A new study conducted by researchers of the Potsdam Institute for
Climate Impact Research (PIK), suggests that man-made climate change repeatedly disturbs
the patterns of atmospheric flow around the globe's Northern hemisphere through a subtle resonance mechanism.
The world has suffered from severe regional weather extremes in recent years, such as the heat wave in the
United States in 2011 or the one in Russia 2010 coinciding with the unprecedented Pakistan flood.
“An important part of the global air motion in the mid-latitudes of the Earth normally takes the form of waves
wandering around the planet, oscillating between the tropical and the Arctic regions.
There are a variety of atmospheric waves, but many require radar to detect them. This NASA satellite image
however shows more obvious gravity waves peaked with clouds off the coast of Australia. Credits: NASA
So when they swing up, these waves suck warm air from the tropics to Europe, Russia, or the US, and when they swing down,
they do the same thing with cold air from the Arctic,” explains lead author Vladimir Petoukhov.
“What we found is that during several recent extreme weather events these planetary waves almost freeze in their
tracks for weeks. So instead of bringing in cool air after having brought warm air in before, the heat just stays.
In fact, we observe a strong amplification of the usually weak, slowly moving component of these waves,” says
Petoukhov. Time is critical here: two or three days of 30 degrees Celsius are no problem, but twenty or more
days lead to extreme heat stress.
Since many ecosystems and cities are not adapted to this, prolonged hot periods
can result in a high death toll, forest fires, and dramatic harvest losses.
Climate change caused by greenhouse-gas emissions from fossil-fuel burning does not mean uniform global
warming – in the Arctic, the relative increase of temperatures, amplified by the loss of snow and ice, is
higher than on average.
This in turn reduces the temperature difference between the Arctic and, for example,
Europe, yet temperature differences are a main driver of air flow. Additionally, continents generally warm and
cool more readily than the oceans.
“These two factors are crucial for the mechanism we detected,” says Petoukhov.
“They result in an unnatural pattern of the mid-latitude air flow, so that for extended periods the slow synoptic
waves get trapped.”
Click on image to enlarge
Meridional windfield over four different timespans. Credits: http://www.pik-potsdam.de
The authors of the study developed equations that describe the wave motions in the extra-tropical atmosphere and
show under what conditions those waves can grind to a halt and get amplified. They tested their assumptions using
standard daily weather data from the US National Centers for Environmental Prediction (NCEP).
During recent periods in which several major weather extremes occurred, the trapping and strong amplification of
particular waves – like “wave seven” (which has seven troughs and crests spanning the globe) – was indeed observed.
The data show an increase in the occurrence of these specific atmospheric patterns, which is statistically
significant at the 90 percent confidence level.
“Our dynamical analysis helps to explain the increasing number of novel weather extremes. It complements previous
research that already linked such phenomena to climate change, but did not yet identify a mechanism behind it,”
says Hans Joachim Schellnhuber, director of PIK and co-author of the study.
“This is quite a breakthrough, even though things are not at all simple – the suggested physical process
increases the probability of weather extremes, but additional factors certainly play a role as well, including
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