Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 286

pressure system stuck in a blocking pattern over Greenland in October 2012 that sent Hur-
ricane Sandy inland. It could well be that the situation that prevailed from the mid-1960s
to the mid-1990s, which saw strong AO + conditions maintaining a strong polar vortex, will
become increasingly rare. Once again, only time will tell if this explanation is the one that
will prevail. Of course, because of the interconnectedness of our global climate system, the
weather we actually experience in mid-northern latitudes is influenced by more than one
long-distance connection or teleconnection.
Near the end of the ozone chapter, we noted that early in 2011, a very severe ozone-
depletion event occurred in the Arctic. The impact of this spread beyond the Arctic. For ex-
ample, in the period including January to March 2011, the UV index over southern Sweden
was 20% above the mean. I have delayed taking a closer look at Arctic ozone depletion
until we had reviewed Arctic meteorological and climate change processes. In the ozone
chapter, we also saw that a trinity of circumstances is needed to occur simultaneously for
ozone depletion to occur in the stratosphere. These are (1) a supply of ozone-depleting sub-
stances, (2) extremely cold temperatures that allow for the formation of polar stratospheric
clouds and (3) solar ultraviolet light (UV). Unlike the Antarctic, the Arctic stratosphere is
usually not sufficiently cold enough at the time of polar sunrise to support the formation of
polar stratospheric clouds and the cascade of depleting chemical reactions will not occur or
will occur only slowly. For this reason, the Antarctic has experienced severe stratospheric
ozone depletion, but the situation in the Arctic, with a generally warmer stratosphere, has
been less severe. Consequently, Arctic UV irradiance has not reached the very high levels
seen in the Antarctic ozone hole. We also saw how implementation of the Montreal Pro-
tocol is making progress in reducing the concentration of ozone-depleting substances in the
stratosphere. But what caused the 2011 event?
Despite the Montreal Protocol, there is still a large reservoir of ozone-depleting sub-
stances aloft. In spring 2011, an AO + condition that generated a robust polar vortex was
set up in March and April. This constrained Arctic air in the region. At polar sunrise, the
trinity of favourable conditions was established and a new record set for Arctic stratospher-
ic ozone loss. A closer look using modelling by Strahan and colleagues published in 2013