New study: Freak summer weather and wild jet-stream patterns are on the rise because of global warming

Simulation of jet stream pattern July 22, 2018. (VentuSky.com)

by Jason Samenow, "The Capitol Weather Gang," The Washington Post, October 31, 2018

In many ways, the summer of 2018 marked a turning point, when the effects of climate change — perhaps previously on the periphery of public consciousness — suddenly took center stage. Record high temperatures spread all over the Northern Hemisphere. Wildfires raged out of control. And devastating floods were frequent.

Michael Mann, climate scientist at Pennsylvania State University, along with colleagues, has published a new study that connects these disruptive weather extremes with a fundamental change in how the jet stream is behaving during the summer. Linked to the warming climate, the study suggests this change in the atmosphere’s steering current is making these extremes occur more frequently, with greater intensity, and for longer periods of time.

The study projects this erratic jet-stream behavior will increase in the future, leading to more severe heat waves, droughts, fires and floods.

[Climate change supercharged a hot and dangerous summer]

The jet stream is changing not only because the planet is warming up but also because the Arctic is warming faster than the mid-latitudes, the study says. The jet stream is driven by temperature contrasts, and these contrasts are shrinking. The result is a slower jet stream with more wavy peaks and troughs that Mann and his study co-authors ascribe to a process known as “quasi-resonant amplification.”

The altered jet-stream behavior is important because when it takes deep excursions to the south in the summer, it sets up a collision between cool air from the north and the summer’s torrid heat, often spurring excessive rain. But when the jet stream retreats to the north, bulging heat domes form underneath it, leading to record heat and dry spells.

[‘Extreme’ jet stream pattern spurred a week of wild weather in U.S. this past summer]

If the excursions in the jet stream endure long enough, it can then set the stage for floods where the jet dips, and wildfires and drought where it ascends.

“What made these events [in the summer of 2018] so devastating was not just the extreme nature of the meteorological episodes but their persistence,” Mann said in a blog post discussing the implications of the new study.

The study, published Wednesday in Science Advances, finds that these quasi-resonant amplification events — in which the jet stream exhibits this extreme behavior during the summer — are predicted to increase by 50 percent this century if emissions of carbon dioxide and other greenhouse gases continue unchecked.

Whereas previous work conducted by Mann and others had identified a signal for an increase in these events, this study for the first time examined how they may change in the future using climate model simulations.

“Looking at a large number of different computer models, we found interesting differences,” said Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research and a co-author of the study, in a news release. “Distinct climate models provide quite diverging forecasts for future climate resonance events. However, on average they show a clear increase in such events.”

In an email, Mann said climate models aren’t fully capturing the phenomenon, and, for this reason, we should expect weather extremes “beyond what is typically projected” into the future.

Mann added the existing analyses that attempt to uncover the role of climate change in recent extreme events “are under-attributing the role that climate change is having … because they are not capturing the key mechanism responsible.”

Mann said in his blog commentary that he was particularly struck by the jet-stream behavior in the summer. “In summer 2018, I would argue, that signal was no longer subtle,” he said. “It played out in real time on our television screens and newspaper headlines in the form of an unprecedented hemisphere-wide pattern of extreme floods, droughts, heat waves and wildfires.”

Although model projections suggest these extreme jet-stream patterns will increase as the climate warms, the study concluded that their increase can be slowed if greenhouse gas emissions are reduced along with particulate pollution in developing countries. “[T]he future is still very much in our hands when it comes to dangerous and damaging summer weather extremes,” Mann said. “It’s simply a matter of our willpower to transition quickly from fossil fuels to renewable energy.”

Dr. Jennifer Francis, a climate researcher at Rutgers University who has published work exhibiting changing jet-stream behavior because of climate change, found the results of this new study compelling. “This work takes a big step toward understanding the spate of deadly extreme weather events during recent summers — heat waves, floods and droughts,” she said in an email.

https://www.washingtonpost.com/weather/2018/10/31/study-freak-summer-weather-wild-jet-stream-patterns-are-rise-due-global-warming/?noredirect=on&utm_term=.7afba03610ea

Northern Hemisphere jet streams stumble as the world warms

The warming of the atmosphere by greenhouse gases is slowing the jet streams which drive the Northern Hemisphere's weather, scientists say. 

by Tim Radford, Climate News Network, April 9, 2017

LONDON – Researchers have once again linked a sequence of devastating climate events to global warming fuelled by prodigal human use of fossil fuels. And this time, they believe they have identified the agency behind the blazing summers that have claimed lives and destroyed livelihoods repeatedly during this century.

They argue in the journal Scientific Reports that human impact on the climate now reaches high into the stratosphere, to influence the behaviour patterns of the giant jet streams that carry heat and moisture around the Northern Hemisphere and keep the weather on the move.

Warming driven by carbon dioxide emissions from car exhausts and power stations, they argue, tends to make these giant oscillating waves stall in their journey around the hemisphere – to create enduring episodes of high and low pressure and lingering hazards of drought and flood.

“The unprecedented 2016 California drought, the 2011 US heatwave and 2010 Pakistan flood as well as the 2003 European hot spell all belong to a most worrying series of extremes,” says Michael Mann from Pennsylvania State University in the US.

“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity”

“The increased incidence of these events exceeds what we would expect from the direct effects of global warming alone, so there must be an additional climate change effect. In data from computer simulations as well as observations, we identify changes that favour unusually persistent, extreme meanders of the jet stream that support such extreme weather events.

“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity.”

Professor Mann has repeatedly confirmed the link between human action and climate change. His co-author Dim Coumou of the Potsdam Institute for Climate Impact Research in Germany and the VU University in Amsterdam in the Netherlands has separately linked storm tracks to surface temperature extremes, made a connection between torrential rains and planetary warming, and confirmed too that less stormy weather is not necessarily a good sign, because it could be the harbinger of heat waves.

And the researchers now have support for their their suspicions: the jet streams that sweep the hemisphere in huge atmospheric waves, plunging between Arctic and tropics, bring changes of weather.

If they should stall, one region may be committed to long drought, dangerous hot weather (as in Russia in 2010 and Texas in 2011) and even forest fires as in California in 2015) – or, in some cases, catastrophic and sustained rainfall of the kind that flooded Pakistan in 2010.

Questions remain

No single extreme event could ever be satisfactorily and conclusively linked to a long-term trend like global warming. But once scientists register an increasing frequency of such events, they can start to use climate simulations to see if such events become more likely in a warming world.

“The more frequent persistent and meandering jet stream state seems to be a relatively recent phenomenon, which makes it even more relevant," said Dr Coumou. “We certainly need to further investigate this – there is some good evidence, but also many open questions.”

And Professor Mann said: “The warming of the Arctic, the polar amplification of warming, plays a key role here. The surface and lower atmosphere are warming more in the Arctic than anywhere else on the globe.

“That pattern projects onto the very temperature gradient profile that we identify as supporting atmospheric waveguide conditions.”

http://climatenewsnetwork.net/jet-streams-stumble-as-world-warms/

Rapid Arctic ice loss linked to extreme weather changes in Europe and US

Arctic warming appears to be the prime reason behind fluctuations in the polar jet stream that is causing unusual weather, study says 

by John Vidal, The Guardian, June 1, 2015

 Arctic is warming faster than elsewhere, triggering changes in the jet stream which will create more extreme weather in western Europe and North America, researchers say. Photograph: Alamy

The string of massive snowstorms and bone-chilling cold on the US east coast, as well as flooding in Britain and record temperatures in Europe, are linked to rapid ice loss in the Arctic, new research appears to confirm.

While the rapidly-thawing Arctic cannot be held responsible for specific weather events like the “snowmageddon” in 2009, Hurricane Sandy, or European heatwaves, researchers at Rutgers university said it appears to be a prime reason why the polar jet stream – a ribbon of winds that encircles the globe – gets ‘stuck’ with increasing frequency.

Western Europe and large parts of North America will experience more extreme weather because of “Arctic amplification” - the enhanced sensitivity of high latitudes to global warming, the team suggested in a paper published in the journal Philosophical Transactions of the Royal Society A. 

“We are seeing these extremes because the Arctic is warming faster than elsewhere. The whole lower atmosphere is heating up but the sea ice is the most observable. This is having this effect on the jet stream, making it extend further south and stay longer,” said co-author Jennifer Francis.

“The jet stream creates weather of all sorts and where you are in relation to it dictates wether it is hot or cold. When we have a ridge, or a big bulge, in the the jet stream, it makes it extend further and stay longer. When that ridge is stronger it tends to be more persistent,” she said.

Deep troughs in the jet stream have been seen regularly in the past few years affecting the east coast of the US, western Europe and central Asia. These have brought prolonged, unusually hot weather to some places, and extended cold or record snowfall to others.

“We are seeing extended periods of extreme weather because when the temperature difference between polar and mid northern latitudes gets smaller [because of global warming] this has the effect of weakening the jet stream , allowing it to be deflects more easily and to meander more. It’s a combination of natural conditions being intensified and global warming,” said Professor Francis.

The authors expect that eventually it will be possible to predict accurately which types of extreme events will be more likely to occur in certain areas but because Arctic amplification has emerged only in the past 20 years it is a challenge to pin down exactly how it affects weather patterns. 

But the impacts could be substantial, they warn. “This new manifestation of of global warming ... may have substantial societal impact as more frequent extreme weather events in mid latitudes will affect billions of people directly through damage to property and infrastructure and indirectly through farming and water supplies,” the authors wrote.

The study builds on other research which shows how the changing Arctic may be affecting weather in mid-latitudes. According to some, a slower jet stream takes a more meandering path as it encircles the northern hemisphere. Other studies have linked ice loss in the Barents and Kara seas to the north of Russia with extremely cold winters in central Asia.

According to the US Snow and Ice data centre, Arctic sea ice extent last month averaged 5.4 million square miles), the second lowest April ice extent in the satellite record. It is 313,000 sq m) below the 1981 to 2010 long-term average of 6 million sq m) and 31,000 sq m) above the previous record low for the month observed in 2007.

http://www.theguardian.com/environment/2015/jun/01/rapid-arctic-ice-loss-linked-to-extreme-weather-changes-in-europe-and-us

Warming Arctic blamed for worsening summer heatwaves

Russia's 2010 heatwave – a harbinger of even more torrid times? (Image: ARTYOM KOROTAYEV/AFP/Getty)

by Fred Pearce, NewScientist, March 12, 2015

It seems our weather is getting slower – and hotter. Arctic warming appears to be aggravating summer heatwaves across Europe and North America, by putting the brakes on atmospheric circulation in mid-latitudes.

The team that uncovered this Arctic effect says it caused the Russian heatwave of 2010, which lasted six weeks, killing crops and causing massive forest fires; the west European scorcher of 2003 that killed an estimated 70,000 people; and possibly the record US heatwave of 2012, which decimated corn crops.

Dim Coumou and colleagues at the Potsdam Institute for Climate Impact Research in Germany studied atmospheric circulation in the Northern Hemisphere from 1979 to 2013. They found longer and more frequent hot spells in mid-latitudes that, they say, are likely to have been triggered by a reduction in the temperature difference between the Arctic, which is warming quickly, and mid-latitudes, where average warming is slower.

The Arctic has in fact warmed twice as fast as the rest of the globe, because of the melting of the ice replaces a reflective surface with dark ocean that absorbs much more solar energy.

Climatologists believe that this temperature difference drives the general west-to-east movement of mid-latitude weather systems, such as the depressions that bring storms and the high-pressure systems that bring hot dry weather in summer and intense cold in winter. A smaller temperature difference slows these systems down, so their associated weather persists for longer.

Bogged down for weeks

Coumou's team found that the frequency of stalled weather systems in summer has doubled since the onset of rapid Arctic warming around 2000. In many cases, they stop moving for weeks at a time. That can mean long spells of hot weather that dry out soils, kill crops, empty rivers, trigger forest fires – and strain the body, with consequences for our health. So even though average weather is not changing much at mid-latitudes, the incidence of heatwaves is increasing fast.

Until recently, says Coumou, researchers had presumed that impact of the Arctic warming on mid-latitudes was greatest in autumn and winter, when heat flows from the oceans into the cooled atmosphere.

The resulting stalled weather systems have been blamed – alongside the changing polar vortex – for the present long cold winter in the eastern US and the one immediately before this. But Coumou says that the summer effects are just as strong.

Jennifer Francis of Rutgers University, New Jersey, says the findings support her hypothesis that Arctic warming is slowing the jet stream, a high-altitude wind that drives atmospheric circulation at ground level in mid-latitudes.

"Coumou and his colleagues are looking at the individual weather systems that ride along the flow of the jet. And they find that as the Arctic warms, the weather systems stall, leading to more persistent heatwaves," she says. "In the case of heatwaves, there is less surface wind to stir up and distribute heat from a scorching sun beating down on dry soil."

With the Arctic melt set to continue, the long-range forecast is looks set to feature more heatwaves and more drought.

Journal reference: Science, DOI: 10.1126/science.1261768

http://www.newscientist.com/article/dn27150-warming-arctic-blamed-for-worsening-summer-heatwaves.html

Just another hot year for the record books? Not quite. How 2014 US temperatures support the case for global weirding

by Alex Carr Johnson, High Country News, January 13, 2015

At first glance, the 2014 US Climate Report released this week by the National Oceanic and Atmospheric Agency (NOAA) might look like the new, hotter normal. Overall, the average annual temperature in the lower 48 states was 0.5 °F above the 20th century average. That makes 2014 the 18th warmer-than-average year in a row and the 34th warmest year on record.

For those of us who live in the West, these numbers come as no surprise. All 11 Western states experienced warmer-than-normal years. Eight of them landed in the  top-ten hottest years on record, with California, Nevada and Arizona all experiencing their hottest years ever. Alaska also had a record-breaking hot year, with Anchorage never dipping below 0 °F at any point in 2014, a historic first.

Looking east of the Rockies, however, the story quickly becomes, well, weirder. South Dakota had the 34th coldest year on record, and Oklahoma the 29th coldest. Seven states in the middle of the country, from Wisconsin to Louisiana, experienced top-ten most frigid years on record. Most notably, a polar vortex brought the Arctic to the Midwest for the first few weeks of 2014, but cooler-than-normal blasts moved through the region multiple times throughout the rest of the year.

While regional weather differences are nothing new, the prolonged hot-cold divide of 2014 was unprecedented. "Never before have such large areas of the country experienced such radically different temperature extremes," Rebecca Lindsey, managing editor for Climate.gov, explained in a post this past August. From January to July, nearly 40% of the country was affected by extreme temperatures, though which extreme depended on which side of the country you were on.

The primary explanation for the temperature divide rests with the jet stream, a narrow high-speed high-altitude current of air which moves eastward across the northern hemisphere, oscillating between mid- and northern latitudes. Normally, the ridges and troughs of the jet stream tend to shift, allowing for warmer air below and colder air above to swing in and out of each region of the country. Beginning in late 2013, however, the jet stream became stuck. Thanks to a high-pressure system that sat over the northern Pacific and a low-pressure system in the upper Midwest, the jet stream spent the first half of 2014 swinging far north into Alaska and Canada, shooting south along the eastern edge of the Rockies into Texas and the Gulf of Mexico, and then returning north along the eastern seaboard. The West, which remained below the jet stream, stagnated with hot, dry air, while the Midwest was left unguarded from Arctic blasts.

Fortunately for still-drought-stricken California, it does not appear as though the jet stream will be returning to its 2014 rut in the near future, with NOAA forecasting above-average precipitation for California in the first few months of 2015. Don't celebrate too soon, however. The jet stream is just one factor in many that influence short-term climate phenomena and long-term climate trends. The U.S. Seasonal Drought Outlook from December indicates drought will likely persist or intensify in the Pacific Northwest, Great Basin, Four Corners region, and east into the lower plains. Current forecasts indicate continued above-average temperatures in the West for the first quarter of 2015.

In regards to the jet stream, this may be just the beginning of many more weird years ahead. Climate scientists are accumulating a growing body of evidence that suggests the quickly-heating Arctic is producing a wavier, stickier jet stream which encourages more persistent weather patterns. This past week, Jennifer Francis of Rutgers University and Stephen Vavrus of University of Wisconsin-Madison published their latest findings on "Arctic amplification" in Environmental Research Letters. "As the Arctic continues to warm faster than elsewhere in response to rising greenhouse-gas concentrations," explain Francis and Vavrus, "the frequency of extreme weather events caused by persistent jet-stream patterns will increase."

For the parched West, let's hope that doesn't mean 2014 on repeat.

http://www.hcn.org/articles/just-another-hot-year-for-the-record-books-not-quite

Arctic warming causing quasi-stationary blocking patterns and extreme weather becoming more common

Rise in blocking-patterns – hot or wet weather remaining stuck over regions for weeks – causing frequent heatwaves or floods

by Damian Carrington, The Guardian, August 11, 2014

A man hangs on to a trash can as rainwater gushes towards Albuquerque in New Mexico, US. Heavy rains caused flash flooding and road closures in the city earlier this month. Photograph: Roberto E. Rosales/AP

Extreme weather like the drought currently scorching the western US and the devastating floods in Pakistan in 2010 is becoming much more common, according to new scientific research.

The work shows so-called “blocking patterns,” where hot or wet weather remains stuck over a region for weeks causing heatwaves or floods, have more than doubled in summers over the last decade. The new study may also demonstrate a link between the UK’s recent flood-drenched winter and climate change.

Climate scientists in Germany noticed that since 2000 there have been an “exceptional number of summer weather extremes, some causing massive damage to society.” So they examined the huge meanders in the high-level jet stream winds that dominate the weather at mid-latitudes, by analysing 35 years of wind data amassed from satellites, ships, weather stations and meteorological balloons. They found that blocking patterns, which occur when these meanders slow down, have happened far more frequently.

“Since 2000, we have seen a cluster of these events. When these high-altitude waves become quasi-stationary, then we see more extreme weather at the surface,” said Dr Dim Coumou, at the Potsdam Institute for Climate Impact Research. “It is especially noticeable for heat extremes.” The intense heatwaves in Russia in 2010, which saw 50,000 people die and the wheat harvest hit hard, and in western Europe in 2003, which saw 30,000 deaths, were both the result of blocking patterns. The Intergovernmental Panel on Climate Change concluded in 2011 that extreme weather would become more common as global warming heats the planet, causing both heatwaves and increasingly severe rain storms.

In 2010, heatwaves caused hundreds of wildfires across Russia. Above, a man tries to stop a fire near Dolginino village. Photograph: Artyom Korotayev/AFP/Getty Images

The rise in blocking patterns correlates closely with the extra heating being delivered to the Arctic by climate change, according to the research which is published in the journal Proceedings of the National Academies of Science (PNAS). Coumou and his colleagues argue there are good physical reasons to think there is a causal link, because the jet streams are driven by the difference in temperature between the poles and the equator. As the Arctic is warming more quickly than lower latitudes, that temperature difference is declining, providing less energy for the jet stream and its meanders, which are called Rossby waves.

Prof Ted Shepherd, a climate scientist at the University of Reading, UK, but not involved in the work, said the link between blocking patterns and extreme weather was very well established. He added that the increasing frequency shown in the new work indicated climate change could bring rapid and dramatic changes to weather, on top of a gradual heating of the planet. “Circulation changes can have much more non-linear effects. They may do nothing for a while, then there might be some kind of regime change.”

Shepherd said linking the rise in blocking events to Arctic warming remained “a bit speculative” at this stage, in particular because the difference between temperatures at the poles and equator is most pronounced in winter, not summer. But he noted that the succession of storms that caused England’s wettest winter in 250 years was a “very good example” of blocking patterns causing extreme weather during the coldest season. 

“The jet stream was stuck in one position for a long period, so a whole series of storms passed over England,” he said.

Flooding in Northmoor Green (Moorland) in Somerset, UK, in February this year. Photograph: David Levene for The Guardian

Coumou acknowledges his study shows a correlation – not causation – between more frequent summer blocking patterns and Arctic warming. “To show causality, computer modelling studies are needed, but it is questionable how well current climate models can capture these effects,” he said.

Prof Tim Palmer, at the University of Oxford, wrote in a PNAS article in 2013 that understanding changes to blocking patterns may well be the key to understanding changes in extreme weather, and therefore to understanding the worst impacts of climate change on society. But he said climate models might have to run down to scales of 1 km to do so. “Currently, national climate institutes do not have the high-performance computing capability to simulate climate with 20-km resolution, let alone 1 km,” he wrote. “[I] look forward to the day when governments make the same investment in climate prediction as they have made in finding the Higgs boson.”

http://www.theguardian.com/environment/2014/aug/11/extreme-weather-common-blocking-patterns

Weather weirding: Is global warming causing extreme weather via jet stream waves?

 A new study investigates how changes to atmospheric winds are making weather more extreme

by John Abraham, "Climate Consensus - The 97%," The Guardian, July 17, 2014

The jet stream that circles Earth's north pole travels west to east. But when the jet stream interacts with a Rossby wave, as shown here, the winds can wander far north and south, bringing frigid air to normally mild southern states. Credit: NASA Goddard Space Flight Center

As I sit here in a northern part of the United States (Minnesota), a rare summer arctic blast barrels down from Canada on what otherwise is one of the warmest days of the year. Global warming? I could use some global warming today, people are saying.

Not only is this a teachable moment, but it coincides with a major new study on climate connections. First, let’s see the current jet stream. It is wildly undulating, first swinging up into northern Canada before curving back and plunging into the central United States. 

Typically, the jet stream represents a separation between cold arctic air and warmer southern air. If you are north of the jet stream, temperatures are cold whereas south of the jet stream it's more likely to be warm.

Image obtained using Climate Reanalyzer (http://cci-reanalyzer.org), Climate Change Institute, University of Maine, USA.

With this in mind, and the jet stream shown, you can almost predict the temperature pattern in the next image. The match is incredible and it is clear that my Minnesota cold-blast is more than balanced out by near 90°F temperatures in northern Canada. With all of this, I want to talk about a new study that looks at these fluctuations on a longer term.

Image obtained using Climate Reanalyzer (http://cci-reanalyzer.org), Climate Change Institute, University of Maine, USA.

Very recently, a paper Amplified mid-latitude planetary waves favor particular regional weather extremes was published in the journal Nature Climate Change. The authors, James Screen and Ian Simmonds, investigated the role that changes to upper level winds in the atmosphere have on the occurrence of extreme weather. What they found was very interesting.

People who follow this site and the climate literature no doubt are aware that a hotly debated topic has arisen in recent years. I have written about studies that have linked loss of Arctic ice and warming of the Arctic region to more severe undulations in the jet stream. That research is still in its infancy and consequently, very exciting. While the idea that global warming increases jet stream undulations have been challenged by others, it is clear that some recent observations support the hypothesis.

The latest study is related to this topic but still unique. The authors don’t ask the question “are humans changing the jet stream patterns?” Instead, they ask, “how do undulations in the jet stream affect weather?” To be fully accurate, the study isn’t just about jet streams, it really deals with mid-latitude planetary waves but for this article, I will use the term “jet stream” as a surrogate for simplicity.

The authors went back into our weather records (1979–2012) and found the 40 months with the most extreme weather (most extreme precipitation and most extreme temperatures). They then evaluated how “wavy” the jet stream was during those extreme months. They found that:

months of extreme weather over mid-latitudes are commonly accompanied by significantly amplified quasi-stationary mid-tropospheric planetary waves. Conversely, months of near average weather over mid-latitudes are often accompanied by significantly attenuated waves.

In common parlance, this means that when the jet stream undulates and travels very slowly, we see more extreme weather. Conversely, when the jet stream travels in a straighter path, the weather is less extreme.

This association itself is not new but it brings the connection of large-scale climatic variations and our local weather to the fore of attention. Perhaps more important, however, are the follow-on observations from the authors. In particular, they report:

Depending on geographical region, certain types of extreme weather (for example hot, cold, wet, dry) are more strongly related to wave amplitude changes than others. The findings suggest that amplification of quasi-stationary waves preferentially increases the probabilities of heat waves in western North America and Central Asia, cold outbreaks in eastern North America, droughts in central North America, Europe, and central Asia, and wet spells in western Asia.

Dr. James Screen, a Research Fellow at the University of Exeter and lead author of the study told me, 

The impacts of large and slow moving atmospheric waves are different in different places. In some places amplified waves increase the chance of unusually hot conditions, and in others the risk of cold, wet or dry conditions.

To my knowledge, previous investigations have not elucidated the geographical locations of extreme weather nor have they identified the location-dependent weather extremes. It will be interesting to watch present and future weather to determine if these relationships between waves and weather continue. 

A note of caution however, we must be mindful that there are other weather factors at play which are superimposed on those discussed in the article. For instance, changes from El Niño to La Niña in the Pacific and long-term variability of sea surface temperatures in the Atlantic are also known to impact our weather extremes. Separating these multiple factors will take skill.

http://www.theguardian.com/environment/climate-consensus-97-per-cent/2014/jul/17/global-warming-causing-extreme-weather-jet-stream-waves