On the left is an image of the global circulation pattern on a normal day. On the right is the image of the global circulation pattern when extreme weather occurs. The pattern on the right shows extreme patterns of wind speeds going north and south, while the normal pattern on the left shows moderate speed winds in both the north and south directions. Graphic: Michael Mann / Penn State University

27 March 2017 (Potsdam Institute for Climate Impact Research) – The increase of devastating weather extremes in summer is likely linked to human-made climate change, mounting evidence shows. Giant airstreams are circling the Earth, waving up and down between the Arctic and the tropics. These planetary waves transport heat and moisture. When these planetary waves stall, droughts or floods can occur. Warming caused by greenhouse-gases from fossil fuels creates favorable conditions for such events, an international team of scientists now finds. “The unprecedented 2016 California drought, the 2011 U.S. 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 the Pennsylvania State University in the U.S., lead-author of the study now to be published in Scientific Reports. “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 favor 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.”

How sunny days can turn into a serious heat wave

“If the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, or lasting rains can lead to flooding”, explains co-author Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK) in Germany. “This occurs under specific conditions that favor what we call a quasi-resonant amplification that makes the north-south undulations of the jet stream grow very large. It also makes theses waves grind to a halt rather than moving from west to east. Identifying the human fingerprint on this process is advanced forensics.” Air movements are largely driven by temperature differences between the Equator and the Poles. Since the Arctic is more rapidly warming than other regions, this temperature difference is decreasing. Also, land masses are warming more rapidly than the oceans, especially in summer. Both changes have an impact on those global air movements. This includes the giant airstreams that are called planetary waves because they circle Earth’s Northern hemisphere in huge turns between the tropics and the Arctic. The scientists detected a specific surface temperature distribution apparent during the episodes when the planetary waves eastward movement has been stalling, as seen in satellite data.

Using temperature measurements since 1870 to confirm findings in satellite data

”Good satellite data exists only for a relatively short time – too short to robustly conclude how the stalling events have been changing over time. In contrast, high-quality temperature measurements are available since the 1870s, so we use this to reconstruct the changes over time,” says co-author Kai Kornhuber, also from PIK. “We looked into dozens of different climate models – computer simulations called CMIP5 of this past period – as well as into observation data, and it turns out that the temperature distribution favoring planetary wave airstream stalling increased in almost 70 percent of the simulations.” Interestingly, most of the effect occured in the past four decades. “The more frequent persistent and meandering Jetstream states seems to be a relatively recent phenomenon, which makes it even more relevant,” says co-author Dim Coumou from the Department of Water and Climate Risk at VU University in Amsterdam (Netherlands). “We certainly need to further investigate this – there is some good evidence, but also many open questions. In any case, such non-linear responses of the Earth system to human-made warming should be avoided. We can limit the risks associated with  increases in weather extremes if we limit greenhouse-gas emissions.” Article: Michael E. Mann, Stefan Rahmstorf, Kai Kornhuber, Byron A. Steinman, Sonya K. Miller, Dim Coumou (2017): Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events. Scientific Reports [DOI: 10.1038/srep45242] Weblink to the article once it is published: www.nature.com/articles/srep45242 Weblink to video that explains the planetary waves phenomenon: https://www.youtube.com/watch?v=MzW5Isbv2A0

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Weather extremes: Humans likely influence giant airstreams

By John Abraham
7 April 2017 (The Guardian) – It was only a few weeks ago that I wrote about changes to extreme weather in a warming world. That prior article dealt with the increase of extreme precipitation events as the Earth warms. I termed the relationship a thermodynamic one; it was driven by local thermodynamic processes. But extreme weather can also occur because of large-scale changes to the atmosphere and oceans. This issue is the topic of another just-published paper that makes a convincing case for a whole new type of influence of humans on extreme weather. In a certain sense, this study confirms what was previously reported here and here. With the march of science, the tools, methods, and evidence get better each year. Before getting into the study, a little background. The jet stream(s) are high-speed rivers of air that flow in the upper atmosphere. There’s more than one jet stream; they blow west to east and they mark the separation of zones of different temperatures. A good primer on jet streams is available here. Using measurements, the authors documented what conditions led to extreme weather patterns that persisted for extended durations. They found that many occur when the jet stream becomes stationary with the undulations stuck in place. They also saw that under certain situations, the jet stream undulations do not dissipate in time; they become trapped in a wave guide. Interestingly, this pattern of a stuck jet stream would occur when the number of undulations was between six and eight. When these circumstances all lined up, according to study co-author Stefan Rahmstorf:

the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, and lasting rains can lead to flooding

And this is also how humans come into the story. As humans emit greenhouse gases, the planet warms. We know that, we predicted it, and it is occurring. However, the warming is not uniform. The Arctic, for instance, is warming more rapidly than the rest of the planet. As a result, the temperature difference between the Arctic and the rest of the world is reducing. It is this temperature difference that maintains the jet stream patterns. [more]

New study links carbon pollution to extreme weather

ABSTRACT: Persistent episodes of extreme weather in the Northern Hemisphere summer have been shown to be associated with the presence of high-amplitude quasi-stationary atmospheric Rossby waves within a particular wavelength range (zonal wavenumber 6–8). The underlying mechanistic relationship involves the phenomenon of quasi-resonant amplification (QRA) of synoptic-scale waves with that wavenumber range becoming trapped within an effective mid-latitude atmospheric waveguide. Recent work suggests an increase in recent decades in the occurrence of QRA-favorable conditions and associated extreme weather, possibly linked to amplified Arctic warming and thus a climate change influence. Here, we isolate a specific fingerprint in the zonal mean surface temperature profile that is associated with QRA-favorable conditions. State-of-the-art (“CMIP5”) historical climate model simulations subject to anthropogenic forcing display an increase in the projection of this fingerprint that is mirrored in multiple observational surface temperature datasets. Both the models and observations suggest this signal has only recently emerged from the background noise of natural variability.

Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events