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, 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 Prediction Center
5200 Auth Road
Washington D. C. 20233
Telephone: (301) 763-8000, ext. 7552
Fax: (301) 763-8125
E-mail: alvin.miller@noaa.gov

Very low total column ozone values (near 100 DU, Dobson Units) were observed over Antarctica during September and October 1995. Ozone depletion of 20 percent to more than 50 percent was observed over Antarctica in 1995, compared to the ozone amounts previous to the early 1980's. Vertical soundings over the South Pole during September and October showed nearly complete destruction of ozone at altitudes between 15 and 20 km. Lower stratosphere temperatures in the winter and spring of 1995 over the Antarctic region were below the long-term average, and sufficiently low for ozone destruction to proceed on polar stratospheric clouds within the polar vortex. Over mid-latitudes, total ozone has declined by about 4 percent per decade since 1979.

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., 1989
		CMDL, 1990

SATELLITE OBSERVATIONS
Parameter	Method	Reference
Total Ozone	NOAA/SBUV/2	Planet et al. 1994
	Nimbus-7 SBUV	Mateer et al., 1971
Temperature Profiles	NOAA/TOVS	Gelman et al., 1986
Aerosols	NOAA/AVHRR	Long & Stowe, 1993

We have used the total column ozone data from the NASA Nimbus-7 SBUV instrument from 1979 through 1988, and the NOAA-11 SBUV/2 from January 1989 to August 1994. The orbital characteristics of the afternoon NOAA polar orbiting satellites are such that the equatorial crossing times, over several years, progress to later in the day. For the SBUV/2 instruments, extremely high solar zenith angles eventually exceed the diffuser's calibrated range. This happened to the NOAA-11 SBUV/2 in late 1994. Fortunately, the NOAA-9 satellite had migrated back into the preferred solar zenith angle range. Recent comparisons with available ground-based observations show that the SBUV/2 NOAA-9 data are, on average, within two to four percent of Dobson total ozone data. Consequently, we use the data from the NOAA-9 SBUV/2 beginning in September 1994.

Figure 1 shows monthly average anomaly values (percent) of zonal mean ozone, as a function of latitude and time, from January 1979 to November 1995. The anomalies are derived relative to each month's 1979-95 average. Certain aspects of the long-term global ozone changes may be readily seen. In the extra-tropics and polar regions, ozone is substantially lower in recent years than in earlier years. Largest anomalies are shown for the winter-spring months in each hemisphere, with positive anomalies of more than 10 percent in the earlier years changing to consistent negative anomalies of greater than 10 percent for recent years. Stolarski et al. (1992), Hollandsworth et al. (1995) and Miller et al. (1995), have indicated that the trends in the mid-latitudes are statistically significant and are about -2 to -4 % per decade. Little or no significant trend has been found over the equatorial region. Large negative anomalies in the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et al., 1993) hav e been related to the Mt. Pinatubo eruption in mid-1991. The anomalies decreased in 1994 along with the diminishing aerosol loading. However, large negative anomalies again developed in 1994-95. In the tropical region, a weak positive anomaly is seen in 1995, as part of a quasi-biennial oscillation of total ozone.

A map of monthly average Southern Hemisphere SBUV/2 total ozone for October 1995 is shown in Figure 2. The region of highest ozone (yellow and red colors) is seen equatorward of the Antarctic region. Very low total ozone values (less than 220 DU, blue and purple) are shown over the Antarctic continent. "Ozone hole" values, below 220 DU , first began to appear over the Antarctic region in the 1980's (Farman et al., 1985). The most extreme low values of total ozone are not shown on this map, because of lack of SBUV/2 data for this time period over the polar region (black area). Figure 3 shows the difference in percent between the monthly mean total ozone for October 1995 and the eight monthly means for October 1979-86 (Nagatani et al., 1988). Decreases in total ozone of greater than 20 percent (green and blue) to more than 50 percent are shown over a large area of Antarctica. Small positive percent anomalies are shown over some areas of the tropics and mid-latitudes, associated this year with th e quasi-biennial increase of total ozone.

Figure 4 shows a comparison of the area covered by ozone values less than 220 DU, the value generally used to denote the "ozone hole". In the Southern Hemisphere winter-spring of 1995, the area of the "ozone hole" was similar to recent record setting years, and substantially larger than the earlier years, such as 1989. There is an uncertainty in the area calculated from the SBUV/2 maps, so small differences in the area for different years may not be significant. This uncertainty arises because of the lack of SBUV/2 data poleward of the latitude of sufficient solar illumination for derivation of good quality SBUV/2 data. Despite uncertainties, the data do indicate that the very low ozone values in 1995 were as widespread over the Antarctic continent as in the record low year of 1993.

The extreme low "ozone hole" values moved in late November away from the South Pole , toward the South America quadrant of Antarctica, and then over the African quadrant, persisting till early December 1995.

The time series in Figure 5 shows total column ozone at the South Pole integrated from balloon-borne ozonesondes. Minimum ozone at the South Pole Station in 1995 occurred on October 5, with a value of 98 DU (+/- 5 DU). This was slightly lower than the 1994 low (102 DU), but not as low as the record 91 DU in 1993. Total ozone at South Pole Station in 1995 was at or above the 1986-1995 average from July to mid-September, and somewhat below average in October and November, but not as low as in 1992 and 1993.

Selected ozone profiles measured at the South Pole are shown in Figure 6. The profile on 5 October 1995, at the time of minimum total column ozone amount at the South Pole for 1995, is compared with profiles on 12 October 1993 and 23 August 1993. The October profiles show nearly complete destruction of ozone between 15 and 20 km. Recovery continued during 1995 in the 10-15 km region, as a result of the diminishing effects of the Pinatubo aerosols. However, ozone depletion in the 22-24 km region, not seen prior to 1992, continued in 1995. The extension of the ozone hole to higher altitudes is believed to be related to increases in chlorine (Hofmann et al., 1995).

Temperatures in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Extremely low stratospheric temperatures (lower than -78 C) 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). PSCs enhance the production and lifetime of reactive chlorine, leading to ozone depletion (WMO/UNEP, 1994).

Daily minimum temperatures over the polar region, 65S to 90S at 50 mb (approximately 19 km) are shown in Figure 7. For most of the southern hemisphere winter and spring of 1995, the minimum temperatures in the polar region were substantially lower than average. 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 8 for the Southern Hemisphere and for the South Polar area. Figure 9 shows temperature anomalies at 50 mb for three latitude regions, 65S-90S, 25S-65S, and 25N-25S (Gelman et al., 1986). For these regions, temperature anomalies during 1995 were near record low values.

Aerosol concentration is another potentially important component of stratospheric variation, and has been suggested as a possible source of ozone depletion (e.g. Hofmann et al., 1992). Aerosol optical thickness from the NOAA/AVHRR instrument in 1994 showed that stratospheric aerosol concentrations continued to diminish from the maximum values observed a few weeks after the eruption of Mount Pinatubo in June 1991. The NOAA 11 AVHRR instrument failed in September 1994. Stratospheric aerosols were at such low levels in 1994, that it was difficult to discern stratospheric aerosols from variations in tropospheric values.

III. SUMMARY Total ozone values over Antarctica during September and October 1995 were extremely low, with the minimum values only slightly higher than the record low values observed in 1993. The area covered by the lowest total ozone values was as large in 1995 as in 1993. "Ozone hole" values persisted into early December 1995. Ozone profiles in the 15-20 km region, show nearly complete destruction of ozone in October 1995, similar to recent years. In 1995 there was some recovery in the ozone at 10 to 15 km altitude, but some enhanced destruction evident at 22 to 24 km. We note that the lower stratosphere temperatures over the Antarctic region in 1995 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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