1. The Secretariat of the World Meteorological Organization (WMO) distributes Bulletins providing information on the state of the ozone layer in the Antarctic during the period August-December each year. Bulletins are distributed via the WMO-Global Telecommunication System (GTS) and are also available on the internet through the WMO web page for the Atmospheric Research and Environment Programme (AREP) at www.wmo.ch/web/arep/00/ozbull1.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. Bulletins are based upon provisional data from the WMO Global Atmosphere Watch (GAW) stations operated in the Antarctic by: Argentina/Finland (Marambio), France (Dumont D'Urville), Germany (Neumayer), Japan (Syowa), Ukraine (Vernadsky), UK (Halley Bay), Uruguay (King George Isl.), USA (South Pole). Satellite ozone data are also used and provided by NASA - Total Ozone Mapping Spectrophotometer (TOMS) and by the 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 the WMO by the National Oceanic and Atmospheric Administration (NOAA) Aeronomy Laboratory in Boulder, Colorado, USA. If this bulletin is quoted, due credit should be given.
3. The purpose of this first Antarctic bulletin for the year 2000 is to describe the early conditions found this year near and over Antarctica, that set the stage for the annual development of the ozone hole. During this very early period, satellite images of ozone distributions near Antarctica provide insights into the development of ozone loss, particularly in the sunlit regions where the losses occur. In addition to sunlight, occasional temperatures sufficiently low to form polar stratospheric clouds (PSCs) are necessary for rapid ozone loss to occur. Threshold temperatures of -78 C can produce PSCs which activate chemical processes that result in rapid ozone losses in the lower stratosphere, while lower temperatures (below -85 C) can further accelerate the chemical processing. The Antarctic polar vortex (the polar night jet) is a region with high velocity winds in the stratosphere that generally circle the Antarctic continent. This vortex region (the vortex and its interior) includes the lowest temperatures and the largest ozone losses that occur anywhere in the world.
4. By the end of the first week of August much of Antarctica still has no sunrise, so the ozone loss there is minimal. The latest observations reveal that the sunlit ring surrounding Antarctica, when compared to the pre-ozone hole period of 1964-76, show an average decrease of about 15% in the total amount of ozone overhead. This decrease is substantially more than last year. Measurements made further North (50 to 60 S) during the months of June and July are also low, indicating 6% and 9% below the 1964-76 reference values. These monthly averages are apparently the lowest values observed during these months since satellite observations began. The ozone observations North of 50 S are typical of previous years.
5. The processes that prime the atmosphere for ozone depletion are well underway. Meteorological data show that minimum temperatures as cold as -93 C can be found daily at 20 km altitude. During late July and early August, temperatures low enough for PSC formation covered more than 25 million square kilometres, about 60% of the vortex area. This is somewhat larger than found in previous years. It is now expected that the lowest temperatures for the season have been reached, and a gradual warming will result as the sun begins to rise over Antarctica. This will result in a corresponding decrease in PSC formation.
6. It appears that the conditions necessary for ozone depletion over Antarctica are now in place, with temperatures sufficiently low to have chemically primed the vortex regions for ozone loss. The sunlit region surrounding Antarctica appears to have less ozone than usual, which may be due to natural variations. In the coming weeks, as the sun rises over Antarctica, chemical ozone loss can be expected to occur, comparable to, or perhaps greater than previous years. Prevailing meteorological conditions in the stratosphere, particularly during the Austral Spring, will strongly influence the extent and intensity of ozone loss.
END of WMO Antarctic Ozone Bulletin 1/2000
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