Stability of atmospheric circulation patterns and its relation to extreme events

• Applicant: Dr. Dim Coumou
• Subject Area: Atmospheric Science
• Term: from 2012 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 225833281

• Final Report Year: 2016

Final Report Abstract

Recent decades show a seemingly accumulation of extreme weather events. The question how climate change affects the frequency and intensity of these events is of crucial importance and has become the topic of an emerging number of scientific publications. Whereas thermodynamic processes are relatively well understood, the dynamical drivers are still associated with large uncertainties. We have proposed and tested a new theory based on equations from fluid dynamics which links extreme weather events to changes in jet circulation as indicated by amplified quasistationary waves. We have shown that the jet substantially weakened in summer associated with a statistically significant increase in the frequency of resonance events since 2000. During these events large meanders form in the jet which can remain in their geographical position for weeks, favoring persistent weather conditions and thus prolonged weather extremes. Changes in the jet are strongly coupled to changes in extratropical storm track activity. Consistently, we find a pronounced weakening of observed summertime storm track activity in the mid-latitudes. In summer, extratropical storms transport moist and cool air from the oceans to the continents bringing relief after periods of oppressive heat. The observed lack of storm activity thus likely favored the occurrence of prolonged heat waves in the US, Europe, Russia st and China. State-of-the-art climate models project a further weakening over the 21 century under a high emission scenario implying an increased risk of summer heat waves in the future. This information is thus of direct interest for governments and decision-makers involved in risk management. We argue that rapid warming in the Arctic might drive the observed changes in circulation. Temperatures in the Arctic rise approximately twice as fast as for the global mean, an effect known as Arctic Amplification. This reduces the temperature difference between the cold polar region and the warm equator. However, it is this temperature difference which drives the motion of air in the troposphere and ultimately fuels the jet and the storm systems in the mid-latitudes. The question of causality in this link is still intensively debated and thus our findings provide valuable information for the ongoing discussion. The Washington Post https://www.washingtonpost.com/news/energy-environment/wp/2015/03/12/first-it-was-crazy-winters-now-global-warming-may-also-be-driving-crazy-summers/ The Guardian http://www.theguardian.com/environment/2014/aug/11/extreme-weather-common-blocking-patterns Science http://www.sciencemag.org/news/2015/03/warming-arctic-may-be-causing-heat-waves-elsewhere-world Several radio interviews, e.g. for NDR Info and Radio 1 http://www.radioeins.de/programm/sendungen/die_profis/archivierte_sendungen/beitraege/sturmaktivitaet-nimmt-ab.html

Publications

• Future changes in extratropical storm tracks and baroclinicity under climate change, Environ. Res. Lett., 9, 2014
  J. Lehmann, D. Coumou, K. Frieler, A. Eliseev and A. Levermann
  
• Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer, Proc. Nat. Ac. Sci., 111(34), p. 12331-12336, 2014
  D. Coumou, V. Petoukhov, S. Rahmstorf, S. Petri and H-J. Schellnhuber
  
• The Relationship between recent Arctic Amplification and Extreme Mid-Latitude Weather. Nature Geoscience, 7, p.627–637, 2014
  J. Cohen, J. Screen, J.C. Furtado, M. Barlow, D. Whittleston, D. Coumou, J. Jones, J. Francis, K. Dethloff, D. Entekhabi and J. Overland
  
• The influence of mid-latitude storm tracks on hot, cold, dry and wet extremes, Scientific Reports 5, 17491, 2015
  J. Lehmann and D. Coumou
  
• The Weakening Summer Circulation in the Northern Hemisphere Mid-latitudes, Science, 348 (6232), p. 324-327, 2015
  D. Coumou, J. Lehmann, J. Beckmann
  
• Record Balkan floods of 2014 linked to planetary wave resonance, Science Advances 15 Apr 2016: Vol. 2, no. 4, e1501428
  L. Stadtherr, D. Coumou, V. Petoukhov, S. Petri, S. Rahmstorf
  
• Weakened Flow, Persistent Circulation, and Prolonged Weather Extremes in Boreal Summer., Chapter 4 in: Climate Extremes: Patterns and Mechanisms, edited by S.-Y. Simon Wang, Jin-Ho Yoon, Christopher C. Funk, Robert R. Gillies. Wiley, 2017. S. 61-74
  D. Coumou, K. Kornhuber, V. Petoukhov, J Lehmann