Cause of 2011 odd Arctic ozone hole found
Maps of ozone concentrations over the Arctic come from the Ozone
Monitoring Instrument on NASA’s Aura satellite. The left image shows
March 19, 2010, and the right shows the same date in 2011. March 2010
had relatively high ozone, while March 2011 has low levels. Credit:
NASA/Goddard
Washington, March 12:
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 short-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.
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