A combination of extreme cold temperatures, man-made chemicals and a stagnant atmosphere caused a significant hole in the Arctic ozone layer in 2011, NASA has found.
Even when both poles of the planet undergo ozone losses during the winter, the Arctic's ozone depletion tends to be milder and shorter-lived than the Antarctic's, researchers said.
This is because the three key ingredients needed for ozone-destroying chemical reactions - chlorine from man-made chlorofluorocarbons (CFCs), frigid temperatures and sunlight - are not usually present in the Arctic at the same time.
Still, in 2011, ozone concentrations in the Arctic atmosphere were about 20 per cent lower than its late winter average, NASA said.
The new study shows that, while chlorine in the Arctic stratosphere was the ultimate culprit of the severe ozone loss of winter of 2011, unusually cold and persistent temperatures also spurred ozone destruction.
Furthermore, uncommon atmospheric conditions blocked wind-driven transport of ozone from the tropics, halting the seasonal ozone resupply until April.
"You can safely say that 2011 was very atypical: In over 30 years of satellite records, we hadn't seen any time where it was this cold for this long," said Susan E Strahan, an atmospheric scientist at NASA Goddard Space Flight Center in Greenbelt.
"Arctic ozone levels were possibly the lowest ever recorded, but they were still significantly higher than the Antarctic's," Strahan said in a NASA statement.
"There was about half as much ozone loss as in the Antarctic and the ozone levels remained well above 220 Dobson units, which is the threshold for calling the ozone loss a "hole" in the Antarctic - so the Arctic ozone loss of 2011 didn't constitute an ozone hole," said Strahan.
The majority of ozone depletion in the Arctic happens inside the so-called polar vortex: a region of fast-blowing circular winds that intensify in the fall and isolate the air mass within the vortex, keeping it very cold.
Most years, atmospheric waves knock the vortex to lower latitudes in later winter, where it breaks up. In comparison, the Antarctic vortex is very stable and lasts until the middle of spring.
But in 2011, an unusually quiescent atmosphere allowed the Arctic vortex to remain strong for four months, maintaining frigid temperatures even after the sun reappeared in March and promoting the chemical processes that deplete ozone.
