 1992).

Daily minimum temperatures over the polar region, 65S to 90S at 50 mb (approximately 19 km) is shown in Figure 5 . We see that for most of the winter and spring of 1994, the minimum temperatures were sufficiently low (lower than -78 C) for polar stratospheric clouds to form and allow enhanced ozone depletion.

Temperature anomalies for the 100-50 mb layer derived from radiosonde data (Angell, 1988) are shown in Figure 6 , and in Figure 7 at 50 mb for three latitude regions, 65S-90S, 25S-65S, and 25N-25S (Gelman et al., 1986). For the Southern Hemisphere as a whole and specifically for mid latitudes and equatorial regions, temperature anomalies for 1994 were near the 1993 record low values.

Aerosol concentration is another potentially important component of stratospheric variation, and, as indicated above, has been suggested as a possible source of ozone depletion (e.g. Hofmann et al., 1992). Aerosol optical thickness from the NOAA/AVHRR instrument ( Figure 8) shows that stratospheric aerosol concentrations continued to diminish from the maximum values observed a few weeks after the eruption of Mount Pinatubo in June 1991. Stratospheric aerosols were at such low levels in 1994, that it was difficult to discern stratospheric aerosols from variations in tropospheric values. AVHRR, however, may not measure all the sulfuric acid aerosol that is present if the size of that aerosol is too small (Rao et al., 1988). The NOAA 11 AVHRR instrument failed in September 1994.

Extending our evaluation of long-term total ozone changes to global scales, we present in Figure 9 monthly average anomaly values (Dobson Units) of zonal mean ozone, as a function of latitude and time. The data base is that of the SBUV on the NASA Nimbus -7 from 1979 to mid-1990 and the NOAA-11 SBUV/2 from January 1989 to September 1994. The anomalies are derived from each month's long-term average. From Figure 9 it is obvious in the extra-tropics and polar regions that ozone is substantially lower in recent years than in earlier years. Stolarski et al. (1992), and more recently Hollandsworth et al. (1994) and Miller et al. (1994) have indicated that the trends in the mid-latitudes are statistically significant and are about -2 to -4 % per decade and that little or no significant trend exists over the equatorial region. One other particular feature is the very large negative anomaly in the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et al., 1993) which is believed to be related to the Mt. Pinatubo eruption in mid-1991. This feature has, in fact, disappeared in 1994 along with the diminishing aerosol loading.

Stratospheric ozone values over Antarctica during September and October 1994 were extremely low, with the minimum values very near the extreme values of around 100 DU observed in 1992 and 1993. In the 15-20 km region, the decline in ozone during September 1994 was similar to that seen in the aerosol perturbed years of 1992 and 1993, even though aerosol particles have diminished markedly in this region since the eruption of Mt Pinatubo in 1991. In the 10-14 km region, where heterogeneous reactions on the volcanic sulfate aerosol particles dramatically reduced ozone in the springs of 1992 and 1993, there was a significant recovery in 1994, reflecting the diminished aerosol loading. We note that the lower stratosphere temperatures over the Antarctic region in September and October 1994 were below the long-term average, and sufficiently low (lower than -78 C) for polar stratospheric clouds to form over a large region. A full explanation of ozone and temperature anomalies must include all aspects of ozone photochemistry and meteorological dynamics.

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