Wednesday, 6 May 2009

December 12, 2000 -- After reaching a record-breaking size in mid-September, the ozone hole over Antarctica has made a surprisingly hasty retreat, disappearing completely by November 19, NASA scientists said.

The attention-grabbing behavior of this year's hole -- both the record size and the quick disappearance -- can be largely attributed to the influence of an atmospheric phenomenon known as "planetary-scale waves," Newman said.

"Just think of (a planetary-scale wave) as being a big low pressure system that almost straddles the entire Southern Hemisphere," Newman said. "These lows and highs ... are so big that you can't see it on a regular weather chart. That's why we call them planetary-scale waves."

These vast pressure waves influence ozone destruction in several ways, but for explaining this year's ozone hole, the most relevant impact of the waves is on the size and stability of the massive jet stream encircling Antarctica called the "Antarctic vortex." The vortex is a fast-moving whirlpool of air that encircles Antarctica during the winter and early spring, effectively sealing it off from the rest of the atmosphere.

The isolation provided by the vortex prevents warmer, ozone-rich air surrounding Antarctica from flowing toward the pole, which would help replace the destroyed ozone and raise temperatures over the continent. Instead, the ozone-rich air -- which is carried toward the pole by the action of the planetary waves -- builds up at the edge of the vortex, forming a "ring" of high ozone concentrations around the continent that can be seen in the satellite images.

Above: Image of the record-size ozone hole taken by NASA satellites on September 9, 2000. Blue denotes low ozone concentrations and yellow and red denote higher levels of ozone. Notice the ring of high ozone concentrations formed when the Antarctic vortex blocks the southerly migration of ozone formed in the tropics. [More images and credits]

Without the warming effect of these waves, the air inside the vortex drops to extremely cold temperatures during the winter's perpetual night. These low temperatures set the stage for ozone destruction, since the chemical reactions that lead to ozone destruction are catalyzed by icy clouds that only form in very cold air.

This year's unusually weak planetary waves allowed the vortex to expand to a greater size. The larger vortex amounted to a larger arena for the destruction of ozone, resulting in the record-size hole.

When the strength of these waves picked up in mid-September, they exerted a force on the vortex which blew it apart earlier than usual. As the vortex broke down, the surrounding warm, ozone-rich air mixed with the air over Antarctica, raising ozone concentrations above the threshold for an ozone "hole."

Across many future climate projections, Australia in winter shows the largest reductions of rainfall of any region in the world. However rainfall in south-western Australia has already decreased faster than predicted, suggesting factors other than those already identified are at work. According to Jones and colleagues, the clues lie 20 km high above Antarctica.

The Antarctic polar vortex is a natural, continent-wide 'tornado' of 200 kph, super-cold winds surrounding the ozone 'hole' from the stratosphere to the surface. It is created by the movement of the globe interacting with temperature differences between the pole and the rest of the Earth's surface. The vortex delivers the winter rain-bearing westerly winds called the 'Roaring Forties', which southern Australia relies on for its water supplies.

However Jones and team have found that global warming and ozone depletion are interacting to shrink and accelerate the vortex, dragging crucial ranfall towards the south pole, away from Australia's landmass.

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