Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 285

atmospheric pressure dominates the Arctic. This helps the subpolar jet stream blow
strongly from west to east with low amplitude meanders and to maintain a strong polar
vortex that locks cold air in the polar region. 17 Is it possible that climate warming may in-
fluence whether we live in an AO + or an AO − world? The answer is “possibly”.
The argument goes like this: The progressive loss of more and more sea ice results in
the excessive heating of the upper ocean relative to historical times. Two positive warming
feedback mechanisms now become active. Some of the excess heat goes into speeding up
warming through our now familiar ice-albedo feedback. What is new for us is the realiza-
tion that much of the remaining excess heat is slowly transferred to the atmosphere through
evaporation and radiation. This increases Arctic atmospheric moisture content. At the same
time, the temperature gradient between the Arctic and middle latitudes is decreased. We
now have just the right conditions to encourage the development of AO − conditions in the
winter. With a weakened jet stream and a weakened polar vortex, cold Arctic air, with its
nowincreasedmoisturecontent,canmoreeasilyescapeintomid-latitudes,bringingintense
cold and snow to the eastern United States and to Western Europe. This is indeed what ap-
pears to have occurred in the winters of 2009-2010, 2011-2012 and 2013-2014. The AO
index in February 2010 of −4.26 was the largest negative anomaly since records began in
1950.
We have here a plausible mechanism that is consistent with present observations and
that links a warming Arctic and diminished ice cover with (as the proponents say) “stack-
ing the deck in favour of severe weather outbreaks” in some mid-latitude regions. Only
time will tell whether this or other plausible explanations best describe the meteorological
consequences that follow from the increasing direct exposure of the Arctic Ocean waters to
the atmosphere.
A weakened jet stream is also a “loopy” jet stream (prominent Rossby waves). This
means it follows a path with greater amplitude meanders, reaching deeply southward with
short distances between peaks that progress much more slowly from west to east and may
even stall. This situation increases the persistence of mid-latitude weather patterns, locking
theminplaceandalsosettingthestageforso-calledmeteorologicalblocking.Itwasahigh-