Ozone hole largest on record
The 1998 Antarctic ozone hole is unusually large and formed very early.
Dr Stephen Wood, NIWA, Scott Base, Antarctica
According to preliminary NASA satellite data it is now the largest on record, last week covering more than 27 million square kilometres, around 5% larger that the previous record set in 1996. Like the 1996 ozone hole, it developed much more rapidly in late August and early September than other years. However, this year the ozone hole has remained stable for longer and is now 20-25% larger than the 1996 ozone hole was on this date.
At Scott Base, NIWA scientist Dr Stephen Wood says that ozone levels at Arrival Heights were consistently low last week, because the size and stability of the vortex kept Arrival Heights well within the ozone hole. The measurement of 139 Dobson Units (DU) on 22 September was the lowest ozone amount for that time of the year in eleven years of observations. Ozone levels there rose to 190 DU over the weekend as the vortex edge moved closer to Arrival Heights, but are now falling again. The lowest value of ozone ever recorded at Arrival Heights was 129 DU in October 1995. Given the way the 1998 hole is developing, this record could well be broken this year.
Most of Antarctica remains under a pool of air with temperature below -80°C, cold enough for the formation of polar stratospheric clouds (PSCs). Chemistry on the surfaces of the cloud particles greatly enhances the effectiveness of chlorine from CFCs in destroying ozone. The ozone hole has probably reached its greatest areal extent for this year, but the lowest amounts of ozone are likely to be seen in the next week. Preliminary NASA satellite measurements show that the minimum value of ozone over the Antarctic has now dropped below 100 DU, close to the all time low of 88 DU measured in 1994.
Ozone values in New Zealand reach an annual maximum in early spring, typically around 350 DU. Dr Brian Connor of NIWA said that this year's ozone values over New Zealand have been similar to other recent years, about 5-6% lower in late winter than was normal in the late '70s and early '80s. This is in contrast to last year when a combination of factors, including the long-term decline in global ozone, resulted in record low seasonal values.
NIWA research into the processes that control the amount of ozone is supported by the New Zealand Foundation for Research, Science and Technology. It involves measurements and mathematical modelling of ozone and a range of trace gases in the stratosphere, both in Antarctica and at Lauder in Central Otago. Understanding the interplay between ozone depletion chemistry and atmospheric circulation is important if we are able to better predict year to year fluctuations in the severity of the Antarctic ozone hole and its effects on Antarctica and the rest of the Southern hemisphere.