World Meteorological Organization

1. The Secretariat of the World Meteorological Organization (WMO) distributes Bulletins providing current Antarctic ozone hole conditions during August-December each year. Bulletins are distributed via the WMO-Global Telecommunication System (GTS) and are also available through the web page for the Atmospheric Research and Environment Programme (AREP) at www.wmo.ch/web/arep/00/ozbull4.html. In addition to the National Meteorological Services, the information in these bulletins should be made available to the national bodies representing their countries with UNEP and that support or implement the Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol.

2. The rapid and early development of the ozone hole that was previously reported continues, as sunlight reaches the South Pole after the total darkness of Winter. Sunlight is required to complete the complex photochemical processes that rapidly destroy the ozone layer over Antarctica each year. Therefore, we are now in the period when historically ozone loss occurred most rapidly. In previous years, ozone has reached its minimum overhead amounts during the first week or two of October. During some years 50% decreases relative to the 1964-76 pre ozone hole norms are found over large areas within the ozone hole and occasionally, decreases of 60% to 70% occur but within a much more limited area.

3. Ground based and satellite measurements both indicate that Antarctic ozone loss this year is presently as great as has been found during any previous ozone hole season. This occurs at least two weeks before minimum overhead ozone values are expected, and during the period with maximum rate of ozone loss. During the past two weeks all twelve Antarctic GAW stations have reported measurements of ozone 50% to 70% below the pre ozone hole norms. For some of the stations, these represent the lowest measured values in their records. Satellite images confirm that the extraordinarily low values are measured when the deepest part of the ozone hole appears above the stations. The latest satellite observations show an average decrease within the ozone hole of about 40-45% below the norms compared with 35% reported two weeks ago.

4. The most recent data indicate that beginning on 18 Sept., the edge of the ozone hole has been over the city of Ushuaia, Argentina (55 S) with ozone values 35% below the 1964-76 norms for that location. South of Australia, ground based measurements at Macquarie Island (55 S) showed a 40% decrease from the pre ozone hole norms. These low values are very unusual, but they are not without precedent and as in the past occur only for brief periods.

5. Natural variations in the meteorological conditions in the stratosphere will significantly affect the size of the ozone hole (area), its depth (low ozone values), and its persistence. If the losses persist as is now expected, we will have the deepest ozone hole on record. Meteorological conditions particularly suitable for intense ozone loss are likely to explain this years early development of the ozone hole. For example, the area of stratospheric temperatures sufficiently low to initiate the ozone loss over Antarctica was somewhat larger during July and August when compared to most previous years. Meteorological data also show that during the first 10 days of Sept. these low temperatures covered a very large area, about 10% to 20% larger than found during early Sept. in the past decade. Both of these conditions could contribute to enhanced ozone loss.

6. Whether the early arrival of this years ozone hole is within the expected natural variation or is in part the result of human activity remains to be determined. Chemicals that result in ozone destruction are no longer increasing in the stratosphere, as the international controls on ozone depleting chemicals continue to work. However, the continued general decrease of ozone in the lower stratosphere and the global increase in greenhouse gases are now believed to result in lower temperatures in the lower stratosphere. These decreases in temperature could expand the period of intense ozone loss during the ozone hole period.

7. Bulletins are based upon provisional data from the WMO Global Atmosphere Watch (GAW) stations operated within or near Antarctica by: Argentina (Comodoro Rivadavia, San Julian, Ushuaia), Argentina/Finland (Marambio), Argentina/Italy/Spain (Belgrano), Australia (Macquarie Island), France (Dumont D'Urville and Kerguelen Island), Germany (Neumayer), Japan (Syowa), New Zealand (Arrival Heights), Russia (Mirny), Ukraine (Vernadsky), UK (Halley, Rothera), Uruguay (King George Island), and USA (South Pole). Satellite ozone data are also used and provided by NASA - Total Ozone Mapping Spectrophotometer (TOMS) and by the National Oceanic and Atmospheric Administration (NOAA) - TIROS Operational Vertical Sounder (TOVS). The ERA-15 and daily T106 meteorological fields of ECMWF are analysed by the Norwegian Institute for Air Research (NILU) Kjeller, Norway, to provide vortex size and extreme temperature information. Ozone data analyses are prepared in collaboration with the WMO World Ozone and Ultraviolet Data Centre in Toronto, Canada through the co-operation and support of the Meteorological Service of Canada (MSC). Graphics support has been provided to WMO by NOAA Aeronomy Laboratory in Boulder, Colorado, USA. If this bulletin is quoted, due credit should be given.

Questions regarding the scientific content of this bulletin should be addressed to

Dr. Michael Proffitt, Senior Scientific Officer of WMO: e-mail proffitt@wmo.ch .

END of WMO Antarctic Ozone Bulletin 4/2000