15 Year-to-year variations (Ozone layer)
Description
This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
15 Year-to-year variations (Ozone layer)
Since ozone is created by solar UV radiation, one expects to see
some correlation with the 11-year solar sunspot cycle. Higher
sunspot activity corresponds to more solar UV and hence more rapid
ozone production. This correlation has been verified, although
its effect is small, about 2% from peak to trough averaged over the
earth, about 4% in polar regions. [Stolarski et al.]
Another natural cycle is connected with the "quasibiennial
oscillation", in which tropical winds in the lower stratosphere
switch from easterly to westerly with a period of about two years.
This leads to variations of the order of 3% at a given latitude,
although the effect tends to cancel when one averages over the
entire globe.
Episodes of unusual solar activity ("solar proton events") can also
influence ozone levels, by producing nitrogen oxides in the upper
stratosphere and mesosphere. This can have a marked, though
short-lived, effect on ozone _concentrations_ at very high altitudes,
but the effect on total column ozone is usually small since most of
the ozone is found in the lower and middle stratosphere. Ozone can
also be depleted by a major volcanic eruption, such as El Chichon in
1982 or Pinatubo in 1991. The principal mechanism for this is _not_
injection of chlorine into the stratosphere, as discussed in Part II,
but rather the injection of sulfate aerosols which change the
radiation balance in the stratosphere by scattering light, and which
convert inactive chlorine compounds to active, ozone-destroying forms.
[McCormick et al. 1995]. This too is a transient effect, lasting 2-3 years.
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