HERE鈥橲 some handy advice for 2001. When the Atlantic off America鈥檚 east coast
is warm in September, northern Europe had better watch out. Expect storms and
widespread floods in the coming winter. For the Middle East the outlook is even
worse: it could mean war. But these aren鈥檛 wild imaginings culled from the pages
of the latest Old Moore鈥檚 Alamanack. The events are all likely
consequences of one of the planet鈥檚 most complex weather phenomena鈥攖he
North Atlantic Oscillation.
The NAO is the Atlantic鈥檚 version of El Ni帽o. It鈥檚 a see-saw flip in
weather systems that, in the depths of winter, is capable of throwing a
successions of storms at the shores of north-west Europe. This autumn, as the
gales howled and flood waters lapped at front doors all over Europe, newspapers
pinned the blame for the bad weather on the oscillation. No need to panic about
global warming, they concluded, it was all the fault of the NAO.
But when weather statistics were finally published for November, the month
that had the worst storms, it emerged that the oscillation had been in the wrong
phase. If anything, it should have been calming the winds and stemming the
floods.
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David Stephenson of the University of Reading, one of the world鈥檚 foremost
authorities on the oscillation, was in no mood to defend the story. 鈥淚 didn鈥檛
say the NAO caused the floods,鈥� he told 快猫短视频. 鈥淚t鈥檚 much more
complicated than that.鈥�
This flurry of misguided interest has raised the public profile of what is
probably the hottest topic among climatologists. The oscillation has been known
about for almost two centuries, but until five years ago it was a research
backwater, yielding about three papers a year on average. Now all that has
changed, and 120 papers were published last year. And six weeks ago, in Orense,
Spain, the American Geophysical Union devoted a four-day conference to the
subject. It is no exaggeration to say that the NAO is inextricably linked to the
future climate of Europe and much of Asia.
Meteorologists follow the NAO using an index that measures the difference in
air pressure between Iceland and the Azores, a group of islands west of Portugal
some 1500 kilometres out into the Atlantic. When the pressure is low over
Iceland and high over the Azores, the index is positive. When the pressure
gradient is reversed, the index is negative.
For much of the year, it scarcely matters whether the index is positive or
negative. But in winter, a high positive index is closely linked to an increase
in the number and intensity of storms across northern Europe. Meanwhile,
southern Europe gets cool, calm and dry weather.
When the index becomes negative, a large area of high pressure spreads
westwards from Siberia, bringing cold, dry and calm weather to northern Europe.
Further south, it turns warm and stormy.
What makes the NAO especially interesting is that the system tends to flip
between these two modes. Almost any winter will see instances of both, sometimes
lasting for only a few days and normally one mode will dominate. In recent
years, however, the NAO has been in a period of unprecedented activity. Its
flips are bigger, and when the index is averaged over the winter it has usually
been positive, often strongly so. So what is changing the index? Bring on the
accused: global warming.
According to data from the past century collected by Phil Jones of the
University of East Anglia, 4 of the 10 most positive winter averages of the
index have occurred in the past 15 years鈥攁 period of record global
temperatures. This winter, if current forecasts are correct, will make this 5
out of 10. What鈥檚 more, in the past 15 years only one winter has had an average
negative index. That was 1995-96, when a winter drought in Britain led to water
shortages the following summer.
The last time the NAO was routinely positive was between 1900 and 1930. That
period also coincided with a spell of planetary warming. Back then the main
cause of warming was probably the solar cycles. This time, it鈥檚 most likely the
greenhouse effect. 鈥淭he period since the early 1970s is the most prolonged
positive phase of the oscillation,鈥� says Tim Hurrell of the National Center for
Atmospheric Research in Boulder, Colorado, who coordinated the Orense
conference. Hurrell admits that this pattern could be random. 鈥淏ut most
researchers no longer believe it. We only really have a record of the NAO going
back 200 years, but it is clear the past 40 years have been quite
别虫肠别辫迟颈辞苍补濒.鈥�
What could be going on? Less than two years ago, Mark Rodwell of Britain鈥檚
Meteorological Office in Bracknell first suggested that the NAO is no random
phenomenon. Rodwell showed that a large, positive winter index was tied to
changes in sea surface temperatures off the US coast the previous September
(快猫短视频, 27 March 1999, p 11). This link has become a basis for
long-range weather forecasts.
At the beginning of December 2000, Mark Saunders and colleagues from
University College London used this link to forecast a 75 per cent chance that
temperature, rainfall, wind speeds and storminess would all be above average
over north-west Europe this winter. It is also likely that the cool and dry
weather in the Mediterranean will extend eastwards across Turkey and into
central Asia.
A recent study by Stephenson found that water flow in the River Euphrates,
which runs from Turkey through Syria and Iraq to the Arabian Gulf, is halved in
years with a strongly positive index. This should set diplomats thinking. The
three countries have a long-running argument over low water levels in the
Euphrates. Turkey and Syria have both dammed the river, and in 1975 Iraq
threatened to bomb a Syrian dam, while Syria blamed Turkey for the water
shortage downstream. Intervention by Saudi Arabia cooled things down, but in
1986 Turkey claimed to have uncovered a Syrian plot to blow up one of its dams,
and two years later Syrian planes shot down a Turkish plane as the simmering
dispute continued. Could a similar drought this year cause tensions to rise
again?
Meanwhile, Rodwell and Saunders cannot say for sure why warm waters in the
western Atlantic should trigger storms over northern Europe. 鈥淎ll we know is
that there is a large exchange of heat from the ocean to the atmosphere there at
that time,鈥� says Saunders. It is possible that storms are a by-product of that
extra energy.
So why is the NAO index becoming more positive? Tim Osborn of the University
of East Anglia recently ran a series of global climate models to simulate 1400
years of 鈥渘atural鈥� weather. None of the winters produced by the model had a
markedly positive NAO index. But as soon as Osborn introduced global warming
into the simulation, the index became strongly positive. 鈥淚n effect, the only
way you can get anything like the recent trend in the NAO is through global
warming,鈥� says Hurrell.
This as yet unpublished finding is bound to generate speculation that the
greenhouse effect is changing the NAO. 鈥淵ou would expect global warming to work
by changing the existing patterns of the atmosphere, such as El Ni帽o and
the NAO,鈥� says Hurrell.
Global warming seems to be closely linked to changes in the NAO. And if that
is true, Europe will surely have to batten down the hatches.
