Summer downpours across
northern Europe linked to
variations of jet stream winds
caused by loss of sea ice.
The six summers from 2007 to 2012
were all wetter than average,
meaning scenes like these in
Brighton yesterday have become
more common.
The unprecedented run of wet
summer weather in recent years
could be due in part to the melting
sea ice in the Arctic, which appears
to be affecting the movement of
high-altitude winds over Britain, a
study has found.
Scientists believe they have
discovered a “causal link” between
the loss of Arctic sea ice in summer
months and variations in the jet
stream that have brought a series
of very wet summers to Britain and
northern Europe.
The six summers from 2007 to 2012
were all wetter than average and,
although none of these individual
events were unprecedented in
themselves, the sequence of six
consecutive wet summers was
“extraordinary” over a 34-year
period from 1979 to 2012, the study
found.
The summer of 2007 was the
wettest over northern Europe
during this period, and the
summers of 2010 and 2011 were the
fifth and sixth wettest respectively.
In contrast, southern Europe
experienced drier-than-usual
conditions, it found.
The study, based on computer
models of the weather, found that
the dramatic loss of summer sea ice
in the Arctic over the past decade
can cause the jet stream – a high
altitude ribbon of fast-moving air –
to shift further south, bring stormy,
wet weather to Britain and north-
west Europe.
“The results of the computer model
suggest that melting Arctic sea ice
causes a change in the position of
the jet stream, and this could help
to explain the recent wet summers
we have seen,” said James Screen of
the University of Exeter, who
carried out the research. “The loss
of Arctic sea ice tends to shift the
jet stream to the south in summer
and so increases the risk of wet
summers in the UK and north-west
Europe.”
He added: “The study suggests that
the loss of sea ice not only has an
effect on the environment and
wildlife of the Arctic region, but has
far-reaching consequence for people
living in Europe and beyond.”
The study, published in the journal
Environmental Research Letters, is
the latest to suggest a possible link
between the loss of summer sea ice
in the Arctic and weather extremes
around the northern hemisphere.
Previous research found that sea-ice
loss could be responsible for
increasing the winter risk of cold,
easterly winds from the Siberian
Arctic blowing across north-western
Europe, as they did in the winters
of 2010 and 2011.
Dr Screen said that there are
several possible factors that could
have influenced recent wet
summers, including the natural
decade-long fluctuations in the sea-
surface temperatures of the Atlantic
Ocean. However, the latest study
suggests that sea ice and the jet
stream could explain up to a third
of the trend towards wetter
summers, he said.
The computer model compared
weather patterns when the summer
sea-ice was relatively extensive, as
it was in the 1970s, to recent years,
when it had contracted by nearly a
half. It found that sea ice alone can
influence the direction in which the
jet stream tends to flow on its
journey from west to east.
When the jet stream veers
northwards between Scotland and
Iceland, which it tended to do this
summer, it usually results in long
periods of dry, sunny weather.
However, when it shifts south, as it
did in the six previous summers, it
brings wet, stormy weather over
Britain from the Atlantic.
“We have some confidence that the
computer models are showing the
real thing. The pattern of summer
rainfall we see in the computer
models, when the only factor we
change is the amount of sea ice in
the Arctic, is very similar to what
we’ve experienced over the past
few years,” Dr Screen said.
Previous studies by Jennifer Francis
of Rutgers University in New Jersey
suggested that the loss of sea ice
could be affecting the temperature
gradients between polar regions
and lower latitudes further south,
which can influence the speed of
the jet stream and so affect how
likely it is to veer further north or
south.
However, Dr Screen said that the
latest study only partly supports
this controversial idea. “The jet
stream is a complex beast,” he said.
http://www.independent.co.uk/news/uk/home-news/melting-arctic-sea-ice-meansits-only-going-to-get-wetter-for-northern-europe-8909587.html