By Paul Homewood
WUWT covered this story last week:
From the UNIVERSITY OF LINCOLN, where they are trying to make the North Atlantic Oscillation bend to their will part of climate change.
Severe snowy weather in winter or extreme rains in summer in the UK might be influenced by warming trends in the Arctic, according to new findings.
Climate scientists from the UK and the US examined historic data of extreme weather events in the UK over the past decade and compared them with the position of the North Atlantic polar atmospheric jet steam using a measure called the North Atlantic Oscillation (NAO) index.
The NAO indicates the position of the jet stream – which is a giant current of air that broadly flows eastwards over mid-latitude regions around the globe – through a diagram which shows ‘negative’ and ‘positive’ spikes, similar to how a heart monitor looks.
The researchers highlight that the exceptionally wet UK summers of 2007 and 2012 had notably negative readings of the NAO, as did the cold, snowy winters of 2009/2010 and 2010/2011, while the exceptionally mild, wet, stormy winters experienced in 2013/2014 and 2015/2016 showed pronounced positive spikes.
The scientists also highlighted a correlation between the jet stream’s altered path over the past decade – so-called jet stream ‘waviness’ – and an increase during summer months in a phenomenon called Greenland high-pressure blocking, which represents areas of high pressure that remain nearly stationary over the Greenland region and distort the usual progression of storms across the North Atlantic.
Increased jet waviness is associated with a weakening of the jet stream, and the accompanying ‘blocking’ is linked to some of the most extreme UK seasonal weather events experienced over the past decade. The strength and path of the North Atlantic jet stream and the Greenland blocking phenomena appear to be influenced by increasing temperatures in the Arctic which have averaged at least twice the global warming rate over the past two decades, suggesting that those marked changes may be a key factor affecting extreme weather conditions over the UK, although an Arctic connection may not occur each year.
Edward Hanna, Professor of Climate Science and Meteorology at the University of Lincoln’s School of Geography, carried out the study with Dr Richard Hall, also from the University of Lincoln, and Professor James E Overland from the US National Oceanographic & Atmospheric Administration Pacific Marine Environmental Laboratory.
Professor Hanna said:
“Arctic warming may be driving recent North Atlantic atmospheric circulation changes that are linked to some of the most extreme weather events in the UK over the last decade.
“In winter, a positive North Atlantic Oscillation (NAO) is linked with a more northward, vigorous jet and mild, wet, stormy weather over the UK, while a negative NAO tends to be associated with a more southerly-positioned jet and relatively cold and dry but sometimes snowy conditions. In summer the jet stream is displaced further north, so a positive NAO is typically associated with warm dry weather, while a negative NAO often corresponds to wetter, cooler UK weather conditions.
“While part of the uneven seasonal North Atlantic Oscillation changes might be due to natural random fluctuations in atmospheric circulation, the statistically highly unusual clustering of extreme NAO values in early winter, as well as extreme high summer Greenland Blocking Index values since 2000, suggest a more sustained, systematic change in the North Atlantic atmospheric circulation that may be influenced by longer-term external factors. This includes possible influences from the tropical oceans and solar energy changes as well as the extreme warming that has recently occurred in the Arctic.
“Of course, weather is naturally chaotic, and extremes are a normal part of our highly variable UK climate, but globally there has recently been an increase in the incidence of high temperature and heavy precipitation extremes. The cold UK winter episodes we noted are not so intuitively linked to global climate change but reflect part of a long-term trend towards more variable North Atlantic atmospheric circulation from year to year during winter months, especially early winter.
“This trend has culminated in the last decade having several record negative and positive December values of the North Atlantic Oscillation, with lots of resulting disruption from extreme weather over the UK. On the other hand there has been no really notably dry, hot, sunny summer in the UK since 2006; summers overall have either been around average or exceptionally wet, and this appears to be linked with strong warming and more frequent high pressure over Greenland in the last decade.”
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Now that the Met Office has published the data for 2017, we can see what the facts tell us.
Wet Summers
First, summer rainfall.
Remember the claim is that:
Severe snowy weather in winter or extreme rains in summer in the UK might be influenced by warming trends in the Arctic, according to new findings.
Summers overall have either been around average or exceptionally wet, and this appears to be linked with strong warming and more frequent high pressure over Greenland in the last decade.
However the data from the Met Office’s England & Wales Precipitation Series give us a totally different story.
Although there have been a couple of wet summers recently, there is absolutely nothing unusual at all about these, when looked at with a longer term perspective.
Indeed, it is plain that wet summers were more common in the 19thC, tending to indicate that a colder Arctic, and correspondingly more southerly jet stream, leads to wetter summers. Not a warmer Arctic, as Hanna suggests.
The facts are even more stark when we look at extreme rainfall months:
Plainly Hanna’s argument falls flat on its face as far as summer rainfall is concerned, but what about his other claim?
Wet Winters
Severe snowy weather in winter or extreme rains in summer in the UK might be influenced by warming trends in the Arctic.
The researchers highlight that the exceptionally wet UK summers of 2007 and 2012 had notably negative readings of the NAO, as did the cold, snowy winters of 2009/2010 and 2010/2011, while the exceptionally mild, wet, stormy winters experienced in 2013/2014 and 2015/2016 showed pronounced positive spikes.
Yes, we have had two wet winters recently. But were they anything more than simple weather events?
Obviously the winter of 2013/14 stands out, but even that weather was not unprecedented. Between November 1929 and January 1930, rainfall totalled 500mm, compared to 455mm in 2013/14.
Of course, November to January is not a season, but the exceptional rainfall then, a record for any three months on the Series, was no less remarkable for that.
[The paper puts a lot of emphasis on weather blocking. In that respect, it is worth noting that the extraordinarily wet weather in 1929/30 actually lasted for four months, not just three. Between October and January, precipitation totals were 624mm.
The wettest four month period in 2013/14 was much less, with 556mm from October to January]
Other than that standout winter of 2013/14, there has been nothing out of the ordinary in the last decade or two.
As for extreme months, the wettest winter months were in the 1860s and 70s, and later in the 1910s.
So, as with summer rainfall, Hanna’s theory simply does not correlate with any of the very clearly known facts.
Cold Winters
Severe snowy weather in winter or extreme rains in summer in the UK might be influenced by warming trends in the Arctic.
The cold UK winter episodes we noted are not so intuitively linked to global climate change but reflect part of a long-term trend towards more variable North Atlantic atmospheric circulation from year to year during winter months, especially early winter.
According to the official Met Office data, however, there has been nothing remotely unusual about recent winters:
December 2010 was notoriously cold, as was the previous winter. However, as Julia Slingo confirmed in a report to Sir John Beddington at the time, neither episode was particularly unusual.
As for cold monthly extremes, again we find that they are not becoming more severe or more frequent.
Final Thoughts
According to Hanna, “we need extended records over at least a further decade to more reliably attribute these changes to global warming”. (Translation – give us some more money!)
He needs no such thing. He already has two centuries of data which wreck his little theory.
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January 9, 2018 at 06:36AM
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