Monday, March 27, 2017

Weather Extremes:  Humans Likely Influence Giant Airstreams

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. (Credit: Michael Mann / Penn State) Click to Enlarge.
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 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."

Read more at Weather Extremes:  Humans Likely Influence Giant Airstreams

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