A scientist at the Royal Netherlands Meteorological Institute (KNMI) adds that the depth of the ozone hole is much smaller than previously seen.

"This breakdown is occurring exceptionally early in the year, about two months earlier than normal", says Henk Eskes, a KNMI senior scientist. "The depth of the ozone hole this year also is unusually small, about half that recorded in 2001."

The KNMI researcher predicted the Antarctic ozone hole will separate into two parts by Wednesday, 25 September, and will weaken even further afterwards. Despite the optimistic forecast, he warned the possibility that one of the two remnants will strengthen and form a new ozone hole "cannot be excluded."

Globally, there has been a slow decrease in the amount of ozone-depleting substances in the atmosphere, Eskes said, due to international treaties to reduce their production.

But, he added, "this decrease is too slow to explain this year's weak ozone hole."

The explanation lies instead with the natural year-to-year variability of atmospheric circulation that influences the size and duration of the ozone hole, according to the Dutch scientist. The ozone hole is surrounded by a vortex of strong winds that block the exchange of air between polar and mid-latitude regions. 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.

"Normally this happens in November-December, but this year we predict it will happen this week," Eskes said.

Since the early 1980s, the Antarctic ozone hole has developed every year starting in August or September.

More than half of the area's natural ozone is depleted eventually, caused by such ozone-depleting substances as chlorofluorocarbons, or CFCs, which had been used as refrigerants, solvents and foam-blowing agents.

The strong ozone depletion occurs only at very low temperatures under the influence of solar radiation. As a result, the ozone hole only appears over the cold region of Antarctica, when the sun returns after the polar winter.

Depletion of the ozone layer also occurs outside the ozone hole at mid-latitudes. In these areas, however, the depletion occurs by slower processes and is less strong than over Antarctica.

KNMI, the Dutch national research and information centre for climate, climatic change and seismology, uses data from ESA's Global Ozone Monitoring Experiment (GOME) instrument onboard the ERS-2 satellite to generate daily global ozone analyses and nine-day ozone forecasts.

The centre's GOME Fast Delivery Service performs near real time processing of the satellite data. GOME ozone observations are assimilated into a tracer transport model, driven by meteorological fields from the numerical weather prediction model generated by the European Centre for Medium-Range Weather Forecasts, a UK-based international organisation for advance weather forecasts.

The GOME Fast Delivery Service was developed under ESA's Data User Programme, an effort dedicated to developing and demonstrating applications services in support of institutional and private user communities.

The GOME instrument is a nadir-scanning ultraviolet and visible spectrometer to monitor atmospheric ozone levels.

Since 1996, ESA has been delivering to KNMI and other users three-day GOME global observations of total ozone, nitrogen dioxide and related cloud data via CD-ROM and the Internet.

Launched earlier this year, ESA's Envisat satellite carries several instruments to assist climate researchers in monitoring ozone levels and other atmospheric conditions.

Enivsat's suite of ten sensors include: the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) instrument to measure trace gases and aerosol concentrations in the atmosphere; the global ozone monitoring by occultation of stars (GOMOS) sensor to observe the concentration of ozone in the stratosphere; and the Michelson interferometer for passive atmospheric sounding (MIPAS) to collect information about chemical and physical processes in the stratosphere, such as those that will affect future ozone concentrations.

ESA, together with the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), also is preparing a series of three satellites called MetOp that will carry follow-on GOME instruments and guarantee at least ten years of continued ozone monitoring from space starting in 2005.

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