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/ozbull5.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. Year to year variability in the intensity of the ozone hole, both in extent (area) and depth (deviations from pre-ozone hole norms) are expected. These variations have been observed for more than 15 years and are not only due to the changing amount of ozone depleting chemicals in the atmosphere, but also to the changes in meteorological conditions in the stratosphere over and near Antarctica. The most important meteorological variable is the lower stratospheric temperature, which must be sufficiently cold to create the polar stratospheric clouds (PSCs) that initiate the chemical processes required for rapid ozone destruction. Ozone loss is restricted to those altitudes where PSCs have occurred, and the period of intense loss is affected by the persistence of these clouds.

3. Near total destruction of ozone occurs over a limited altitude range and has been observed for a number of years using instruments carried by balloons. Since the middle of Sept., GAW Antarctic stations have observed these very low values in a layer from 15 to 20 km. In some cases, the loss appears to be total over this entire range. The near total destruction has occurred earlier than in previous years.

4. Ground based and satellite measurements both indicate that the depth (deviation from pre-ozone hole norms) of the Antarctic ozone hole during the last 10 days of September was the greatest on record. By comparing satellite averages over 10 day intervals throughout the historical record we find that the lowest ozone values of the season have consistently occurred during early (1-10) or mid (11-20) October, as was the case in 1998, the previous record year. It might be anticipated that further decreases will continue to occur, wherever ozone destruction has not been complete and PSCs have initialised the chemistry. NOAA reports from SBUV/2 satellite data that the extent of the ozone hole was also record breaking in early Sept., but it now has decreased in area to near the average for the past decade. Presently, ozone is more than 50% below the 1964-76 pre-ozone hole norms throughout most of the ozone hole. The average ozone during the entire month of Sept. was also exceptionally low, with values from 50% below norms within the hole at 75 - 80 S, to 15% below norms at 60 - 55 S, a region normally outside the hole. Values near the norms continue to persist at mid-latitudes.

5. Variations in the meteorological conditions in the stratosphere affect the extent of the ozone hole, its depth, and its persistence. In particular, in earlier Bulletins we reported that 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, possibly explaining the early development of the ozone hole. The ozone hole has also intensified since 1995, and during this period, the area with temperatures low enough for PSC formation during October is double that found during any earlier 5 year period. It remains to be seen whether the trend towards colder temperatures in the lower stratosphere during October will continue, and possibly extend the depth and the persistence of this years ozone hole and those in the future. This downward trend may result from the continued general decrease of ozone in the lower stratosphere and the global increase in greenhouse gases, both believed to result in lower temperatures in the lower stratosphere.

6. 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 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. 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 5/2000