The ozone layer is a layer of the atmosphere which sits about 10 kilometres above the Earth's surface and protects life from ultraviolet radiation from the Sun.
This panoramic image from February 14, 2014, shows parts of Chile and Argentina from the International Space Station.
"This study adds to growing evidence showing the profound effectiveness of the Montreal Protocol". Not only has the treaty spurred healing of the ozone layer, it's also driving recent changes in Southern Hemisphere air circulation patterns. Ultimately, ozone depletion has shifted the midlatitude jet stream and the dry regions at the edge of the tropics toward the South Pole.
But the recovering ozone later is allowing for the jet stream over the Antarctic to move further up in the atmosphere, according to new computer simulations from the University of Colorado Boulder. In this study, Banerjee and her co-authors have shown that around the year 2000, the circulation of the Southern Hemisphere also stopped expanding polewards-a pause or slight reversal of the earlier trends.
Ian Rae, an organic chemist from the University of Melbourne who was not involved in the study, said: "The "weather bands" that bring our cold fronts have been narrowing towards the south pole, and that's why southern Australia has experienced decreasing rainfall over the last thirty years or so".
The researchers put the restrengthening of the ozone layer down to the 1987 Montreal Protocol which was an global treaty that banned chemicals such as chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODSs) which were found to be weakening the atmospheric layer. Beginning around 2000, concentrations of those chemicals in the stratosphere started to decline and the ozone hole began to recover.
"The challenge in this study was proving our hypothesis that ozone recovery is in fact driving these atmospheric circulation changes and it isn't just a coincidence", Banerjee said.
The researchers achieved this by employing a two-step statistical method known as detection and attribution: detecting whether specific patterns of wind changes noticed are unlikely to be caused by natural variability alone and, if so, whether the variations could be due to human-induced factors, like emissions of ozone-depleting chemicals and CO2. Then, they isolated the effects of greenhouse gases and ozone separately, demonstrating that while an increase in Carbon dioxide emissions has continuously expanded the near-surface circulation (including the jet stream) polewards, only the ozone changes could be attributed to a break in circulation trends. Prior to 2000, both ozone depletion and rising Carbon dioxide levels pushed the near-surface circulation poleward. Since 2000, Carbon dioxide has constantly pushed this circulation poleward, thus balancing the opposing effect of the ozone recovery.
With ozone recovering and Carbon dioxide levels continuing to climb, the future is less certain, including for those Southern Hemisphere regions whose weather is affected by the jet stream and those at the edge of the dry regions.
Atmospheric chemist Antara Banerjee from the University of Colorado Boulder: "We term this a "pause" because the poleward circulation trends might resume, stay flat, or reverse". "It's the tug of war between the opposing effects of ozone recovery and rising greenhouse gases that will determine future trends".