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Science Forum Index » Environment Forum » Cyclonic Storms Over Western Europe
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| Author |
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| David Naugler |
 Posted: Tue Dec 16, 2003 11:09 am |
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Guest
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From:
http://www.vision.net.au/~daly/guests/max-beran.htm
Cyclonic Storms Over Western Europe
by Max Beran
In a BBC radio interview on Tuesday 9 December, Geoff Jenkins of the
Hadley Centre informed listeners that his organization had identified
a significant trend in UK storminess over the past half-century or so.
He explained this in terms of storm tracks in the Atlantic region and
insisted that it could not be explained other than via man-made
climate change.
My ears pricked up at this as I recalled a salutary tale about time
series and trends by Sir John Mason, at the time Director General of
the Met Office, which also concerned UK wind storms. He first showed a
pillar-graph of storm frequency by decade (starting, I think, from the
1950s). This revealed a very clear upward trend within the span of the
diagram. He then added the data from the previous decade which
entirely quashed that original impression.
At that time (early 1980s I guess), the meteorological establishment
was sceptical about man-made global warming, 180 degrees away from
their current stance. So I was motivated to follow up on these new
data.
A bit of digging led to the conference of the parties (COP9) in Milan
and a report for it prepared by the Met Office
(you can read it at
www.met-office.gov.uk/research/hadleycentre/pubs/brochures/2003/global.pdf
..)
The relevant diagrams and text make it clear that the pronouncements
about trend and attribution were not based on wind data (too many
disturbances) but on a storm index derived from changes in 3-hour air
pressure data from 28 UK locations. Details of the construction of the
index are not provided. However the key phrase is "the average number
of storms (per station) shows a significant increase in the United
Kingdom winter period" and this, coupled with the positive attribution
to man's activities in causing a shift in storm tracks, was also the
take-home message from the interview.
The diagrams in the COP9 report reveal very noisy time series with a
positive skewness (upward spikes taller than downward). An upward
trend is indeed discernible, assisted by the addition of trend and
smoothing lines. What interested me was the significance of the trend
especially as the passage from storminess index to wind speed will
inject further scatter. So I digitised the graphs to obtain the
annual time series. In what follows I tended to stick to the Jan-Mar
data as I couldn't be sure which winter the Oct-Dec data referred to
(lagged correlations were higher than current-year correlation).
The initial analysis was a regression of storminess index on year
number. The regression coefficient implies a 17% increase in
storminess index per decade. At nearly 2.4 times its own standard
error of estimate, this appears highly significant and remains so even
after allowing for a null hypothesis that doesn't predetermine the
sign of the trend (something which is frequently forgotten by
researchers eager to establish the existence of upward trends). This
conclusion is also not altered by transforming the data to eliminate
the asymmetry between upward and downward spikes. Of course this does
not exhaust the scope for inflating the significance through selection
of season, base time period, and spatial boundaries, but a large
reduction in the number of degrees of freedom would be required to
bring a multiple of 2.4 out of the range of statistical significance.
Establishing a trend is of course not the same as establishing
attribution. Specifically it does not examine the possible influence
of natural fluctuations that are known to operate on the decadal time
period. In climatological studies for the UK area, they are
frequently represented by the North Atlantic Oscillation (NAO) index
which is itself based on the meridional air pressure gradient in the
North Atlantic.
The Met Office's COP9 report in fact does discuss the possible
relevance of the NAO as a natural contributor to the trend but
dismisses it as being of secondary importance. It was noticeable
though that this dismissal was rather uncharacteristically hedged with
several small provisos so one sensed that the issue might not be as
clear cut as the bold take-home message implied. This prompted a more
detailed analysis of the time series adding in NAO data as an
explanatory variable (NAO monthly data are available on the Climatic
Research Unit website at http://www.cru.uea.ac.uk/ftpdata/nao.dat).
The first calculation was to compare the simple pairwise correlations
between the variables. It transpired from this that the variance in
storminess index explained by the NAO was considerably greater that
that explained by the trend term (over 25% for NAO versus less than
10% for year number) a finding which is itself at variance with the
implication of the statement in the report for COP9 that "although
there is a similar upward trend in the NAO, there is quite a poor
correlation between this figure and the storm rate ….". However,
because of interactions between them, pairwise comparisons are
insufficient to separate the variance explained by two contending
variables. This can be done using a linear regression between the
storminess index as dependent variable and year number and NAO as
independent variables. The analysis confirmed the initial finding
based on the pairwise correlations in that the burden of explained
variance fell on the NAO term. In fact the regression coefficient of
the secular trend term dropped from 2.4 to 1.4 times its standard
error which takes it out of the range of acceptable significance; as
many as one trend-free sample in five could throw up a positive or
negative trend of this size.
See:
http://www.vision.net.au/~daly/guests/max-beran.htm
for graph.
The weakness of the secular trend is illustrated in the above graph
which shows the time trend (or lack of) in the storminess index once
the effect of NAO is removed. There is a hint of a quadratic term
visible on the diagram and adding such a term in an analysis of
variance shows the linear and quadratic terms to be of approximate
equal "power", though neither are significant.
I suspect that "Still Waiting for the Greenhouse" readers will not
need a moral to this tale – secular trend disappears once one allows
for non-stationary fluctuations operating in the background - it is
all too common in global change science to find little or nothing of
substance once one takes the time and effort to dig behind the
headlines. |
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