As anticipated in July ('Stop Press: Large ozone hole predicted', Australian Antarctic Magazine 5:25), a large hole formed in the ozone layer over Antarctica during the spring of 2003. At its maximum extent on 24 September, the hole equalled in size the record set on 10 September 2000. This situation strongly contrasted with the behaviour seen in 2002, when the ozone hole was the smallest in over a decade ('Unusual behaviour of the Antarctic ozone hole', Australian Antarctic Magazine, 5:24).
At Davis, atmospheric measurements by the AAD and Bureau of Meteorology using balloons and LIDAR followed the evolution of the ozone hole as part of a study into the microphysics of stratospheric ozone. These measurements also contributed to the first year of an international program which is investigating polar ozone loss.
Because of below average temperatures in the polar stratosphere over the 2003 winter, and the relative stability of the polar vortex, the ozone hole grew rapidly during August, attaining a size and depth not previously witnessed for that time of year. Significantly, the region over Antarctica where more than 50% of the total column ozone was destroyed reached an area about 20% higher than the previous record set in 2000. The growth phase peaked in early September, and the areal extent of the hole remained near record levels for much of the month. During October and November, the hole decayed relatively quickly, following a trend similar to that observed in 2000. Interestingly, the ‘filling-in’ of the hole was more rapid than the decay of the polar vortex, and is probably related to influences on stratospheric circulation from the tropics.
Despite the large extent of the ozone hole in 2003, solar ultraviolet (UV) levels near the surface were not unduly elevated. The only populated region to lie under the hole was the southern tip of South America, and this region was exposed on four occasions in September and early October. During these times, low solar elevation angles combined with cloud cover restricted UV exposure to levels comparable with mid-latitude summer sites.
What will happen in 2004? The winds in the equatorial stratosphere should reverse in direction to flow eastward during the year, and this may enhance the size of the ozone hole by restricting the poleward flow of ozone from the tropics. A similar situation occurred in 2002, however other meteorological factors disturbed the Antarctic atmosphere and created an abnormally small hole. We'll have to wait and see, as there are still many subtleties in global atmospheric dynamics that remain to be understood.
For more information, see:
AAD ozone hole press release: http://www.aad.gov.au/default.asp?casid=11817
Andrew Klekociuk, Space and Atmospheric Sciences Program, AAD