The influence of Arctic amplification on mid-latitude summer circulation in a warming climate. This diagram shows a schematic representation of proposed dynamical mechanisms in summer. a weakening of storm tracks, b latitudinal-shift in jet positions, and c amplification of quasi-stationary waves. Graphic: Coumou, et al., 2018 / Nature Communications

By Jonathan Watts
20 August 2018

(The Guardian) – Summer weather patterns are increasingly likely to stall in Europe, North America, and parts of Asia, according to a new climate study that explains why Arctic warming is making heatwaves elsewhere more persistent and dangerous.

Rising temperatures in the Arctic have slowed the circulation of the jet stream and other giant planetary winds, says the paper, which means high and low pressure fronts are getting stuck and weather is less able to moderate itself.

The authors of the research, published in Nature Communications on Monday, warn this could lead to “very extreme extremes”, which occur when abnormally high temperatures linger for an unusually prolonged period, turning sunny days into heat waves, tinder-dry conditions into wildfires, and rains into floods.

“This summer was where we saw a very strong intensity of heatwaves. It’ll continue and that’s very worrying, especially in the mid-latitudes: the EU, US, Russia, and China,” said one of the coauthors, Dim Coumou from the Potsdam Institute for Climate Impact Research. “Short-term heatwaves are quite pleasant, but longer term they will have an impact on society. It’ll have an affect on agricultural production. Harvests are already down this year for many products. Heatwaves can also have a devastating impact on human health.”

Circulation stalling has long been a concern of climate scientists, though most previous studies have looked at winter patterns. The new paper reviews research on summer trends, where it says there is mounting evidence of planetary wind systems – both low-level storm tracks and higher waves in the troposphere – losing their ability to shift the weather. […]

As a result, there is less relief in the form of mild and wet air from the sea when temperatures accumulate on land, and less relief from the land when storms build up in the ocean. Last year, Hurricane Harvey had a devastating impact on Texas because it was parked an unusually long time on the coast, where it kept drawing up moisture from the sea and dumping it in the form of the greatest deluge ever recorded in the US. Scientists had previously noted that hurricanes are slowing and bringing more rain. […]

“What happens in the Arctic doesn’t stay in the Arctic. By upsetting the energy balance of the planet we are changing the temperature gradient between the equator and the pole. This in turn sets in motion major reorganisations of the flow patterns of the atmosphere and ocean,” said Chris Rapley, professor of climate science at University College London. “The consequences are emerging and they are disruptive, and likely to become even more profoundly so. We are on a journey and the destination doesn’t look good.” [more]

Summer weather is getting 'stuck' due to Arctic warming

Observed and projected changes in the mid-latitude Northern Hemisphere summer storm tracks and westerlies. The percentage change in summer storm tracks (vertical axis) and westerlies (horizontal axis) in future (2081–2100, under scenario RCP8.5) relative to 1981–2000 for individual CMIP5 climate models is shown, and their linear fit (solid black line). Observed changes based on ERA-Interim data are given for the 1979–2013 period. Graphic: Coumou, et al., 2018 / Nature Communications

20 August 2018 (PIK) – Be it heavy downpours or super-hot spells, summer weather becomes more persistent in North America, Europe and parts of Asia. When those conditions stall for several days or weeks, they can turn into extremes: heatwaves resulting in droughts, health risks and wildfires; or relentless rainfall resulting in floods. A team of scientists now presents the first comprehensive review of research on summer weather stalling focusing on the influence of the disproportionally strong warming of the Arctic as caused by greenhouse-gas emissions from burning fossil fuels. Evidence is mounting, they show, that we likely meddle with circulation patterns high up in the sky. These are affecting, in turn, regional and local weather patterns – with sometimes disastrous effects on the ground. This has been the case with the 2016 wildfire in Canada, another team of scientists show in a second study.

“Giant airstreams encircle our globe in the upper troposphere – we call them planetary waves,” explains Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research (PIK) and co-author of the second paper. “Now evidence is mounting that humanity is messing with these enormous winds. Fueled by human-made greenhouse-gas emissions, global warming is probably distorting the natural patterns.” Usually the waves, conveying chains of high- and low-pressure domains, travel eastwards between the equator and the North Pole. “Yet when they get trapped due to a subtle resonance mechanism,” says Schellnhuber, “they slow down so the weather in a given region gets stuck. Rains can grow into floods, sunny days into heat waves, and tinder-dry conditions into wildfires.”

Investigating the Arctic Factor and connecting the dots

“While it might not sound so bad to have more prolonged sunny episodes in summer, this is in fact a major climate risk,” says Dim Coumou from the Potsdam Institute for Climate Impact Research (PIK) and Vrije Universiteit Amsterdam, lead-author of the review paper and co-author of the wildfire case study. “We have rising temperatures due to human-caused global warming which intensifies heat waves and heavy rainfall, and on top of that we could get dynamical changes that make weather extremes even stronger – this is quite worrying.” This summer is an impressive example of how stalling weather can impact societies: persistent hot and dry conditions in Western Europe, Russia and parts of the US threaten cereal yields in these breadbaskets.

Tons of studies have appeared on this topic in recent years, sometimes with seemingly conflicting results. For the paper now published in Nature Communications, an international team of scientists set out to review the existing research and tried to connect the dots, with a focus on the Arctic factor. Under global warming, the Arctic warms more than the rest of the Northern hemisphere. This reduces the temperature difference between the North Pole and the equator, yet this very difference is a main driving force for the airstreams. “There are many studies now, and they point to a number of factors that could contribute to increased airstream stalling in the mid-latitudes – besides Arctic warming, there’s also the possibility of climate-change-induced shifting of the storm tracks, as well as changes in the tropical monsoons,” says Simon Wang from Utah State University in the US, a co-author of the review paper.

“Under global warming, the Indian summer monsoon rainfall will likely intensify and this will also influence the global airstreams and might ultimately contribute to more stalling weather patterns. All of these mechanisms do not work in isolation but interact,” says Wang. “There is strong evidence that winds associated with summer weather systems are weakening and this can interact with so-called amplified quasi-stationary waves. These combined effects point towards more persistent weather patterns, and hence more extreme weather.”

The case of the Canadian wildfire disaster

The wildfire in Canada’s Alberta region in 2016 is one stark example for the potentially disastrous impact of planetary-waves slow-down and the resulting summer-weather stalling. In a study now published in Scientific Reports, the other research team shows that indeed the blaze has been preceded by the trapping of a specific kind of airstreams in the region. In combination with a very strong El-Nino event this favored unusually dry and high-temperature conditions on the ground, entailing an increased fire hazard here. It took two months before the officials eventually could declare the fire to be under control. This was the costliest disaster in Canadian history with total damages reaching 4.7 billion Canadian Dollars.

“Clearly, the planetary wave pattern wasn’t the only cause for the fire – yet it was an additional important factor triggering a deplorable disaster,” says Vladimir Petoukhov from PIK, lead-author of the case study. “In fact, our analysis reveals that beyond that single event, actually from the 1980s on, planetary waves were a significant factor for wildfire risks in the region. Since it is possible to detect the wave patterns with a relatively long lead-time of ten days, we hope that our findings can help forest managers and fire forecasters in the future.”

A phenomenon that sounds funny but isn’t: “extreme extremes”

“Computer simulations generally support the observations and our theoretical understanding of the processes, so this seems pretty robust,” concludes Coumou. “However, the observed changes are typically more pronounced than those seen in climate models.” So either the simulations are too conservative, or the observed changes are strongly influenced by natural variability. “Our review aims at identifying knowledge gaps and ways forward for future research,” says Coumou. “So there’s still a lot to do, including machine learning and the use of big data. While we do not have certainty, all in all the state of research indicates that changes in airstreams can, together with other factors, lead to a phenomenon that sounds funny but isn’t: extreme extremes.”

Nice sunny days can grow into heat waves – and wildfires: summer weather is stalling

ABSTRACT: Accelerated warming in the Arctic, as compared to the rest of the globe, might have profound impacts on mid-latitude weather. Most studies analyzing Arctic links to mid-latitude weather focused on winter, yet recent summers have seen strong reductions in sea-ice extent and snow cover, a weakened equator-to-pole thermal gradient and associated weakening of the mid-latitude circulation. We review the scientific evidence behind three leading hypotheses on the influence of Arctic changes on mid-latitude summer weather: Weakened storm tracks, shifted jet streams, and amplified quasi-stationary waves. We show that interactions between Arctic teleconnections and other remote and regional feedback processes could lead to more persistent hot-dry extremes in the mid-latitudes. The exact nature of these non-linear interactions is not well quantified but they provide potential high-impact risks for society.

The influence of Arctic amplification on mid-latitude summer circulation



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