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Ozone hole about to split

ozone hole 19 September

Antarctic ozone hole detected on the 19th September 2002. (Pic: ESA/KNMI)

The ozone hole above the Antarctic is about to split in two, months earlier than usual, according to Dutch calculations - but it's weather rather than good environmental management that is the cause.

Henk Eskes, a senior scientist at the Royal Netherlands Meteorological Institute (KNMI), has predicted that the ozone hole will separate by today, Wednesday 25th September 2002.

"It comes and goes every year, it's a seasonal phenomenon," said Dr Paul Fraser, the Chief Research Scientist at CSIRO Atmospheric Research. "It's just that this year it looks like it will break up early."

KNMI is the national meteorological centre of the Netherlands. The prediction comes from data the European Space Agency has supplied using the Global Ozone Monitoring Experiment - a nadir-scanning ultraviolet and visible spectrometer that monitors atmospheric ozone levels.

"The hole is already elongated to such an extent that the minimum ozone hole has broken into two already," Dr Fraser said yesterday.

The ozone hole has been sitting in the shape of a 'W'. "The question is will it separate into two Vs - into two holes," he explained.

The ozone hole began forming each August or September in the early 1980's. It usually breaks up in November or December. "It's a spring phenomenon," said Dr Fraser.

Initiatives like banning chlorofluorcarbons have reduced the amount of ozone depleting substances in the atmosphere, and the scientific community is hoping that eventually the ozone hole will no longer appear annually over Antarctica.

"What we are hoping for in 2050 is that it will never occur again," said Dr Fraser.

But this year's early splitting of the ozone hole is more to do with weather than environmental management. "It's not to do with the amount of chemicals in the atmosphere," said Dr Fraser. "It's air movements more than anything else."

The ozone hole appears over the cold region of Antarctica when the sun returns after the polar winter - strong ozone depletion occurs with the combination of very low temperatures and high solar radiation.

The size and the duration of the ozone hole relies on natural year-to-year variability of atmospheric circulation. The ozone hole is surrounded by a vortex of strong winds that stops air moving between polar regions and warmer mid-latitude regions, keeping the hole in the ozone stable.

During the South Pole's spring and summer, the temperature increases and the winds weaken. As a result, ozone-poor air inside the vortex mixes with the ozone-richer air outside, and the ozone hole dissipates.

Depletion of the ozone layer also occurs happens at mid-latitudes outside the ozone hole. But the depletion occurs by slower processes so it is weaker than over Antarctica and doesn't lead to a hole.