Response of the wintertime Northern Hemisphere atmospheric circulation to current and projected Arctic sea ice decline: a numerical study with CAM5
Yannick Peings* and Gudrun Magnusdottir (Department of Earth System Science, University of California, Irvine, CA, U.S.A.)
Abstract
The wintertime Northern
Hemisphere (NH) atmospheric circulation response to current (2007-2012)
and projected (2080-2099) Arctic sea ice decline is examined with the
latest version of the Community Atmospheric Model (CAM5). The numerical
experiments suggest that the current sea ice conditions force a remote
atmospheric response in late winter that favors cold land surface
temperatures over mid-latitudes, as has been observed in recent years.
Anomalous Rossby waves forced by the sea ice anomalies penetrate into
the stratosphere in February and weaken the stratospheric polar vortex,
resulting in negative anomalies of the Northern Annular Mode (NAM) that
propagate downwards during the following weeks, especially over the
North Pacific. The seasonality of the response is attributed to the
timing of the phasing between the forced and climatological waves. When
sea ice concentration taken from projections of conditions at the end of
the 21st century is prescribed to the model, negative anomalies of the
NAM are visible in the troposphere, both in early and late winter. This
response is mainly driven by the large warming of the lower troposphere
over the Arctic, as little impact is found in the stratosphere in this
experiment. As a result of the thermal expansion of the polar
troposphere, the westerly flow is decelerated and a weak but
statistically significant increase of the mid-latitude meanders is
identified. However, the thermodynamical response extends beyond the
Arctic and offsets the dynamical effect, such that the stronger sea ice
forcing has limited impact on the intensity of cold extremes over
mid-latitudes.
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