April 26, 2011 | A new study in Geophysical Research Letters looks at how the anticipated recovery of the ozone hole over Antarctica and simultaneous increase in greenhouse gas concentrations will combine to affect weather and climate in the Southern Hemisphere. It concludes that over the coming half century, ozone recovery will result in a nearly complete cancellation of the effects of increased greenhouse gases on atmospheric circulation.
The Southern Hemisphere’s prevailing atmospheric circulation pattern is the Southern Annular Mode (SAM), also known as the Antarctic Oscillation. In the mode's positive phase, a stronger and more southerly vortex encircles the pole, leading to fewer intrusions of Antarctic air into the southern oceans. The negative phase features a weaker, more variable vortex and a greater risk of Antarctic outbreaks of cold air heading north.
SAM has trended toward positive values since the 1960s, a development that scientists attribute to the effects of both stratospheric ozone depletion above Antarctica and increases in greenhouse gas emissions. The ozone hole stopped growing in about 2000, however, due to a ban on ozone-depleting substances. It is expected to recover completely sometime after the middle of this century, leaving scientists wondering whether atmospheric circulation patterns will return to conditions prior to the formation of the ozone hole or if increasing greenhouse gas concentrations will counter the effects of the recovery.
For the study, a team of scientists that includes NCAR’s Clara Deser used the Community Atmosphere Model (CAM), a component of NCAR’s Community Earth System Model, to run different simulations for the period 2000-2060. The results confirm that ozone recovery and greenhouse gas forcings will have opposite effects, with the increase in greenhouse gas concentrations countering ozone recovery and preventing SAM from returning to its pre-1960s patterns.
Polvani, L. M., M. Previdi, and C. Deser (2011), “Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric circulation trends,” Geophysical Research Letters, 38, L04707, doi:10.1029/2011GL046712