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The sporadic bursts of cold weather that occur in the eastern half of
the US are not enough for some. Meteorologists who have spent several decades
attempting to predict the vagaries of winter weather are hoping for more
icy blasts to rescue what could be the best seasonal forecast so far. It
is the fruit of 40 years of detailed observation of solar activity and the
behaviour of winds in the upper atmosphere. But the basis for this forecast
hangs by a thread. Only two years ago, during the winter of 1988/89, there
was a similarly ‘correct’ combination of solar and upper atmosphere conditions.
The predicted cold winter obstinately failed to materialise, and meteorologists
were left wondering where they had gone wrong.

The lack of a reliable method for long-term seasonal forecasting is
more than a minor inconvenience. History provides numerous examples of the
devastating effects of unexpectedly severe weather. Moreover, its economic
consequences are still keenly felt. The sustained cold of December 1976
and January 1977 in the US, for example, caused massive economic disruption
as supplies of oil and natural gas ran short. Some estimates of the cost
ran as high as $40 billion.

This was not an isolated occurence. Abnormally cold spells have hit
the eastern half of the US in nine of the past 15 years. In northwestern
Europe, despite the mild winters of the past three years, there were six
extremely cold winters in the 10 years from 1978 to 1987. Even last winter,
which was mild overall, parts of the eastern half of the US had their coldest
December for a century. Oil prices rose by more than 20 per cent as a result,
in response to the increased demand for domestic heating oil.

If only meteorologists could predict with some certainty the likelihood
of an exceptionally cold winter, at least the problems of inadequate fuel
stocks could be avoided. But such a prediction is no simple matter. The
numerical models that meteorologists use to predict day-to-day weather will
never be accurate for more than 10 to 14 days ahead. Instead, forecasters
of seasonal variations must interpret past observations of changes in weather
patterns around the world.

Since the middle of the 19th century, researchers have tried to show
that changes in the weather are connected in some way with sunspots, the
outward sign of an increase in the activity of the Sun. Over the past few
years, meteorologists Harry van Loon and Karin Labitzke have compiled statistical
evidence for such a link (see ‘When the wind blows’, 8 September 1988).
This link appears to involve winds that blow in the stratosphere above the
equator, reversing their direction regularly, from east to west or from
west to east, every 12 to 15 months. The phenomenon is called the quasi-biennial
oscillation. Meteorologists have known about the QBO since the 1950s, but
until the 1980s no one recognised a subtle but statistically significant
link between the QBO and certain patterns of weather. When the stratospheric
winds blow from the west, and coincide with a period of high solar activity
– these occur about every 11 years, and correspond to the appearance of
a large number of dark spots on the surface of the Sun – winters in the
eastern and central US are very cold.

On this basis, some meteorologists, among them Anthony Barnston and
Robert Livezey from the Climatic Analysis Center of the US National Weather
Service, predicted that between December 1988 and February 1989, the US
would suffer some severe weather. As we know, the winter of 1988/89 was
mild overall, although February was cold. It looked as though their attept
to make the connection between the Sun and the weather on Earth had failed.
But those researchers who remained convinced that such a link existed began
to search for reasons why.

In late 1989, Barnston and Livezey thought they had the answer. They
said their prediction had failed to take account of another important element
in the climate: the more or less regular pattern of fluctuations in the
tropical Pacific. When the temperature of the surface of the Pacific is
abnormally high – the phenomenon called El Nino – the chances of cold winter
weather over North America increase.

But the opposite situation, where surface temperatures are well below
normal – La Nina – is far less common. So much so that until late 1988,
no one had seen the combination of La Nina, westerly stratospheric winds
and high solar activity. Barnston and Livezey proposed that La Nina was
responsible for the failure of the predictions. They reasoned that La Nina
had cancelled out the effect of the other two climatic factors.

Success this year will help to back up the theory. But it will also
highlight two important problems with a forecasting method based on the
theory of a link between the Sun and the weather on Earth. First is the
inherent weakness of the link as a forecasting tool. If fluctuations elsewhere
in the climate system can throw the influence of the Sun and the weather
patterns of the upper atmosphere so completely out of gear, then perhaps
the link is not such a reliable indicator as the meteorologists thought.
Secondly, and more importantly, meteorologists have so far been unable to
come up with a sound physical explanation of precisely how changes in the
number of sunspots combine with stratospheric winds to influence the weather
at lower levels.

Despite observational evidence for the link, it is still possible that
this is nothing more than a statistical coincidence, reflecting the natural
variability of the climate. At best, it may be too soon to say. Meteorologists
have seen the correlation for only three solar cycles. And computer models
of long-term atmospheric patterns suggest that the atmosphere may have its
own in-built fluctuations that repeat every 10 or 12 years. So the statistics
may simply represent the interaction between two normal fluctuations, neither
of which happens at a fixed frequency. This means that after a few cycles
the correlation with solar activity could shift, or even disappear completely.

Finding a convincing physical mechanism that will explain the connection
between solar activity, the behaviour of the upper atmosphere and the weather
on the surface of the Earth is tricky. The day-to-day weather is driven
almost entirely by solar energy absorbed or near the suface of the Earth,
in the lowest part of the atmosphere called the troposphere. But changes
in the energy output of the Sun are much too small to explain the longer-term
chagnes in climate that we experience on Earth. No one has yet found a thermodynamically
acceptable explanation of how such small changes in the energy of the stratosphere
could influence the troposphere.

In 1988, Brian Tinsley at the National Science Foundation in Washington
reviewed the possible mechanisms by which solar changes in the stratosphere
could be amplified in the troposphere. There is clear evidence that this
amplification could happen. Only about a hundredth of the Sun’s output is
in the far ultraviolet (very short wavelength, high energy) region of the
electromagnetic spectrum but about a fifth of the observed change in solar
output is at these wavelengths. Such changes could strongly affect the concentration
and the distribution in the upper atmosphere of minor components such as
ozone. Small changes in the concentration of these chemical species may
induce large changes in the energy balance of the Earth. In other words,
altering the chemistry of the upper atmosphere alters the balance between
what the Earth’s atmosphere absorbs and what it radiates. This in turn could
alter the circulation patterns that determine our weather.

The same effect could result from the reversal in the Sun’s magnetic
field that accompanies each 11-year cycle of solar activity. Magnetic changes
affect the way in which cosmic rays are funnelled down to the upper ultraviolet
atmosphere around the poles. Like variations in ultraviolet radiation, changes
in the pattern of this ionising radiation could alter the chemical make-up
of the upper atmosphere, especially reactive chemical species such as free
radicals.

Blocking anticyclones

Computer models are not yet advanced enough even to simulate the QBO
in the stratosphere. It will be a long time before they can hope to take
account of subtle variations in chemistry, then correlate these with unexpected
weather patterns. In so doing they will need to forge a link with a well-known
meteorological phenomenon called ‘blocking’. Blocking systems now seem to
be associated with variations in both high-altitude winds – (the jet stream)
and in tropical ocean temperatures. In middle latitudes, westerly winds
fluctuate between a vigorous, almost circular pattern and a weaker, meandering
one. These alternative circulation patterns are most easily seen in the
pressure patterns of the middle troposphere – the part of the atmosphere
that lies between 3 and 7 kilometres above the Earth. As the flow of these
upper winds becomes more contorted they occasionally split into two branches.
Sometimes these two branches trap a stationary region of high pressure,
called a blocking anticyclone.

Such blocking anticyclones usually last for about two weeks, but sometimes
for much longer. Their position in the atmosphere depends partly on where
they are in relation to continents and mountains in the northern hemisphere.
The Himalays and the Rockies in particular seem to play an important part
in setting up unusual ciculation patterns. There is some evidence that this
is true in that blocking is less common in the southern hemisphere, where
middle latitudes are almost entirely covered by oceans. Most blocking anticyclones
appear close to the Greenwich meridian, with about half as many in the eastern
part of the North Pacific. Occasionally, these two blocks are formed simultaneously,
as in the extreme winters of 1963 and 1979. During these two winters there
were blocking anticyclones over Britain and the West coast of the US.

Until the early 1980s, many researchers thought blocking was unconnected
with events in the tropics. In 1983, Mikuro Miyakoda and his colleagues
at the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey reinforced
this assumption when they examined the pattern of weather in January 1977.
They found an exceptionally strong Pacific block – and the eastern half
of the US shivered through the coldest month on record.

Myakoda’s conclusions came as a pleasant surprise to meteorologists.
The model had produced a remarkably accurate forecast of the month’s unusual
weather patterns, using only information about the atmosphere on 1 January.
It had assumed normal values for other things that might have influenced
the weather, such as snow cover and the temperature of the sea surface.
It seemed that, when the westerly winds in the upper troposphere reached
a certain critical combination of speed and direction, they would set up
an exaggerated standing wave pattern between the Himalayas and the Rockies.
This would ‘lock’ the weather patterns for some time, sweeping mild air
north towards Alaska, and allowing Arctic air to surge south across the
eastern region of North America. At last they had begun to make headway
in explaining the extremes of weather.

But subsequent efforts to make monthly forecasts based on these results
were much less successful. By the late 1980s, researchers had come to the
conclusion that focusing on the middle latitudes was far too narrow a view.
The much-hailed success of 1983 had, it seemed, been a fluke. It was impossible
to expand the method used to produce this single forecast into a general
one. There were two main reasons for this shift of opinion. The first and
most dramatic was the El Nino of 1982/83. The second was the realisation
that certain weather patterns in the tropics, which repeated every 40 or
50 days, were not simply local disturbances: they had far-reaching effects
on climate all over the world.

The record-breaking El Nino of 1982/83 changed the course of meteorological
thinking. For many years, meteorologists had accepted that changes in the
temperature of the tropical oceans must affect the world’s climate. They
knew, for example, that these oceans provide much of the heat energy that
drives the circulation of the atmosphere. Some of this heat is released
as the rising plume of moist air above the tropical regions condenses as
water vapour. But the evidence of a link between El Nino and unusual weather
conditions outside the tropics was hard to find. Nevertheless, following
the coincidence between the extreme winter weather of 1976/77 and El Nino,
a few researchers proposed a physical connection between the two. This particular
debate continues, but since then the events of the 1980s have led researchers
to accumulate more evidence to support the theory that an El Nino or a La
Nina will have an important influence on wave patterns in the atmosphere
of middle latitudes, especially in winter. Certainly, this would fit in
with Barnston and Livezey’s explanation of why their forecast for the winter
of 1988/89 failed.

In the early 1980s work at Kyoto University and the University of Wisconsin
produced evidence of weather cycles that repeat much more often. These studies
support the wider ‘global’ view of the reasons for extreme weather. Roaland
Madden and Paul Julian of the National Center for Atmospheric Research in
Boulder, Colorado, had first discovered short-term oscillations in 1971
in the winds that blow 15 kilometres above Caton Island in the Central Pacific.
But their work went largely unnoticed until geostationary weather satellites
recorded unusual bands of cloud. These observations confirmed Madden and
Julian’s theory that variations in stratospheric winds were linked with
the weather patterns of the troposphere. The patterns took the form of cloud
development above the Indian Ocean every 40 or 50 days. The clouds would
intensify, sweep eastwards across the Pacific at 30 kilometres per hour,
and peter out before they reached South America.

Now many researchers think these oscillations have much more than local
effects. It seems that they are connected in some way with the timing and
strength of the monsoon over India. More importantly, perhaps, strong short-term
oscillations seem to produce more frequent blocking in middle latitudes.
And the strength of these short-term oscillations seems to be related to
the QBO in the stratosphere. So far, the explanation for these links has
eluded researchers. They suspect that, as with El Nino, the key lies in
alterations in the way energy is transported by the atmosphere from the
tropics to high latitudes.

Meteorologists still lack a physical explanation for what they observe.
But their observations suggest that it may be possible to predict accurately
at least some longer-term features of the weather. Such predictions must
be based on a better understanding of the way climate changes over months,
even years. This requires a model for the way the various components of
world climate interact: a tall order, perhaps, but the economic benefits
are undeniable.

* * *

History leaves forecasters out in the cold

Before the Industrial Revolution, extreme weather in Europe had serious
consequences, not just for people but for economics. As rivers froze, water
mills stopped and flour production ceased, making bread scarce. More recently,
the icy conditions prevailing in Europe may have been the most important
reason why the Phoney War was prolonged until the spring of 1940.

Goebbels recorded at length in his diary how the cold had a major impact
on the industrial output of Germany. Stocks of coal diminished and railways
could no longer meet demand. The cold also put a stop to normal military
operations, preventing Hitler from launching an attack on the ill-prepared
Anglo-French forces until the spring.

In 1947 the fuel crisis caused by extreme winter weather almost brought
Britain to its knees. Seven weeks of unexpected and intense cold from late
January led to severe restrictions on the use of power. The electricity
and coal industries had not yet recovered from the underinvestment of the
war years. Stocks of coal were too low to meet demand and the railways could
not move coal from the mines. Industrial production fell by a quarter in
February compared with other winter months. Although industry recovered
quickly, Britain lost an estimated 200 million Pounds in exports, a major
contribution to the total deficit of 630 million Pounds in 1947. This led
directly to the devaluation of sterling in the following year.

In January 1963 electricity and gas consumption in Britain rose by 15
to 20 per cent, while economic output dropped by 7 per cent. A similar economic
impact was recorded across northern Europe from Poland to France.

It was not only advanced industrial countries that were affected by
the cold weather of 1979/80. The coldest winter in 30 years caused massive
fuel shortages, frequent power cuts and widespread economic disruption in
Turkey. This precipitated a dramatic shift in the country’s economic policies
and heralded and long period of austerity.

William Burroughs is a writer on weather and climate. His latest book
Watching the World’s Weather will be published in March by Cambridge University
Press.