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 Atmospheric Research and Environment Programme web page at www.wmo.ch/web/arep/ozone.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. Meteorological Conditions: The area with temperatures low enough to form polar stratospheric clouds (PSCs) has dropped to near 15 million square kilometres (million km2) at an altitude of 16-17 km during the past few weeks. Although this is considerably smaller than in August and September, this area still remains a significant portion of the polar vortex, presently with an area of about 35 million km2. Last year the vortex was frequently oblong, usually extending toward South America, and reached 40 million km2 in area, the largest in recent years. However, it weakened and dissipated earlier than usual, beginning in mid-October. The polar vortex this year has been generally circular and centred on the South Pole, suggesting a more stable and persistent vortex. The extremely low ozone values that characterise the ozone hole are generally confined to the interior of the vortex.

3. Observed ozone changes: Ground-based measurements since 1 October have averaged 50% below the pre-ozone-hole norms at Arrival Heights, Halley, and Syowa stations, and about 25% below norms at Dumont d'Urville, Marambio and Vernadsky. Satellite measurements confirm these values and that early October ozone values over central Antarctica are more than 60% below norms. With a more circular vortex that is roughly centred on the pole, the populated southern regions of South America, King George Island, and Kerguelen Island have remained outside the ozone hole and consequently have not experienced the very low ozone hole values. Ozone measurements made from balloons flown out of Belgrano, Neumayer, Syowa and South Pole stations during the past week have a revealed 4-5 km layer between 14 and 21 km where more than 95% of the ozone has been destroyed, similar to recent years. Balloon measurements from Marambio station have been somewhat higher, since they have been near the edge of the ozone hole during this period

4. Ozone hole characteristic: In the past few days, the area of the ozone hole has remained large at 23 to 24 million km2, only a little smaller than its maximum during September of 25 million. A measure of the amount of ozone loss within the ozone hole on any day can be expressed in millions of tons (Mt) of ozone and estimated from the daily column ozone data available from satellites and ground stations. This "ozone mass deficit" is calculated within the area that is more than 10% below pre-ozone-hole norms and provides a means of measuring the progress of the depletion, and for comparing the depth of ozone holes from year to year. Last years ozone hole reached its maximum depth of 57 Mt by the second week in September, a value higher than all previous years, but quickly decreased to less than 40 Mt by 1 October. This year the ozone mass deficit reached about 54 Mt in mid September, and remains very high during October, with some daily values of 53 Mt and with an early October average value of 50 Mt. With a circular vortex and associated ozone hole that have persistently been centred on the South Pole, the ozone hole is expected to remain intact for some weeks to come.

5. Acknowledgements: These Bulletins use provisional data from the WMO Global Atmosphere Watch (GAW) stations operated within or near Antarctica by: Argentina (Comodoro Rivadavia, San Julian, Sobral, 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). Potential vorticity maps are provided by ECMWF and their ERA-15 and daily T106 meteorological fields are analysed by the Norwegian Institute for Air Research (NILU) Kjeller, Norway, to provide vortex extent 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. Supporting graphics can be found at http://exp-studies.tor.ec.gc.ca/cgi-bin/selectMap (MSC) and http://www.nilu.no/projects/nadir/o3hole (NILU) .

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 4/2001