Articles | Volume 9, issue 2
Earth Syst. Dynam., 9, 359–382, 2018

Special issue: The Earth system at a global warming of 1.5°C and 2.0°C

Earth Syst. Dynam., 9, 359–382, 2018

Research article 16 Apr 2018

Research article | 16 Apr 2018

Midlatitude atmospheric circulation responses under 1.5 and 2.0 °C warming and implications for regional impacts

Camille Li1,2, Clio Michel1,2, Lise Seland Graff3, Ingo Bethke4,2, Giuseppe Zappa5, Thomas J. Bracegirdle6, Erich Fischer7, Ben J. Harvey5, Trond Iversen3, Martin P. King4,2, Harinarayan Krishnan8, Ludwig Lierhammer9, Daniel Mitchell10, John Scinocca11, Hideo Shiogama12, Dáithí A. Stone8,13, and Justin J. Wettstein14,1,2 Camille Li et al.
  • 1Geophysical Institute, University of Bergen, Bergen, Norway
  • 2Bjerknes Centre for Climate Research, Bergen, Norway
  • 3Norwegian Meteorological Institute, Oslo, Norway
  • 4Uni Climate, Uni Research, Bergen, Norway
  • 5Department of Meteorology, University of Reading, Reading, UK
  • 6British Antarctic Survey, Cambridge, UK
  • 7Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland
  • 8Lawrence Berkeley National Laboratory, Berkeley, CA, USA
  • 9German Climate Computing Center (DKRZ), Hamburg, Germany
  • 10School of Geographical Sciences, University of Bristol, Bristol, UK
  • 11Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Victoria, Canada
  • 12National Institute for Environmental Studies, Tsukuba, Japan
  • 13Global Climate Adaptation Partnership, Oxford, UK
  • 14College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, USA

Abstract. This study investigates the global response of the midlatitude atmospheric circulation to 1.5 and 2.0 °C of warming using the HAPPI (Half a degree Additional warming, Prognosis and Projected Impacts) ensemble, with a focus on the winter season. Characterising and understanding this response is critical for accurately assessing the near-term regional impacts of climate change and the benefits of limiting warming to 1.5 °C above pre-industrial levels, as advocated by the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC). The HAPPI experimental design allows an assessment of uncertainty in the circulation response due to model dependence and internal variability. Internal variability is found to dominate the multi-model mean response of the jet streams, storm tracks, and stationary waves across most of the midlatitudes; larger signals in these features are mostly consistent with those seen in more strongly forced warming scenarios. Signals that emerge in the 1.5 °C experiment are a weakening of storm activity over North America, an inland shift of the North American stationary ridge, an equatorward shift of the North Pacific jet exit, and an equatorward intensification of the South Pacific jet. Signals that emerge under an additional 0.5 °C of warming include a poleward shift of the North Atlantic jet exit, an eastward extension of the North Atlantic storm track, and an intensification on the flanks of the Southern Hemisphere storm track. Case studies explore the implications of these circulation responses for precipitation impacts in the Mediterranean, in western Europe, and on the North American west coast, paying particular attention to possible outcomes at the tails of the response distributions. For example, the projected weakening of the Mediterranean storm track emerges in the 2 °C warmer world, with exceptionally dry decades becoming 5 times more likely.

Short summary
This study investigates the midlatitude atmospheric circulation response to 1.5°C and 2.0°C of warming using modelling experiments run for the HAPPI project (Half a degree Additional warming, Prognosis & Projected Impacts). While the chaotic nature of the atmospheric flow dominates in these low-end warming scenarios, some local changes emerge. Case studies explore precipitation impacts both for regions that dry (Mediterranean) and regions that get wetter (Europe, North American west coast).
Final-revised paper