Paul Kushner
University of Toronto

"Probing the drivers of the Northern Hemispheric response to Arctic sea ice loss"

Abstract: In several modelling studies, consistent coupled atmosphere-ocean general circulation model (AOGCM) responses to induced Arctic sea ice loss have begun to emerge. Robust features to Arctic sea ice loss include tropical and extratropical warming in addition to warming throughout the Arctic troposphere; and an intensification of the wintertime Aleutian Low with an expansion of the Siberian High. If the effects of tropical adjustment (Deser et al.’s so-caled ‘mini global warming’) are accounted for, the isolated circulation response to sea ice loss becomes even clearer.

We suggest that midlatitude ocean warming, that is itself associated with sea ice loss, can account for several of the more robust aspects of the extratropical atmospheric response. This process is probed by extracting the midlatitude sea surface temperature (SST) response to sea ice loss from AOGCM simulations and imposing it as a separate boundary forcing in an atmospheric general circulation model (AGCM), in addition to the loss of sea ice itself. Sea ice loss in isolation drives warming that is confined to the Arctic lower troposphere and only a weak atmospheric circulation response. When the extratropical SST response caused by sea ice loss is also included in the forcing, the warming extends into the Arctic mid-troposphere during winter and the circulation response strengthens in a manner more consistent with AOGCM experiments. This result is robust to different estimation methods for the SST response to sea ice loss.

These results help explain why AGCM experiments that are driven by sea ice loss, even if they include local SST warming in the sea ice loss regions, underestimate Arctic free tropospheric warming seen in coupled atmosphere-ocean GCM simulations. Important questions remain about the dynamical drivers of the extratropical SST warming used as a forcing here, which point to the need for careful experimental design in future model studies.

Biography: Paul J. Kushner has been at the Department of Physics at the University of Toronto since 2004, where he served as Associate Chair for Undergraduate Studies from 2012-2015. He is the principal investigator of the Canadian Sea Ice and Snow Evolution Network (CanSISE, www.cansise.ca), a research network studying snow, sea ice and related climate processes in the Arctic and the Western Cordillera region of Canada. He is a member of the Scientific Steering Committee for the Community Earth System Model of the National Centre for Atmospheric Research, and has served as an editor of the Journal of Climate of the American Meteorological Society. Before joining the faculty at the University of Toronto, he was a research scientist in the U.S. National Oceanographic and Atmospheric Administration in Princeton NJ, and a lecturer in the Dept. of Geosciences at Princeton University.

Kushner's research group uses computer models and techniques of theoretical physics to learn about the atmosphere's circulation and how it will respond to climate change. The group's current research projects focus on climate variability from interannual to multidecadal scales, on what controls the jet streams, on Arctic and high latitude processes (including how snowfall and atmospheric circulation are linked together), and on how the stratosphere (the atmosphere above 10 km) is linked to the troposphere (the atmosphere below 10 km).