The Atmospheric Response to Realistic Reduced Summer Arctic Sea Ice Anomalies - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 105

above the ice edge. Glowienka-Hense and Hense [1992]
forced a GCM with a polynya in the Kara Sea. Their ice
anomaly was in the pack ice far from the ice edge and their
response was weak local heating with a general weakening
of the Atlantic storm track, very similar to our Sum95e re-
sponse. They argue that open water in the ice pack yields a
different response than at the ice edge.
Perhaps, a parallel can be drawn from the better under-
stood topic of the atmospheric response to midlatitude SSTs,
where it has been shown that the atmospheric response is
highly sensitive to the location of the SST forcing with re-
spect to the climatological flow [see Kushnir et al. , 2002,
and references therein]. A conundrum in our results is that
the partial ice anomaly experiments (Sum95ke, Sum95le,
and Sum95be) all yield very similar patterns to each other
and the full ice anomaly. This finding would be consistent
with the response projecting on a key mode of natural vari-
ability. So, having examined the first four EOF patterns, it is
unclear at this point whether a less dominant mode of vari-
ability is being excited by the sea ice anomalies.
The atmospheric circulation response to extreme sea ice
anomalies is explored in the context of how they may feed
back onto the sea ice. A strong negative feedback was sug-
gested in the winter sea ice forcing GCM studies [ Alexander
et al. , 2004; Deser et al. , 2004] where the atmospheric re-
sponse was of the opposite sign to the circulation that ini-
tially forced the sea ice anomalies. The exchanges of latent
heat between the Arctic north of 70°N and the midlatitudes
are largest during August as shown in a study of the ob-
served energy budget of the Arctic [see Serreze et al. , 2007,
Figure 6]. Increased moisture in the Arctic has been shown
to enhance downward longwave fluxes and possibly impact
the sea ice [ Francis and Hunter , 2006]. Figure 2 presents the
ensemble averaged vertical profiles of meridional moisture
transport in Sum95e (grey line) and Cntle (dark line) at 70°N
averaged over all longitudes (plain lines) and in the Pacific
sector (lines with dots) for 160°-200°E. The global average
moisture transports into the Arctic cap do not differ much
between Cntle and Sum95e. However, in the Pacific sector
the poleward moisture transport is enhanced notably in the
lower 500 hPa. This increase of moisture would trap more
longwave radiation and would work to delay ice formation,
suggesting a positive feedback.
Observed atmospheric circulations present during reduced
Arctic sea ice summers resemble the model response found
in our study. During August 1995 the observed SLP field
displays a negative anomaly over the Arctic and an anoma-
lous high over the North Pacific (Plate 10a), which compares
favorably with the model response to reduced sea ice. Plate
10b presents a 7-year composite of August 500-hPa anoma-
lies based on summers with anomalously low sea ice in the

Kara Sea. The anomalous high in the North Pacific is strik-
ingly similar to the model response at 500 hPa (Plate 3b).
This pair of panels was chosen to illustrate that the model
response compares well with observations during 1995 as
well as in a more robust measure based on composites. The
similarity between the observations and the model results
suggests that realistic Arctic sea ice decreases may force cir-
culation changes in the North Pacific and warrants further
examination.

Acknowledgments. This work benefited from discussions with H.
Nakamura, W. Robinson, S. Peng, R. Grotjahn, N. mölders, and I.
Polyakov. S. Bourne is thanked for a careful reading of the manu-
script. We deeply appreciate the through critiques received from an
anonymous reviewer and E. DeWeaver that improved this paper.
This research was supported by a grant from the NOAA's Arctic
Research Office issued through the International Arctic Research
Center (IARC), the Frontier Research System for Global change
through IARC, and by the Geophysical Institute. Support was also
provided by the National Science Foundation through grant ARC-
0327664. We also thank Steve Worley at NCAR for providing
the HadISST data set for N. Raynor of the Hadley Centre. This
work was supported in part by a grant of HPC resources from the
Arctic Region Supercomputing Center at the University of Alaska
Fairbanks as part of the Department of Defense High Performance
Computing modernization Program. We thank G. Robinson, C.
Swingley, and W. Chapman for their assistance with various com-
puter issues. Plots have been prepared using the open source soft-
ware packages NCL (www.ncl.ucar.edu) and GrADS (www.iges.
org/grads/).

REFERENCES

Alexander, M. A., U. S. Bhatt, J. Walsh, M. Timlin, and J.
Miller (2004), The atmospheric response to realistic Arctic
sea ice anomalies in an AGCM during winter, J . Clim. , 17 ,
890-905.
Bitz, C. m., J. C. Fyfe, and G. m. Flato (2002), Sea ice response to
wind forcing from AMIP models, J. Clim. , 15 , 522-536.
Briegleb, B. P., and D. H. Bromwich (1998a), Polar radiation budg-
ets of the NCAR CCm3, J . Clim. , 11 , 1246-1269.
Briegleb, B. P., and D. H. Bromwich (1998b), Polar climate simu-
lation of the NCAR CCm3, J . Clim. , 11 , 1270-1286.
Comiso, J. C., C. L. Parkinson, R. Gersten, and L. Stock (2008),
Accelerated decline in the Arctic sea ice cover, Geophys . Res .
Lett., 35 , L01703, doi:10.1029/2007GL031972.
Deser, C., and H. Teng (2008), Recent trends in Arctic sea ice and
the evolving role of atmospheric circulation forcing, 1979-2007,
this volume.
Deser, C., J. E. Walsh, and M. Timlin (2000), Arctic sea ice vari-
ability in the context of recent atmospheric circulation trends, J.
Clim. , 13 , 617-633.
Deser, C., G. Magnusdottir, R. Saravanan, and A. Phillips (2004),
The effects of North Atlantic SST and sea ice anomalies on the